9 Topics from Chapter 9: Ocean, Cryosphere and Sea Level Change
Oceans
Andrea Richter-Sanchez
The IPCC report is a document that provides governments scientific information in order for them to use that information to develop climate change policies. The first paper was written in 1990 by the Intergovernmental Panel on Climate Change and the report has continued to come out every six to seven years since then. The following paper will provide a summary of the IPCC Chapter 9: Ocean, cryosphere, and sea level. It will assess past and projected changes by looking at models, reconstructions, and observations.
9.2.1 Ocean surface
9.2.1.1 Sea Surface Temperature (SST)
The AR5 (IPCC Fifth Assessment Report) showed that it is certain that global sea-surface temperatures (SST) have increased since the 20th century. In the Arctic the average SST increase in the last twenty years is slightly higher than the global average. This new report has improved the SST collection methods by enhancing the treatment of sea ice and by increasing their buoy-based observations. There has been a “high-confidence” positive SST trend globally since 1950. It is important to note that there is very high confidence that the Indian Ocean, western equatorial Pacific Ocean, and western boundary currents have warmed faster than the global average while the Southern Ocean, the eastern equatorial Pacific, and the North Atlantic Ocean have warmed more slowly than the others or they have slightly cooled.
The tropical oceans have been warming faster than other regions since 1950, with the fastest warming occuring in the tropical Indian and western Pacific Oceans. It is believed that this increased warming is due to local atmosphere-ocean coupling, the Indonesian throughflow, and trends in the Walker circulation. The models are “virtually certain” that SST rates will keep increasing in the 21st century and that the rate at which they increase will be determined by future emissions. These rates are projected by the CMIP6 (SSP1-2.6 and SSP5-8.5) models to be between 0.86° C to 2.89° C. An increase in sea surface temperatures can cause an increase in extreme weather events and can also cause habitat loss for marine life.
9.2.1.2 Air-sea fluxes
Air-sea fluxes are crucial because they give us the best indicator as to how oceans affect climate and how the atmosphere forces ocean variability. These fluxes help us understand the difference in heat between the atmosphere and the ocean. The amount of precipitation and evaporation determines the freshwater flux into the ocean.
The new report finds that an increase in radiative forcing is likely causing an energy increase of the upper 700 m in the ocean. Since the AR5, researchers have used new surface flux products to help improve uncertainty values which now imply that global energy imbalances often exceed the observed ocean warming. The globally integrated and large scale fluxes are more consistent with heat content and salinity change. It is difficult to look at regional trends in air flux because they tend to be underestimated or the models disagree with each other. There is a high confidence that air-sea heat flux and stress biases are reduced in coupled models with high ocean resolution over low resolution models.
9.2.1.3 Upper Ocean Stratification and Surface Mixed Layers
Upper-ocean stratification is the rate of density change from surface to deep ocean. The increase in stratification is primarily due to the increase in surface temperature. More heat in the ocean allows for more convection in the atmosphere above, with more moist air that increases condensation, and ultimately leads to diabatic heating and increased energy for storms. This explains why more stratified oceans potentially host better conditions for intense storms. The SROCC found it very likely that density stratification increased by 0.46-0.51% every decade from the parameters of 60°S and 60°N in 1970 to 2017. The projections state that shoaling of mixed layer depth will happen in our century but it will only occur if there are strong emissions and only happen in some locations. Scientists worry about the stratification because if the water is not mixed properly, then the warm water will not be able to absorb as much oxygen and will affect marine life by decreasing the nutrient levels in the water. The CMIP6 climate models project a shallowing of the mixed layer in summer and winter by 2100 due to increased radiative forcing. The shallowing will not happen in high latitude areas such as the Arctic because of the sea-ice retreat which deepens the mixed layer.
Box 9.2 Marine Heatwaves
Ocean heat has been increasing since 1970 and will continue to increase even under low emission scenarios. Models have shown that the ocean surface temperature is expected to increase by an average of 0.86 Celsius by the year 2100. When it comes to deep ocean warming, the long term models imply that deep ocean warming will be irreversible for centuries to come. Oceans have a greater heat capacity than land and the water is recycled on the order of thousands of years. The long and intense holding of heat in the oceans is one of the reasons behind why ocean warming will last so long. Marine heatwaves are periods of unusually high surface temperatures in the oceans and these heat waves can lead to impacts on marine ecosystems. These impacts include ocean warming, coral bleaching, loss of biodiversity, and an increase in storms due to warm waters. It is believed that the biggest change will happen in the tropical oceans and the Arctic. Another ocean warming effect is the stratification of the ocean. The models show that upper-ocean stratification will continue to increase throughout the 21st century.
9.2.2 Changes in Heat and Salinity
9.2.2.1 Ocean Heat Content and Heat Transport
Ocean warming causes coral bleaching and leads to habitat loss for marine species. The ocean has stored 91% of the total energy gained from 1971 to 2018 which is one of the reasons why the model deemed it “extremely likely” that anthropogenic forcing was the main cause of ocean heat content (OHC). Ocean warming usually occurs in the upper 700m, but the AR5 and SROCC found that the deep ocean (deeper than 2000m) has been warming since 1992. Even though generally the ocean has warmed, there are some regions that have experienced slight cooling. The SROCC estimated that the Southern Ocean took 75% of the global heat uptake from the late 1800s to 1995. They believe that this interhemispheric asymmetrical global heat uptake is due to the large concentrations of aerosols in the Northern Hemisphere compared to concentrations in the Southern Hemisphere. The Southern Ocean has had more deep ocean warming than any other ocean because of how the Antarctic Bottom Water is heating and spreading. The distribution of heat circulating in the oceans causes non-uniform ocean warming, however there is high confidence that the ocean on average is warming as CO2 emissions increase. There is clear data that proves these occurrences are attributed to the anthropogenic heat stored in our oceans.
9.2.2.2 Ocean Salinity
Ocean salinity is the amount of salt dissolved in ocean water. This measurement is important because, like temperature, it affects seawater density. The density of the water then determines aspects of the global ocean circulation. A change in salinity can also affect marine life in the ocean because it disrupts their homeostasis. Another reason why ocean salinity is important is because it has a direct correlation to the water cycle by controlling the amount of freshwater that leaves the ocean by precipitation and evaporation. This year’s report agrees with the last assessment’s result that surface salinity contrasts are increasing. The Pacific and Southern Ocean have become less saline while the Atlantic Ocean has become more saline.
