5 Topics from Chapter 5: Global Carbon and Other Biogeochemical Cycles and Feedbacks

Introduction

Daisy Aguilar

Since the beginning of the Industrial era, the three major long-lived heat-trapping gases, carbon dioxide (CO₂), methane (CH₄) and nitrous oxide (N₂O), have increased substantially. This chapter details the atmospheric budgets, variability, trends of the past, and projections into the future for these three gases.

Atmospheric CO₂ levels are not just determined by emissions; both the ocean and the terrestrial biosphere respond by taking up CO₂. These strength of these processes are connected to both chemical properties and biological responses, and the term “biogeochemistry” is used to encompass the interactions among these systems. The chapter also discusses future projections of ocean acidification and its effects on marine life.

Since the ocean and terrestrial sinks both remove CO₂ from the air, they are negative feedbacks. The airborne fraction (AF) of CO₂ measures the fraction of emissions that stays in the air, and had a mean of 44% between the years of 1959-2019.

Climate models can help assess future projections of carbon and biogeochemical cycles and improve understanding of carbon-climate feedbacks. A crucial question is the remaining carbon budget for stabilization at particular temperature targets like 1.5 or 2^o C above preindustrial. The transient temperature response to cumulative CO₂ emissions (TCRE) is a useful metric in this discussion.

While the focus of the chapter is primarily on the last 60-year period, information from climates of the past are also useful to understand carbon-climate feedbacks. The Cenozoic Era consists of the last 66 million years of Earth’s history, and has some dramatic changes in the carbon cycle. For instance, during the Paleocene-Eocene Thermal Maximum (PETM) nearly 56 million years ago, CO₂ concentrations increased roughly 900 to 2000 ppm in only 3,000-20,000 years, creating a hot, short-lived climate. Despite the rapidity of this change, it is estimated that the current rate of increase is 4-5 times faster than in the PETM.

The Mid-Pliocene warm period 3 million years ago is thought to have similarly high CO₂ concentrations as today, with feedbacks that differed from today due to a stronger ocean circulation and a decrease in carbon storage in the deeper part of the ocean.

The past, present, and future each provide a unique scenario of the major biogeochemical processes on Earth. A better understanding can be gathered by evaluating controls on concentrations by different underlying source and sink processes over different timespans. There are various options to be considered when looking into carbon dioxide removal. When considering these options, something to keep in mind is the biogeochemical effects and how feasible it is to achieve these goals.

Section 5.2: Historical Trends, Variability and Budgets of CO₂, CH₄, and N₂O

Ryan Boyd

Section 5.2 of the IPCC AR6 Report summarizes the three main long-lived greenhouse gases in our atmosphere: carbon dioxide (CO₂), methane (CH₄) and nitrous oxide (N₂O). The budgets, growth rates, emissions, sinks, spatial variability, lifetimes, and many other factors concerning the characteristics of these gases in our atmosphere were covered in depth over this section. These gases are so important due to the radiative forcing that they cause. Radiative forcing is essentially how much extra energy is trapped by a gas. In response to a positive radiative forcing, the Earth’s temperature must rise. Understanding where these gases come from, the interactions they have in our atmosphere, and other factors can help govern the decisions being made about climate change.

The largest cause of the radiative forcing from greenhouse gases has been carbon dioxide, the budget of which is controlled by several factors. A mass balance of any atmospheric component can be considered as the sum of sources set equal to the sum of the sinks. The sources of CO₂ would include fossil fuels and carbonate emissions (abbreviated [latex]E_{FOS}[/latex]) and CO₂ fluxes from forestry and other land use (abbreviated [latex]E_{LULUCF}[/latex]). The sinks of CO₂ include the growth rate of CO₂ in the atmosphere (abbreviated [latex]G_{ATM}[/latex]), the ocean sink ([latex]S_{OCEAN}[/latex]), the land sink ([latex]S_{LAND}[/latex]), and a term to account for the independent estimates for some of these terms ([latex]B_{IMB}[/latex]). Combined, these components give a budget for carbon dioxide consisting of: [latex]E_{FOS} + E_{LULUCF} = G_{ATM} + S_{OCEAN} + S_{LAND} + B_{IMB}[/latex]. Impacting one of the components of this equation can have varying effects in the global climate system.

The vast majority of changes to carbon dioxide emission rates since the pre-industrial era have been due to anthropogenic activities. The two main sources are emissions from fossil fuel production and consumption, and land usage, land-use change and forestry, abbreviated LULUCF in the report. Fossil fuel emissions are estimated by different sources by considering economic activity and numerous other factors, with an uncertainty in global emissions of ± 5%. Over the past decade CO₂ emission growth has declined slightly, which is attributed coal being used less for energy production. The LULUCF source deals with CO₂ release from converting and managing land to be used for agriculture or other practices. Comparing the emission rates of these two sources in Figure 5.5, fossil fuels have emitted roughly 6 times more carbon annually than LULUCF. However, since the 1980s, there is low confidence in LULUCF trends due to differences between models.

Figure 5.6 provides atmospheric evidence for the increases in CO₂ due to fossil fuel emissions. In Figure 5.6a, since CO₂ measurements began at the South Pole and Mauna Loa observatory in 1957 and 1958 there has been an annual growth rate of 1.56 ± 0.18 ppm. Additionally, the amount of δ₁₃C isotope has decreased steadily since measurements began, via Figure 5.6b. Fossil fuels are low in the δ₁₃C isotope, so a decrease in the value of this isotope and an increase in atmospheric CO₂ would indicate that the source of this CO₂ increase is fossil fuels.

Ocean and land sinks have seen increases in their sink capacity as atmospheric levels of CO₂ have increased; however, this growth is not enough to keep up with the growth in atmospheric CO₂. CO₂ in the air heavily influences the ocean sink, as well as the transfer of CO₂ between the atmosphere and the ocean. It was estimated the ocean takes in about 23% of anthropogenic emissions, and that natural CO₂ emissions from the ocean occur in lower latitudes, with much of the oceanic CO₂ uptake occurring in the mid-latitudes. The recycling of ocean water through the meridional overturning circulation disproportionately affects the ocean carbon sink in certain regions of the globe, such as it being 20% less in the Indian and Pacific parts of the Southern Ocean.

