Editing IPCC:AR6/WGI/Chapter-7 (section)

===== 7.4.2.4.3 Synthesis for the net cloud feedback =====

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The understanding of the response of clouds to warming and associated radiative feedback has deepened since AR5 (Figure 7.9 and FAQ 7.2). Particular progress has been made in the assessment of the marine low-cloud feedback, which has historically been a major contributor to the cloud feedback uncertainty but is no longer the largest source of uncertainty. Multiple lines of evidence (theory, observations, emergent constraints and process modelling) are now available in addition to ESM simulations, and the positive low-cloud feedback is consequently assessed with ''high confidence'' .

The best estimate of net cloud feedback is obtained by summing feedbacks associated with individual cloud regimes and assessed to be α C = 0.42 W m <sup>–2</sup> °C <sup>–1</sup> . By assuming that the uncertainties of individual cloud feedbacks are independent of each other, their standard deviations are added in quadrature, leading to the ''likely'' range of 0.12 to 0.72 W m <sup>–2</sup> °C <sup>–1</sup> and the ''very likely'' range of –0.10 to +0.94 W m <sup>–2</sup> °C <sup>–1</sup> (Table 7.10). This approach potentially misses feedbacks from cloud regimes that are not assessed, but almost all the major cloud regimes were taken into consideration ( [[#Gettelman--2016|Gettelman and Sherwood, 2016]] ) and therefore additional uncertainty will be small. This argument is also supported by an agreement between the net cloud feedback assessed here and the net cloud feedback directly estimated using observations. The observational estimate, which is sensitive to the period considered and is based on two atmospheric reanalyses (ERA-Interim and MERRA) and TOA radiation budgets derived from the CERES satellite observations for the years 2000–2010, is 0.54 ± 0.7 W m <sup>–2</sup> °C <sup>–1</sup> (one standard deviation; [[#Dessler--2013|Dessler, 2013]] ). The observational estimate overlaps with the assessed range of the net cloud feedback. The assessed ''very likely'' range is reduced by about 50% compared to AR5, but is still wide compared to those of other climate feedbacks (Table 7.10). The largest contribution to this uncertainty range is the estimate of tropical high-cloud amount feedback which is not yet well quantified using models.

In reality, different types of cloud feedback may occur simultaneously in one cloud regime. For example, an upward shift of high-clouds associated with the altitude feedback could be coupled to an increase/decrease of cirrus/anvil cloud fractions associated with the cloud amount feedback. Alternatively, slowdown of the tropical circulation with surface warming ( [[IPCC:Wg1:Chapter:Chapter-4#4.5.3|Section 4.5.3]] and Figure 7.9) could affect both high and low-clouds so that their feedbacks are co-dependent. Quantitative assessments of such covariances require further knowledge about cloud feedback mechanisms, which will further narrow the uncertainty range.

In summary, deepened understanding of feedback processes in individual cloud regimes since AR5 leads to an assessment of the positive net cloud feedback with ''high confidence'' . A small probability (less than 10%) of a net negative cloud feedback cannot be ruled out, but this would require an extremely large negative feedback due to decreases in the amount of tropical anvil clouds or increases in optical depth of extratropical clouds over the Southern Ocean; neither is supported by current evidence.

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'''Table 7.9''' '''|''' '''Assessed sign and confidence level of cloud feedbacks in different regimes in AR5 and AR6.''' For some cloud regimes, the feedback was not assessed in AR5, indicated by N/A.

{| class="wikitable"
|-
| Feedback

| AR5

| AR6

|-
| High-cloud altitude feedback

| Positive ( ''high confidence'' )

| Positive ( ''high confidence'' )

|-
| Tropical high-cloud amount feedback

| N/A

| Negative ( ''low confidence'' )

|-
| Subtropical marine low-cloud feedback

| N/A ( ''low confidence'' )

| Positive ( ''high confidence'' )

|-
| Land cloud feedback

| N/A

| Positive ( ''low confidence'' )

|-
| Mid-latitude cloud amount feedback

| Positive ( ''medium confidence'' )

| Positive ( ''medium confidence'' )

|-
| Extratropical cloud optical depth feedback

| N/A

| Small negative ( ''medium confidence'' )

|-
| Arctic cloud feedback

| Small positive ( ''very low confidence'' )

| Small positive ( ''low confidence'' )

|-
| Net cloud feedback

| Positive ( ''medium confidence'' )

| Positive ( ''high confidence'' )

|}

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