The Human Perturbation of the Carbon and Biogeochemical cycles
Global mean concentrations for well-mixed GHGs (CO2, CH4 and N2O) in 2019 correspond to increases of about 47%, 156%, and 23%, respectively, above the levels in 1750 (representative of the pre-industrial). Current atmospheric concentrations of the three GHGs are higher than at any point in the last 800,000 years, and in 2019 reached 409.9 ppm of CO2, 1866.3 ppb of CH4, and 332.1 ppb of N2O. Current CO2 concentrations in the atmosphere are also unprecedented in the last 2 million years. In the past 60 Myr, there have been periods in Earth’s history when CO2 concentrations were significantly higher than at present, but multiple lines of evidence show that the rate at which CO2 has increased in the atmosphere during 1900–2019 is at least 10 times faster than at any other time during the last 800,000 years, and 4-5 times faster than during the last 56 million years.
Contemporary Trends of Greenhouse Gases
It is unequivocal that the increase of CO2, CH4, and N2O in the atmosphere over the industrial era is the result of human activities. This assessment is based on multiple lines of evidence including atmospheric gradients, isotopes, and inventory data. During the last measured decade, global average annual anthropogenic emissions of CO2, CH4, and N2O, reached the highest levels in human history at 10.9 ± 0.9 PgC yr-1 (2010–2019), 335–383 Tg CH4 yr-1 (2008–2017), and 4.2–11.4 TgN yr-1 (2007–2016), respectively.
The CO2 emitted from human activities during the decade of 2010–2019 (decadal average 10.9 ± 0.9 PgC yr-1) was distributed between three Earth system components: 46% accumulated in the atmosphere (5.1 ± 0.02 PgC yr-1), 23% was taken up by the ocean (2.5 ± 0.6 PgC yr-1) and 31% was stored by vegetation in terrestrial ecosystems (3.4 ± 0.9 PgC yr-1 ). Of the total anthropogenic CO2 emissions, the combustion of fossil fuels was responsible for 81–91%, with the remainder being the net CO2 flux from land-use change and land management (e.g., deforestation, degradation, regrowth after agricultural abandonment or peat drainage).
Over the past six decades, the average fraction of anthropogenic CO2 emissions that has accumulated in the atmosphere (referred to as the airborne fraction) has remained nearly constant at approximately 44%. The ocean and land sinks of CO2 have continued to grow over the past six decades in response to increasing anthropogenic CO2 emissions. Interannual and decadal variability of the regional and global ocean and land sinks indicate that these sinks are sensitive to climate conditions and therefore to climate change.
Recent observations show that ocean carbon processes are starting to change in response to the growing ocean sink, and these changes are expected to contribute significantly to future weakening of the ocean sink under medium- to high-emission scenarios. However, the effects of these changes is not yet reflected in a weakening trend of the contemporary (1960-2019) ocean sink.
Atmospheric concentration of CH4 grew at an average rate of 7.6 ± 2.7 ppb yr-1 for the last decade (2010–2019), with a faster growth of 9.3 ± 2.4 ppb yr-1 over the last six years (2014–2019). The multi-decadal growth trend in atmospheric CH4 is dominated by anthropogenic activities, and the growth since 2007 is largely driven by emissions from both fossil fuels and agriculture (dominated by livestock) sectors. The interannual variability is dominated by El Niño–Southern Oscillation cycles, during which biomass burning and wetland emissions, as well as loss by reaction with tropospheric hydroxyl radical OH play an important role.
Atmospheric concentration of N2O grew at an average rate of 0.85 ± 0.03 ppb yr-1 between 1995 and 2019, with a further increase to 0.95 ± 0.04 ppb yr-1 in the most recent decade (2010–2019). This increase is dominated by anthropogenic emissions, which have increased by 30% between the 1980s and the most recent observational decade (2007–2016). Increased use of nitrogen fertilizer and manure contributed to about two-thirds of the increase during the 1980–2016 period, with the fossil fuels/industry, biomass burning, and wastewater accounting for much of the rest.
Ocean Acidification and Ocean Deoxygenation
Ocean acidification is strengthening as a result of the ocean continuing to take up CO2 from human caused emissions. This CO2 uptake is driving changes in seawater chemistry that result in the decrease of pH and associated reductions in the saturation state of calcium carbonate, which is a constituent of skeletons or shells of a variety of marine organisms. These trends of ocean acidification are becoming clearer globally, with a very likely rate of decrease in pH in the ocean surface layer of 0.016 to 0.020 per decade in the subtropics and 0.002 to 0.026 per decade in subpolar and polar zones since the 1980s. Ocean acidification has spread deeper in the ocean, surpassing 2000 m depth in the northern North Atlantic and in the Southern Ocean. The greater projected pH declines in CMIP6 models are primarily a consequence of higher atmospheric CO2 concentrations in the Shared Socio-economic Pathways (SSPs) scenarios than their CMIP5-RCP analogues.