In terms of the land sink of CO₂, there is a high level of confidence that the land CO₂ sink has increased over roughly the last 60 years, via various vegetative and atmospheric models. Overall, the Northern Hemisphere acts as a greater land sink of atmospheric CO₂, due simply to there being more land area compared to the Southern Hemisphere. The ecological biomes that serve as the greatest CO₂ sinks are forests, because of the longevity of carbon in trees compared to other forms of vegetation.

There is also growing evidence that the accumulation of atmospheric CO₂ is potentially impacting water cycling in plants and drought. It has been reported that plants are increasing their water-use efficiency through the closure of their leaf stomata, which is how they regulate their water retention. This closure of plant leaves can inadvertently increase the availability of water on land and in soil, as plants will not be transpiring. CO₂ increases also have other effects, which ultimately lead to greater evapotranspiration, potentially offsetting the more available water from stomata closure. Evapotranspiration is the process by which water moves from soil to the atmosphere through plant transpiration. This process can vary strongly with greater atmospheric CO₂, as greater CO₂ amounts can increase photosynthesis, plant growth, and the vegetative growing season. Additionally, these drought impacts are affecting the ability of the land to act as a carbon sink, as forested areas are negatively impacted by drought, leading to die-offs and greater threat of fire.

Compared to carbon dioxide, methane (CH₄) is a stronger greenhouse gas. More radiative forcing can be caused per methane molecule than per CO₂ molecule, and additionally can undergo chemical reactions with other components of the atmosphere, making it an incredibly important greenhouse gas to account for.

When determining the importance of these atmospheric molecules, it is important to discuss their lifetimes. A chemical lifetime is how long the molecule may last in a system before being depleted through various loss processes. Methane has an atmospheric lifetime of roughly 9.1 ± 0.9 years in the troposphere, with 90% of its loss process being through oxidation with the OH radical, and an additional 5% due to oxidation via bacterial processes. In the lower troposphere, a small fraction is lost through reaction with chlorine, excited oxygen atoms (O¹D), and other reactions with OH. Additional loss of a similar scale occurs through transport to the stratosphere, as shown in Figure 5.14.

The IPCC claims with virtual certainty that methane emissions outweigh the loss, and that the near doubling in methane emission rates are primarily anthropogenic. Changes to the strength and consistency of methane emission processes have led to variations in methane’s growth rate over the past few decades. According to Figure 5.13a, since measurements started being recorded in the 1970s methane has increased from as low as 1600 ppm to over 1900 ppm in just four decades. Ppm, meaning parts per million, is a measure of the concentration of a specific molecule in our atmosphere. This increase of roughly 300 ppm is characterized by steady increases from 1980-2000 and from 2007-2016. This increase, while strong, is categorized by a plateau in emissions in the early 2000s, largely attributed to emission declines in certain fossil fuel sectors and increased loss in the atmosphere. The resumption of methane concentrations increases in 2007 is up for debate. The fraction of emissions contributed to fossil fuels is likely decreasing however, evidence for which is once again provided by the δ₁₃C isotope, which saw a sharp increase in its emissions during the early 2000s plateau in Figure 5.13c.

Methane has both natural and anthropogenic emission processes, and one of the key challenges scientists face is determining if changes in emissions are directly from humans, indirectly from humans via climate change, or naturally. Anthropogenic emission processes include agricultural practices and livestock, landfill leakage, waste facility leakage, and fossil fuel extraction processes. Many of the anthropogenic methane emissions are primarily sourced in the Northern Hemisphere due to the heavy industry common in the United States, parts of Europe, and China. It is estimated that of the fossil fuel sources, coal mining alone has been responsible for 35% of those emissions; China in particular has seen a strong increase in methane emissions due to coal mining in the past decade. Agricultural activities and livestock management is another main source of anthropogenic methane. When livestock manure is stored, biological processes in oxygen-deficient conditions result in methane formation. Landfills are another significant source and globally their emission have been increasing since 1970 despite landfill emission decreasing in heavily industrialized western nations. Furthermore, burning of biomass (both a natural and human process) and the consumption of human-made biofuel account for approximately 5% of global anthropogenic methane emissions. Droughts can increase methane emissions by increasing fire potential, a threat that may worsen with global warming.

Natural emissions of methane are primarily from wetlands but are also from inland freshwater sources, ocean water sources, and various geological processes. Wetlands produce methane not through vegetation but rather by biological processes within their soils. Vegetation does play a role in methane transport, especially in lower latitudes. Modelling wetlands can be challenging due to the exact spatial bounds of wetlands being poorly defined sometimes. This, combined with the fact that their emissions have high seasonal variability due to factors such as temperature and sunlight make any wetland emission estimates inherently uncertain. Methane emissions due to water sources and various geological processes are also clouded by uncertainty. Coastal oceans, fjords, and mud volcanoes are all characterized by poor methane measurements, giving rise to uncertainty in these emission estimates. Inland water methane sources, which consist of freshwater sources such as lakes and rivers, are often unintentionally counted or measured twice when estimating emissions, contributing to their uncertainty. There is also medium confidence that anthropogenic activities and global warming result in greater methane emissions from inland water sources.

Combining these various emission rates and sinks gives an atmospheric growth rate for CH₄ of 2 ± 4 ppb/yr in the decade of 2000-2009, and 7 ± 3 ppb/yr between 2008-2017, according to Table 5.2. The strong growth rate in the last decade compared to the plateaued, highly uncertain growth in the decade prior shows with virtual certainty that the emissions and sinks of methane have changed considerably in recent decades and can easily be further perturbed by human activities. This is shown visually in Figure 1 of Cross-Chapter Box 5.2, where the imbalance between sources and sinks is lowest during the plateau of the 2000s, and much higher in other decades.

The third and final greenhouse gas discussed by the IPCC report in this section is nitrous oxide, or N₂O. Not only is N₂O a greenhouse gas capable of radiative forcings, but it also plays a role in stratospheric ozone loss and other chemical reactions in the atmosphere. Increases in nitrous oxide are due to anthropogenic processes, and sinks are through naturally occurring processes in the soil and atmosphere.

