Highlights

🐻‍❄️The Arctic warms more quickly than the Antarctic due to differences in radiative feedbacks and ocean heat uptake between the poles.

🌊Because of existing greenhouse gas concentrations, energy will continue to accumulate in the Earth system until at least the end of the 21st century, even under strong emissions reduction scenarios.

☁️The net effect of changes in clouds in response to global warming is to amplify human-induced warming. Compared to the last IPCC report, major advances in the understanding of cloud processes have increased the level of confidence in the cloud feedback cycle.

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Earth’s Energy Budget

Since AR5, the accumulation of energy in the Earth system, quantified by changes in the global energy inventory for all components of the climate system, has become established as a robust measure of the rate of global climate change on interannual-to-decadal timescales. Compared to changes in global surface air temperature (GSAT), the global energy inventory exhibits less variability, which can mask underlying climate trends. Compared to AR5, there is increased confidence in the quantification of changes in the global energy inventory due to improved observational records and closure of the sea level budget. Energy will continue to accumulate in the Earth system until at least the end of the 21st century, even under strong mitigation scenarios, and will primarily be manifest through ocean warming and associated with continued sea level rise through thermal expansion.

Since AR5, the accumulation of energy in the Earth system, quantified by changes in the global energy inventory for all components of the climate system, has become established as a robust measure of the rate of global climate change on interannual-to-decadal timescales. Compared to changes in global surface air temperature (GSAT), the global energy inventory exhibits less variability, which can mask underlying climate trends. Compared to AR5, there is increased confidence in the quantification of changes in the global energy inventory due to improved observational records and closure of the sea level budget. Energy will continue to accumulate in the Earth system until at least the end of the 21st century, even under strong mitigation scenarios, and will primarily be manifest through ocean warming and associated with continued sea level rise through thermal expansion.

The global energy inventory increased by 435 [325 to 545] Zettajoules (ZJ) for the period 1971–2018 and 153 [101 to 206] ZJ for the period 2006–2018. This corresponds to an Earth energy imbalance of 0.57 [0.43 to 0.72] W m-2 for the period 1971–2018, increasing to 0.79 [0.52 to 1.06] W m-2 for the period 2006– 2018, expressed per unit area of Earth’s surface. Ocean heat uptake is by far the largest contribution and accounts for 91% of the total energy change. Compared to AR5, the contribution from land heating has been revised upwards from about 3% to about 5%. Melting of ice and warming of the atmosphere account for about 3% and 1% of the total change respectively. More comprehensive analysis of inventory components and cross-validation of satellite and in situ-based global heating rates lead to a more confident assessment relative to AR5.

Improved quantification of effective radiative forcing, the climate system radiative response, and the observed energy increase in the Earth system for the period 1971–2018 demonstrate improved closure of the global energy budget compared to AR5. Combining the likely range of ERF with the central estimate of radiative response gives an expected energy gain of 340 [47 to 662] ZJ. Combining the likely range of climate response with the central estimate of ERF gives an expected energy gain of 340 [147 to 51 527] ZJ. Both estimates are consistent with an independent observation-based assessment of the global energy increase of 284 [96 to 471] ZJ, (very likely range) expressed relative to the estimated 1850-1900 Earth energy imbalance.

Since AR5, additional evidence for a widespread decline (or dimming) in solar radiation reaching the surface is found in the observational records between the 1950s and 1980s, with a partial recovery (brightening) at many observational sites thereafter. These trends are neither a local phenomenon nor a measurement artefact. Multi-decadal variation in anthropogenic aerosol emissions are thought to be a major contributor, but multi-decadal variability in cloudiness may also have played a role. The downward and upward thermal radiation at the surface has increased in recent decades, in line with increased greenhouse gas concentrations and associated surface and atmospheric warming and moistening.

