3 Topics from Chapter 3: Human Influence on the Climate System
3.4: Human Influence on the Cryosphere
Emma Holt-Toman
Many aspects of the Earth’s cryosphere have been affected by human activity, including sea ice, snow cover, glaciers, and the ice sheets of Greenland and Antarctica. Since the 1970s, anthropogenic forcing has been the primary cause of sea ice loss in the Arctic. Human influence has also very likely contributed to declines in spring snow cover throughout the Northern Hemisphere since the 1950s, as well as world wide glacier retreat. Many aspects of these changes could not occur without human influence from an increase in greenhouse gasses caused by burning coal, oil, and gas. CMIP5 and CMIP6 models both show that anthropogenic forcings have very likely contributed to these changes to the cryosphere.
Sea Ice
It is very likely that anthropogenic forcing is the primary cause Arctic sea ice loss that has been observed since the 1970s. This has mainly been due to increases in greenhouse gases from burning fossil fuels. Multiple studies and simulations conducted since 2013 have shown that the observed changes can only be reproduced in models that include anthropogenic forcings as a contributing factor to the Arctic sea ice loss since 1979, which has seen substantial reductions throughout the years with the largest reductions observed in September. There is high confidence that the Arctic sea ice loss that is observed in the summers since 1850 is abnormal. In 2017, a study that observed sea ice extent (SIE), or areas in which at least 15% of the surface is covered by ice, in Septembers between 1979 and 2012 found that “anthropogenic signals…were separable from the response to natural forcings due to solar irradiance variations and volcanic aerosol.”
While the Arctic has lost sea ice in summer months likely due to anthropogenic forcing, Antarctic sea ice has shown slight increases in summer and winter months between 1979 and 2017. These increases were very small however, and not statistically significant. Furthermore, in the austral spring of 2016, extremely low SIE was observed, which is likely due to warm surface waters in Antarctica and increased fresh water fluxes.
Snow Cover
Human influence has very likely contributed to declines in spring snow cover throughout the Northern Hemisphere since the 1950s, which is a key component of the climate in the northern land areas. When spring snow melt occurs earlier than it typically does, it causes warming and soil to dry out. CMIP5 models show that there is a strong relationship between snow cover extent (SCE) in the Northern Hemisphere and annual-mean surface temperature, and determine that it is likely that human influence is responsible for the decline in snow cover. Multiple CMIP5 and CMIP6 models have routinely associated the decline in snow cover in the Northern Hemisphere to anthropogenic forcings, due to how the changes cannot be replicated when only including natural variability alone. Likewise, decreases spring snow thickness have also been observed in the Northern Hemisphere, which has been attributed to an increase in anthropogenic greenhouse gases by multiple CMIP6 studies. Additionally, decreases in snow cover extent have been attributed to greenhouse gases since 1925, which cannot be explained by natural forcings alone. To conclude, because models consistently show that the warming of annual-mean surface air temperatures are increasing due to human influenced greenhouse gases increasing, we can conclude that it is very likely that human influence has contributed to a decrease in snow cover in the Northern Hemisphere.
Glaciers
Glaciers are melting rapidly across the globe, and human influence has very likely contributed to the worldwide glacier retreat that has been observed since the mid-twentieth century due to the warming of the atmosphere. A study of 85 glacier systems in the Northern Hemisphere found that anthropogenic influence contributed to a decrease in glacier mass loss. When simulating different forcings on glaciers, “natural-only forced simulations” showed a net growth of glaciers, while simulations which included anthropogenic forcings showed a net loss of glaciers. The changes in glacier mass would not have occurred without anthropogenic climate change. One study estimated that 85% of the overall change in glacier mass since 1850 has been caused by human induced greenhouse gas emissions. Therefore, it can be concluded that anthropogenic influence is the main cause behind the global reduction in glacier mass that has occurred since the late 1900s.
Ice Sheets
It is very likely that human influence has contributed to the decline in mass of the Greenland Ice Sheet over the last twenty years. There is medium confidence that in the last 350 years, the Greenland Ice Sheet has experienced unprecedented melting during the summer months at a rate of two to five times the pre-industrial level, suggesting that anthropogenic forcing has likely contributed to the melting. Furthermore, we can state with high confidence that the loss of ice mass has significantly increased since the early 2000s. While there is medium confidence that it is very likely that human influence has caused the melting of the Greenland Ice Sheet over the last twenty years, there is insufficient evidence that human influence is responsible for the changes in the Antarctic ice sheet. This is due to the high natural variability in the melt rate. The West Antarctic Ice Sheet is warming, while East Antarctica is not. Because West Antarctica is under sea level, the warmer ocean contributes to increased melting, but there is no general agreement if the ice loss in the recent years is due to natural internal variability or anthropogenic forcing. However, there is medium agreement that human influence is contributing to the melt of the Antarctic ice sheet due to increased greenhouse gases.
