Feedback dynamics

The response of the climate system to an initial forcing is increased by positive feedbacks and reduced by negative feedbacks.^[94] The main negative feedback to global temperature change is radiative cooling to space as infrared radiation, which increases strongly with increasing temperature.^[95] The main positive feedbacks are the water vapour feedback, the ice–albedo feedback, and probably the net effect of clouds.^[96] Uncertainty over feedbacks is the major reason why different climate models project different magnitudes of warming for a given amount of emissions.^[97]

As air gets warmer, it can hold more moisture. After an initial warming due to emissions of greenhouse gases, the atmosphere will hold more water. As water is a potent greenhouse gas, this further heats the climate: the water vapour feedback.^[96] The reduction of snow cover and sea ice in the Arctic reduces the albedo of the Earth's surface.^[98] More of the Sun's energy is now absorbed in these regions, contributing to Arctic amplification, which has caused Arctic temperatures to increase at more than twice the rate of the rest of the world.^[99] Arctic amplification also causes methane to be released as permafrost melts, which is expected to surpass land use changes as the second strongest anthropogenic source of greenhouse gases by the end of the century.^[100]

Cloud cover may change in the future. If cloud cover increases, more sunlight will be reflected back into space, cooling the planet. Simultaneously, the clouds enhance the greenhouse effect, warming the planet. The opposite is true if cloud cover decreases. It depends on the cloud type and location which process is more important. Overall, the net feedback over the industrial era has probably been positive.^[101] An analysis of satellite data between 1983 and 2009 reveals that cloud tops are reaching higher into the atmosphere and that cloudy storm tracks are shifting toward Earth's poles, suggesting clouds will be a positive feedback in the future.^[102]

Roughly half of each year's CO₂ emissions have been absorbed by plants on land and in oceans.^[103] Carbon dioxide and an extended growing season have stimulated plant growth making the land carbon cycle a negative feedback. Climate change also increases droughts and heat waves that inhibit plant growth, which makes it uncertain whether this negative feedback will persist in the future.^[104] Soils contain large quantities of carbon and may release some when they heat up.^[105] As more CO₂ and heat are absorbed by the ocean, it is acidifying and ocean circulation can change, changing the rate at which the ocean can absorb atmospheric carbon.^[106]

A concern is that positive feedbacks will lead to a tipping point, where global temperatures transition to a hothouse climate state even if greenhouse gas emissions are reduced or eliminated. A 2018 study tried to identify such a planetary threshold for self-reinforcing feedbacks and found that even a 2 °C (3.6 °F) increase in temperature over pre-industrial levels may be enough to trigger such a hothouse Earth scenario.^[107]