Effective Radiative Forcing

For carbon dioxide, methane, nitrous oxide and chlorofluorocarbons, there is now evidence to quantify the effect on ERF of tropospheric adjustments (e.g., from changes in atmospheric temperatures, clouds and water vapour). The assessed ERF for a doubling of carbon dioxide compared to 1750 levels (3.93 ± 0.47 W m-2) is larger than in AR5. Effective radiative forcings (ERF), introduced in AR5, have been estimated for a larger number of agents and shown to be more closely related to the temperature response than the stratospheric-temperature adjusted radiative forcing. For carbon dioxide, the adjustments include the physiological effects on vegetation.

The total anthropogenic ERF over the industrial era (1750–2019) was 2.72 [1.96 to 3.48] W m-2. This estimate has increased by 0.43 W m-2 compared to AR5 estimates for 1750–2011. This increase includes a +0.34 W m-2 from increases in atmospheric concentrations of well-mixed greenhouse gases (including halogenated species) since 2011, a +0.15 W m-2 from upwards revisions of their radiative efficiencies and a +0.10 W m-2 from re-evaluation of the ozone and stratospheric water vapour ERF. The 0.59 W m-2 increase in ERF from greenhouse gases is partly offset by a better-constrained assessment of total aerosol ERF that is more strongly negative than in AR5, based on multiple lines of evidence. Changes in surface reflectance from land-use change, deposition of light-absorbing particles on ice and snow, and contrails and aviation-induced cirrus have also contributed to the total anthropogenic ERF over the industrial era, with –0.20 [–0.30 to –0.10] W m–2, +0.08 [0 to 0.18] W m–2 and +0.06 [0.02 to 0.10] W m-2, respectively.

Anthropogenic emissions of greenhouse gases (GHGs) and their precursors contribute an ERF of 3.84 [3.46 to 4.22] W m-2 over the industrial era (1750–2019). Most of this total ERF, 3.32 [3.03 to 3.61] W m-2 , comes from the well-mixed greenhouse gases, with changes in ozone and stratospheric water vapour (from methane oxidation) contributing the remainder. The ERF of GHGs is composed of 2.16 [1.90 to 2.41] W m-2 from carbon dioxide, 0.54 [0.43 to 0.65] W m-2 from methane, 0.41 [0.33 to 0.49] W m-2 from halogenated species, and 0.21 [0.18 to 0.24] W m-2 from nitrous oxide. The ERF for ozone is 0.47 [0.24 to 0.71] W m-2. The estimate of ERF for ozone has increased since AR5 due to revised estimates of precursor emissions and better accounting for effects of tropospheric ozone precursors in the stratosphere. The estimated ERF for methane has slightly increased due to a combination of increases from improved spectroscopic treatments being somewhat offset by accounting for adjustments.

Aerosols contribute an ERF of –1.3 [–2.0 to –0.6] W m-2 over the industrial era (1750–2014). The ERF due to aerosol–cloud interactions (ERFaci) contributes most to the magnitude of the total aerosol ERF and is assessed to be –1.0 [–1.7 to –0.3] W m-2, with the remainder due to aerosol–radiation interactions (ERFari), assessed to be –0.3 [– 0.6 to 0.0] W m-2. There has been an increase in the estimated magnitude but a reduction in the uncertainty of the total aerosol ERF relative to AR5, supported by a combination of increased process-understanding and progress in modelling and observational analyses. ERF estimates from these separate lines of evidence are now consistent with each other, in contrast to AR5, and support the assessment that it is virtually certain that the total aerosol ERF is negative. Compared to AR5, the assessed magnitude of ERFaci has increased, while the magnitude of ERFari has decreased. The total aerosol ERF over the period 1750–2019 is less certain than the headline statement assessment. It is also assessed to be smaller in magnitude at –1.1 [–1.7 to –0.4] W m-2, primarily due to recent emission changes.

Climate Feedbacks and Sensitivity

The net effect of changes in clouds in response to global warming is to amplify human-induced warming, that is, the net cloud feedback is positive (high confidence). Compared to AR5, major advances in the understanding of cloud processes have increased the level of confidence and decreased the uncertainty range in the cloud feedback by about 50%. An assessment of the low-altitude cloud feedback over the subtropical oceans, which was previously the major source of uncertainty in the net cloud feedback, is improved owing to a combined use of climate model simulations, satellite observations, and explicit simulations of clouds, altogether leading to strong evidence that this type of cloud amplifies global warming. The net cloud feedback, obtained by summing the cloud feedbacks assessed for individual regimes, is 0.42 [–0.10 to 0.94] W m-2 °C–1. A net negative cloud feedback is very unlikely.

