Cloud feedback
[1]: 2224 On their own, clouds are already an important part of the climate system, as they consist of water vapor, which acts as a greenhouse gas and so contributes to warming; at the same time, they are bright and reflective of the Sun, which causes cooling.
[7]: 95 However, some cloud types are more difficult to observe, and so climate models have less data about them and make different estimates about their role.
[16] Clouds have two major effects on the Earth's energy budget: they reflect shortwave radiation from sunlight back to space due to their high albedo, but the water vapor contained inside them also absorbs and re-emits the longwave radiation sent out by the Earth's surface as it is heated by sunlight, preventing its escape into space and retaining this heat energy for longer.
At the top of the atmosphere, it can be described by the following equation[20] The net cloud radiative effect can be decomposed into its longwave and shortwave components.
The longwave effect is calculated by the next following equation Where σ is the Stefan–Boltzmann constant, T is the temperature at the given height, and F is the upward flux in clear conditions.
Putting all of these pieces together, the final equation becomes Under dry, cloud-free conditions, water vapor in atmosphere contributes 67% of the greenhouse effect on Earth.
[3]: 1022 Climate change increases the amount of water vapor in the atmosphere due to the Clausius–Clapeyron relation, in what is known as the water-vapor feedback.
(AR6 WG1, Ch1, 223) When the Intergovernmental Panel on Climate Change had published its Sixth Assessment Report (AR6) in 2021, the uncertainty range regarding cloud feedback strength became 50% smaller since the time of the AR5 in 2014.
ECS is an estimate of long-term (multi-century) warming in response to a doubling in CO2-equivalent greenhouse gas concentrations: if the future emissions are not low, it also becomes the most important factor for determining 21st century temperatures.
[11] These model results had attracted considerable attention when they were first published in 2019, as they would have meant faster and more severe warming if they were accurate.
Climate models generally assume that aerosols increase liquid water path, which makes the clouds even more reflective.
[16] However, satellite observations taken in 2010s suggested that aerosols decreased liquid water path instead, and in 2018, this was reproduced in a model which integrated more complex cloud microphysics.
[31] Moreover, large-scale observations can be confounded by changes in other atmospheric factors, like humidity: i.e. it was found that while post-1980 improvements in air quality would have reduced the number of clouds over the East Coast of the United States by around 20%, this was offset by the increase in relative humidity caused by atmospheric response to AMOC slowdown.
First, it had increased greatly from 1950s to 1980s, largely due to the widespread burning of sulfur-heavy coal, which caused an observable reduction in visible sunlight that had been described as global dimming.
[40] Climate models do account for the presence of aerosols and their recent and future decline in their projections, and typically estimate that the cooling they provide in 2020s is similar to the warming from human-added atmospheric methane, meaning that simultaneous reductions in both would effectively cancel each other out.
[45] It was suggested that this finding could help explain past episodes of unusually rapid warming such as Paleocene-Eocene Thermal Maximum[46] In 2020, further work from the same authors revealed that in their large eddy simulation, this tipping point cannot be stopped with solar radiation modification: in a hypothetical scenario where very high CO2 emissions continue for a long time but are offset with extensive solar radiation modification, the break-up of stratocumulus clouds is simply delayed until CO2 concentrations hit 1,700 ppm, at which point it would still cause around 5 °C (9.0 °F) of unavoidable warming.
