2 Topics from Chapter 2: Changing State of the Climate System

Findings from IPCC (AR6) Chapter 2.1 to 2.3.1

Connor Oman

2.1
Chapter two of the IPCC report comprehensively covers evidence that indicates large changes in components of Earth’s climate system. This evidence takes the form of remote sensing from satellites, ocean sediment samples, ice cores, fossilized organisms, and climate model simulations. All of this evidence combined will serve to prove that the Earth is undoubtedly warming.

2.2

The second section of chapter two specifically looks at the changes in climate drivers. These climate drivers are essentially climate attributes (like land usage and greenhouse gases) that have a direct impact on the average global temperature. More scientifically, each of these drivers have a specific radiative forcing on the planet, which can be measured by watts per square meter. Radiative forcing can be either positive or negative. A positive radiative forcing will lead to warming, whereas a negative radiative forcing will lead to cooling. In section 2.2, we will be looking at the significance of each of these drivers and how they have transformed over time.

2.2.1

One climate driver we as humans have very little control over is solar and orbital forcing. Sunspots, which are essentially spots on the sun that are less bright, can impact solar forcing. The variation of incoming solar energy also arises from variations in the Earth’s orbit and these orbital variations can be precisely calculated for tens of millions of years in the past.

However, the total solar radiative forcing fluctuations over the past 9 kiloyears never exceeded 1 watt per meter squared. In terms of a more recent time frame, the variability in solar forcing since the Maunder Minimum (which was 306 years ago) has been less than 0.1%. The report proceeds to conclude that the increase in radiative forcing due to insolation changes over the last 306 years has likely been 0.05 W/m² to 0.10 W/m². However, there was likely a decrease in solar forcing of about 0.04 W/m² between the years of 1986 and 2008 due to a decreased sunspot cycle.

A new reconstruction based on data including updated isotope data has found that solar forcing has been unusually high over the second half of the 20th century. That being said, there is no evidence that this unusual high has been experienced over the last 9 kiloyears. However, the IPCC report states that there has been very little variation due to these sunspots over the last millennium. Overall, solar radiative forcing has been relatively high recently, but not in the context of the past 9 kiloyears. The global-mean radiative forcing due to changes in insolation is currently anywhere from -0.06 W/m² to 0.08 W/m².

2.2.2

Volcanic eruptions can lead to a great negative forcing on the climate. However, their greatest impact on climate can be seen directly 2 to 5 years after a strong eruption event because they inject short-lived aerosols into the atmosphere. These aerosols are too heavy to exist in the atmosphere for an extended period of time which is why the strongest impact of volcanic eruptions only lasts for a couple of years. Scientists can measure this residence time through the use of satellites, but the uncertainty is about 15% to 25%. These aerosols cause a negative radiative forcing because they reflect incoming solar radiation from the sun. Similar to variations in insolation, the forcing due to volcanic activity is not consistent because large volcanic events only occur every 35 to 40 years. When these events do occur, they can have radiative forcings of greater than – 1 W/m². The IPCC can reconstruct the pattern of volcanic activity in the past by looking at sulfate concentrations in the ice of the Greenland ice sheet. Using this data, scientists determined volcanic eruptions have occurred every 3 to 130 years over the last 2.5 kiloyears (the average being 43 years). The years 1450 to 1850 had unusually high concentrations of aerosols found in the ice and the reason for this difference is relatively unexplained. In all, volcanos can exert a strong negative forcing for a couple of years, but once the aerosols fall out after a couple of years, its forcing becomes less pronounced. The modern concentrations of these aerosols are relatively consistent with the projected measurements of aerosols for the last 2.5 kiloyears.

2.2.3

Compared to those short-lived volcanic aerosols, well-mixed gas molecules like CO2 can exist in the atmosphere for much longer. Many gases that are able to trap terrestrial infrared radiation (greenhouse gases) will exist in the atmosphere for an extended period of time. The main three gases this section of the chapter will focus on are carbon dioxide (CO₂), methane (CH₄), and nitrous oxide (N₂O). In 2011, the concentration of CO₂ was 390.5 individual molecules of CO₂ per million molecules of air (ppm). Whereas, CH₄ was 1803.8 ppb and N₂O was 324.2 ppb. With great confidence, it has been concluded that these concentrations of greenhouse gases are much higher than the concentrations of greenhouse gases found in ice cores dating back to 800 kiloyears ago.