To understand anthropogenic N₂O production, it is important to understand the various complicated processes that produce nitrous oxide. The main emission sources of anthropogenic N₂O include the chemical industry’s production of synthetic fertilizers, agricultural practices involving fertilizer and manure, wastewater, and fossil fuel emissions. Fertilizers often contain nitrogen (a plant nutrient), and the production of these fertilizers by the chemical industry will often involve the emission of N₂O. There is high confidence that since the 1980s, there has been a roughly 45% increase in agricultural N₂O emissions, largely due to the increasing usage of these synthetic fertilizers. Many bacteria also produce nitrous oxide as a by-product. N₂O is often released as a result of combustion following the burning of biomass and of course the ever-present combustion of fossil fuels.

N₂O is also released via natural sources, some of which are potentially being influenced by human actions. Some of the largest contributors of natural N₂O emissions are microbial processes in soil. Nitrification and denitrification are natural biological processes performed by bacteria in which varying forms of nitrogen are interconverted, often releasing nitrous oxide in the process. These bacterial soil processes also play a small role in the natural N₂O sink. The ocean is another natural N₂O source. Ekman transport is the process by which ocean water is displaced at the surface through a combination of forces. When Ekman transport occurs off of the western coasts of continents, deep ocean waters rise to the surface, in a process known as upwelling. This ocean water is often rich in nutrients and can release nitrous oxide into the air. The IPCC points out that this N₂O source is often not included in climate models, potentially affecting ocean N₂O estimates. Oxygen is present in roughly 97% of the ocean, and where it is, nitrification is the primary N₂O source. N₂O is also released in inland waters, where specifically the nitrification and denitrification processes play a role. Human-made canals, lakes, and other bodies of water can potentially impact these emissions, and the emissions from this source may change more as humans further change their environment.

Figure 5.17 gives a cohesive look at the overall nitrous oxide budget of the earth and its atmosphere. The largest sink of N₂O is stratospheric loss, where N₂O is lost to various chemical reactions. Estimates of nitrous oxide emissions are improving due to better models and understanding of their sources and sinks, and it is estimated that roughly 40% of all N₂O emissions are anthropogenic in origin. The primary anthropogenic source, agriculture, has been trending up since 2007-2016.

These three greenhouse gases are incredibly influential on both present and future climates because of their differing effects on radiative forcing. To understand their differing effects however, one must first understand each’s lifetime. As discussed previously, methane’s lifetime is rather short-lived, only about 9 years. However, CO₂’s lifetime is incredibly variable, ranging from one year to thousands, and N₂O’s average lifetime is 116 years. So even if these gases’ emissions were immediately shut off, they would still exist in the atmosphere to disturb Earth’s energy balance for decades. Figure 5.18 shows the effective radiative forcing of these greenhouse gases, with CO₂ significantly outweighing the effects of CH₄ and N₂O over the last 150 years.

Thus, it is imperative that changes to all sectors of the economy and society begin soon so as not to worsen the damage that has been done soon. Clean and renewable energy sources need to be implemented sooner rather than later. Changes to industry and how various goods and materials are produced is necessary. For example, there are changes the chemical industry could implement to reduce N₂O emissions that are not only efficient but also inexpensive. The warming the Earth will experience over the coming decades is inevitable, but actions to prevent any additional damage should be taken sooner rather than later. Currently, the evidence is there, but the necessary response is not.

Section 5.3: Ocean Acidification and Deoxygenation

Nicole Ferrie

Assessing ocean acidification and deoxygenation events in paleoclimates or climate of the recent past can help understand current and future ocean acidification and deoxygenation events. From the onset of the Industrial Revolution to today our oceans have absorbed 25% of anthropogenic CO₂ emissions. Increasing CO₂ emissions within our oceans has led to ocean acidification and deoxygenation. There is robust evidence that acidification is due to increased hydrogen concentration from carbonate buffering reactions. There is high confidence that this acidification is impacting marine organisms. There is a 99% probability that anthropogenic emissions of greenhouse gases have substantially contributed to ocean warming. Ocean warming decreases stratification within the upper ocean and lends to slowing ocean circulation. Slowing ocean circulation leads to decreased ocean ventilation. Ocean ventilation is the process that transports carbon dioxide and other important greenhouse gases dissolved in the surface layer of the ocean into the interior of the ocean. Ocean warming and decreased ventilation causes deoxygenation, which affects marine life.

5.3.1 Paleoclimate Context
The Paleocene-Eocene Thermal Maximum (PETM) was a period of global warming 55.9-55.7 million years ago that exceeded pre-Industrial temperatures by 4-8°C. It was caused by a large pulse of CO₂ delivered to the ocean-atmosphere system in a geologically short time span. As one of the largest causes of ocean warming is due to CO₂ emissions on a very short time scale, there may be a parallel between the ocean’s change during PETM to today. The PETM climate proxy shows that there was ocean acidification. A climate proxy is a physical characteristic or item from a past period that can be used to construct past climates. For example, marine fossils can help to construct past sea levels. There is medium confidence among models that the increased CO₂ concentrations decreased the depth at which calcium carbonate is dissolved. However, there is low confidence that the rates of ocean acidification is a magnitude lower than today’s rates. Deoxygenation is more certain, with models showing medium confidence that it was widespread, and some areas were virtually anoxic. Feedback mechanisms that caused recovery of acidification and deoxygenation in the ocean are not well constrained, but may be due to the recovery of ocean and land carbon sinks, and silicate weathering – which, in simplified terms, is the uptake of inorganic carbon by rocks.

5.3.2 Historical Trends and Spatial Characteristics in the Upper Ocean

Over recent decades, there is a 99% agreement that the ocean has undergone acidification globally due to CO₂ uptake. Ocean pH has dropped by -0.017 to -0.027 every decade since the late 1980’s. However, this acidification is not spatially or temporally uniform. The central and eastern tropical pacific have undergone the fastest acidification, the western tropical pacific the slowest, and the subtropical ocean in between. The subpolar and polar oceans have a wider range of pH change (-0.03 to -0.026). The spatial variations in acidification are due in part to physical variations such as differing rates and patterns of ocean circulation, as well as different seawater temperatures. The spatial variation is also due to different biological activity. For example, the tropics have a higher rate of biological activity than the subpolar and polar regions. Temporally, ocean acidification rates differ due to internal climate variability. Internal climate variability is variations in climate overtime not caused by humans. Examples of this are monsoons in Asia, or the Pacific Decadal Oscillation which causes variations in sea surface temperatures over 20 to 30 year periods.