Human Influence on the Ocean
The ocean is a vital part of the Earth’s climate, due to how it stores and transports heat, freshwater, and carbon in the atmosphere. Since the middle of the twentieth century, human activity has caused major changes to the ocean’s temperature, salinity, sea level, and circulation. It is extremely likely that anthropogenic forcings have caused the ocean to increase in temperature since the 1970s, including the surface and the deeper ocean. Salinity of the ocean’s surface and subsurface have also been affected, which is extremely likely to be caused by human activity from changes in the water cycle such as increased precipitation in some areas, and decreased precipitation in others. Furthermore, due to increased glacier melting, anthropogenic forcing has also very likely contributed to global sea level rise that has been observed since the 1970s.
Ocean Temperature
It is extremely likely that anthropogenic forcings have caused the ocean to increase in temperature since the 1970s, which includes the surface and the deeper ocean. Human influence has very likely contributed to ocean warming above 700 meters, and it is virtually certain has experienced significant warming since 1971, and very likely that it has warmed significantly between 700 and 2000 meters as well. The ocean absorbs heat from the sun and from increased anthropogenic greenhouse gases, and recent studies have shown that the ocean has absorbed and stored more than 90% of the heat gained by the Earth since the early 1970s, mainly due to increased fossil fuel consumption. Furthermore, the rate of warming has doubled since 1993, and over one-third of this increased heat is stored below 700 meters. However, this can vary between oceans. In the Pacific and Indian Oceans have warmed more in the upper layers, while the southern oceans have experienced more warming in the deeper layers of the ocean. Models show that the amount of observed rise in ocean temperature cannot be attributed to natural forcings alone. Simulations only replicate the amount of observed warming through a combination of natural and anthropogenic forcings. In conclusion, it is very likely and with high confidence that the change in ocean temperature extending to the deeper layers of the ocean that has been observed since the 1970s has been caused by anthropogenic forcings, rather than natural forcings alone.
Ocean Salinity
The salinity level of the ocean is important for many reasons. Because saltwater is denser than freshwater, salinity plays a role in the ocean’s circulation and how heat from the ocean is carried across the globe. Salinity is also linked to the water cycle due to how it effects precipitation and evaporation. Salinity of the ocean’s surface and subsurface have changed since the mid twentieth century, which is extremely likely due to human activity. Fresh water regions have become fresher, and salt water regions have become saltier. Salinity changes can occur due to changes in the water cycle and ocean atmosphere fluxes. In areas where more rainfall occurs than evaporation, rainfall is increasing, and in areas where evaporation occurs at a higher rate than precipitation, rainfall is decreasing, and these changes to the water cycle are having an effect on the ocean’s salinity. Furthermore, the melting of mountain glaciers and ice sheets are also contributing to the changes through an influx of freshwater pouring into the ocean. Although some changes in ocean salinity can be explained through natural variation due to oscillation patterns and volcanic eruptions, the changes that have been observed since the 1960s cannot be explained by natural variation alone, and it is very likely that anthropogenic forcings have significantly influenced the observed salinity changes in parts of the ocean. CMIP5 and CMIP6 models are only able to replicate these observed changes when including anthropogenic greenhouse gas increases.
Sea Level
The average sea level across the globe is increasing due to melting ice, including ice sheets and mountain glaciers, and the expansion of warming ocean water. It has been concluded with high confidence that an increase in human induced greenhouse gas emissions is very likely the primary cause of global sea level rise that has been observed since the 1970s. A study from 2014 demonstrated the significance of anthropogenic forcings such as greenhouse gases and aerosol forcings and the effect they have had on the rising sea level. About 87% of the 0 to 700 meter thermosteric sea level rise that has been observed since the 1950s can be attributed to human influenced forcings. CMIP5 models show that the observed changes did not occur from natural forcings alone, and could only be replicated when including anthropogenic forcings in the simulations.
Ocean Circulation
Ocean circulation is an important component in the transport of heat and freshwater across the globe. The Atlantic Meridional Overturning Circulation (AMOC) is a system of ocean currents that moves warm water northward to the surface and colder, dense water southward. If this system is interrupted or weakened, it would lead to extreme cold temperatures in Europe and the east coast of North America, and disrupt seasonal monsoons. Although it is generally agreed that human influenced greenhouse gasses could weaken the AMOC, solar and volcanic emissions can also play a role. Therefore, there is low confidence that anthropogenic forcings influenced the slowdown of the AMOC that was observed from the mid 2000s to the mid 2010s. However, as human induced climate change increases, reducing the density of surface waters via warming, increased precipitation and meltwater, it is suggested that the AMOC will weaken or even shut down over the next 100 years.
