Direct and Indirect Evidence

All indications are that the earth's atmosphere is warming. We might expect to observe other signs around the world that are consistent with this conclusion. Global ocean temperature and sea level are beginning to rise in small but measurable amounts. Rapid melting of snow and ice observed in many parts of the world provides a more dramatic signal that global warming is underway. This chapter will explore the signs of warming that are beginning to show up in various places around the world. Here are a few brief examples:

• Glaciers and ice caps. Each year, enough fresh meltwater is brought into the world's oceans to raise sea level by 0.8 mm.

• Snow cover. Every 10 years, there is 2 percent less snow cover in the northern hemisphere than there was a decade before.

• Lakes and rivers. Each year, the world's lakes and rivers are covered by ice 12 fewer days during the winter months than they were150 years ago.

• Permafrost. There is 7 percent less permanently frozen areas (permafrost) than there was in 1900.

• Spring. Leaves unfold, birds migrate, and birds lay eggs earlier every year.

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