5.3.3 Ocean Interior Change

There is a 99% chance that the interior of the ocean is going through acidification due to CO₂ uptake at the surface. This is caused by ocean ventilation and circulation. However, the amount of acidification due to anthropogenic CO₂ decreases with depth. This varies spatially, with the subpolar, North Atlantic and Southern Ocean presenting the deepest acidification due to their ocean circulation patterns. The spatial variability in ocean circulation and ventilation leads to low confidence on the extent of ocean acidification in many regions. Along with acidification, the ocean interior is experiencing deoxygenation. This is caused in part due to a decrease of oxygen solubility, a decrease in ventilation, and slowing ocean circulation. However the spatial and temporal distribution due to physical and biological parameters means that deoxygenation is not uniform. The North Pacific, Southern Ocean, and South Atlantic have seen the greatest deoxygenation. Oxygen loss may reduce nitrous oxide flux from the ocean into the atmosphere, creating an ocean-climate feedback mechanism. While nitrous oxide emissions are expected to decline within the ocean, there is low confidence in the decrease of the nitrous ocean-atmosphere flux.

Chapters 5.3.4-5.3.5

Ryan Boyd

The trends of ocean acidification and deoxygenation are progressively getting more severe, and scientific developments since the last IPCC report have allowed for more accurate predictions for future ocean acidification and its spatial variability. Deoxygenation is the process by which oxygen levels in the ocean are reduced. Eutrophication is one of the main causes of this, which is when runoff causes excess nutrient levels. Additionally, the impact of acidification and deoxygenation on coastal oceans should be addressed, due to high confidence of these areas being impacted as well.

First, when compared to AR5, ocean pH is expected to decrease more than what was predicted in that report. pH is a measure of the ocean’s acidity, with a more acidic ocean resulting in a lower pH. A decrease of -0.16 ± 0.002 pH compared to the previous value of -0.14 ± 0.001 pH is reported, with this change being attributed to greater levels of CO₂ entering the atmosphere. Further, due to the meridional overturning circulation (MOC) discussed in the prior section, ocean acidification is predicted to extend into the depths of the ocean, albeit of a lower magnitude than the above changes. Due to the lengthy timescales for the CO₂ to be dispersed within the ocean through the MOC, these changes will be near irreversible, even if geoengineering methods such as carbon dioxide removal are performed on a mass scale.

In terms of the coastal ocean, it is reported with high levels of confidence that ocean acidification and deoxygenation play a strong role in changes to coastal waters. In these areas, the fluxes of CO₂ between the water and the atmosphere are highly variable, due to varying factors including but not limited to primary production (including photosynthesis and respiration) and eutrophication.

Spatially, there is high differentiation in the responses to ocean acidification and deoxygenation in coastal waters. The process of acidification is time-consuming, and there is high confidence that subtropical to temperate coastal waters have an emergence for acidification of more than two whole decades. Furthermore, factors such as eutrophication can vary from region to region. Eutrophication is highly prevalent in areas affected by high anthropogenic activities that produce waste such as freshwater, nutrients, organic matter, and other discharges from industry. Deep coastal waters also often see lower pH values. The lack of sunlight lowers the rate of photosynthesis and with cellular respiration ongoing, CO₂ levels then rise and lower the pH. Additionally, regions of heavy industry have strong effects on nearby coastal waters, often saturating them with CO₂, negatively affecting the biology of these areas. These biological effects are only made worse by temperature fluctuations, hypoxic conditions, and the introduction of toxic elements that come from anthropogenic activity. Hypoxic conditions are often related to other factors, such as seasonal circulations and stratification of the coastal waters, which can cause organic matter build-up, worsening the hypoxic conditions. There is a medium level of agreement that hypoxic conditions may be ameliorated by reducing the leakage of nutrients due to human activities.

The effects of ocean acidification and deoxygenation are far reaching. As discussed, ocean acidification is occurring globally, even in areas like the deep ocean. Ocean acidification and deoxygenation can have detrimental effects on marine life and biology, a prime example being coral bleaching events. Reducing fossil fuel emissions is an important step in ensuring the health of the globe’s oceans.

Section 5.4: Biogeochemical Feedbacks on Climate Change

Nicole Ferrie

Biogeochemical feedbacks are poorly constrained in climate models and one of the biggest uncertainties in climate change projections. Biogeochemistry is an interface between abiotic and biotic processes that focuses on the study of how chemical materials cycle between living things and the Earth’s nonliving spheres: atmosphere, hydrosphere, and lithosphere. The biogeochemical feedbacks of Earth can either amplify or diminish climate forcings that lead to climate change.

Specifically, biogeochemical feedback affects greenhouse gas forcings through things like carbon dioxide and nitrogen cycling where Earth’s reservoirs can act as sources or sinks for these compounds.

5.4.1: Direct CO₂ Effect on Land Carbon Uptake

Both the IPCC Fifth Assessment Report (AR5) and the Special Report on Climate Change and Land (SRCCL) concluded with high confidence that ecosystem carbon storage increases with increasing atmospheric CO₂. This relationship, which is a negative feedback, is due to increased photosynthetic flux at the leaf-level. A negative feedback diminishes a process. Here the ecosystem takes CO₂ out of the atmosphere in response to increasing anthropogenic CO₂ emissions. However, the magnitude of this negative feedback depends upon a multitude of other factors in plant ecosystems – such as: seasonal drought, where plants chose to use the carbon they absorb, changes in plant community composition (i.e. changes in the species of plants within the ecosystem), disturbance (i.e. fires or storms), natural plant mortality, nutrient availability, acclimation of photosynthesis to long-term CO₂ exposure, and growth temperature (the temperature range that the plant are able to grow in). The effect of these other factors on CO₂ uptake by ecosystems is unknown and poorly constrained in climate models. The exception to this statement is nutrient availability. In recent years, field studies have shown that despite increasing atmospheric CO₂ concentrations photosynthesis will decline with lack of sufficient nutrients. However, the magnitude of this effect is unknown.