Globally, the mean salinity contrast at near-surface between high- and low- salinity regions increased by 0.14 from 1950 to 2019. Surface fluxes (evaporation minus precipitation) have also been linked with “medium confidence” as to why the Atlantic is saltier than the Pacific. Ocean circulation changes are affected by salinity but can also affect salinity, otherwise known as having “simultaneity”. An example of this would be “how in the subpolar North Atlantic, increasing northward transport of ‘Atlantic waters’ entering the subpolar gyre from the South have compensated for the salinity decrease expected from increased Greenland meltwater flux since the early 1990s.” New evidence from the regional climate models confirm that in general the fresh water gets fresher and the salt water gets saltier during the 21st century. This is similar to the concept of how dry regions will get drier and wet regions will get wetter due to climate change. Climate changes can intensify the natural processes that are already occurring.
9.2.3 Regional Ocean Circulation
9.2.3.1 Atlantic Meridional Overturning Circulation
The Atlantic Meridional Overturning Circulation (AMOC) is a system of ocean currents that are driven by temperature and salinity of the water (water’s density). This system is often compared to a global conveyor belt because of its motion that carries currents. It helps keep the ocean well mixed and is one of the forces that drives our climate because of how it distributes heat and energy around the Earth. As CO2 forcing increases with climate change, the AMOC will weaken because North Atlantic surface waters are less dense and will not sink as well as they used to. A weakening in the AMOC will not change the global heat transport significantly, but can change the delivery of heat to the higher latitudes. This is another reason why we see more warming in the Arctic than Antarctica because there is more heat transport in the north as global warming intensifies.
The Atlantic Multidecadal Variability (AMV) is a North Atlantic basin-wide sea surface temperature fluctuation on multi-decadal time scales and it is driven by the AMOC. There is new evidence that shows how aerosols contribute to AMV changes. It is crucial that we do not underestimate this change because it can also lead to underestimating internal variability in subpolar SSTs that feedback on the North Atlantic.
9.2.3.5 Eastern Boundary Upwelling Systems
Eastern boundary upwelling systems (EBUS) are where trade winds draw cold and generally low pH and low oxygen waters upward. Upwelling is important for the nutrient transport cycle in the oceans because it helps the upper and lower levels of the ocean have the nutrients needed for high productivity. The SROCC found with “high confidence” that three out of the four major EBUS have had large-scale wind intensification for the last sixty years. They assess that only the California current system has experienced large scale favorable upwelling wind intensification. Coastal warming and wind intensification may cause more upwelling intensification in these local regions. While generally, changes in upwelling winds are due to the change in temperature between ocean and land, research shows that poleward expansion of Hadley cells could also be a cause. The related poleward migration of subtropical highs produce patterns of reduced upwelling at low latitudes and stronger upwelling at high latitudes because of poleward expansion of these cells. There is “medium confidence” that upwelling wind changes in EBUS will remain moderate in the 21st century (within ±10-20% from present-day values).
9.2.4 Steric and dynamic sea-level change
9.2.4.1 Global mean thermosteric sea-level change
Steric sea-level change is the variation of the ocean volume due to density changes caused by ocean salinity and ocean temperature differences. The changes in ocean heat content cause global thermosteric sea-level change. Short lived climate forcers are correlated with sea level rise because ocean heat content and ocean temperature responses last significantly longer in the ocean than the atmospheric forcing does. While these climate forcers do last a long time, they do not last as long as the sea-level rise associated with CO2 emissions. An example of this process is how 70% of thermosteric sea-level rise associated with methane forcing would last 100 years after methane emissions stopped. The IPCC model and paleo observations show a “medium confidence” that there will be a 0.113 ± 0.013 m/YJ (meters per yottajoule of ocean heat content change) or a 0.617 ± 0.071 m/oC (meters per degree rise) in sea-level change.
9.2.4.2 Ocean dynamic sea-level change
Dynamic sea-level change is the sea level deviation from the geoid and is strongly affected by internal variability. Unlike the models used for the other sections in this chapter, high-resolution models are not used in sea level projections because these models have a limited range of forcing scenarios that describe sea-level rise. Projections for regional sea-level change in CMIP6 and it’s past versions all show a large spread. The difference is these models’ outcomes are due to changes in surface fluxes and in ocean response. The spread is largest in regions with large projected variations in ensemble-mean ocean dynamic sea-level change such as the Southern Ocean dipole with sea-level rise north of the ACC and a fall to the south, Atlantic dipole with a sea-level rise north of 40oN and a fall in 20-40oN, the NW Pacific dipole, and the large sea-level rise which is the Arctic.
Many climate and ocean models agree that the AMOC weakening is associated with significant thermosteric sea-level rise along the American coast around 40oN which leads to large sea level rise in that region. Sea-level change has been one of the most felt effects of climate change throughout the world. Many areas are starting to experience flooding more often and people are using adaptation tactics to protect their homes or they are moving elsewhere. The majority of the world’s population lives on coastal areas due to the economic benefits of living near the coast. The plan is for policy makers to look at the projections these models make to assess what policy is needed to limit sea-level rise and how to protect the people who are in danger of these effects.
The oceans cover 70% of the Earth and hold 96.5% of all of Earth’s water. The work that these models do is crucial for understanding the gravity of the issues we will face. All of these sections are connected to one another and affect each other. That is why it is important to have a fundamental knowledge of these processes in order to understand the bigger picture. The next section will cover sea ice. Sea ice is decreasing in volume as emissions have increased. The melting of sea ice will affect salinity, ocean temperature, and sea-level rise. As scientists we must work together with policy makers to find feasible solutions and educate the public on what is happening.
Sections 9.3 Sea Ice and 9.4 Ice Sheets
Katie Love
Sea ice is floating, frozen ocean water and is measured as the actual area of the ocean in a region that is covered by sea ice. Sea ice grows in the winter and shrinks in the summer and accounts for 7% of the Earth’s surface at its maximum extent. Sea ice is typically 0.5 to 3 meters thick. If all the sea ice were to melt from Earth’s surface, no actual sea level rise would occur since the ice is floating on the surface, so it has already displaced the water. However, melting of sea ice has many other serious effects, such as a decreased albedo of the once sea ice covered water, which increases the absorption of heat. There are two main areas of sea ice, in the Arctic and Antarctic, which are discussed in detail below.