Ocean sediments can help us understand what climate was like in the more distant past. In terms of sediment sampling, there have been significant improvements in the resolution and accuracy of the boron isotope measurements in ocean sediments. The boron isotope data can be used to corroborate data with CO₂ concentrations in ice cores. This improves the accuracy/reliability of each dataset and deepens our understanding of the past trends in atmospheric CO₂ concentration. Other methods of corroborating more evidence of past atmospheric CO₂ concentration include stomatal density inside of fossilized plants and paleosol carbonates, but these methods are harder to use on longer time scales. Using boron isotope and ice core data, we know that CO₂ concentrations have decreased 16 ppm per million year for the last 50 million years. The last measured occurrence of an atmospheric concentration above 1000 ppm was before the Eocene transition. As a result of this gradual decrease over a gigantic period of time, it’s extremely likely that we haven’t experienced a greater concentration of atmospheric CO₂ since 2 million years ago.

Interestingly, CH₄ concentrations have been higher in the Northern Hemisphere than the Southern Hemisphere over the past 110 kiloyears. This likely has to do with the imbalance of land between the Southern and Northern Hemisphere. However, the concentration is still closely correlated to the centennial/millennial timescales and the glacial cycle. During the most recent ice age (otherwise known as the last glacial maximum or LGM), CH₄ had a concentration of 390.5 ppb which is roughly 20% of the modern concentration. Methane has also noticeably increased after the Industrial Revolution where the pre-industrial values were measured to be 729.2 ppb. It should be noted that methane had increased before the Industrial Revolution, but this is due to the trapped CH₄ that is released when the climate is exiting a glacial period. Likewise, new records show that N₂O concentrations are also positively correlated to the glacial cycle. In a span of 200 years right after the last glacial termination, N₂O concentrations increased by 30 ppb. However, unlike methane, N₂O has been increasing at a slower rate than CH₄ after the Industrial Revolution. Nitrous oxide had a pre-industrial concentration of 270.1 ppb which is roughly 80% of the modern concentration.

Overall, the gas that has experienced the greatest boom in concentration is CO₂. According to table 2.2, between the years 2011 and 2019, CO₂ has experienced a 5% increase in ppm. Compare that to CH₄ which experienced a 3.5% increase in ppb and N₂O which experienced a 2.4% increase in ppb. Carbon Dioxide emission has also been increasing. The rate of CO₂ increase in 2000 to 2011 was around 2.0 ppm per year compared to the rate of CO₂ increase in 2011 to 2019 which was around 2.4 ppm per year. Part of the reason CO₂ is accumulating at a greater rate than other gases is because it has a relatively long residence time in the atmosphere. Methane has a significantly shorter residence time of about 10 years. Nitrous oxide has a residence time of over a century, but has a smaller atmospheric accumulation rate because it’s being emitted into the atmosphere at a lower rate.

2.2.4

Halogenated greenhouse gases are similar to your more common greenhouse gases like CO₂. They still trap infrared radiation and are often more efficient at doing so. They have a more complicated chemical structure and are less likely to chemically react with other molecules. Since they are less likely to react, they have exceedingly large residence times meaning they can persist in the atmosphere for 50 to hundreds of years. Halogenated gases like CFCs and HFCs are created through industrial processes and are not created naturally. However, the abundance of CFCs in the atmosphere has decreased over time as mandated by the Montreal Protocol on Substances that Deplete the Ozone Layer. That being said, HCFCs and HFCs are still increasing along with PFCs and SF₆. The positive radiative forcing due to CFC concentrations has decreased by 9% from its maximum in 2011. The rate of HCFC (primarily used in foam blowing and refrigeration) emissions has decreased, but the total concentration is still increasing.