5.4.2 Direct CO2 Effects on Projected Ocean Carbon Uptake

From the onset of the industrial revolution our oceans’ have acted as an integral negative feedback to growing CO₂ rates. Currently, oceans act as a sink for 25% of atmospheric CO₂. A sink takes up more carbon from the atmosphere than it releases to the atmosphere. Climate models conclude that as atmospheric CO₂ increases ocean uptake will also increase. This pattern will continue until 2050 when the ocean sink stops growing due to warming temperatures, slowing ocean circulation, wind patterns, marine organisms activity, and carbonate buffering. The ocean absorbs carbon through a buffer reaction. A buffer reaction tries to resist changes in pH (potential hydrogen) through chemical reactions. As CO₂ is added to the ocean the buffer reaction within the ocean produces acidic hydrogen ions (𝐻+), carbonate ions (𝐶𝑂₃²⁻), carbonic acid (𝐻₂ 𝐶𝑂₃ ), and bicarbonate ions (𝐻𝐶𝑂₃−) known as dissolved inorganic carbon (DIC). Eventually the ocean will absorb too much CO₂, creating too much dissolved inorganic carbon which causes the acidity to drop. The reaction will then move backwards, no longer accepting CO₂. Aside from the CO₂ concentration itself, warming temperatures reduce CO₂’s solubility in the ocean. Slowing ocean circulations, wind patterns and temperature affects the movement of the carbon in the ocean. The ocean takes up carbon mostly at the surface. The amount of carbon it takes up is based on a gradient from the air into the ocean. If all of the carbon is sitting at the surface of the ocean because ocean circulation has slowed, the ocean will take up less carbon. Due to increasing ocean acidity the calcification rates of marine organisms decrease, which also decreases carbon uptake by the ocean. Overall, as more CO₂ is added to the ocean the rate of ocean CO₂ uptake will decrease.

5.4.3 Climate Effect on Land Carbon Uptake

Earth’s plants and soils act as carbon dioxide sinks for atmospheric CO₂. However climate change can mitigate the magnitude or sign of this feedback. Climate models predict with medium confidence that land carbon uptake will decrease due to climate change from increased fires, changes in soil carbon storage, and plants physiological response to warming temperatures. Warming temperatures affect the speed of photosynthesis and the water gradient that drives water loss from plants. Whether these processes will increase or decrease the rate of photosynthesis is vital in understanding plants magnitude as a carbon sink — yet these processes are poorly constrained. Models predict with high confidence that fire prone weather will increase throughout the world, while fires are predicted to increase in both occurrence and severity in tropical, Arctic, and mid-latitude climates. Climate models conclude with low confidence chance that fires are a positive carbon feedback. Soils which currently act as a negative feedback are expected to become a positive carbon feedback with the changing climate due to increased decomposition from warming temperatures. Decomposition of organic matter from living organisms releases CO₂ from the soil. Permafrost, a frozen soil in high latitudes, is a large sink of carbon, as the temperatures warm and the permafrost melts, models predict a large release of carbon. The magnitude of the carbon-climate effect for fire, plants, and soils is poorly constrained, and further work is required to understand these mechanisms.

5.4.4 Climate Effects on Future Ocean Carbon Uptake

Anthropogenic climate change will affect both the physical (i.e. heat distribution, ocean circulation speed) and biological (i.e., marine life) processes that are responsible for carbon uptake in the oceans. Physical drivers are primarily responsible for the present-day anthropogenic carbon-ocean sink, with the paramount process being CO₂ uptake by air-sea fluxes. An air-sea CO₂ flux is a large-scale structure that is controlled by CO₂ gradients between the air and sea. CO₂ can move in between the air and sea through wind-driven gas exchange, as well as vertically in the ocean through sinking down (subduction) and coming back up (upwelling). Climate change affects these processes in three ways, one being ocean warming. Ocean warming reduces the solubility of CO₂, and reduces CO₂ movement in the ocean. Consequently more CO₂ will remain in the surface layer than the deeper layers of the ocean. Second, climate change affects wind and storms. Changing wind and storms will decrease the ocean CO₂ mixing, and wind-driven gas exchange. The third process is climate’s effect on ocean circulation. The ocean has an inter ocean wide circulation that moves around CO₂, heat, salt, and other materials. Ocean circulation is expected to slow down with climate change. This increases CO₂ stratification in the ocean, decreasing the air-sea carbon dioxide flux. Each of the three physical processes here act as positive feedback mechanisms, decreasing the amount of carbon the ocean will take up. However, models do not know the extent of these processes in weakening the ocean as a carbon sink.

Biological processes are responsible for natural (non-human) carbon storage and control the carbon sink on geological time scales (millions of years) through the biological carbon pump (BCP). The BCP sequesters carbon from organic matter like phytoplankton or shelled organisms. It controls much of the vertical gradient of DIC. Ocean warming will limit the supply of nutrients to the upper ocean due to stratification. This will cause a decrease in photosynthetic organisms in low latitudes. Climate change will alter the magnitude and efficiency of the BCP but the sign of the feedback and the magnitude is not known.

5.4.5 Carbon Cycle Projections in Earth Systems Models

Earth System Models simulate the evolution of carbon sources and sinks on land and in the ocean up to a century in the future. They include changes in feedback mechanisms. It is important to test the accuracy and limitations of these models, chiefly for land models. Land feedbacks within models are more uncertain. This is often done by looking at the model’s results compared to observational data. The models results are compared to observational benchmarks from the international land model benchmarking system (ILAMB). ILAMB is a reference system that has standard values of carbon-climate feedback processes from observational studies. When comparing ILAMB metrics against the models used in AR5 to the models used in the IPCC Sixth Assessment Report (IPCC6), the new models are found to outperform or perform equivalently to the AR5 models for both land and ocean for all benchmarks. However, there are still limitations within the IPPC6 models.

Another method to test the accuracy of the models in land and ocean cumulative carbon uptake is to compare the results of the IPCC6 models ran from 1850-2014 against observation-based estimates from the Global Carbon Project (GCP) over the same time period. Part of the GCP involves creating a global budget for Earth’s current carbon dioxide concentration including resolving its sources, sinks, and fluxes. Results show that IPCC6 models may slightly underestimate carbon uptake by oceans. Another concern within climate models is the accuracy of carbon sinks and sources over different latitudes. To test the latitudinal accuracy of these models we can compare the results of our models to atmospheric inversion models. Atmospheric inversion models estimate surface-to-atmosphere net carbon fluxes using an atmospheric transport model. Results show that IPCC6 models attribute more of the global land carbon sink to the tropics than the midlatitudes, with a large underestimation in Northern latitudes from 30-70 degrees. In comparison, models estimate ocean carbon uptake across all latitudes in agreement with the atmospheric inversion models.