The combined effect of all known radiative feedbacks (physical, biogeophysical, and non-CO2 biogeochemical) is to amplify the base climate response, also known as the Planck temperature response (virtually certain). Combining these feedbacks with the base climate response, the net feedback parameter based on process understanding is assessed to be –1.16 [–1.81 to –0.51] W m-2 °C–1, which is slightly less negative than that inferred from the overall ECS assessment. The combined water vapour and lapse rate feedback makes the largest single contribution to global warming, whereas the cloud feedback remains the largest contribution to overall uncertainty. Due to the state-dependence of feedbacks, as evidenced from paleoclimate observations and from models, the net feedback parameter will increase (become less negative) as global temperature increases. Furthermore, on long time scales the ice sheet feedback parameter is very likely positive, promoting additional warming on millennial time scales as ice sheets come into equilibrium with the forcing.

Radiative feedbacks, particularly from clouds, are expected to become less negative (more amplifying) on multi-decadal timescales as the spatial pattern of surface warming evolves, leading to an ECS that is higher than was inferred in AR5 based on warming over the instrumental record. This new understanding, along with updated estimates of historical temperature change, ERF, and Earth’s energy imbalance, reconciles previously disparate ECS estimates. However, there is currently insufficient evidence to quantify a likely range of the magnitude of future changes to current climate feedbacks. Warming over the instrumental record provides robust constraints on the lower end of the ECS range, ​​but owing to the possibility of future feedback changes it does not, on its own, constrain the upper end of the range, in contrast to what was reported in AR5. Based on multiple lines of evidence the best estimate of ECS is 3°C, the likely range is 2.5°C to 4°C, and the very likely range is 2°C to 5°C. It is virtually certain that ECS is larger than 1.5°C. Substantial advances since AR5 have been made in quantifying ECS based on feedback process understanding, the instrumental record, paleoclimates and emergent constraints. There is a high level of agreement among the different lines of evidence. All lines of evidence help rule out ECS values below 1.5°C, but currently it is not possible to rule out ECS values above 5 °C. Therefore, the 5°C upper end of the very likely range is assessed to have medium confidence and the other bounds have high confidence.

Based on process understanding, warming over the instrumental record, and emergent constraints, the best estimate of TCR is 1.8°C, the likely range is 1.4°C to 2.2°C and the very likely range is 1.2°C to 2.4°C

On average, CMIP6 models have higher mean ECS and TCR values than the CMIP5 generation of models. They also have higher mean values and wider spreads than the assessed best estimates and very likely ranges within this Report. These higher ECS and TCR values can, in some models, be traced to changes in extra-tropical cloud feedbacks that have emerged from efforts to reduce biases in these clouds compared to satellite observations. The broader ECS and TCR ranges from CMIP6 also lead the models to project a range of future warming that is wider than the assessed warming range, which is based on multiple lines of evidence. However, some of the high-sensitivity CMIP6 models are less consistent with observed recent changes in global warming and with paleoclimate proxy data than models with ECS within the very likely range. Similarly, some of the low-sensitivity models are less consistent with the paleoclimate data. The CMIP models with the highest ECS and TCR values provide insights into high-risk, low-likelihood futures, which cannot be excluded based on currently-available evidence.

Climate Response

The total human-forced GSAT change from 1750–2019 is calculated to be 1.29 [0.99 to 1.65] °C. This calculation is an emulator-based estimate, constrained by the historic GSAT and ocean heat content changes from Chapter 2 and the ERF, ECS and TCR from this chapter. The calculated GSAT change is composed of a well-mixed greenhouse gas warming of 1.58 [1.17 to 2.17] °C, a warming from ozone changes of 0.23 [0.11 to 0.39] °C, a cooling of –0.50 [–0.22 to –0.96] °C from aerosol effects, and a –0.06 [–0.15 to +0.01] °C contribution from surface reflectance changes from land-use change and light absorbing particles on ice and snow. Changes in solar and volcanic activity are assessed to have together contributed a small change of –0.02 [–0.06 to +0.02] °C since 1750.

