7.8: Future Geographies - The Evidence for Climate Change

It is clear from past research that Earth's climate has varied significantly over time, the causes of which are many and complex. Even over the span of human history (i.e. several thousand years), the Earth has undergone significant periods of warming and cooling. Present day global warming however is not, to most scientists, a consequence of natural climate variability, but of human impact on the Earth system.

Evidence for Global Warming

There is clear consensus that present day warming is real and humans are driving it. Evidence for global warming can be found in every part of the Earth system. Besides the well documented changes and broken records in air temperature, global warming is

• heating the world's oceans
• reducing sea ice extent, especially in the Arctic
• melting glaciers
• causing sea level to rise
• altering habitats and,
• affecting plant and animal distributions

Evidence from the oceans

Global warming has already had a significant impact on the hydrosphere, especially glaciers and oceans. Tide gauge measurements show a worldwide increase of sea level of 15-20 cm (6-8 inches). According to the Intergovernmental Panel on Climate Change (IPCC) suggest the rise is due to the expansion of ocean water due to rising temperatures (contributing about 3 - 7 cm) and the melting of mountain glaciers and small ice caps (contributing about 2 - 4 cm). Sea level is rising so high in locations like the arctic coast of Canada that small villages are threatened.

Arctic sea ice has experienced a significant decrease in areal extent over the past few decades. Such ice thinning and retreat has impacts ocean salinity, heat balance and animal habitat. Polar bear populations are on the decline as ice thins making for precarious hunting conditions. Ice pack break up leaves polar bears stranded further from land. Polar bears are being forced to swim longer distances between ice floes and drowning during the journey.

Video: Arctic Sea Ice, September 1979 to September 2014 (Courtesy NSIDC)

Evidence from glaciers

Most of the world's glaciers are in retreat as a result of global warming. Evidence for the loss of ice is documented in the journals of the first explorers to Alaska. In the 1790's travelers to the area near Glacier Bay reported only a small embayment of coastline with a large glacier occupying the basin of Glacier Bay. By the 1890's the glacier was in retreat as 40 miles of coastline was now exposed. Today, visitors can still marvel at the great tidewater glaciers, but for how long? Glacier Bay now extends for 60 miles.

Bruce Molinia of the United States Geological Survey has documented the retreat of Alaskan glaciers by comparing present day conditions to photographs taken by geologists and visitors to the region over a century before. The dramatic evidence is seen in photographs of Muir Glacier taken from the same position in 1899 and 2003.

Figure \(\PageIndex{1a}\): Muir Glacier, 1899 (Courtesy USGS)

Figure \(\PageIndex{1b}\): Muir Glacier, 2003 (Courtesy USGS)

Evidence from plant and animal distributions

Evidence of global warming is appearing in the distribution of plant and animal species. Growing seasons in many parts of the globe have shifted as a result of warmer temperatures. Scientists have observed that plants have been slowly moving toward higher elevations in some mountain regions. This "escalator effect" as it has been called, has pushed some plant species up as much as ten feet (3 meters) in some mountain regions. Because some plant species migrate faster than others, established ecosystems could be severely altered.

Figure \(\PageIndex{2}\): Great Tit (Parus major) (Source: Wikimedia)

Animal species are also responding to habitat changes as a result of global warming. As spring arrives earlier in the year, migrating animals start moving earlier in search of food sources. A recent study of the great tit (Parus major) reported that the birds are laying eggs nearly two weeks earlier than they have over the past fifty years. Doing so enables them to hatch earlier and take advantage of caterpillars which are also appearing two weeks earlier due to warmer spring temperatures.

Geographic Patterns of Global Warming

The effects of global warming will not be the same in all places. The smallest changes in temperature are to occur in tropical regions, while the Arctic and Antarctic will experience considerable changes. The Arctic regions are seen as the "bell weather" of what global warming will bring. [ See "The Arctic: Our Global Thermostat" from Scientific American Frontiers].

Figure \(\PageIndex{3}\): House damaged by melting permafrost. (Source: Select Committee on Energy Independence and Global Warming)

The arctic is already experiencing the impact of global warming. The melting tundra presents an engineering nightmare to those living in this environment. Coastal erosion of permfrost is destabilizing slopes causing them to fail. Climate scientists fear that the melting permafrost will release millions of tons of stored carbon back into the atmosphere further fueling global warming.

For detailed information on geographic patterns of global warming, see the "Future Geographies ..." sections at the end of each chapter.