Earth System Models can be used to look at the evolution of carbon sources and sinks under different shared socio-economic pathways (SSPs). SSPs are scenarios of different anthropogenic greenhouse gas productions up to 2100. They are based on altering pathways of socioeconomic change and climate policy. Over all SSPs, land shows a higher inter-annual variability than oceans due to anthropogenic land change, plant growth seasonal variability, and changes in albedo. Carbon uptake on land primarily takes place in present day forests. In mid and high latitudes a new carbon sink is projected due to increased CO₂ and warming. Hereafter the similarities between the SSPs stop as land and ocean carbon uptake are driven primarily by increases in atmospheric CO₂. Under high emissions scenarios the land and ocean carbon sinks take up a larger gross amount of carbon but the ratio of uptake to production of CO₂ declines over the 22nd and 21st century. Conversely, under low emission scenarios the land and ocean carbon sinks take up a smaller gross amount of carbon but the ratio of uptake to production of CO₂ grows over the 22nd and 21st century. Overall, the uncertainty surrounding future CO₂ is dominated by changes in anthropogenic emissions of CO₂ instead of carbon-climate feedbacks.

5.4.6 Emergent Constraints to Reduce Uncertainties in Projections

Emergent constraints help to reduce uncertainty within climate models. Uncertainty is any climate process within a model that is not completely known. For example, discussed earlier in this report is the nutrient-limiting effect on photosynthesis. As this constraint is poorly represented in models there is uncertainty in how plants will respond to increasing CO₂ because we have not accurately represented the effects of limited nutrients. Natural variability (i.e. day to day or season to season variations caused by natural weather patterns) and lack of knowledge of what future climate will looks like (i.e. SSPs) also contribute to uncertainty within models. When analyzing a group of climate models with a wide range of results sometimes a trend or relationship can rise between two variables (i.e., CO₂ causes warming). These trends that appear are called emergent constraints.

5.4.7 Climate Feedbacks from CH4 and N2O

Climate change and increasing atmospheric CO₂ has direct and indirect effects on the sources and sinks of methane (CH4) and nitrous oxide (N2O). A direct effect is climate forcing, for example CO₂ leads to warmer temperatures. An indirect effect is in response to a forcing, for example, increasing temperatures from CO₂ stresses plants leaving them more vulnerable to disease. While these effects are not yet fully understood, strides have been made in the IPCC6 report since the release of the AR5 report. The alteration of wetlands through land use, such as rice farming or wetland draining may produce a methane feedback. There is high agreement among models that methane from wetland emissions will increase with a changing climate, but the magnitude is not well understood – it is dependent on the rate at which microorganisms produce methane. Permafrost thaw, which releases large amounts of CH4 upon melting, as well as fires, freshwater bodies, and landfills are all predicted to grow as methane sources under climate change. Models predict permafrost thaw and alterations of freshwater bodies due to climate change will produce a positive feedback for methane production. Also producing a positive, but smaller magnitude feedback, is wildfires. There is medium confidence that wildfire methane release may increase by up to a factor of 1.5 during the 21st century. Along the same vein, models predict with high agreement that nitrous oxide emission will increase from terrestrial sources. This is consistent with observational evidence from past climates. However, the magnitude of this feedback depends on future nitrogen availability which is poorly constrained. Permafrost thaw, boreal, and Arctic ecosystems as a nitrogen source have not been accounted for within models but could serve as a huge emission source for nitrous oxide. Ultimately, there is medium confidence that the land N₂O feedback is positive, but a low confidence in magnitude. Conversely, there is high agreement between a limited number of models, that the ocean will provide a negative nitrous oxide feedback with climate change. This is due to increased ocean stratification, and decreased productivity. Overall, there is medium confidence that the combined feedback from CH4 and N2O is positive, but there is low confidence (a two out of ten chance of being correct) in the magnitude.

5.4.8 Combined Biogeochemical Climate Feedback

Both biogeochemical non-CO₂ feedbacks, like methane or nitrous oxide, and biogeochemical CO₂ feedbacks are important to the remaining global carbon budget and the transient climate response to cumulative carbon emissions (TCRE). The global remaining carbon budget is how much carbon we can emit before we warm the world by a certain amount. For example, the remaining global carbon budget for 1.5°C was 296 Gigatons of CO₂ at the start of 2021. The TCRE is the ratio of the globally averaged surface temperature per unit of carbon dioxide emitted. This section aims to understand the magnitude of biogeochemical feedbacks on the TCRE and the remaining global carbon budget. There is low confidence from models in how biogeochemical non-CO₂ feedbacks will affect the climate system, TCRE, and the remaining carbon budget. In both biogeochemical non-CO₂ and CO₂ feedbacks there is low confidence in the magnitude of the feedbacks.

5.4.9 Abrupt Changes and Tipping Points

Abrupt changes and tipping points within biogeochemical cycles lead to additional uncertainties for future greenhouse gas concentrations. Abrupt changes are large climatic shifts that happen quickly on geologic time scales and persist for years or longer, effecting large areas of the globe. An example of an abrupt change is the Dust Bowl drought of the 1930s that contributed to the Great Depression. A tipping point is a critical threshold, that when reached, a climatic system must reorganize, often quickly and irreversibly. To illustrate this, imagine a child going down a slide. There is a certain point where the child can no longer climb back up to the top of the slide and must continue down to the bottom. The bisophere is known to have both abrupt changes and tipping points, such as tropical, boreal, and arctic forest dieback, or the release of large amounts of carbon and methane from permafrost melt. Abrupt changes and tipping points of forest dieback could be linked to temperature and precipitation extremes, possible enhancement in fire activity, drought, or insect outbreak. However, forest dieback is not expected to change atmospheric CO₂ concentration significantly, due to projected increase in forest growth up north, and temperate forests invading into boreal forests. While there is a large uncertainty in permafrost melt or thaw, permafrost release of methane and carbon are not expected to modify global temperatures substantially compared to anthropogenic greenhouse gas production. Overall, while abrupt changes and tipping points produce uncertainty in future climate and are not well constrained in models, anthropogenic release of greenhouse gases leave us with a much larger uncertainty in future climate.