Hydrofluorocarbons (HFCs) essentially serve the same purpose as HCFCs and CFCs and its concentration in the atmosphere has been increasing. If we refer back to table 2.2, we can see an alarming example where the concentration of HFC-32 has increased by 300% from 2011 to 2019. In general, all of the concentrations of the HFC variants increased significantly with the lowest increase being 71% (HFC-134a). SF₆ and NF₃ are both chemicals used for electrical equipment and like HFCs, SF6 had increased 36% and NF3 had increased 147%. Perfluorocarbons like CF₄ and C₂F₆ had also increased by 8.2% and 16.3% respectively, but it should also be noted that CF₄ has some natural sources unlike C₂F₆ which is only man-made. The total forcing caused by these halogenated gases is 0.41 W/m² which is noticeably smaller than the forcing due to methane and carbon dioxide.

2.2.5

The water vapor is a strong greenhouse gas, and concentrations in the stratosphere are connected to human emissions of methane. Between 1980 to 2010, the concentration of water vapor increased by about 1.0 ppm or 6% in the 16 km to 26 km layer in measurements over Boulder, Colorado. However, there have been efforts to study stratospheric water vapor trends with satellites, but the results suggest little global net change.

With the prevalence of the aforementioned CFCs, global ozone concentration significantly dropped in the 1980s by about 3.5%. However, because of the Montreal Protocol, global concentrations of ozone had increased very slightly between 2000 and 2017. The largest event of ozone loss occurs over Antarctica in the spring, but ozone can still be destroyed globally in the stratosphere.

Tropospheric ozone increased by 2-7% per decade in the northern mid-latitudes, by 2-17% per decade in the tropics, and by less than 5% per decade in southern mid-latitudes. The total tropospheric radiative forcing due to ozone from 1750 to 2019 is around 0.47 W/m².

2.2.6

Aerosol optical depth is a measure of how much light is scattered by aerosols in an air column. Determining a global trend in aerosol optical depth is tricky, but AR5 concludes with high confidence that the aerosol optical depth in Europe and Eastern USA has decreased, whereas the aerosol optical depth in Asia has increased. The concentration of aerosols is strongly related to industrial waste practices. In European countries and the USA, there are tons of restrictions on what you can release into the environment, whereas in other countries those restrictions don’t exist. We can also use ice cores to find past concentrations of aerosols like black carbon and sulfates. However, unlike the trend for the greenhouse gases and hydrofluorocarbons, most aerosols do not exhibit a linear path of increase. Aerosols typically show an increase from 1700 to the end of the 20th century and then a decrease right after. Aerosols create a negative radiative forcing because the particles reflect incoming solar radiation in two ways. Firstly, the aerosol can directly reflect the incoming solar radiation which provides a negative forcing of about -0.45 W/m². Secondly, the aerosol can act as cloud condensation nuclei which provides a negative forcing of about -0.45 W/m². However, both of these forcings have large amounts of uncertainty and the report states that this uncertainty is the greatest contributor to the overall radiative forcing uncertainty. Also, it should be stated some aerosols like black carbon do not reflect solar radiation and provide a positive forcing.

2.2.7

Land use change has two separate effects on climate. Flattening land and removing forests can increase albedo which reduces the incoming insolation which provides a negative forcing of about -0.15 W/m². This falls in the category of biophysical effects, along with roughness and evapotranspiration. However, the biogeochemical effects of land cover change (increasing CO₂) far outweighs the increase in albedo because destroying trees and vegetation releases a lot of carbon dioxide into the atmosphere. Slash and burn is often a practice that’s utilized to level a forest for cropland/livestock. However, this process has especially large biogeochemical emissions because burning vegetation and trees releases more CO₂.

Humans are responsible for 60% of land use change since the 1980s and the reasons for this include deforestation for lumber, cropland intensification, livestock intensification, and urbanization. Ward et al. (2014) determined that combined radiative forcing of both the biophysical effect and the biogeochemical effect totals 0.9 W/m² with a rather large uncertainty of 0.5 W/m².

2.2.8

Throughout section two of chapter 2, the report analyzes the magnitude of radiative forcing from different climate drivers to ultimately determine that the Earth is experiencing a positive net radiative forcing. The climate driver that contributes the most to this positive net forcing is well mixed greenhouse gases.

