The Earth’s Energy Budget, Climate Feedbacks And Climate Sensitivity

Abstract

This chapter assesses the present state of knowledge of Earth’s energy budget: that is, the main flows of energy into and out of the Earth system, and how these energy flows govern the climate response to a radiative forcing. Changes in atmospheric composition and land use, like those caused by anthropogenic greenhouse gas emissions and emissions of aerosols and their precursors, affect climate through perturbations to Earth’s top-of-atmosphere energy budget. The effective radiative forcings (ERFs) quantify these perturbations, including any consequent adjustment to the climate system (but excluding surface temperature response). How the climate system responds to a given forcing is determined by climate feedbacks associated with physical, biogeophysical and biogeochemical processes. These feedback processes are assessed, as are useful measures of global climate response, namely equilibrium climate sensitivity (ECS) and the transient climate response (TCR). This chapter also assesses emissions metrics, which are used to quantify how the climate response to the emissions of different greenhouse gases compares to the response to the emissions of carbon dioxide (CO2). This chapter builds on the assessment of carbon cycle and aerosol processes from Chapters 5 and 6, respectively, to quantify non-CO2 biogeochemical feedbacks and the ERF for aerosols. Other chapters in this Report use this chapter’s assessment of ERF, ECS and TCR to help understand historical and future temperature changes (Chapters 3 and 4, respectively), the response to cumulative emissions and the remaining carbon budget (Chapter 5), emissions-based radiative forcing (Chapter 6) and sea level rise (Chapter 9). This chapter builds on findings from the IPCC Fifth Assessment Report (AR5), the Special Report on Global Warming of 1.5°C (SR1.5), the Special Report on the Ocean and Cryosphere in a Changing Climate (SROCC) and the Special Report on climate change, desertification, land degradation, sustainable land management, food security, and greenhouse gas luxes in terrestrial ecosystems (SRCCL). Very likely ranges are presented unless otherwise indicated.

Generated Summary

This document is a chapter from the IPCC Sixth Assessment Report (AR6) that assesses the major physical processes affecting Earth’s energy budget and how they relate to climate change. It integrates information from previous IPCC reports and community-led assessments, focusing on the Earth’s energy budget, radiative forcing, climate feedbacks, and climate sensitivity. The study builds on AR5, the Special Report on Global Warming of 1.5°C, the Special Report on the Ocean and Cryosphere in a Changing Climate, and the Special Report on climate change, desertification, land degradation, sustainable land management, food security, and greenhouse gas fluxes in terrestrial ecosystems to improve understanding of climate change. The chapter assesses changes in the global energy inventory, the role of effective radiative forcing, climate feedbacks, and metrics to evaluate emissions. The goal is to better understand climate models and improve projections of future climate change.

Key Findings & Statistics

• The global energy inventory increased by 282 [177 to 387] Zettajoules (ZJ; 1021 Joules) for the period 1971–2006 and 152 [100 to 205] ZJ for the period 2006–2018.
• Ocean heat uptake accounts for 91% of the total energy change.
• The total anthropogenic ERF over the industrial era (1750–2019) was 2.72 [1.96 to 3.48] W m–2.
• Anthropogenic emissions of greenhouse gases and their precursors contribute an ERF of 3.84 [3.46 to 4.22] W m–2 over the industrial era (1750–2019).
• The ERF of greenhouse gases 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.
• Aerosols contribute an ERF of –1.3 [–2.0 to –0.6] W m–2 over the industrial era (1750–2014).
• 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.
• 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.
• 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.
• The total human-forced GSAT change from 1750 to 2019 is calculated to be 1.29 [0.99 to 1.65] °C.
• 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.
• The ERF of greenhouse gases (excluding ozone and stratospheric water vapour) over 1750–2019 is assessed to be 3.32 ± 0.29 W m–2.
• The ERF for ozone is 0.47 [0.24 to 0.70] W m–2 for 1750–2019.
• The ERF from land-use change is assessed to be –0.20 ± 0.10 W m–2.
• The assessed ERF over 1750–2019 of contrails and aviation-induced cirrus is 0.06 [0.02 to 0.10] W m–2.
• The overall assessed ERF of light-absorbing particles on snow and ice is +0.08 [0.00 to 0.18] W m–2.
• The best estimate solar ERF is 0.01 W m–2, with a likely range of –0.06 to +0.08 W m–2.
• The ERF for 1750–2019 from aerosols is –1.1 [–1.7 to –0.4] W m–2.