ARCTIC SEA ICE (9.3.1)
Arctic Sea-Ice Coverage
In the Arctic Sea, sea ice typically has significant ice coverage year round, even in the summer. However, it has been decreasing every month of the year since 1979. Losses in sea ice are highest in the Northern Hemisphere summer and early autumn due to increased temperatures and incoming solar radiation. From 2010 to 2019, two million square kilometers were lost in the months of August, September, and October. The loss was more than that from 1979 to 1988, showing an increase in sea ice loss over time. This trend of increased sea ice loss in the Arctic has been seen in the past few years and is expected to continue so long as temperatures and greenhouse gas emissions continue to rise. This past summer, Arctic Sea extent ice was 40% lower than that of 30 years ago. Figure 9.13 (below) shows historical trends of Arctic sea ice as well as projections for the future.
The seasonal ice zone is growing, which although initially sounds like there is more ice, it means quite the contrary. Since loss in the summer is greater than that in the winter, the sea ice is retreating more in the summer and then part is freezing again in the winter. The more melting that occurs in the summer, there is less ice able to grow back in the following winter. This leads to sea ice being younger and thinner. Old ice is almost completely gone from the Arctic Sea. That area where ice is only present in the winter is growing since more ice is melting each year. Areas that used to be year-round ice are now experiencing seasonal losses. This expands the area and lengthens the time of open water season in the seasonal sea-ice zone.
While there are trends of decreasing sea ice mass and a growth in the seasonal ice zone, there have been observed fluctuations in the trends of sea ice cover over time. These fluctuations are largely attributed to the changes in natural external forcings and anthropogenic forcings, as well as internal variability and internal feedback. Examples of these fluctuations include the Allerod warm period, the extensive sea ice cover during the Younger Dryas, the decrease in sea ice during the early Holocene (due to a summer insolation that was higher than that of today), and the increase in sea ice cover throughout the middle and late Holocene. These fluctuations support the idea of co-variability of sea ice cover and temperature fluctuations on the millennial scale. However, they do not disprove the fact that the changes we are witnessing today are widely accepted to be driven by anthropogenic causes.
Besides the above fluctuations on the millennial scale, recent sea ice loss is caused largely by increased greenhouse gas emissions, an external forcing. The area of sea ice is strongly correlated with global temperatures, carbon dioxide concentrations, and cumulative anthropogenic carbon dioxide emissions. It is estimated that half of summer sea ice loss is anthropogenic. The more greenhouse gasses emitted, the higher the rates of sea ice loss experienced in the Arctic are predicted to be.
In addition to these large anthropogenic drivers, there is, however, internal variability that causes fluctuations on a smaller, seasonal scale. There are atmospheric temperature fluctuations, such as cyclone activity, and internal variability across decades related to oceanic heat transport changes. These factors contribute to 30-50% of Arctic summer sea-ice loss observed since 1979. These drivers are harder to predict and control than those associated with anthropogenic emissions.
There is potential for a complete loss of Arctic summer sea ice, which is defined as a reduction of sea ice area below 1 million square kilometers in September. This is significantly more probable in climate prediction projects for a global mean warming of 2 degrees Celsius, than in scenarios with a mean warming of 1.5 degrees Celsius. In the scenario of 2 degrees Celsius of warming, some years would be completely free of Arctic summer sea ice. In scenarios of 3 degrees Celsius of warming, the Arctic would be practically sea ice free in most years, and not just in September. Not all take into account factors such as decreases in carbon dioxide emissions, the loss of winter sea ice being reversible, ice-albedo feedback, and increased emission of longwave radiation from open water, which explains part of their variability. Figure 9.14 (below) shows the sea ice area in March and September for each of the three models, all of which show some amount of decrease.
A loss of all Arctic Sea ice would have numerous consequences. Shipping routes in the region will change and open up the area to travel in ways never experienced before. There are oil reserves beneath the ice that oil companies would like to explore, much to the dismay of an array of stakeholders. Additionally, the loss of sea ice increases the albedo of the region, absorbing heat that would have been reflected. This feedback amplifies warming. This has potential effects on weather systems, reduces the pole-to-equator temperature gradient which results in warmer latitudes warming, makes weather patterns more extreme, increases coastal erosion in areas that were once buffered from waves by sea ice, and alters ecosystems and habitats. Some models predict a complete loss of sea ice so the conversation of what we will do in response is becoming ever more urgent.
Arctic Sea-Ice Volume and thickness
Along with Arctic sea ice extent loss, Arctic sea ice has been experiencing decreases in volume and thickness. There is high confidence that Arctic sea ice has gotten thinner since 1979 and decreased in volume. However, there is only low confidence for the corresponding amounts.
This is due to a lack of reliable, long term data and a substantial spread in data. Data we do have is from submarines and satellites. These pieces of technology have not been around nearly as long as the sea ice so are unable to paint the full picture of sea ice throughout history. Unlike glaciers and permafrost where you can take core samples, sea ice is constantly moving, growing, and shrinking, making collecting historical data nearly impossible, especially in the seasonal ice zone. The best estimates are that there has been a 55-65% decrease in volume from 1979 to 2010 and a 72% decrease in volume from 1979 to 2016. With better monitoring technology, we will be able to get accurate modern data, but these massive losses are significant enough to signal the alarm.
ANTARCTIC SEA ICE (9.3.2)
Antarctic sea-ice coverage
Unlike massive losses seen in the Arctic Sea, there has not been a significant trend in the annual mean of Antarctic sea ice cover from 1979 to 2019. There was a substantial decrease in sea ice extent from 1979 to 2017, but then there was an increase in sea ice extent since then. These opposing regional trends have canceled each other out, yielding no significant trend in the data. There are two likely causes for this fluctuation: 1) wind driven changes related to meridional wind trends since the advection of the sea ice is strongly correlated to winds and cyclones, and 2) increased near surface ocean stratification, which cools the surface ocean and has lead to an overall long-term surface cooling trend of the Southern Ocean since 1979. There is uncertainty regarding the precise relative contribution of each of the above drivers, but both likely contribute to the trends we see. Figure 9.15 (below) shows the historical records and projections of Antarctic sea ice. Overall, we see that Antarctic Sea ice is not melting. This is mostly due to the fact that the ocean around Antarctica is not warming very much at all.
There are a lot of unknowns in Antarctic Sea ice largely because of a lack of data. There are discrepancies between models and the observed evolution of Antarctic sea ice. This could be due to stratification, freshening of the ocean by ice shelf melt water, clouds, and wind and ocean driven processes. Additionally, the analysis and understanding of long-term evolution of Antarctic sea ice is hindered by the lack of records. Before satellites, there were not the observational records available today, and there are limited paleo records. The discrepancies and uncertainties yield limited evidence for larger scale fluctuations. For example, when looking at ice cores of the area, results indicate that sea ice in the Ross Sea is increasing and the sea ice in the Bellingshausen Sea is decreasing as part of centennial trends. These observations are consistent with modeled simulations and provide contrasting evidence for trends of sea ice melting and relate to the direct role of ozone depletion.