2.3.1

In order to construct a temperature profile of the past, we must rely on proxy data such as ice cores, ocean sediments, and fossilized organisms. Using proxy data, AR5 found the global mean surface temperature (GMST) during the Paleocene-Eocene Thermal Maximum (PETM) was 4 to 7 degrees Celsius warmer than the pre-PETM climate. Compared to the GMST in 1850 to 1900, the PETM’s GMST was 10 to 25 degrees warmer. The temperatures were so extreme during the PETM that multiple species of aquatic organisms went extinct. The Early Eocene Climatic Optimum was 10 to 18 degrees Celsius warmer than 1850-1900, the Miocene Climatic Optimum was not projected, the Mid-Pliocene Warm Period was 2.5 to 4.0 degrees Celsius warmer, and the Last Interglacial was 0.5 to 1.5 degrees Celsius warmer. Although these changes in temperature may seem drastic, the leap between each time period is thousands to millions of years. The GMST of the Last Glacial Maximum was estimated to be 5 to 7 degrees Celsius cooler compared to 1850 to 1900. This temperature is important because it gives us some understanding of what the global mean surface temperature would have to be for the climate to enter an ice age.

Findings from IPCC(AR6) Chapter 2.3.1.1.3 to 2.3.2.5

Dennis Duffin

2.3. Changes in Large-Scale Climate: New Estimates of Global Warming, 2-35 to 2-62

At a glance: global warming estimates have improved and updated from the previous IPCC report, released in 2014. This is due to advances in the following: (1) new observational datasets, (2) recent record warm periods and reevaluation of previous IPCC metrics. These changes have implications for the following estimates: threshold crossing times, remaining carbon budgets, and impacts assessments. Below we provide a summary of these changes, and of these implications.

Dataset Innovations

Comparing the AR6 to the previous report (AR5), all major datasets used for assessing observed temperature change based on global mean surface temperature have been updated and improved. These improvements can be explained through advances in technology from 2013, as well as through new interpolations (bringing in climate data from various sources for warming estimates). Consider how far personal technology has come since 2013 in the form of computational power (phones, laptops, etc.). These changes have also occurred at a climate modeling level, allowing for advancements in data collection, assessment, and adjustment (not to mention collection capability of temperature stations themselves). Without going into the weeds too much, these innovations result in a more accurate sampling of datasets, notably in the Arctic, which has now eliminated a cool bias from previous reports, and has improved confidence in models since IPCC5.

Effects of warming since AR5 + Reevaluation of previous IPCC metrics

To represent change over time, previous IPCC reports have used linear trendlines to compare 1850 to 2020. Since much of the warming has occurred since 1970, in AR6, this methodology has been improved upon, and instead focuses on change-based estimates over trend-based estimates. This change in methodology only impacted the warming estimates by 5% (about -0.03C), but it is important to cite, as it is a major improvement from AR5.

With that said, AR6 has confidence that each year from 2015 to 2020 has been warmer than any prior year on record. The 2011-2022 decade is believed to have been 0.19^o C warmer than the 2003-2012 period, which is approx. 0.34^o Fahrenheit. What is significant here is that 2016 was an El Nino event, which is a weather event associated with warming, where 2020 was a La Nina event, associated with cooling. The global mean temperatures of 2016 and 2020, however, seemed quite on par with one another. These data points indicate strong warming in recent years, and thus explain the calibration in values from AR5.

Updates to Global Warming Level Crossing Times

The global warming level crossing time is, historically, what makes the press from IPCC reports. This term refers to the threshold for where warming becomes quite dangerous, popularly set at 1.5^o C global warming. More specifically, this estimate is defined as the midpoint of a twenty-year period where the average global surface temperatures exceed this warming threshold. In the majority of AR6 scenarios, this crossing time now lies in the early 2030s – about a 10 year shift towards the present from previous estimates. This change in estimate results from a higher confidence in values (as we discussed previously), which ultimately shortened the estimated band of values for AR6.

Updates to Remaining Carbon Budgets

The Earth’s carbon budget is a concept cited in economics, energy policy, and climate science alike. This is because carbon dioxide, when emitted into the atmosphere, has a heat trapping effect. Thus, it has been well documented that increases in carbon emittance is correlated with global warming. Specifically, Earth’s carbon budget is a term referring to how much CO₂ humanity has left to emit, while remaining within a particular warming metric (traditionally 1.5^o C). The AR6 report indicates that the remaining carbon budget to remain under 1.5^o C of warming is approximately 400 billion tonnes CO₂, or approximately 50 tonnes per person on Earth at current population levels. Comparing these values to current CO₂ emissions (approximately 36 billion tonnes per year), Earth’s carbon budget will only last until the early 2030s. Such a calculation is a driver for the energy transition in the public and private sectors alike.

