If global temperatures are actually increasing as demonstrated here, and contrary to what climate change skeptics such as George Will and Fred Singer assert, then there should be other symptoms, such as melting glaciers and rising oceans. In fact, this is certainly the case. Sea levels have been rising slowly since 1880 and

much more rapidly since about 1940 (Figure 1.7). The rate of rise has been about 1.7 mm per year over the last 100 years, but, as shown in the insert, has been approximately 3.5 mm per year (about 1.4 inches per decade) from 1993 through 2009 (31). The cause of more than half of this rise in sea level is due to thermal expansion of the ocean from the warmer temperatures. The rest is from melting of glaciers and ice caps (about 30%) and Greenland and Antarctic ice sheets (about 15%) (24).

Greenland ice has been shrinking at 50-100 Gt/yr 8 from 1963 to 1993 and at even higher rates from 2003 to 2005 (37). A recent analysis of several different methods to measure ice loss shows that Greenland lost ice at the rate of 263 ±30 Gt/yr between 2005 and 2010 (38). Antarctica has a number of ice shelves that have been receding since the late 1980s, mostly in the Antarctic Peninsula and West Antarctica (39). The Antarctic Peninsula has been warming much more rapidly than the rest of Antarctica, but recent analysis of Antarctic temperatures show that West Antarctica has warmed by 0.17 ± 0.06°C per decade between 1957 and 2006, with the peninsula warming by 0.11 ± 0.04°C per decade, and continent-wide warming of 0.12 ± 0.07°C per decade (40). East Antarctica has actually gained ice because of higher precipitation while the Antarctic Peninsula and West Antarctica have lost ice. The net Antarctica ice sheet loss was 81 ± 37 Gt/yr between 2005 and 2010, for a combined Greenland and Antarctica ice sheet loss of 344 ± 48 Gt/yr (38).

Ice sheet dynamics play a critical role in the loss of ice from Greenland and Antarctica. The ice on Greenland is about two miles thick in the center and tapers

off at the edges. As surface ice melts, it forms large lakes on the surface. Sometimes enormous moulins or holes form in the ice where the water disappears to the base of the ice sheet and flows to the sea (41). These rivers of water lubricate the base of the ice and can increase the rate of flow of the ice into the sea but the details are not well understood. Antarctica is covered by a sheet of ice that averages about one and a half miles thick but is two and a half miles thick at its maximum and contains about 10 times as much ice as Greenland. The pressure of this ice sheet forms glaciers, or rivers of ice, that slowly flow to the sea. Floating ice shelves, such as the Ross and the Larsen ice shelves, form where glaciers enter the sea. These ice shelves butt up against the glaciers and retard the flow of glacial ice into the sea. In recent years, enormous sections of the Larsen and Wilkins ice shelves have disintegrated, removing the pressure against the glaciers so they flow more rap­idly into the sea. These ice sheet dynamics in both Greenland and Antarctica can potentially lead to much more rapid ice loss than is considered in the IPCC esti­mations of ice loss (18, 42), which would raise estimates of predicted sea level rise.

Melting of the Arctic ice does not contribute to rising sea levels, since the Arctic ice is floating in the ocean already. But it is an indicator of global warm­ing. According to the National Snow and Ice Data Center, the maximum extent of winter Arctic sea ice occurs in March and has been declining at a rate of 2.6% per decade since 1979 (when satellite measurements began). The summer Arctic sea ice melt season now lasts nearly a month longer than it did in the 1980s. The minimum Arctic sea ice occurs in mid-September, and it has been declining at 13% per decade compared to the average from 1979-2000. The least Arctic sea ice extent since 1979 occurred in September 2012 and was 16% lower than the previ­ous low in 2007 (43). The loss of sea ice has a positive feedback on global warming because ice reflects the sun while the dark ocean absorbs it. So the loss of summer sea ice causes even greater warming due to this albedo effect.

Glaciers have been melting worldwide for most of the last century, as dramati­cally illustrated by Al Gore (44) and as shown in Figure 1.8. Not all glaciers are melting, and a few glaciers are actually increasing due to increased precipitation in some areas, but worldwide the melting trend is clear and accelerating. Not only is this a concern for long-term rise in sea level, but of even greater concern is the fact that water for the major rivers in Asia comes from glaciers in the Himalayas, which are melting as part of the overall glacial decline. About 2 billion people in more than a dozen Asian countries depend on rivers fed from glaciers and snow in the Himalayas and the Tibetan Plateau. The Tibetan Plateau is heating up twice as fast as the global average, leading Chinese scientists to believe that 40% of the glaciers could disappear by 2050 (45).