The Physical Environment

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 Climate Systems

Climate Change: Global Warming

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.

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.

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 200-2005 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 CS.46 Changes in Sea Ice, 1979 - 2003
Courtesy NASA (Source)

Evidence from glaciers

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%.

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.



Figure CS.47a Muir Glacier, 1899
 (Courtesy USGS)

Figure CS.47b Muir Glacier, 2003
 (Courtesy USGS)

The BBC's David Shukman reports that scientists have found that the rate of melting on the Greenland ice cap is far greater than what normally occurs during the summer. Sea level is expected to rise by 7 meters should the ice cap melt, drowning coastlines worldwide. Video icon - BBC report - Greenland ice cap

A first ever gravity survey of Antarctica recently revealed that it had lost a substantial amount of mass. Researchers found that the ice sheet covering Antarctica lost 152 (plus or minus 80) cubic kilometers of ice annually between April 2002 and August 2005. The estimated mass loss was enough to raise global sea level about 1.2 millimeters (0.05 inches), about 13 percent of the overall observed sea level rise for the survey period.

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. [readings icon  Video link: The Arctic  See "The Arctic: Our Global Thermostat" from Scientific American Frontiers]

Temp change

Figure CS.48 Projected future regional patterns of warming based on three emissions scenarios (low, medium, and high growth). Source: NASA Earth Observatory, based on IPCC Fourth Assessment Report (2007)

Tropical Regions

Most model predictions indicate the smallest change to temperature will occur in tropical latitudes. Depending on model assumptions and location, annual changes on the order of .1 oC to 3 oC are predicted. Analysis indicates that there may be significant differences within the tropics, especially in Asia, depending on proximity to the sea. Warming is projected to be least in the islands and coastal areas throughout Indonesia, the Philippines, and coastal south Asia and Indo-China and greatest inland. Even with relatively small temperatures, they can be devastating. A 3-degree Celsius (5.4 Fahrenheit) rise in temperature would result in a 60 percent reduction in the arabica coffee area in Brazil, the world's largest producer.

Subtropical Regions

Though most geoscientists have felt that the Arctic will show the first true signs of a future climate, dramatic changes have been recorded in subtropical regions. Recent analysis of satellite data has found that each hemisphere's jet stream has moved poleward by about 1 degrees of latitude or 70 miles. Jet streams are found on the poleward limit of the tropics which means they are getting wider. Continued movement would mean the spread of subtropical deserts like the Sahara. Regional climate predictions for southern Africa during summer suggest a warm season increase of 2oC to 4oC over the subcontinent, with the doubling of carbon dioxide. Current climate models project regional temperature increases between 4o and 10o F by 2100, with an 8o to 15oF increase in the average summer heat index for the southeastern United States.

The Midlatitudes

Climate change is expected to increase the frequency of extreme events in midlatitude regions like the midwest United States. A severe drought in 1988, heat waves in 1995 and 1996, flooding on the Mississippi in 1993 (100-year flood) and 2002, and numerous tornadoes and severe thunderstorms can be expected in the future. Illinois will be become warmer, especially in the summer having temperatures more like present-day Oklahoma or Arkansas. Wisconsin temperatures could rise 5o-10oF in the winter and by 8o - 17o F during the summer by 2100. Extreme heat will be more common than today. Southern Ontario's winter temperatures are expected to increase by 3o - 7oC and summer's to be 4-8C warmer. More southerly states like Illinois will experience less warmer. Winter temperatures are expected to increase by 5o-7F during the winter in Indiana and summer temperatures increasing by 8o - 10o F. Growing seasons could be 4 to 7 weeks longer in Wisconsin and 3 to 6 weeks longer to the south in Illinois. Under a medium-high emissions scenario, the IPCC predicts a 5.5o-7.9oF change in statewide in California.

Arctic regions House damaged by permafrost

The arctic regions appear to be impacted the most. Observations of mean annual surface air temperature over the past 50 years has increased 3.6oF to 5.4°F in Alaska and Siberia and decreased by 1.8°F over southern Greenland. Mean annual surface air temperature over the Arctic region (north of 60° latitude) is projected to increase 3.6°F by 2050 and 8°F by 2100.

Figure CS.49 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. Climate scientists fear that the melting permafrost will release millions of tons of stored carbon back into the atmosphere further fueling global warming.

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For Citation: Ritter, Michael E. The Physical Environment: an Introduction to Physical Geography.
2006. Date visited.  http://www.uwsp.edu/geo/faculty/ritter/geog101/textbook/title_page.html

© 2003-2008
Michael Ritter (tpeauthor@mac.com)
Last revised 06/21/07