Updates to Assessments of Impacts, Adaptation

The AR6 has recalibrated the historical warming due to several factors, including improved reconstructions and a shift to the use of surface air temperature instead of surface temperature. These changes imply that the world has warmed more already than previously thought. Although AR6 presents a vision that Earth today is more impacted by anthropogenic warming than in previous reports, this finding itself does not generally imply that projected climate impacts are expected to occur sooner. On the level of “where are we today,” the variations from AR5 to AR6 are more recalibrations of values, and do not inherently serve as practical justification for the urgency of adaptation action.

Thus, for the most part, risks previously associated with 1.5^o C warming in AR5 have not changed in magnitude, but are instead associated with warming of 1.58^o C in IPCC6. Alternatively, warming impacts of 1.5^o C in AR6 are equivalent to those at 1.42^o C in AR5.

2.3.1.1 Surface Temperatures during the Instrumental Period

When discussing climate models, the instrumental period is often discussed. This period refers roughly to the mid-19th century, and extends to present day (considered the period where regular measurements of temperatures were taken). Thus, there is typically a much higher degree of certainty with trends in this period, as opposed to prior periods. AR5 concluded with certainty that global mean surface temperature had increased since the late 19th century, although the degree of warming held a level of uncertainty. AR6, with its improvements in datasets and metrics, have reduced these uncertainties both over land and sea.

2.3.1.2 Free Atmosphere Temperatures during the Instrumental Period

Climate scientists are not only concerned with changes in surface temperatures when discussing climate change, but they are also interested in changes in upper atmospheric temperatures, primarily in the troposphere (the lowest region of the atmosphere) and the stratosphere (second lowest). Scientists are also interested in the barrier between these two atmospheric regions (deemed the tropopause). Although not certain, tropopause height may impact the position of the jet stream, the strength and scale of storms in the tropics and extratropics, and other weather events, thus amplifying the impacts of global warming on the hydrologic cycle.

AR5 concluded with certainty that tropospheric temperatures had risen, and stratospheric temperatures had fallen, since the mid-20th century: however, there was low confidence in the vertical structure of temperature trends, specifically in regards to tropopause height and on temperature trends in the tropical upper troposphere. AR6 did not change the story much: there remains uncertainty in the quantitative impacts on atmospheric layers.

2.3.2 Global Hydrological Cycle

Paleo Perspective of Global Hydrological Cycle

When discussing global warming, it is important to consider the impacts of warming on the hydrologic cycle. The hydrologic cycle can be viewed as a feedback to global warming, amplifying the impacts of warming. As the air warms, it can hold more water vapor. Thus, a warmer climate can have more extreme precipitation, amplifying the amount of rain that falls during heavy precipitation events. However, unlike say temperature (which is more directly impacted by changes in Earth’s energy budget), precipitation is a more difficult metric to predict in climate models. Thus, it is important to examine historical periods of Earth, and find correlations between temperature and precipitation. AR5 assessed trends in hydroclimate, and found that Earth periods that were warmer were wetter, whereas periods that were colder were dryer. AR6 made considerable progress in assessing these trends from AR5, which can be expected, since paleo perspectives on climate is a relatively new field (since the 1990s). The emphasizing theme here is that many changes in the data from AR5 to AR6 are results of better science, and not as much a result of larger anthropogenic influence.

Surface Humidity

When referring to humidity, there are two ways to measure it, especially when considering impacts of global warming on the hydrologic cycle. There is the overall amount of water vapor in the air, the specific humidity. Then there is the proportion of water vapor in the air respective to its maximum capacity, which is relative humidity. An analogy for this is to think of a sponge: ringing the sponge completely out would be a measure of its specific humidity, where measuring how much water the sponge is holding as a percentage would be its relative humidity. If regions have higher specific humidities, then that means there is a higher potential downpour, in cases of rainfall. Whereas when regions warm, the capacities to hold water vapor increases, and thus the relative humidity could decrease. AR6 has high confidence that surface specific humidity over land and ocean has increased since the 1970s, followed by a decrease in relative humidity over most global land areas since the 2000s. These values also provide evidence for more intense rainfall events since the 1970s.