Antarctic sea-ice thickness
Similar to sea ice coverage, there is not enough data to estimate long term trends in Antarctic sea ice thickness. There is a low confidence in any long term trend due to the fact that some models indicate an increase and some models indicate a decrease, so the results are unclear.
9.4 Ice Sheets
Ice sheets are defined as a permanent layer of ice covering a large amount of land. There are two ice sheets in the world: Greenland and Antarctica. Both ice sheets are massive and cumulatively cover 0.35% of Earth’s surface. Unlike sea ice, ice sheets have the ability to impact sea levels because they are not floating in the water. The Greenland ice sheet has the potential to raise sea levels by 7 meters and the Antarctic ice sheet has the potential to raise sea levels by 61 meters. These two ice sheets represent the largest potential contributors of melting ice to sea level rise so it is therefore crucial to understand the evolution they are experiencing.
GREENLAND ICE SHEET (9.4.1)
Recent observed changes
The Greenland ice sheet is losing mass through changes in the mass balance. The mass balance is defined as the gains by snow on the top of the ice sheet and losses by melting and calving of the ice sheet. The Greenland ice sheet is experiencing increases in both the amount of snow and the amount of loss by melting and calving. Overall, the losses outweigh the gains though so the ice sheet is experiencing a massive decrease in mass.
The Greenland ice sheet has lost a significant amount of mass already. The IPCC report focuses on three large outlet glaciers: the Jakobshavn, Kangerlussuaq, and Helmein. These three glaciers alone account for 12% of the drainage of the ice sheet surface. From 1880 to 2021, they lost 22 Gt/yr, with most of the loss occurring in the 1900s (at a rate of 120 Gt/yr when peripheral glaciers are included). Most of this loss occurs in the summer, with unprecedented levels of melting since 1900. The rate of melting has continued to increase and from 2010 to 2019, 243 Gt of the Greenland ice sheet was lost on average each year.
The Greenland ice sheet surface elevation is lowering in all regions, widespread calving fronts are retreating, and there are no glaciers advancing. This area is struggling with warming temperatures and poses one of the greatest risks related to sea level rise. Currently, the melting of the Greenland ice sheet accounts for 25% of sea level rise. Figure 9.17 (below) shows cumulative mass change and potential contributions to sea level rise.
There are a few main drivers to the mass loss from the Greenland ice sheet: 1) Large scale variation in atmospheric circulation is thought to drive changes seen in surface mass balance. This includes changes in precipitation and melt rates. These circulations vary on daily and seasonal timescales and are associated with the Greenland Blocking Index. 2) Higher incident shortwave radiation in combination with reduced cloud cover causes higher melt rates. This basically means that more energy from the sun is reaching and warming the Earth’s surface. 3) Positive albedo feedback has contributed to recent melting. Snow has a very high albedo and reflects even more light than ice so changes in surface features and bare ice extent have led to a decreased albedo and higher melting rates. 4) A decrease in meltwater infiltration due to ice slabs have led to an increase in runoff. 5) Higher ocean temperatures have caused an increase in ice discharge, glacier retreat, rapid submarine melting (which promotes calving), and the thinning and breaking up of ice. Most of the mass loss, particularly from 2000 to 2010 was due to this last driver, the thinning and breakup of ice.
While certain regions of ice and specific glaciers may respond differently to a climate forcing, such as those listed above, based on relative geometry and bedrock topography, the Greenland ice sheet as a whole has been experiencing increased melting. It is very likely that humans have contributed to the increased surface melting and there is medium confidence that anthropogenic causes resulted in the recent mass loss.
Projections to 2100
There are many different projections as to how much increased melting of ice sheets could impact sea level rise. The IPCC does state that it is virtually certain under all emission scenarios the Greenland ice sheet will continue to lose mass through 2100 and those mass losses will increase as emissions do. The melting of the Greenland ice sheet will likely contribute 0.06 to 0.13 meters to sea level rise by 2100, although there is deep uncertainty within that range. Over 2 degrees Celsius of temperature rise would eventually melt the Greenland ice sheet.
Projections beyond 2100
Similar to projections to 2100, projections beyond 2100 differ greatly in magnitude and rate. Many models cross the threshold for irreversible loss, with some doing so in as early as 400 years. There is high confidence that such a threshold for irreversible loss exists for the Greenland ice sheet in a warmer climate, but low agreement on the nature of those thresholds and related tipping point.
IPCC Report Chapter 9, Sections: 9.4.2 – 9.5.2
Connor Lewis-Smith
Introduction:
Chapter 9 of the IPCC Report, Oceans Cryosphere and Sea Level Change, continues with section 9.4.2 on the Antarctic Ice Sheet and section 9.5.2 on Glaciers. Like preceding sections, the report frequently refers to the previous Assessment Report 5 (AR5) from 2014 along with the Special Report on the Ocean and Cryosphere in a Changing Climate (SROCC) from 2019. These reports are referenced in the summary to help contextualize the latest findings. What follows is a high-level summary of the observed changes to the Antarctic Ice Sheet and glaciers along with evaluations of modelling these parts of the cryosphere and their future projected changes.
Section 9.4.2 Antarctic Ice Sheets
The Antarctic Ice Sheet (AIS) covers the continent of Antarctica and is the single largest mass of ice on Earth taking up 2.7% of the planet’s surface area. The AIS is by far the single largest store of freshwater on the planet. If all the AIS melted, sea levels would rise by 61.1 meters. This section of the IPCC report provides information on recent observed changes, assesses modeling of the AIS and summarizes projections of changes to the AIS in the future.
9.4.2.1 Recent observed changes
Observations of the AIS have continued to benefit from advances in remote sensing techniques, especially from those using satellites. These observations provide insight into the ice sheet mass balance of the AIS, which is the net balance between inputs of mass into the ice sheet and the outputs of mass out. Ultimately, this balance can provide insight into the AIS’s impact on sea level rise as the climate warms. Agreements of estimates from a review of post-AR5 studies indicate that the AIS has lost 2670 [1800-3540] gigatons of mass from 1992 to 2020, which is the equivalent to 7.4 [5.0-9.8] millimeters of global mean sea level rise. Yet this mass loss has not been uniform across space and time.