5.4.10 Long Term Response Past 2100

There is very high confidence that the land and ocean will continue to change in response to a changing climate and rising CO₂ concentrations. There is medium confidence that after 2100 land will stop acting as a terrestrial carbon sink, and instead become a carbon source. There is low confidence in the timing and magnitude of this feedback. This response is shown under both very high emission scenarios of greenhouse gases and low/negative emission scenarios. There is high confidence that the ocean as a carbon sink will weaken past 2100 but will remain a sink, even under long-term, high-emission scenarios.

5.4.11 Near-Term Prediction of Ocean and Land Carbon Sinks

Changes in the strength of terrestrial and oceanic carbon sinks in the near-term are dependent on natural interannual variability. An example of natural variability is the El Niňo Southern Oscillation (ENSO). ENSO is a periodic event that changes winds and sea-surface temperatures around the tropical eastern Pacific, influencing global ocean circulation, storms, and precipitation. ENSO actually drives seasonal changes of air-land CO₂ flux for up to 6-8 months. Short-term changes in the terrestrial carbon sink is also due to predictable changes in photosynthesis and respiration, which can be viewed as CO₂ uptake and CO₂ release. Photosynthesis decreases in winter when many plant species die off and increases in spring when many plant species grow. Ocean carbon sinks have less variability than terrestrial sinks as oceans change on much longer time scales. However, near-term prediction of ocean sinks is based on the predictability of air-sea CO₂ flux, which is based on the dissolved inorganic carbon gradient from the near-surface ocean to the near-surface air. Temperature variations largely control this gradient in the short term, affecting physical parameters such as stratification.

5.5 Remaining Carbon Budgets

Daisy Aguilar

Carbon budgets are essential to consider in any climate policy, but this may become challenging as there is so much uncertainty when estimating the remaining carbon budgets. Any remaining carbon budgets is better understood as a distribution that reflects the probability of meeting a set goal. In other words, this goal would refer to the smallest amount of CO₂ that could be emitted as the probability of meeting the temperature target exceeds a certain threshold.

The amount of CO₂ emissions that is estimated in order to stay in line with specific levels to that of global warming is measured through the transient climate response to cumulative emissions (TCRE). In several In several contexts, the TCRE has been shown to be approximately constant, meaning temperature change is proportional to the cumulative emissions of CO₂.

The remaining carbon budget utilizes this relation to relate temperature change on Earth to the collective CO₂ emissions that can be emitted. This essentially serves a way to predict any global warming by the cumulative CO₂ emissions through various scenarios.

Research on the TCRE has helped to provide understanding on why the ratio between emissions and temperature remains constant over a wide range of scenarios. The most fundamental line of reasoning is the following: in response to a constant concentration of CO₂, the Earth continues to warm because the ocean still uptakes heat. However, when emissions stop, CO₂ concentrations decrease slightly, as the land and ocean continue to uptake carbon. This decreases radiative forcing, and offsets the fact that the Earth would continue to warm if CO₂ remained constant.

Another critical aspect is that as emissions increase, there is also an increase in the airborne fraction of CO₂ emissions. This is due to the fact that the increase in emissions diminishes the land and ocean carbon uptake. This in itself increases sensitivity, but there is also a decreased sensitivity of radiative forcing to CO₂ concentration (decreasing sensitivity).

Uncertainties in climate feedbacks cause different climate sensitivity, and potentially affect TCRE. CO₂ concentrations are not the only thing that drive changes in climate – other climate feedbacks may also cause uncertainties, including water vapor, albedo, or clouds; basically anything that may cause the Earth to warm. Water vapor is a climate feedback because as the temperature on Earth increases then the amount of water vapor in the atmosphere would also increase which would then cause the greenhouse effect to increase as well, but different models produce different strengths of this feedback. 

To best estimate the value of TCRE it requires Earth system models (ESMs). In order to gain a better understanding of the effects of greenhouse gas on Earth, models have been created to represent any radiative forcing caused by greenhouse gases, the global carbon cycle, and the dynamics of the atmosphere and oceans. The analysis put into CMIP5 or CMIP6 conclude that the main contributor to the uncertainty of TCRE is the lack of uncertainty in land carbon feedback and the ocean heat uptake. The IPCC AR6 report displays a table (Table 5.7) of an overview with various studies conducted estimating the TCRE. These studies are based off modeling and theoretical studies. The TCRE assessment assists in providing for what warming would be if CO₂ concentrations are doubled. Based off the AR6 TCRE assessment, the TCRE is estimated to be in between 1.0-2.3 K/EgC. One exagram of carbon is equal to one trillion tons of carbon emissions, or 3.67 trillion tons of CO₂ emissions.

The remaining carbon budget refers to the amount of CO₂ that can be emitted to stabilize warming at a specific temperature. For the target global average warming levels to stay within 1.5 to 2.5^o C there are various factors that this depends on. These factors would consist of anything that isn’t CO2 related that may also contribute to the climate, TCRE limitations and ZEC. According to figure 5.31, the assessment of the remaining carbon budget shows how historical CO₂ emissions to the present consist with the limit global warming according to specific levels. This is done by taking the ZEC and any non-CO₂ warming and combining that with the global warming intended with human-induced warming. It is vital that when estimating remaining carbon budgets as it can be relatively similar to the value of present-day warming.

The carbon budget estimate does not take into consideration the radiative forcing that is caused by non-CO₂ emissions. Instead, when estimating the remaining carbon budget it looks into the historical warming, TCRE, ZEC, estimates of non-CO₂ emissions, and any not represented feedbacks. Since it is unknown how non-CO₂ emissions will change, it is difficult to understand its contribution to the relation between warming and the cumulative emissions.

Unrepresented Earth system feedbacks are not all implemented when taking the estimates of TCRE. Thawing permafrost is an example of an unrepresented Earth system feedback, as well as tropospheric ozone and methane lifetime feedbacks. Thawing permafrost is Earth system feedback is because as temperatures increase, there is an increase in the amount of permafrost that thaws. As it is thawing, it can potentially release more greenhouse gases. With all these unrepresented feedbacks, it would change the feedbacks to about 7 ± 27 PgC K-1. It’s thus still unknown whether these would reduce or increase the remaining carbon budget. According to the AR5, it did not account for the unrepresented Earth system climate feedbacks. Gathering all of these uncertainties into one estimate is truly impossible because they some of them depend on one another and are not solely independent.