Uncertainties regarding the true value of ECS and TCR are the dominant source of uncertainty in global temperature projections over the 21st century under moderate to high GHG emission scenarios. For scenarios that reach net zero carbon dioxide emissions, the uncertainty in the ERF values of aerosol and other short-lived forcings contribute substantial uncertainty in projected temperature. Global ocean heat uptake is a smaller source of uncertainty in centennial-time-scale surface warming.

The assessed historical and future ranges of GSAT change in this Report are shown to be internally consistent with the Report’s assessment of key physical-climate indicators: greenhouse gas ERFs, ECS and TCR. When calibrated to match the assessed ranges within the assessment, physically based emulators can reproduce the best estimate of GSAT change over 1850–1900 to 1995–2014 to within 5% and the very likely range of this GSAT change to within 10%. Two physically based emulators match at least two-thirds of the Chapter 4-assessed projected GSAT changes to within these levels of precision. When used for multi scenario experiments, calibrated physically based emulators can adequately reflect assessments regarding future GSAT from Earth system models and/or other lines of evidence.

It is now well understood that the Arctic warms more quickly than the Antarctic due to differences in radiative feedbacks and ocean heat uptake between the poles, but that surface warming will eventually be amplified in both poles. The causes of this polar amplification are well understood, and the evidence is stronger than at the time of AR5, supported by better agreement between modelled and observed polar amplification during warm paleo time periods. The Antarctic warms more slowly than the Arctic owing primarily to upwelling in the Southern Ocean, and even at equilibrium is expected to warm less than the Arctic. The rate of Arctic surface warming will continue to exceed the global average over this century. There is also high confidence that Antarctic amplification will emerge as the Southern Ocean surface warms on centennial time scales, although only low confidence regarding whether the feature will emerge during the 21st century.

The assessed global warming potentials (GWP) and global temperature-change potentials (GTP) for methane and nitrous oxide are slightly lower than in AR5 due to revised estimates of their lifetimes and updated estimates of their indirect chemical effects. The assessed metrics now also include the carbon-cycle response for non-CO2 gases. The carbon cycle estimate is lower than in AR5, but there is high confidence in the need for its inclusion and in the quantification methodology. Metrics for methane from fossil fuel sources account for the extra fossil CO2 that these emissions contribute to the atmosphere and so have slightly higher emission metric values than those from biogenic sources.

New emission metric approaches such as GWP* and the combined-GTP (CGTP) are designed to relate emission rates of short-lived gases to cumulative emissions of CO2. These metric approaches are well suited to estimate the GSAT response from aggregated emissions of a range of gases over time, which can be done by scaling the cumulative CO2 equivalent emissions calculated with these metrics by the transient climate response to cumulative emissions of carbon dioxide. For a given multi-gas emission pathway, the estimated contribution of emissions to surface warming is improved by either using these new metric approaches or by treating short- and long-lived GHG emission pathways separately, as compared to approaches that aggregate emissions of GHGs using standard GWP or GTP emission metrics. By contrast, if emissions are weighted by their 100-year GWP or GTP values, different multi-gas emission pathways with the same aggregated CO2 equivalent emissions rarely lead to the same estimated temperature outcome.

The choice of emission metric affects the quantification of net zero GHG emissions and therefore the resulting temperature outcome after net zero emissions are achieved. In general, achieving net zero CO2 emissions and declining non-CO2 radiative forcing would be sufficient to prevent additional human-caused warming. Reaching net zero GHG emissions as quantified by GWP-100 typically results in global temperatures that peak and then decline after net zero GHGs emissions are achieved, though this outcome depends on the relative sequencing of mitigation of short-lived and long-lived species. In contrast, reaching net zero GHG emissions when quantified using new emission metrics such as CGTP or GWP* would lead to approximate temperature stabilization.