Mass loss from the AIS has been observed since 2000 and primarily takes the form of ice discharge, which is the transfer of land-ice into the ocean. Most of the observed mass loss is linked to changes occurring at the ice shelf, which is the area where the ice sheet meets the ocean. The primary processes driving the reduction of mass are basal melt and by ice shelf disintegration preceded by strong surface melt. While there has been overall mass loss from the AIS, recent work cited in the report suggests that the net loss has not increased further since 2016 due to mass gains in Eastern Antarctica, specifically in the regions of Dronning and Maud Land. In contrast, there is very high confidence that overall mass loss has been primarily driven by the Western and Peninsular regions of the AIS.
The reason why mass loss has increased in the Western AIS is still not fully understood. One of the primary forms of mass loss is basal melt, which is the thinning of the ice shelves. This thinning has been observed at the Western AIS outlet glaciers and the SROCC stated with high confidence that this thinning of the ice shelves was driven by warmer ocean waters. Yet, what caused the warmer waters is still not well defined. At present, there is still only limited evidence suggesting that increased greenhouse gas forcing has slightly modified mean local winds, which would in turn facilitate intrusion of Circumpolar Deep Water heat and increase ice-shelf melting. The uncertainty is partially due to there being too few observations of the ice-shelf boundary layer. The report restates the SROCC’s statement that there is limited evidence and medium agreement for anthropogenic forcing driving the mass balance changes to the AIS, regardless of additional process-based evidence since the previous report.
The mass loss at the ice shelves driven by glacier flow acceleration has partly been offset by increased snowfall. Snowfall is the main driver of variability for the surface mass balance as the primary input of mass into the AIS. As the climate warms, atmospheric moisture content overall will increase. This means that for a given amount of upward motion, there will be more precipitation. Over space and time, snowfall across the AIS has varied and has been observed to dominate AIS mass balance variability. Yet snowfall since 1979 has not allowed for significant surface mass balance trend to be inferred.
9.4.2.2 Model Evaluation
There is medium confidence in the capacity of climate models to simulate surface mass balance changes and climatology in Antarctica. This is in part due to the low confidence in simulations of Southern Ocean temperature addressed earlier in Chapter 9 of the IPCC report. It is also difficult for models to resolve ice shelf cavities, which are sections of the ice shelf that extend out over water. Continued research on sub-shelf melt rates will help improve modelling but the current confidence is low. Two other important drivers of future AIS mass change that remain hard to model are physical processes related to ice shelf disintegration and ice sheet instabilities. However, the models have been improving and now include ice shelf and grounding line evolution along with parametrization for sub-shelf melt. The models have also benefited from improved bedrock topography that is becoming available.
9.4.2.6 Projections to 2100 and Beyond
Projections of how AIS mass change will impact sea level has varied across the reports. The 2100 projections from AR5 ranged from slightly negative to slightly positive (-0.04 to 0.16 meters) Sea Level Equivalent (SLE) change in AIS mass under an emissions scenario. SROCC’s 2100 projections for the same emissions scenario ranged in the slightly positive levels of SLE (0.01-0.11 meters). Many of the new projections after the SROCC indicate that the AIS will overall lose mass and contribute to sea level rise, under all emissions scenarios. Most of the thinning to the AIS is modeled to occur in the Amundsen Sea sector in the West and the Totten Glacier in the East.
Across the studies, the relationship between emissions scenario and AIS response varies greatly. The upper end of the projections are not well constrained due to different assumptions about the future sensitivity of sub-shelf basal melting to ocean warming along with the potential of tipping points in ice sheet structure triggering ice shelf disintegration. Based on the most recent models, the AIS will continue to lose mass under all emission scenarios leading up to 2100. There is medium confidence that AIS losses driven by ice shelf disintegration and ocean warming will not be cancelled out by increasing snowfall.
Beyond 2100, three studies have made projections about the AIS to 2300 since the SROCC. One found that if warming exceeds ~3°C above the pre-industrial era, the AIS mass loss SLE could be 6-12 meters. Still, deep uncertainty remains in the role of Antarctic ice sheet instabilities under very high emissions.
9.5.1 Glaciers
Glaciers are huge masses of ice that move slowly over land like frozen rivers. While ice sheets like those that cover Antarctica and Greenland are also considered glaciers, this section of the IPCC report is specifically concerned with mountain glaciers and both Greenland and Antarctica’s glaciers on the periphery of their ice sheets. Mountain glaciers are permanent throughout the year and are found in alpine areas. As glaciers melt, they release freshwater. Therefore, understanding how glaciers are responding to climate change is important for forecasting sea level rise. However, not all of Earth’s glacial mass has sea level rise potential since 15 percent of the total glacier volume is already below sea-level and would not contribute to sea-level rise if melted. This part of Chapter 9 begins by providing an overview of observations and mass changes to glaciers, then evaluates models and summarizes future projections.
9.5.1.1 Observed and reconstructed glacier extent and mass changes
Data for glacier mass used in IPCC reports has been primarily compiled from the Randolph Glacier Inventory (RGI), which inventories glacier outlines. Since the last Assessment Report, RGI data inventory has expanded and improved in resolution. Models also now have access to data on glacier thickness, which is managed by the global Glacier Thickness Database. Some significant updates to the understanding of the distribution of glacial mass from expanded data and observations were a decreased estimate of volume in High Mountain Asia, the Southern Andes, and the Arctic. These decreases in global glacier mass were partially made up for by the increases now defined in Antarctic peripheral glaciers.
Glacial mass change around the globe has been observed as varying greatly over space and time. Yet, the IPCC reports a coherent declining trend of 210 +/- 90 gigatons per year (~16% of starting 1901 glacier mass), or 170 +/- 80 gigatons per year from 1971 to 2019 (excluding glaciers peripheral to ice sheets). In terms of sea level equivalent, glaciers have lost 17.1 [12.7-21.5] millimeters from 1993 to 2019. The report also states with very high confidence that the global glacial mass loss rate has been increasing since 2000. There is also new evidence that from 2010 to 2019, glaciers lost more mass than any decade since observations were recorded.