Human Influence on the Biosphere
The amount of atmospheric CO2 has been rising since the beginning of the Industrial Revolution, and the carbon cycle has consequently been altered, increasing in magnitude since the beginning of systematic monitoring. There is medium confidence that the increasing accumulation of atmospheric CO2 is enhancing plant growth, and therefore increasing the amplitude of the seasonal cycle of atmospheric CO2. There is low confidence that this CO2 fertilization has been the main reason for the detected greening due to land management in some areas that have probably contributed to the greening. The accumulating excess CO2 is also being absorbed into the ocean, which is a significant carbon sink, and altering the ocean’s chemistry. It is virtually certain that global ocean acidification is due to the increased uptake of anthropogenic CO2.
Terrestrial Carbon Cycle
There is medium confidence that the increasing accumulation of atmospheric CO2 is enhancing plant growth, and therefore increasing the amplitude of the seasonal cycle of atmospheric CO2 over the last 20 to 30 years. This change is connected to changes in land use and management, as well as “increased CO2 fertilization, nitrogen deposition, increased diffuse radiation, and climate change.” There are three main indicators of how climate change is affecting the terrestrial carbon cycle, including “atmospheric CO2 concentration, atmosphere-land CO2 fluxes, and leaf area index.” In general, the terrestrial carbon sink has more yearly variability than the ocean. Depending on the area, the strength of the terrestrial carbon sink changes due to water availability and temperature driven by seasonal changes. In the summer and spring, carbon uptake is increased, while in the autumn and winter carbon is released through respiration, and this cycle of atmospheric CO2 has increased since it has been monitored. The increase of excess carbon in the atmosphere has led to an increase in plant growth in warmer months, and subsequently an increase in plant respiration in colder months of the Northern Hemisphere. Furthermore, it was estimated that leaf area index has increased between 25% and 50%, and 70% of this greening is believed to be caused by increased CO2 fertilization. However, some models that are used to find causes in these trends do not include nitrogen fertilization, changes to crop cultivars, irrigation effects. Therefore, there is medium confidence that human influence is the main driver in the increased amplitude in the seasonal atmospheric CO2 cycle.
Ocean Biogeochemical Variables
It is virtually certain that global ocean acidification is due to the increased uptake of increasing anthropogenic CO2. The ocean is a significant carbon sink, accounting for about 30% of the CO2 that is emitted into the atmosphere. The dissolving carbon in the ocean is decreasing the pH, and making the normally slightly alkaline ocean more acidic. Since the mid 1700s, the pH of the surface of the ocean has decreased by 0.1, which equals a 30% increase in hydrogen ions. This change has disrupted the balance of carbonate ions, which are essential for many marine organisms that rely on calcium carbonate for shell and skeleton production, such as corals. Furthermore, warmer oceans caused by anthropogenic greenhouse gases have contributed to a decline in phytoplankton, subsequently leading to a decrease in the ocean’s carbon uptake ability. In the last 40 years, the ocean’s pH has decreased not only in the surface layer, but also in the middle layer of the ocean and up to 3000 meters deep in some areas. These changes are virtually certain to be caused by human influence through an increase in anthropogenic CO2. In addition to acidification, the concentration of oxygen in the ocean has decreased since the 1960s, partially due to human influence with medium confidence.
3.7: Human Influences on Modes of Climate Variability
Rory Spurr
Introduction
Modes of climate variability are best described as naturally occurring, dynamic, large-scale trends in the climate that occur in timescales ranging from several weeks to decades. A familiar and rather famous example of a mode is the El Niño-Southern Oscillation (ENSO). These modes are in part determined by atmospheric and oceanic circulations, the interaction between the two, and the interactions both the atmosphere and ocean have with the land surface. Climate variability that occurs on seasonal to decadal time scales can be attributed to these modes, as well as interacting effects between multiple modes. The effects of these modes are often termed “teleconnections”, a concept that refers to the ability for these modes to influence climate in remote regions through oceanic and atmospheric pathways. Due to teleconnections, regional climates around the world are determined by both local physical processes, and non-local, large-scale climate phenomena. Thus, to have a complete grasp on how humans influence the global climate, insights into how humans are altering modes of climate variability are of high importance.
