Unit 12: Earth's Changing Climate // Section 7: Observed Impacts of Climate Change
Human-induced climate change has already had many impacts. As noted above, global average surface temperatures rose by 0.6°C +/- 0.2°C and sea levels rose by 0.12 to 0.22 meters during the 20th century. Other observed changes in Earth systems that are consistent with anthropogenic climate change include:
- Decreases by about two weeks in the duration of ice cover on rivers and lakes in the mid- and high latitudes of the Northern Hemisphere over the 20th century;
- Decreases by 10 percent in the area of snow cover since satellite images became available in the 1960s;
- Thinning by 40 percent of Arctic sea ice in late summer to early autumn in recent decades, and decrease by 10 to 15 percent in extent in spring and summer since the 1950s (Fig. 12);
- Widespread retreat of non-polar glaciers;
- Increases by about 1 to 4 days per decade in growing seasons in the Northern Hemisphere, especially at higher latitudes, during the last 40 years; and
- Thawing, warming, and degrading of permafrost in some regions (footnote 11).
Figure 12. Arctic sea ice coverage, 1979 and 2003
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Source: © National Aeronautics and Space Administration.
The Earth is not warming uniformly. Notably, climate change is expected to affect the polar regions more severely. Melting snow and ice expose darker land and ocean surfaces to the sun, and retreating sea ice increases the release of solar heat from oceans to the atmosphere in winter. Trends have been mixed in Antarctica, but the Arctic is warming nearly twice as rapidly as the rest of the world; winter temperatures in Alaska and western Canada have risen by up to 3–4°C in the past 50 years, and Arctic precipitation has increased by about 8 percent over the past century (mostly as rain) (footnote 12).
Observed climate change impacts are already affecting Earth's physical and biological systems. Many natural ecosystems are vulnerable to climate change impacts, especially systems that grow and adapt slowly. For example, coral reefs are under serious stress from rapid ocean warming. Recent coral bleaching events in the Caribbean and Pacific oceans have been correlated with rising sea surface temperatures over the past century (footnote 13). Some natural systems are more mobile. For example, tree species in New England such as hemlock, white pine, maple, beech, and hickory have migrated hundreds of meters per year in response to warming and cooling phases over the past 8,000 years (footnote 14). But species may not survive simply by changing their ranges if other important factors such as soil conditions are unsuitable in their new locations.
Insects, plants, and animals may respond to climate change in many ways, including shifts in range, alterations of their hibernation, migrating, or breeding cycles, and changes in physical structure and behavior as temperature and moisture conditions alter their immediate environments. A recent review of more than 40 studies that assessed the impacts of climate change on U.S. ecosystems found broad impacts on plants, animals, and natural ecosystem processes. Important trends included:
- Earlier spring events (emergence from hibernation, plant blooming, and onset of bird and amphibian breeding cycles);
- Insect, bird, and mammal range shifts northward and to higher elevations; and
- Changes in the composition of local plant and animal communities favoring species that are better adapted to warming conditions (higher temperatures, more available water, and higher CO2 levels).
Because many natural ecosystems are smaller, more isolated, and less genetically diverse today than in the past, it may be increasingly difficult for them to adapt to climate change by migrating or evolving, the review's authors concluded (footnote 15). This is especially true if climate shifts happen abruptly so that species have less response time, or if species are adapted to unique environments (Fig. 13).
Figure 13. Polar bear hunting on Arctic sea ice
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Source: © Greenpeace/Beltra.