The highest regional glacial mass loss was observed in New Zealand, Alaska, Central Europe, Iceland and the Southern Andes. This upper end loss rate was greater than 720 kg per m2 per year. The regions on the low end of observed mass loss, which was below 250 kg per m2 per year, include the Russian Arctic, the periphery of Antarctica, and High Mountain Asia. In fact, in the Karakoram in High Mountain Asia, glaciers have had a slight increase in mass since the 2000s, which has been dubbed the “Karakoram Anomaly.” New evidence suggests that this mass gain is occurring due to a combination of the low-temperature sensitivity of debris-covered glaciers, a decrease of summer air temperatures and an increase in snowfall possibly related to agriculture practices. There is little evidence that the “Karakoram Anomaly” will persist into future decades. Even if it were to persist, overall glacier mass records show with very high confidence that the global rate of glacier mass loss is increasing. Additionally, recent improvements in the understanding of historic glacier size and variation provide medium confidence that the global glacier recession so far this century is unprecedented in the last 2000 years.
There are many factors that drive glacial mass change. Historical data confirms the dominant role of orbital forcing on the millennial-scale fluctuations in global glacial mass, which is the effect on climate due to changes in the earth’s orbit. Other forcings, like those from volcanic activity and ocean circulation are important drivers of glacial mass fluctuation at shorter time scales. There are also other drivers such as shallow debris cover, which can significantly increase melt by reducing a glacier’s albedo, which is the ability to reflect sunlight. As dark objects have a lower albedo than the glacial ice, they absorb more shortwave radiation in the form of sunlight and increase melting. After debris cover reaches a thickness of two to five centimeters, it can actually slow down melting by blocking the ice from shortwave radiation. About four to seven percent of earth’s total glacier area is debris covered, yet many of the lower reaches of glaciers tend to be heavily covered. This creates many differences in glacier responses to warming. Glacier internal dynamics can also drive mass change. For example, glacial structural collapse could dramatically expedite melting by exposing more glacial ice volume to air and sunlight. Glacier calving and collapse can be driven by glacial lakes, which form at the end of retreating glaciers and absorb and hold heat. These lakes, which have increased in volume globally by about 48 percent from 1990 to 2018, can be positive feedbacks to glacier melting as glacial lake volume increases with increased melting. Geothermal energy fluxes can also dramatically increase glacial melting by heating the glaciers from beneath.
The report concludes that anthropogenic warming of the atmosphere is very likely the main driver of the recent glacier retreat observed since the 1990s. Along with warming temperatures that directly increase melting, there is high confidence that other factors associated with the increased temperatures, like precipitation changes and internal glacier dynamics have modified glacier mass.
9.5.1.2 Model evaluation
Modeling glaciers is difficult because there are large data gaps involving individual glacier thickness and surface mass balance. However, these gaps are starting to get filled. AR5 used simplistic models so the results were in the medium confidence range with regard to timing and magnitude of projected glacier mass loss. Regardless, AR5 and SROCC models overall found high confidence that glaciers would lose substantial mass by the end of the century. Since SROCC, only two of the existing models account for frontal glacial mass loss, also known as frontal ablation. Global glacier models also do not include more fine scale factors such as glacier surface albedo changes due to light absorbing particles or dynamic instabilities like glacier collapse. This can cause both underestimated and overestimated sensitivity to warming in current glacier models. Therefore, there is only medium confidence in the capability of current models to simulate glacier mass changes as a response to climatic forcing.
9.5.1.3 Projections
Understanding the mass balance of glaciers is crucial for projecting the future of glacier mass. The mass balance is the difference between the mass that accumulates and adds to the glacier and the mass lost. There is high confidence that there is an imbalance in the world’s glaciers given the recent decades of warming, meaning that ablation rates are higher than accumulation rates. Since glaciers take time to respond and rebalance, the observed retreat is only a partial response as the glaciers are committed to losing even more mass in the future even without additional warming. One model estimates that 36 +/- 8 percent of the global glacial mass is committed to be lost due to greenhouse gas emission warming that has already occurred. The adjustment time to rebalance glacier mass, also known as response time, is widely variable.
Given the uncertainties with modeling glaciers summarized above, projections are still not very high in their resolution. The models do agree that regions already with relatively little glacier coverage will experience the greatest relative decrease in glacier mass. Regions in the Arctic and Antarctic are expected to be the larger contributors to sea level rise beyond 2100 due to their significantly larger glacier surface coverage. There is high confidence as we look to the future that higher emission scenarios will have larger glacier mass loss.
9.5.2 Permafrost
Kathryn Husiak
IPCC Chapter 9 9.5.2 – 9.7
This section of the IPCC reports on the physical characteristics of permafrost, as well as its role in the climate cycle and projections based on a variety of models. Permafrost is defined as the layer of ground where water within the layer is frozen. It occurs where local surface/ground temperatures are consistently below freezing (primarily in the Arctic), terrestrially as well as below bodies of water (also called subsea permafrost). Close to the surface, the water content freezes and thaws in annual cycles, this layer is called the active layer. The volume of ice in permafrost is variable (up to 90% in some areas), depending on localized climatic conditions. It is difficult to directly quantify permafrost because of its subsurface nature. While there is a lack of studies that directly connect anthropogenic forcing to changes in permafrost, it is known that air and surface temperature along with physical soil disturbance do change permafrost. Since it is widely accepted that anthropogenic forcing has been directly tied to Arctic air and surface warming, there is high confidence in the relationship between anthropogenic climate change and permafrost changes. The permafrost changes used in this section are as follows; extent (land area coverage), temperature, and active layer thickness (ALT).
9.5.2.1 Observed and Reconstructed Changes
SROCC estimates that if all the permafrost on the planet thawed and flowed into the oceans, there would not be a noticeable change in sea level, so sea-level models exclude permafrost. There has been an increase in permafrost temperatures over the past 30-40 years, which is discussed more thoroughly in Chapter 2. A global picture of permafrost is difficult to create due to the low geographical density of observation sites, and localized thermal conditions of the soil and air in the Arctic. Considering all of this, SROCC states a medium confidence that ALT (the depth of thawing and freezing) is increasing across the Arctic. In high elevation areas on mountain slopes, a connection between permafrost degradation and slope instability is noted, resulting in increased rock avalanches. SROCC is highly confident that subsea permafrost, permafrost under Arctic shelves, has drastically decreased. Some isolated permafrost areas have completely thawed in recent decades.
9.5.2.2 Evaluation of permafrost in climate models
There is a high level of variability in coupled climate models due to a lack of thermal surface properties and processes in land models. Including more accurate snow insulation models fixes the modelling issues slightly. When organic matter decays due to increased temperatures and warms permafrost, it creates a positive feedback loop in permafrost thaw. This loop has been recently discovered, but including it in models still doesn’t improve permafrost extent modelling.