Ocean deoxygenation is projected to continue to increase with ocean warming. Earth system models (ESMs) project a 32–71% greater subsurface (100–600 m) oxygen decline, depending on scenario, than reported in the Special Report on the Ocean and Cryosphere (SROCC) for the period 2080– 2099. This is attributed to the effect of larger surface warming in CMIP6 models, which increases ocean stratification and reduces ventilation. There is low confidence in the projected reduction of oceanic N2O emissions under high emission scenarios because of greater oxygen losses simulated in ESMs in CMIP6, uncertainties in the process of oceanic N2O emissions, and a limited number of modelling studies available.
Future Projections of Carbon Feedbacks on Climate Change
Oceanic and terrestrial carbon sinks are projected to continue to grow with increasing atmospheric concentrations of CO2, but the fraction of emissions taken up by land and ocean is expected to decline as the CO2 concentration increases. ESMs suggest approximately equal global land and ocean carbon uptake for each of the SSPs scenarios. However, the range of model projections is much larger for the land carbon sink. Despite the wide range of model responses, uncertainty in atmospheric CO2 by 2100 is dominated by future anthropogenic emissions rather than uncertainties related to carbon–climate feedbacks.
Increases in atmospheric CO2 lead to increases in land carbon storage through CO2 fertilization of photosynthesis and increased water use efficiency. However, the overall change in land carbon also depends on land-use change and on the response of vegetation and soil to continued warming and changes in the water cycle, including increased droughts in some regions that will diminish the sink capacity. Climate change alone is expected to increase land carbon accumulation in the high latitudes (not including permafrost), but also to lead to a counteracting loss of land carbon in the tropics. More than half of the latest CMIP6 ESMs include nutrient limitations on the carbon cycle, but these models still project increasing tropical land carbon, and increasing global land carbon through the 21st century.
Future trajectories of the ocean CO2 sink are strongly emissions-scenario dependent Emission scenarios SSP4-6.0 and SSP5-8.5 lead to warming of the surface ocean and large reductions of the buffering capacity, which will slow the growth of the ocean sink after 2050. Scenario SSP1-2.6 limits further reductions in buffering capacity and warming, and the ocean sink weakens in response to the declining rate of increasing atmospheric CO2. There is low confidence in how changes in the biological pump will influence the magnitude and direction of the ocean carbon feedback.
Beyond 2100, land and ocean may transition from being a carbon sink to a source under either very high emissions or net negative emissions scenarios, but for different reasons. Under very high emissions scenarios such as SSP5-8.5, ecosystem carbon losses due to warming lead the land to transition from a carbon sink to a source, while the ocean is expected to remain a sink. For scenarios in which CO2 concentration stabilizes, land and ocean carbon sinks gradually take up less carbon as the increase in atmospheric CO2 slows down. In scenarios with moderate net negative CO2 emissions and CO2 concentrations declining during the 21st century (e.g., SSP1-2.6), the land sink transitions to a net source in decades to a few centuries after CO2 emissions become net negative, while the ocean remains a sink. Under scenarios with large net negative CO2 emissions and rapidly declining CO2 concentrations (e.g., SSP5-3.4-OS (overshoot)), both land and ocean switch from a sink to a transient source during the overshoot period.
Thawing terrestrial permafrost will lead to carbon release, but there is low confidence in the timing, magnitude and the relative roles of CO2 versus CH4 as feedback processes. CO2 release from permafrost is projected to be 3–41 PgC per 1ºC of global warming by 2100, based on an ensemble of models. However, the incomplete representation of important processes such as abrupt thaw, combined with weak observational constraints, only allow low confidence in both the magnitude of these estimates and in how linearly proportional this feedback is to the amount of global warming. It is very unlikely that gas clathrates in terrestrial and subsea permafrost will lead to a detectable departure from the emissions trajectory during this century.
The net response of natural CH4 and N2O sources to future warming will be increased emissions. Key processes include increased CH4 emissions from wetlands and permafrost thaw, as well as increased soil N2O emissions in a warmer climate, while ocean N2O emissions are projected to decline at centennial time scale. The magnitude of the responses of each individual process and how linearly proportional these feedbacks are to the amount of global warming is known with low confidence due to incomplete representation of important processes in models combined with weak observational constraints. Models project that over the 21st century the combined feedback of 0.02–0.09 W m-2 °C-1 is comparable to the effect of a CO2 release of 5-18 PgCeq °C-1.
The response of biogeochemical cycles to the anthropogenic perturbation can be abrupt at regional scales, and irreversible on decadal to century time scales. The probability of crossing uncertain regional thresholds (e.g., high severity fires, forest dieback) increases with climate change. Possible abrupt changes and tipping points in biogeochemical cycles lead to additional uncertainty in 21st century GHG concentrations, but these are very likely to be smaller than the uncertainty associated with future anthropogenic emissions.