Humans can alter modes of climate variability by affecting the way they behave, or more specifically, the temporal (occurrence, variability, seasonality, persistence, etc.) or spatial characteristics of the modes and associated teleconnections that define them. Anthropogenic forces describe the ways in which humans can alter the climate. These anthropogenic forces include greenhouse gas emissions, deforestation and more. For example, higher concentrations of greenhouse gas emissions warm the Earth by radiating long wave radiation that earth emits from its surface back down. This causes long wave radiation to stay in Earth’s atmosphere, and slowly warms Earth’s climate over time. Changes in Earth’s atmospheric, surface, and oceanic temperatures impact how atmospheric and oceanic circulations interact. This provides a mechanism for how humans can affect the atmospheric and oceanic circulations, and thus a mechanism for how humans can alter the modes of climate variability.
To determine if humans have altered the modes of climate variability, historical model simulations are run to determine if climate variability has changed significantly from historical patterns. The models used to do this in the IPCC sixth assessment report are the CMIP (coupled model intercomparison project) models, which are climate models focused on global warming predictions. Typically, modes are described by sea surface temperature (SST), sea level pressure (SLP) or other climate variables within certain latitude-longitude boxes. SST or SLP data are collected and compiled for each mode of climate variability, these data are used in climate models to show historical trends in climate variability. In model simulations, models keep track of the climatic conditions inside latitude longitude boxes, while also recording how they change via math that simulates basic physical processes of our climate. By keeping a record of how the conditions in these latitude-longitude boxes change, climate models give us a picture of how modes of climate variability have acted over a certain time period. Historical trends in climate variability can then be compared to modern trends, to see if patterns are shifting over time. If it is known that a trend is shifting over time, we can then use the model to evaluate if any anthropogenic forcings are the cause of the shifted pattern.
The IPCC 6th Assessment report discusses human impacts on five interannual modes of climate variability and two decadal or multidecadal modes of climate variability. By testing 7 different modes that encompass regional climates all over the Earth across two different timescales, we gain insight into how humans may be altering long term averages in Earth’s climate. To try and focus this report on the most important conclusions from the IPCC report, two modes of climate variability that showed to be significantly affected by anthropogenic forces will be described.
Southern Annular Mode
The Southern Annular Mode (SAM), also termed the Antarctic Oscillation (AAO), is the mode comprising the large-scale atmospheric circulation variability found in the high latitudes of the Southern hemisphere. SAM is typically defined as the difference in SLP between 40oS (Southern tips of South America and Australia, solid magenta circle in Figure AIV.2, a) and 65oS (outer edges of Antarctica, dashed magenta circle in figure AIV.2, a). SAM has two phases, a positive phase where SLP over Antarctica is relatively low and a negative phase where SLP over Antarctica is relatively high (Figure 1). SAM oscillates between phases on timescales of a few weeks to several months. The positive phase of SAM is associated with stronger, poleward shifted storm tracks in the midlatitudes of the Southern hemisphere. This in turn leads to colder conditions over much of Antarctica, as well as colder SST in the waters surrounding Antarctica. During its positive phase, teleconnections from SAM result in drier conditions in Chile and Argentina, as well as wetter conditions in South Africa, Madagascar, and Southeast Australia. During the negative phase of SAM these storm tracks shift equatorward. This brings low SLP systems to the southern tip of South America and Australia, resulting in opposite effects as the positive phase.
Overall, SAM has been trending towards its positive phase since the 1970s, meaning that SAM conditions have been in the positive phase more often and for longer periods of time than historical variability has shown. This trend is visualized in Figure 3.35, where the indices (which describes what phase SAM is currently in, with positive indices indicating SAM is in a positive phase) are trending in a positive direction as we move forward in time. The indices become especially positive as we approach 2000. It is important to note here that the positive trends are also climbing out of the natural variability of SAM, providing evidence that SAM trends are influenced by anthropogenic forces.
From 1970 to 1990 this trend is likely caused by ozone depletion, although this effect has slowed in the early 2000’s. Ozone depletion is a phenomenon where chemicals released into the atmosphere called chlorofluorocarbons (CFCs) break down ozone molecules in the lower stratosphere (the layer of Earth’s atmosphere above the troposphere, the troposphere being the bottom layer that humans reside in). Around 1997-2000 the concentrations of ozone depleting chemicals being released into the air decreased dramatically, due to various policies put in place to restrict the use of these chemicals in industry. Ozone in the stratosphere absorbs UV radiation from the sun, thus with less ozone in the stratosphere, the stratosphere will cool as less UV is being absorbed there. This cooling of the stratosphere, which is usually relatively warm compared to the upper troposphere, leads to a cooling of the troposphere through heat transfer as well as a drop in the height of the troposphere. These lead to SAM trends toward the positive phase, less due to the resulting temperature change over Antarctica, but more due to the influences the temperature changes in the stratosphere and troposphere have on atmospheric circulation patterns.