9.5.2.3 Projected permafrost changes
Pan-Arctic permafrost thaw depth is increasing, near-surface permafrost extent is decreasing, and that high elevation permafrost thaw is increasing as climate change advances; although the magnitude of these changes is unsure due to high variability in the modelling.
9.5.3 Seasonal snow cover
In the Northern Hemisphere (NH), the maximum average seasonal snow coverage is 45%, while in the Southern Hemisphere (SH), 2% is covered. Terrestrial snow cover is analysed using the following variables; areal snow cover extent (SCE), time period of continuous snow cover/snow cover duration (SCD), and snow accumulation/depth (SD). Snow cover accounts for the largest surface area coverage of all the cryosphere components, and due to its high albedo, is a major player in the positive cooling feedback loop.
9.5.3.1 Observed changes of seasonal snow cover
Seasonal snow cover (SCE) in mountain areas has declined since the mid-20th century. In the NH, simple snow models and coarse resolution surface temperature satellite data show an average decrease in NH spring SCE since 1950. Using surface observations, remote sensing, and land surface models, the spring snow off date on a continental-scale has been occurring earlier in the year. It is more difficult to observe and predict changes in NH autumn months with altimetry, due to increased cloud cover and low illumination levels of the earth’s surface based on the earth’s tilt away from the sun. Even with these data collection issues, models and observations of the physical characteristics of spring snowpack, general regional and hemispheric spring trends, and anthropogenic influences on temperature have shown a decrease of spring SCE in the NH. NH trends in Figure 9.23a) and c), show an observed decrease in NH seasonal SCE for 1981-2016.
9.5.3.2 Evaluation of seasonal snow in climate models
In the past, models have underestimated observed seasonal snow decreases in the NH due to misrepresentation of snow albedo feedbacks, temperature sensitivity, and snow processes. Considering these properties and processes allowed for more reliable model outputs. Even still, CMIP6 doesn’t accurately represent changes in the thermal properties of snow such as black carbon altering albedo and melt rates, vegetation covering snow, and snow insulation metrics.
9.5.3.3 Projected snow cover changes
As global climate warming persists, all global climate models that accurately evaluate the snow cover and temperature relationship agree that the NH and SH SCE and snow cover duration will decrease as temperatures increase. While snow cover changes are reversible if large cooling events occur, there are no large cooling events predicted to occur in any global climate models; it is very unlikely that this snow reversal will occur in the future.
9.6 Sea Level Change
As sea level has been defined and discussed earlier in Chapter 9, this section will be focusing on global mean sea level (GMSL), how it’s measured, and the Earth’s energy budget.
9.6.1.1 Global mean sea-level change budget in the pre-satellite era
Satellites weren’t used for GMSL data collection until the early 1990s, and before then only regional tidal gauges were used to observe sea level. Satellite altimetry is very useful because it provides almost global coverage, but because the data only extends to 1993, there is a limit to its potential trend predictions. The AR5 assessment of GMSL used a single, simple tidal gauge reconstruction so there was little uncertainty in its assessment. The majority of tidal gauge reconstructions show robust acceleration of sea level rise (SLR) due to changes in SH winds which increase ocean heat uptake, and the melting of Greenland ice.
9.6.1.2 Global mean sea-level change budget in the satellite era
During the GRACE/Argo satellite period of 2006-present, data collection has led to reduced uncertainty in changes to GMSL. Still, all the pre-satellite and satellite data contributions show that acceleration of GMSL rise has been occurring since the 1960s, and satellite data shows that melting ice sheets account for 35% of this acceleration.
9.6.1.3 Regional sea-level change in the satellite era
Altimetry observations show that 98% of the ocean surface has experienced significant SLR, varying regionally by ocean and hemisphere. Regional sea level budgets are only accurate on ocean basin scales, with increased uncertainty on smaller scales. Vertical land movement is a huge cause of small scale variability.
9.6.2 Paleo context of global and regional sea-level change
Paleo sea level records and reconstructions are valuable because they give information about changes in ice sheets and can help create and test models of warm periods and equilibrium responses. When looking at paleo/historical records, GMSL is oftentimes reported by the proxy value of global mean surface temperature (GMST). Polar amplification is an important player in equilibrium balancing as [CO2] and temperatures change; polar temperatures were shown to be double the GMST were predicted to be. In the following subsections, information from Table 9.6 is integrated in respective time period subsections for greater comprehension.
Last Interglacial (LIG)
129-116 ka, [atm CO2] = 266-282 ppm
0.5+1.5°C GMST change compared to 1850-1900. The AR5 found that the GMSL was 5-10 m higher than today. High summer insolation caused a polar amplification of sea surface temperature (SST) and surface air temperature. There is uncertainty of the magnitude and timing of Antarctic ice sheet mass loss.
Holocene
Early: 11.65-6.5 ka, Mid: 6.5-5.5 ka, Last Millennium: 805-1850CE, total [atm CO2] = 250-285 ppm
In the early Holocene, 50-60 m of GMSL rise occurred since the ice sheet maximum of the LGM. The early and mid Holocene changes in GMSL were caused by abrupt meltwater discharges and steady ice sheet melting. Between 1820 and 1860, sustained increases in GMSL caused the fastest acceleration of GMSL rise in the past 3,000 years.
9.6.3 Future sea-level changes
This section shares likely GMSL projections based on Representative Concentration Pathways (RCPs). RCPs are climate change mitigation targets that describe 4 different levels of CO2 emissions for modelling; 2.6, 4.5, 6.0, and 8.5 W/m2. They complement Shared Socioeconomic Pathways (SSPs) in modelling. High emission projections accompanied by low confidence projections are important in climate modelling, particularly SLR, because under high emissions projections, there is a much greater chance that climate change effects will occur above the likely range.
9.6.3.1 Global mean sea level projections based on the Representative Concentration Pathways
Through 2050, there is little variation between projected SLR between all of RCPs. But for 2100, there is low agreement and high variation between RCPs due to increasing uncertainty in projections the further forward you look.
9.6.3.2 Drivers of projected sea-level change
Table 9.7 shows the drivers of GMSL and regional SLR and the methods used to create the projections. Table 9.8 provides the data on individual contributions to SLR. The most impactful drivers will be explained below.
Global Mean Thermosteric Sea-Level Rise
This describes the physical property of water to expand when warm, and contract when cool. As oceans continue to absorb the majority of the earth’s atmospheric heat and there continues to be more heat due to anthropogenic forcings, thermal expansion is one of the main causes of GMSL rise. RCP 2.6 projects a 0.14 m SLR relative to 1995-2014 by 2100 due to thermal expansion.