Global Precipitation

Global precipitation may seem like a natural result of increases in specific humidity, however, this is not necessarily the case. In instances of rainfall, yes, global warming will likely result in increased rainfall due to more moisture in the air. However, precipitation events globally are constrained by multiple factors, including the energy required for evaporation, which is not directly a consequence of global temperature. With that said, AR6 found that globally averaged land precipitation has likely increased since the 1950s, with this increase accelerating since the 1980s. However, these estimates are not stated with high confidence, and there is even less certainty over the global oceans.

2.3.3 Atmospheric Circulation

Hadley and Walker circulations

When discussing climate change, often you will hear about the amplification of extreme weather events (wet regions will get wetter, dry regions will get drier). This is a direct result of increasing humidity in the air. However, circulation also affects rainfall. In particular, one may hear of the convergence of moisture in rainy tropical bands, and the removal of moisture from desert regions at particular latitudes. These are a result of Hadley Circulations – low-latitude overturning circulations that have air rising near the equator and sinking in the subtropics. AR6 finds likely evidence of a widening of Hadley circulation since the 1980s, with less certainty to the extent of the widening. This is followed by a strengthening of the circulation, particularly in the Northern Hemisphere. The potential strengthening would take even more moisture towards the wet tropical regions, while the desert regions will have more moisture carried away.

Another circulation to note is the Walker circulation. Instead of the Hadley cell (which, as discussed, causes rainfall to increase near the equator), the Walker cell results in air rising over the Western Pacific. Thus, as you travel across the Western Pacific, you’ll also experience higher rainfall averages. AR6 finds likely evidence of a strengthening of Walker circulations, resembling a La-Niña-like pattern, with a western shift, with less certainty to the extent of these changes.

2.3.4 Cryosphere

Sea ice can be distinguished from icebergs in their origin: sea ice is frozen seawater that floats on the ocean’s surface, whereas icebergs originate from glaciers. Sea ice properties impact shipping routes, marine animal behaviors, and climate by reflecting radiation back to space.

Arctic Sea Ice: Annual mean Arctic sea ice extent was found to have likely decreased by 3-4% each decade between 1979 and 2012, with confidence that the larger decrease in September had not been seen in the past 1000 years. Additionally, Arctic sea ice was shown to be thinner year round, correlated with higher rates of sea ice drift speed. Sea ice in the Arctic seems to play a more important role in regulating climate.

Antarctic Sea Ice: Antarctic sea ice data remains imprecise, due to a high variability in measurements from the area during the instrumental record. The satellite era (1979) kicked off more reliable Antarctic recordings, and from this period onward found both increases and decreases in sea ice area. While important to biodiversity, Antarctic ice plays less of a role in regulating climate, and thus these values being variable should not contribute highly to variations in the climate models.

Snow cover is an important component of Earth’s energy budget. Unpolluted, freshly fallen snow reflects radiation back into the atmosphere (referred to as albedo), and results in a net cooling effect. Thus, as terrestrial snow melts, it amplifies the impacts of global warming. Since 1978, there have been substantial decreases in spring snow coverage in the Northern Hemisphere.

Glacier mass is a critical measure, since melting glaciers add to rising sea levels, contributing to more intense coastal storms. AR6 finds with very high confidence that worldwide, glaciers have retreated in mass since 1950, and continue on that trajectory. Although there is history of glacial retreats in the pre-instrumental period, what is occurring in this period is highly unusual, as almost all glaciers are simultaneously retreating. Such a phenomenon has not occurred in the past 2000 years, with high confidence. Glacier mass retreating has increased since the 1970s.

The Changing of Oceans

Arief Suryo

I. Introduction and Background
The five big oceans of the world, the Atlantic Ocean, Pacific Ocean, Indian Ocean, Arctic Ocean, and Southern Ocean are having bigger roles than ever before in the energy budget of the Earth system. This subchapter assesses many changes that has happened historically in each component of the ocean and the relations among them.

a. Ocean Surface and shallow temperature
Since the last assessment report (AR5) most observation show the increasing of global ocean warming for all depths in the ocean. The rate of warming over the period of 1993 -2017 had likely more than doubled compared with the period of 1969 -1992. The AR5 shows different certainty of this conclusion at different depths. The scientists are mostly certain for the upper 700 m depth, but only slightly likely certainty for layer from 700m to 2000m depth and below that there exists rising of ocean temperature.