Other Important Findings

• There has been increased confidence in the quantification of changes in the global energy inventory due to improved observational records and closure of the sea level budget.
• Since AR5, research and new observations have improved scientific confidence in the quantification of changes in the global energy inventory.
• Improved understanding of adjustments to radiative forcing and of aerosol–cloud interactions have led to revisions of forcing estimates.
• New approaches to the quantification and treatment of feedbacks have improved the understanding of their nature and time-evolution, leading to a better understanding of how these feedbacks relate to equilibrium climate sensitivity (ECS).
• 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.
• The combined effect of all known radiative feedbacks is to amplify the base climate response, also known as the Planck temperature response.
• 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 choice of emissions metric affects the quantification of net zero GHG emissions and therefore the resulting temperature outcome after net zero emissions are achieved.
• The total anthropogenic ERF over the industrial era (1750–2019) is estimated as 2.72 [1.96 to 3.48] W m–2.
• For the period 1971–2018, the total energy gain was 434.9 [324.5 to 545.3] ZJ, with an equivalent Earth energy imbalance of 0.57 [0.43 to 0.72] W m–2.
• There is high confidence that the global energy budget is closed for 1971–2018 with improved consistency compared to AR5.
• Based on multiple lines of evidence, it is virtually certain that the total aerosol ERF is negative.
• It is very likely that the warming in the Arctic will be more pronounced than the global average over the 21st century.
• High-ECS models are useful for exploring low-likelihood, high-impact futures.
• The best estimate of ECS is 3°C, and the likely range is 2.5°C to 4°C, and the very likely range is 2°C to 5°C.
• For the process-based assessment of ECS the best estimate is 3.4°C with a likely range of 2.5 to 5.1 °C and a very likely range of 2.1 to 7.7 °C.
• The best estimate TCR is 1.8°C, and the likely range is 1.4°C to 2.2°C, and the very likely range is 1.2°C to 2.4°C.

Limitations Noted in the Document

• The study acknowledges uncertainties related to the measurement of the global energy imbalance, particularly in absolute terms, even with the improved precision of satellite observations.
• Regional surface energy budget estimates and their representation in climate models remain uncertain.
• The contribution of aerosol and clouds to dimming and brightening is still debated.
• The models often do not fully reproduce the observed dimming and brightening, potentially due to inadequate representations of aerosol mediated effects or related emissions data.
• The relative influence of cloud-mediated aerosol effects versus direct aerosol radiative effects on dimming and brightening in a specific region may depend on the prevailing pollution levels.
• Estimating the uncertainty in sensible and latent heat fluxes over land is difficult because of the large temporal and spatial variability.
• There is low confidence in the magnitude of the tropospheric adjustments, and the assessment of this can be difficult.
• It is difficult to make a robust assessment of the ERF from ozone changes over the 1750–2019 period.
• There is low confidence in the stratospheric-temperature adjustment terms.
• The impact from LAPs is uncertain.
• The relative influence of cloud-mediated aerosol effects versus direct aerosol radiative effects on dimming and brightening in a specific region may depend on the prevailing pollution levels.
• The results are somewhat dependent on the applied statistical methods.
• The value of α’ may depend on historical non-CO2 forcings.
• Low confidence results from the studies based on glacial–interglacial periods.

Conclusion

The chapter provides a comprehensive assessment of the Earth’s energy budget and its role in climate change, providing insights into the causes of warming and projecting future climate changes. It builds on previous IPCC reports, integrating knowledge from diverse sources such as satellite data, ocean temperature measurements, and models. The study demonstrates that human activities have created an energy flow imbalance, leading to a warming planet, with oceans absorbing the majority of the excess energy. It emphasizes the importance of understanding radiative forcing, climate feedbacks, and emissions metrics in order to better understand the causes of climate change and project future climate change accurately. The research highlights the significance of cloud changes in amplifying warming. The study finds there is high confidence that polar amplification will continue, with the Arctic warming more than the global average. There is robust evidence, and high agreement, that ECS is greater than 1.6°C. The study also reveals the importance of state-dependent feedbacks and highlights the need to consider these for accurate climate projections. It emphasizes the complexity of climate processes and the importance of further research to refine our understanding of the Earth’s climate system. This study underscores the need for continued efforts to reduce emissions to prevent additional human-caused warming.

DOI

10.1017/9781009157896.009