Climate Systems

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, 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. Most recognize that the present change to a warmer climate is a product of human impact on the Earth system.

Watch "Global Warming" Courtesy of NASA

Evidence for Global Warming

Though a few scientists remain skeptical, there is a a growing consensus that the 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 well documented changes 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.

A 2005 study provided "compelling" evidence that ocean warming over the past 40 years is linked to the industrial release of carbon dioxide. Scientists from Scripps Institution of Oceanography used a variety of scenarios to reproduce the observed rise of ocean temperatures over the last four decades. The rise of ocean temperatures could not be accurately explained by computer models based on the natural climate variability of solar radiation and volcanic emissions. "What absolutely nailed it was greenhouse warming," said Dr Barnett, lead scientist on the study. Their model reproduced the observed ocean temperatures with a statistical confidence of 95%.

Arctic sea ice has experienced a significant decrease in areal extent over the past few decades. Recent NASA research indicates that arctic sea ice is shrinking at a rate of 9.6 percent per decade.  September ice coverage between 2000-2005 were 20 percent below the period 1979 - 2000. 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.

Figure 9.46 Arctic Sea Ice, September 1979 to September 2014
(Courtesy NSIDC)

Evidence from glaciers

Evidence for warming is occurring in all parts of the Earth system. 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.