The measurement of ice core and sediment cores has been used as a passive tracker for the increase of ocean temperature all over the world. The rise from the Last Glacial Maximum to the Holocene has been recorded at rate of 2.57^o C ± 0.24^o C. Ocean heat content (OHC) measurements have been derived from ocean-climate model with much needed bias adjustment to reduce the uncertainty. The AR5 found that there is a sustained increase in global OHC and closely related with global thermosteric sea level (ThSL). Current multidecadal rates of increasing OHC is greater than the last deglaciation and has a tight link with the change in surface temperature over the centuries.

b. Ocean Thermohaline Circulation
One part of large-scale ocean circulation is the thermohaline circulation. This circulation is driven by the different gradients of global density that move hot and cold water of the ocean through all over the big 5 oceans. This circulation has direct impact to the global climate and is one of the main feedback to the climate change.

i. Atlantic Meridional Overturning Circulation
The Atlantic Meridional Overturning Circulation (AMOC) is a large system of ocean circulation in the Atlantic that acted as conveyor belt of water and heat between the equator and the Arctic. In AR5 report, there was no reported evidence of a trend in the AMOC system due to insufficient observation at that time. But the Special Report on the Ocean and Cryosphere in a Changing Climate (SROCC) has reported that there has been a weakening of AMOC during 2004 -2017 compared with 1850-1900. There are multiple lines of observational evidence since the 1980s that help to show the change in AMOC.

ii. WBCs
Western boundary currents (WBCs) are categorized as narrow, deep-reaching, and fast-flowing currents that are an integral part of oceanic circulation and gyres. There is medium to high confidence that WBCs has been undergone an intensification, warming and poleward expansion except for the Gulf Stream and Kuroshio current. The poleward shift of subtropical gyres is on the order of 0.1^o ± 0.040^o per decade over the period of 1993 to 2018.

c. Sea Level Rise
For the period of 1902 to 2010, SROCC has concluded that the global mean sea level (GMSL) has been increasing for 1.1 – 1.9 mm/yr. Its rate of increase is larger than the previous two millennia. Uncertainty also arises for the GMSL variability over regions. The vertical land motion, the distribution of ice sheets and isostasy driven by sediment are main key things that added to the large uncertainties.

II. The Changing Chemistry of the Ocean
The ocean chemistry has been affected by emissions directly and by global warming. This could lead to increased vulnerability of marine ecosystems.

a. Ocean Salinity and Acidification
The last AR5 report showed that subtropical regions have been more saline, while tropics and high latitudes area have becoming fresher since 1950s. The mean surface contrast between high and low area of salinity has increased by 0.13 [0.08 to 0.17] PSS-78. Meanwhile the Atlantic has become more saltier, and Pacific and Southern Ocean have become fresher. With newer observational data gathered from Argo floats, the ocean reanalysis model has shown more robust patterns of surface salinity with the hydrological cycle. With high certainty, scientists have concluded the salinity change also extends to the ocean interior along ventilation pathways.

Meanwhile the acidity of the ocean (pH) has decreased since preindustrial times mainly because of the ocean uptake of CO₂ from atmosphere. From paleo proxy measurements, the surface pH has steadily increased over the last 50 Myr. Since the 1980s, the decline of pH has been observed in all the oceans. In summary, ocean pH has been decreased globally over the last 40 years by 0.003 – 0.026 pH per decade. Meanwhile the surface open ocean pH is at its lowest for at least 26 kyr.

b. Oxygen content reduction in the ocean
SROCC has claimed that there is high confidence in the loss of oxygen over all ocean depths since 1960. The rate of deoxygenation is between 0.3 to 2.0%, meanwhile in the upper 1000m depth of the ocean, the decline is between 0.5 – 3.3%. The result of the measurement is mainly limited to regional scale assessment. It’s because there is difference in depth range, time-period, baseline climatology, methodology, and the use of different units. In summary the deoxygenation has been closely related with the warm climate.