The changes in SAM found in the IPCC report will have important and far-reaching effects on climate. As stated before, a positive phase SAM will lead to drier conditions in Chile and Argentina, which on average will lead to glacier recession. This can be seen as the storm tracks during a positive phase have shifted southwards toward Antarctica, thus the high moisture winds of the storm track are not hitting Chile and Argentina. During a negative phase, these winds will hit the Andes mountains (a mountain range located in Chile and Argentina), and the moist air will start to rise over the mountains and becomes cooler in the process. This process of rising motion and cooling of moist air leads to precipitation, in this case in the form of snow. Without this precipitation, the glaciers are not accumulating snow. The lack of precipitation combined with the dry conditions of the positive SAM phase lead to glacier recession. Similarly, the positive SAM phase leads to strengthening of westerly winds near Antarctica, which strengthens ocean circulation patterns near the Antarctic peninsula. This strengthening of the ocean circulation allows a patch of warmer water, called Circumpolar Deep Water (CDW) to up well. This relatively warm water meets the ice shelves of the Antarctic peninsula, causing melting and receding of the glaciers. Combining both effects, it is seen that a positive trend in the SAM leads to increased glacial melting and receding, which in turn contributes to global sea level rise. It should be noted that the magnitude of global sea level rise caused by the positive SAM phase is not known and warrants further research.
Atlantic Multidecadal Variability
Atlantic Multidecadal Variability (AMV) is a climate mode that describes fluctuations in the surface temperatures of the Northern Atlantic Ocean. Teleconnections from this mode are particularly pronounced in the surrounding continents and the Arctic, thus this mode largely describes climate patterns of the Atlantic Ocean, Arctic, Eastern United States and Western Europe. As the name suggests, this mode oscillates between warm and cold phases on time scales of decades to multiple decades. This mode is usually assessed by using SST anomalies in the Northern Atlantic, and changes in the average behavior of this mode have great implications for Atlantic Meridional Overturning Circulation (AMOC).
The AMOC describes the large-scale, three-dimensional circulation of water in the Atlantic Ocean that also drives ocean circulation across the globe. In the North Atlantic the water is very cold and salty. Cold, salty water is very dense, causing it to downwell (sink) in the Northern Atlantic. This in turn causes a northward flow of warm surface water from the equator. As this water travels poleward some of it evaporates, causing its salinity, and thus density, to rise. The water that sinks in the Northern Atlantic then travels southward towards Antarctica at depth. Eventually this cold, deep water will upwell in the Indian or Pacific Oceans, becoming warmer. This water will eventually make its way back up to the Northern Atlantic over a period of about a thousand years. Thus, the AMOC brings heat northward in the form of warm surface water, while also driving ocean circulation across the globe. It is important to evaluate anthropogenic influences on the AMV and AMOC as a warming of the Northern Atlantic Ocean would decrease the density of the water there. This decrease in density causes less sinking in the Northern Atlantic, eventually resulting in a slowdown of the AMOC.
The IPCC report found that the CMIP6 model shows robust evidence that external forcings have modulated the AMV over the last century. It is believed that anthropogenic as well as volcanic aerosols contributed to the cooling the AMV has encountered from 1960-1990. Aerosols are small particles that become suspended in the air and reflect incoming solar radiation. This reflection of solar radiation prevents it from reaching Earth’s surface and ultimately leads to a cooling of the climate. The big caveat to this is that the degree to which the anthropogenic forces contributed to the resultant cooling of the AMV are not robust. This is largely due to the large timescale that the AMV works on, and thus it is hard to have the data and modeling certainty to reliably estimate historical AMV patterns.
The big takeaway from the conclusion is the fact that there is a large amount of evidence saying that the AMV is impacted by external forcings. Thus, although we do not know the degree to how much anthropogenic forces have influenced the AMV so far, it is apparent that a large degree of warming can have intense effects on the AMV. SST observations and model predictions show a high amount of warming (0.5o C) in the Northern Atlantic (Figure 3.40). This means that despite the seemingly inconclusive model evaluations of anthropogenic forces on the AMV from the IPCC report, it is not out of the question to assume that increased anthropogenic warming in general will lead to increased warming patterns in the AMV, and thus increased warming in the Northern Atlantic. Nonetheless, more research is needed to fill in the gaps left by the newest generation of climate models, and the absence of historical data in evaluating trends in the AMV.