Reveal Earth's Atmosphere

Climate Change -UNEP

Climate Change - What is it?

Effects of Climate Change on the Atmosphere and Hydrosphere

No single event can be attributed unequivocally to global warming: notice melting, not an increase in hurricane intensity, not the bleaching of coral reefs. It is the sum of all of these changes collectively those points very strongly to a world in which global warming is having an increasing effect. The most dramatic impacts being felt so far in the atmosphere and hydrosphere are the melting cryosphere, rising seas, and the rise in extreme weather events.

Many of the observations presented were described in the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment. Much of what is presented in the report, and much of what is known about changes caused by warming temperature, comes from studies in the Northern Hemisphere because that is where the scientists are concentrated. Europeans, in particular, have been collecting information over decades and centuries that is useful today.

Cryosphere

Warming at the poles has been much greater than warming in other parts of the globe, a phenomenon due largely to positive feedback mechanisms. The cryosphere is especially sensitive to warming temperature because of the water-ice transition. If the temperature in a tropical forest increases by a few degrees, the forest only becomes warmer. But where water is near its freezing point, a small temperature increase converts solid ice into liquid water. This, in turn, drastically reduces albedo, which further increases warming. Ice does not accumulate as easily on open water as it does on or near other ice. Each winter there is less ice, and the ice that forms is thinner, which makes increased melting likely when summer arrives.

Recent high latitude temperature increases have reduced ice and snow cover in the Arctic region. Satellite mapping of the extent of Arctic sea ice in September shows a 20% drop off since 1979, the first year satellite mapping was done, and an even greater decrease in the past few years. Scientists report that their climate models cannot explain the great loss of sea ice without factoring in human- induced greenhouse gas emissions.

Antarctic sea ice has shown no consistent change in extent at this time. Yet, in parts of Antarctica, ice shelves are collapsing. The largest collapse since the end of the last glacial advance occurred off the Antarctic Peninsula in 2002. The Larsen B ice shelf was 1,255 square miles (3,250 sq. km), about the size of Rhode Island, and650 feet (200 m) thick. Its collapse followed that of the Larsen A in 1995.

Glaciers and ice sheets have been in retreat at least since 1961. Beginning in 2000, the melting rate was 1.6 times more than the average rate of the 1990s, and three times the rate of the 1980s. Glaciers in the low latitudes are retreating most rapidly. Mount Kilimanjaro glacier, immortalized in Ernest Hemingway’s short story “The Snows of Kilimanjaro,” has capped the equatorial African mountain for the past 11,700 years. But this glacier has been retreating for at least a century, perhaps due to a decrease in atmospheric moisture over that part of Africa. More recently, melting due to global warming has added to and speeded up the process. In all, the ice cap shrank from 4.71 square miles (12.1 sq. km) in 1912 to 0.68 square miles (1.76 sq. km) in 2006. Ohio State University’s Lonnie Thompson has witnessed the acceleration of the rate of ice loss and predicts the end of the snows of Kilimanjaro at around 2015. At that time, all that remains of Kilimanjaro glaciers will be in an Ohio State University freezer.

Snowfall has increased in the interior of Greenland and portions of Antarctica, yet ice sheets in both locations are shrinking back. Warmer temperatures melt the ice sheets at their edges, while melt water traveling between the ice sheet and the underlying rock causes the ice to slip at its base and enter the melting zone more rapidly. Between 2003 and 2005, Greenland’s low coastal areas lost about three times more weight in ice than the interior accumulated as snow. The net annual ice melt is equal to the volume of water that flows through the Colorado River in 12 years.

Northern Hemisphere permafrost is thawing, turning portions of the Arctic that were frozen for thousands of years into wetlands. (Wetlands are poorly drained landscapes that are covered all or a large portion of the year with fresh or salt water.)

There is evidence that the southern extent of permafrost in the Yukon of Canada has moved pole ward a distance of 60 miles (100 km) since 1899, although accurate records go back only 30 years. The loss of permafrost intensifies Arctic runoff and erosion because frozen lands are more stable. (Erosion is the transport of sediments from their original location by wind, precipitation, ice, or gravity.) Melting permafrost is a positive feedback mechanism for global warming because it releases methane and other hydrocarbon greenhouse gases into the atmosphere.

Water Cycle and Water Resources

Less water trapped in ice means that more water winds up in the other reservoirs, such as the atmosphere, streams and lakes, and the oceans. Generally, the water cycle is becoming more extreme: Wet regions are becoming wetter, and dry regions are becoming drier. Europe is wetter and is experiencing increased runoff and stream flow. The United States has weathered a 20% increase in blizzards and heavy rainstorms since 1900; the total amount of winter precipitation is up 10%.

By contrast, dry areas have more than doubled in size since the 1970s. Arid and semiarid regions, such as Africa’s Sahel, are experiencing increased drought. Reduced rainfall in the southwestern United States has lowered Colorado River flow to less than it was in the Dust Bowl years of the mid-1930s. For five millennia, the Hamoun wetlands, covering 1,500 square miles (4,000 sq. km) and containing ample water, fish, and game, were a place of refuge for the people of Central Asia. The removal of water for irrigation before it could enter the wetlands, coupled with intense droughts, turned the area into a region of salt flats in 2002.

Warmer air has increased the temperature of surface water in the Northern Hemisphere’s lakes and rivers by about 0.3 to 3.6°F (0.2 to 2°C) since the 1960s. The ice on large lakes and rivers in the mid and high latitudes now freezes nine days later, breaks up 10 days earlier, and is thinner and less extensive than in the past. In some East African lakes, deep water has also warmed, which can affect deep aquatic life. This trend will likely be seen in other lakes.

Warmer temperatures change the thermal structure of lakes. A warm surface layer is not dense enough to sink, so its ability to mix with the colder deeper layers is reduced. This keeps oxygen out of the deep layers of the lake and causes aquatic life to suffer. Water quality also decreases in the lake surface (where most organisms live) as solids, salts, and pollutants collect and are no longer mixed throughout the lake.

Rivers are also experiencing changes due to rising temperatures. Due to shorter winters, snow melts earlier in spring, and river flow peaks earlier in the year. Because communities typically need more water in summer, when there is less rainfall, this shift puts a strain on water supply systems.

Water systems will soon be strained by shrinking glaciers. The people of the Andes Mountains of South America rely on snow and ice melt for their water during the dry summers. Runoff is currently high because the glaciers are melting back at about 328 feet (100 m) per decade. By the end of this decade, however, some glaciers will be gone or too small to provide much melt water. Himalayan glaciers are also melting. These glaciers feed seven rivers that provide more than half the drinking water for 40% of the world’s people.