Remaining Carbon Budgets to Climate Stabilization
There is a near-linear relationship between cumulative CO2 emissions and the increase in global mean surface air temperature (GSAT) caused by CO2 over the course of this century for global warming levels up to at least 2°C relative to pre-industrial. Halting global warming would thus require global net anthropogenic CO2 emissions to become zero. The ratio between cumulative CO2 emissions and the consequent GSAT increase, which is called the transient climate response to cumulative emissions of CO2 (TCRE), likely falls in the 1.0°C–2.3°C per 1000 PgC range. The narrowing of this range compared to AR5 is due to a better integration of evidence across the science in this assessment. Beyond this century, there is low confidence that the TCRE remains an accurate predictor of temperature changes in scenarios of very low or net negative CO2 emissions because of uncertain Earth system feedbacks that can result in further warming or a path-dependency of warming as a function of cumulative CO2 emissions.
Mitigation requirements over this century for limiting maximum warming to specific levels can be quantified using a carbon budget that relates cumulative CO2 emissions to global mean temperature increase. For the period 1850–2019, a total of 655 ± 65 PgC (2390 ± 240 GtCO2) of anthropogenic CO2 has been emitted. Remaining carbon budgets (starting from 1 January 2020) for limiting warming to 1.5°C, 1.7°C, and 2.0°C are 140 PgC (500 GtCO2), 230 PgC (850 GtCO2) and 370 PgC (1350 GtCO2), respectively, based on the 50th percentile of TCRE. For the 67th percentile, the respective values are 110 PgC (400 GtCO2), 190 PgC (700 GtCO2) and 310 PgC (1150 GtCO2). These remaining carbon budgets may vary by an estimated ± 60 PgC (220 GtCO2) depending on how successfully future non-CO2 emissions can be reduced. Since AR5 and SR1.5, estimates have undergone methodological improvements, resulting in larger, yet consistent estimates.
Several factors affect the precise value of remaining carbon budgets, including estimates of historical warming, future emissions from thawing permafrost, and variations in projected non-CO2 warming. Remaining carbon budget estimates can increase or decrease by 150 PgC (550 GtCO2, likely range) due to uncertainties in the level of historical warming, and by an additional ± 60 PgC (±220 GtCO, likely range) due to geophysical uncertainties surrounding the climate response to non-CO2 emissions such as CH4, N2O, and aerosols. Permafrost thaw is included in the estimates together with other feedbacks that are often not captured by models. Despite the large uncertainties surrounding the quantification of the effects of additional Earth system feedback processes, such as emissions from wetlands and permafrost thaw, these feedbacks represent identified additional amplifying risk factors that scale with additional warming and mostly increase the challenge of limiting warming to specific temperature thresholds. These uncertainties do not change the basic conclusion that global CO2 emissions would need to decline to at least net zero to halt global warming.
Biogeochemical Implications of Carbon Dioxide Removal and Solar Radiation Modification
Land- and ocean-based carbon dioxide removal (CDR) methods have the potential to sequester CO2 from the atmosphere, but the benefits of this removal would be partially offset by CO2 release from land and ocean carbon stores. The fraction of CO2 removed that remains out of the atmosphere, a measure of CDR effectiveness, decreases slightly with increasing amount of removal and decreases strongly if CDR is applied at lower CO2 concentrations.
The century-scale climate–carbon cycle response to a CO2 removal from the atmosphere is not always equal and opposite to the response to a CO2 emission. For simultaneously cumulative CO2 emissions and removals of greater than or equal to 100 PgC, CO2 emissions are 4 ± 3% more effective at raising atmospheric CO2 than CO2 removals are at lowering atmospheric CO2. The asymmetry originates from state-dependencies and non-linearities in carbon cycle processes and implies that an extra amount of CDR is required to compensate for a positive emission of a given magnitude to attain the same change in atmospheric CO2. The net effect of this asymmetry on the global surface temperature is poorly constrained due to low agreement between models.
Wide-ranging side-effects of CDR methods have been identified that can either weaken or strengthen the carbon sequestration and cooling potential of these methods and affect the achievement of sustainable development goals. Biophysical and biogeochemical side-effects of CDR methods are associated with changes in surface albedo, the water cycle, emissions of CH4 and N2O, ocean acidification and marine ecosystem productivity. These side-effects and associated Earth system feedbacks can decrease carbon uptake and/or change local and regional climate, and in turn limit the CO2 sequestration and cooling potential of specific CDR methods. Deployment of CDR, particularly on land, can also affect water quality and quantity, food production and biodiversity, with consequences for the achievement of related sustainable development goals. These effects are often highly dependent on local context, management regime, prior land use, and scale of deployment. A wide range of co-benefits are obtained with methods that seek to restore natural ecosystems or improve soil carbon. The biogeochemical effects of terminating CDR are expected to be small for most CDR methods.
Solar radiation modification (SRM) would increase the global land and ocean CO2 sinks but would not stop CO2 from increasing in the atmosphere, thus exacerbating ocean acidification under continued anthropogenic emissions. SRM acts to cool the planet relative to unmitigated climate change, which would increase the land sink by reducing plant and soil respiration and slow the reduction of ocean carbon uptake due to warming. SRM would not counteract or stop ocean acidification. The sudden and sustained termination of SRM would rapidly increase global warming, with the return of positive and negative effects on the carbon sinks.