Evidence
Carbon Dioxide Emissions
The primary greenhouse gases in the earth’s atmosphere are water vapor (H20), carbon dioxide (CO2), methane (CH4), chlorofluorocarbons (CFC-11, 12, 113, CCl4), nitrous oxide (N2), ozone, (O3), and aerosols. After water vapor, which is not directly affected by human activities, carbon dioxide is the greenhouse gas most prevalent in the atmosphere. Because of its large volume, carbon dioxide plays a primary role in climate change. Scientists have investigated carbon dioxide’s behavior in the atmosphere, its past and present concentrations, and its relation to human industrial activity.
Evidence for the increase of human-made carbon dioxide in the atmosphere is well established. This evidence is one of the foundations for climate change science. Measuring stations in Hawaii and Alaska have reported increasing carbon dioxide emissions—and rising atmospheric concentrations of carbon dioxide—since 1957.
Using paleoclimatolgical evidence, scientists compared current CO2 concentrations with past concentrations. They found that the present levels of carbon dioxide in the atmosphere are the highest in half a million years.
Based on the heat trapping properties of carbon dioxide, scientists predict the increased atmospheric concentrations of carbon dioxide will lead to a rise in the average global temperature.
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Temperature Increases
The direct evidence for positive climate change will not reveal itself in something as straightforward as warmer summers. Rather, it will reveal itself in small changes in the global average surface temperature (mean surface temperature). Scientists collect temperature data from measuring stations all over the world to calculate the mean surface temperature. Evidence points to a small increase in global temperatures. However, major climate events have been separated by minor changes in global mean temperatures. The last ice age was only estimated to have a mean temperature that was 5 degrees Celsius lower than current temperature levels.
Increases in mean temperatures alone are not enough to prove the connection between human carbon dioxide emissions and global climate change. Scientists have also called for experimental measurements of the heat absorption by the ocean and the cooling potential of ocean cloud-cover and atmospheric aerosols (small, dust like particles that are suspended in the air). Better knowledge of these processes would allow scientist to create more accurate computer climate models.
Records of global climate are not as well established as the records of past carbon dioxide concentrations, but they are still largely uncontroversial.More about Temperature Increases…
Cloud Cover and Atmospheric Feedbacks
Carbon dioxide levels impact climate change by absorbing radiation and causing the temperature of the atmosphere to increase. The increased temperatures vaporize water—a more powerful greenhouse gas. Some scientists predict that a rise in temperature will result in higher rates of ocean evaporation and cloud formation. The clouds will then trap more heat by absorbing outgoing radiation (like carbon dioxide). However, other scientists believe that the clouds will help to cool the atmosphere. They believe that the clouds will increase the albedo, or the amount of solar radiation reflected by clouds back into space without penetrating the lower atmosphere. Clouds have both potentially positive (warming) and negative (cooling) feedback effects. Since the atmosphere is a complex and variable system, it is challenging to observe and measure the operation of such atmospheric feedback effects. The role of cloud and atmospheric moisture remains one of the least understood feedback mechanisms involved in climate change.
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Computer Simulated Climate Forecasts
Since the 1980’s, scientists have tried to develop computer models capable of forecasting general climate trends based on known information. These computer models have been used to predict the global and regional effects of climate change, including changes in precipitation levels, growing seasons, and sea levels. These climate models have not been heavily relied on in the past because of difficulties in accounting for effects of cloud cover, water vapor, the albedo effect, and ocean circulation. Recent climate models have corrected many of these flaws and increased the credibility of climate models.
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Environmental Observation
Many observed environmental changes provide further evidence of a warming trend. Satellite data from the late 1960’s reveals that there is an overall reduation in the area covered by snow. The Arctic sea ice is thinning. The spring melting of ice in rivers and lakes has been ocurring earlier and earlier. The sea levels are rising at a rapid rate. The growing season has become longer. Fish, plans, insects, and birds are expanding their habitats to higher elevations and higher latitudes. Ice shelves in Antarctice are shrkinking and collapsing.
The range of evidence described has done much to establish beyond question the fact that the earth’s atmosphere is indeed warming. “The effect of anthropogenic greenhouse gases is detected,” as the IPCC concludes its Third Assessment. It also suggests some of the ecological changes that may accompany global warming.