Are there natural processes that can amplify or limit global warming?
Yes. Changes in one part of the climate system trigger processes that may either amplify the initial change or counteract it. With a positive climate feedback, warming triggers changes that cause more warming. With a negative climate feedback, warming triggers changes that lead to cooling.
The most fundamental negative (cooling) feedback is that the Earth radiates heat into space based on its temperature. The relationship between temperature and radiated heat is such that an increase in temperature is accompanied by an even bigger increase in radiated heat. The feedback does not prevent temperature from rising, but it slows the rate of temperature increase (or decrease) that a given energy imbalance can cause. The radiative feedback allows the Earth to achieve a new balanced (equilibrated) state in response to a change in surface temperature.
The other key feedbacks are water vapor, snow and ice, and clouds. Warming temperatures increase the amount of water vapor in the atmosphere. Because water vapor is a powerful greenhouse gas, it amplifies warming. Decreases in snow and ice make the Earth less reflective to incoming sunlight, also amplifying warming. Changes in clouds may either amplify or limit global warming, depending on where (latitude and altitude) and when (time of year) changes occur. Nearly all climate models scientists use today predict that net cloud feedbacks will either be neutral or positive (warming), but such predictions are still uncertain.
Numerous other feedbacks also exist. Warmer temperatures may decrease the rate at which the ocean absorbs carbon dioxide. Global currents that distribute heat among the world’s oceans may change because of temperature and salinity changes. Expansion or contractions of global vegetation can influence the reflection and absorption of incoming sunlight, the flow of energy and moisture between the surface and the air, and the carbon cycle. With the exception of not knowing precisely how much humans will do to control greenhouse gas emissions in coming decades, the strength of climate feedbacks—especially cloud feedbacks—is the biggest source of uncertainty in predictions of future climate.
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