Greenland and Antarctic ice sheets
Ice sheets are huge, continuous areas of ice that form on land and at the poles. As of 2021, there are only 2 ice sheets on Earth. The Greenland ice sheet is averaged to be 1.6 km thick, covers 0.35% of the earth’s surface area, and if completely melted it would contribute 7.2 m of SLR. The Antarctic ice sheet is averaged to be 2.2 km thick, covers 2.7% of the earth’s surface area, and if completely melted it would contribute 61.1 m of SLR. RCP 2.6 projects a sum total of 1.1m SLR relative to 1995-2014 by 2100 for both ice sheets. Ice sheet melting is the second main cause of GMSL rise.
Glaciers
Glaciers are permanently frozen ice that is over land and on mountains. When glaciers calve (break apart) the ice that falls into the ocean raises the sea level, since that ice is displacing water. Glaciers can move slowly over land and alter the topography of the earth’s surface and this movement is visible as glacial striations, which are huge scratches on rock surfaces. Through 2100, glacial calving and melting is predicted to contribute 0.09-0.18 m of GMSL rise.
Ocean dynamic sea level
Ocean dynamic sea level is the sea level deviation from geoid (irregular surface shape of the earth) and water piling on Western coasts from ocean gyre rotation. Coarse satellite resolution of 100 m doesn’t capture all processes that influence GMSL, but more fine resolutions of regional projections are more accurate of GMSL changes.
9.6.3.3 Sea-level projections to 2150 based on SSP scenarios
As previously mentioned in section 9.6.3.1, change in GMSL will likely stay between 0.19-0.23 m through 2050 regardless of emission levels. Through 2150 it is likely that GMSL acceleration could slow if lower emission scenarios are achieved. Now that GMSL projections are possible through 2150, it is important to report on these predictions since society as a whole is planning long-term infrastructure that will be affected by GMSL.
9.6.3.4 Sea-level projections up to 2100 based on global warming levels
The cryosphere is predicted to have very different responses based on global warming acceleration (the change in rate of global warming), and due to the almost infinite variability in how this warming could occur, it is difficult to predict GMSL with certainty. Table 9.9 is helpful in demonstrating how uncertainty in GMSL rise increases as emissions levels increase through 2100. This occurs because as emissions increase, there is a greater likelihood of low confidence cryosphere dynamical changes such as random calving occurring; as mentioned previously, ice dynamics are very difficult to predict. This can also be visualized in Figure 9.26; comparing between SSP1-2.6 and SSP5-8.5, the uncertainty (represented in the percentile whiskers) increases greatly for the Antarctic ice sheet even though the median 2100 predictions are similar. To continue with this example, the collapse of marine-based sections of the Antarctic ice sheet alone could cause 0.1-0.4 m GMSL rise above the likely range. These types of low confidence predictions could account for 0.6-1.6 m increase in GMSL by 2100.
9.6.3.5 Multi-century and multi-millennial sea-level rise
The anthropogenic-attributed emissions that have already been introduced into the atmosphere since the Industrial Revolution through 2016 will likely lead to 0.7-1.1 m increase in GMSL through 2300, regardless of future emissions. Once thermal expansion of ocean water stops occurring (which will be 2,000 years after human’s stop emitting high levels of CO2), SLR will continue for 10,000 years. This is because ice sheets will continue to calve and melt, due to the low reversibility of ice sheet formation. Looking at paleo records of ice formation, it takes a very short time (on the geological timescale) to melt glaciers, but a huge amount of time to reform glaciers. One way to predict far into the future is to observe what has occurred far in the past; paleo global surface air temperature and GMSL of warm periods such as the LIG (discussed in section 9.6.2) are useful metrics for predictions. Ice coring is also a very valuable tool used to learn about the earth thousands of years in the past; as global warming progresses, this valuable paleo data is melting away at the same time. During the warm (+0.5-1.5oC) LIG, GMSL was 5-10m higher than today, and this paleo sea level is consistent with our 10,000 year predictions of SLR.
9.6.4 Extreme sea levels: Tides, surges and waves
This section focuses on past and future events of extremely high or low sea surface height, which are usually temporary surges or changes in tidal patterns.
9.6.4.1 Past changes
Anthropogenic altering of coasts and relative sea level rise (the height of water compared to the land at a specific location) are the two most impactful drivers of tidal changes at tide gauge stations. Regional morphological processes such as erosion and sedimentation, and tide patterns are also used to evaluate past extreme sea level events. The poleward shifts of cyclones and increase in intensity of cyclones due to climate change have shown to have significant influence on tidal surges. Thus, as climate change advances, so will extreme storm surges and resultant flooding and erosion at coasts.
9.6.4.2 Future changes
The predicted main driver of future regional extreme sea level rise is changes in relative sea level. Tidal amplitude changes will be most affected by GMSL rise because as GMSL increases, so will the baseline tidal level. Looking more locally, the development of coastlines with hard and soft infrastructure will cause regional fluctuations in the probability of extreme sea level events.
9.7 Final Remarks
This section of the IPCC report has improved from previous reports by using models and including processes that reduce uncertainty for particular aspects of climate change. In summary, the Antarctic and Greenland ice sheets still impose the highest level of uncertainty through 2010, and beyond. This uncertainty is further exacerbated when higher emission scenarios are considered. This only furthers the assertion that anthropogenic fossil fuel emissions have not only short term consequences, but severe long term consequences that are most likely irreversible. The most certain claim considered in this section is that GMSL will continue to rise, because all of the factors that influence GMSL will continue rising, such as thermal expansion and mass loss from glaciers and ice sheets. Considering that 40% of the global human population live within 100km of a coastline, SLR is extremely important to predict, and take adaptive and mitigative climate action using accurate predictions.
Due to constraints set on the assignment, the sections and subsections above were prioritized over the sections mentioned below for reasons such as extremely low confidence in projections, or overly specific or technical focus on a section topic.
● Mid-Pliocene Warm Period (MPWP)
● Marine Isotope Stage 11 (MIS 11)
● Last Glacial Maximum (LGM)
● Last Deglacial Transition: Meltwater pulse 1A (MWP-1A)
● 9.6.1.3 Regional sea-level change in the satellite era
● 9.6.1.4 Attribution and time of emergence of regional sea-level change
● Cross Chapter Box 9.1
● Gravitational, rotational, and deformational (GRD) effects
● Glacial isostatic adjustment and other drivers of vertical land motion
● Box 9.4