Some people are surprised that human-induced changes to the atmosphere can have such significant effects on climate. One reason the effects are so large is the operation of climate “feedbacks,” many of which greatly amplify small changes. For example, human activities increase global temperature through the addition of the heat-trapping gases described on the previous page. Then, the warmer atmosphere causes more evaporation, which increases the amount of water vapor, causing more warming. This leads to more evaporation, even more warming, and so on, in a self-reinforcing cycle.
A second important feedback in the climate system involves the melting of snow and ice. As the world warms, snow and ice, which are white and thus reflect heat away from the planet, begin to melt. This reveals darker land and water underneath, causing the Earth to absorb rather than reflect much of the Sun’s heat, and increasing the initial warming.
A third feedback involves carbon released from soils and from the oceans as climate warms. For example, enormous amounts of carbon are stored in frozen soils in the Arctic. As this “permafrost” thaws, it will release ever-greater amounts of carbon dioxide and methane. Wetter soil conditions tend to produce methane, while drier conditions produce carbon dioxide. As these greenhouse gases are released, they will cause more warming, which causes more thawing, in another feedback loop.