5.6 Biogeochemical Implications of Carbon Dioxide Removal and Solar Radiation Removal

Daisy Aguilar

Carbon dioxide removal (CDR) is known as a type of geoengineering which is a negative CO₂ emission and acts as the process by which CO₂ is removed from the atmosphere and stored for long periods of time. Carbon moves through the carbon cycle around the globe. As this happens, the carbon goes through the atmosphere, ocean, land and other geological reservoirs. The process of carbon dioxide removal is better explained by having carbon dioxide removed from the atmosphere as the air goes through a sort of large filter which is then stored somewhere underground.

Solar radiation modification (SRM) is defined as a type of solar geoengineering which would allow sunlight to be reflected back into space. SRM methods has its own effects on the carbon cycle. One of those reducing the amount of sunlight that reaches the surface since it is being reflected which causes there to be a reduction in photosynthesis leading to an effect on plants. This can have a negative effect on plants due to lack of photosynthesis but may also help decrease heat stress. The SRM theorizes ways to limit global warming or reduce any impacts done to the climate and result in a quicker way to help decrease the global temperatures.

Some examples by which carbon dioxide is removed from the atmosphere are forests, farms, air capture, carbon storage, or ocean-based removal. Forests in particular do a really good job of storing carbon that is removed form the atmosphere. This process is done by photosynthesis, as it removes CO₂ from the air and stores carbon in the wood and into the soil. Farms are very helpful towards storing carbon because the soil stores the carbon removed from the atmosphere, but this also becomes very beneficial to the overall agricultural health because it increases the productivity of crop and the overall health of soil. Carbon removal with ocean-based techniques are ways that theorize the acceleration of the carbon cycle in the ocean and in the end could result in benefits to the ocean. For example, seaweed would help remove the carbon by helping restore the ecosystem and store carbon in the ocean.

Any land-based biological CDR methods can have multiple effects on climate change. Some of these methods can include afforestation, reforestation, and forest management. Afforestation for example, can change the albedo and evapotranspiration. The albedo would be affected by the surface land absorbing more solar radiation. In regions where there is seasonal snow cover afforestation will decrease the surface albedo and cause a warming effect. Afforestation may also affect the evapotranspiration because of the surface cooling like in the tropical region where the latent heat flux is increased due to evapotranspiration. This would imply that the location from which these CDR methods are taken from are a crucial aspect towards global warming. As for ocean-based CDR methods, it is impossible to be able to alter the entire ocean so these methods are based more off its productivity. The coastal wetlands and seagrass are known to have the highest productivity level of carbon uptake. To reduce the carbon uptake the restoration of vegetated coastal ecosystems also known as blue carbon increases the rate of carbon uptake by burying the carbon into the seagrass and coastal wetlands. Enhanced weathering-based CDR would fall under the geochemical CDR methods and are based on natural weathering processes of soils, rivers, and the ocean surface that remove CO₂ from the atmosphere.

The pH levels and the alkalinity increase as the ocean increases its rate of CO₂ uptake and may be beneficial to ocean acidification as it would increase the about of carbon storage in the ocean that could also lead to an increase productivity level in seagrass. The chemical CDR methods has two techniques of direct air carbon capture with carbon storage which are carbon storage and the capture of CO₂ directly from the air. The main concerns with the chemical CDR methods are that when CO₂ is being captured from the air, it could potentially leak out and have a negative impact towards the productivity levels of agriculture. On the other hand, methane removal methods have only been theorized as potential proposals and not actually tested. Methane removal could have more of a drastic impact on reducing warming. To remove methane from the atmosphere would require it to be done microbially.

When analyzing carbon dioxide removal there are various methods that are categorized based off the different carbon cycle processes. It considers any system feedbacks that either increase or decrease the process of capturing and storing CO₂ into the atmosphere and how effective it can be in terms of limiting climate change. Table 5.9 displays a better picture of the many possibilities of carbon dioxide removal methods. This table breaks down exactly what type of category the CDR method is apart of, the type of process it goes through, the time it will take to store the carbon as well as any other effects. The methods that are categorized as enhanced biological production and storage which tend to occur in land, soil, or other geological formation tend to have CO₂ removed by a biological or organic process as well as the methods categorized in a more biological production that occur in the coastal region and the oceans. Whereas the methods that are categorized as chemical and any enhanced geochemical processes on land and in ocean have a process that is geochemical or inorganic.

The purpose behind CDR is to compensate for any reductions from emissions that are insufficient through the carbon sinks process. Its success rate would be based off biogeochemical limitations. Because there are so many uncertainties it’s difficult to be able to exactly know how much CO₂ emissions would be offset by CDR. Although, for CDR to have a significant in the reduction of CO₂ in the atmosphere then it would have to be over a century at a large scale but there is also the probability that CDR would outweigh the CO₂ in the ecosystems of land and ocean. Knowing this means that the climate effects would be at a slow rate and therefore CDR would prevent climate change at a slow rate. The concern when theorizing any methods like CDR are the possible side effects it could have on carbon or other biogeochemical processes. The probability that any side effects would occur on biogeochemical cycles is low. This is due to CDR is correlated to the surface albedo and N₂O emissions. Although the SRM methods are a way to help limit global warming it is not something to consider as a possible effect to the carbon cycle, but it will affect the biogeochemical cycles through its climate effects. SRM would have a much more rapid climate effect within a 5-year time scale. When comparing this scale to the SRM methods and CDR methods, CDR wouldn’t be beneficial as a source of reducing climate change.

One of the consequences to the carbon cycle by the CDR methods is the rebound effect. The rebound effect is caused by the effect of having CO₂ released by the terrestrial biosphere and the ocean because of the decrease in CO₂ in the atmosphere due to the Industrial era releasing fossil fuel emissions into the atmosphere and were later on taken up by the ocean and land. Some CDR will be outgassed of CO₂ by the ocean and land ecosystems. CDR becomes less effective when the land and carbon cycles release CO₂ back into the atmosphere thus defeating the purpose behind this method.