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Transportation and Climate Change Clearinghouse

Overview of Climate Change

An Introduction to the Science of Climate Change | Our Changing Atmosphere | Our Changing Climate

An Introduction to the Science of Climate Change

The earth's climate is predicted to change over time, in part because human activities are altering the chemical composition of the atmosphere through the buildup of greenhouse gases - primarily carbon dioxide, methane, and nitrous oxide. Click here for more information about greenhouse gases. The heat-trapping property of these gases is undisputed. Although uncertainty exists about exactly how the earth's climate responds to these gases, global temperatures are rising.

Climate change may result from:

  • Natural factors, such as changes in the sun's intensity or slow changes in the earth's orbit around the sun
  • Natural processes within the climate system (e.g. changes in ocean circulation)
  • Human activities that change the atmosphere's composition (e.g. through burning fossil fuels) and the land surface (e.g. deforestation, reforestation, urbanization, desertification, etc.)

Our Changing Atmosphere

The Greenhouse EffectD

Energy from the sun drives the earth's weather and climate and it heats the earth's surface; in turn, the earth radiates energy back into space. Atmospheric greenhouse gases (water vapor, carbon dioxide, and other gases) trap some of the outgoing energy, retaining heat somewhat like the glass panels of a greenhouse.

Without this natural "greenhouse effect," temperatures would be much lower than they are now, and life as is known today would not be possible.

Since the beginning of the industrial revolution, atmospheric concentrations of carbon dioxide have increased nearly 30 percent, methane concentrations have more than doubled, and nitrous oxide concentrations have risen by about 15 percent. These increases have enhanced the heat-trapping capability of the earth's atmosphere.

Why are greenhouse gas concentrations increasing? Scientists generally believe that the combustion of fossil fuels and other human activities are the primary reason for the increased concentration of carbon dioxide. Plant respiration and the decomposition of organic matter release more than 10 times the CO2 released by human activities. However, these releases have always been in balance with the carbon dioxide absorbed by plant photosynthesis. What has changed in the last few hundred years is the amount of additional carbon dioxide released by human activities. Energy burned to run cars and trucks, heat homes and businesses, and power factories is responsible for about 80 percent of the U.S. society's carbon dioxide emissions, about 25 percent of U.S. methane emissions, and about 20 percent of global nitrous oxide emissions. Increased agriculture, deforestation, landfills, industrial production, and mining also contribute a significant share of emissions. The United States emits about 25 percent of the total global greenhouse gases.

Our Changing Climate

Global mean surface temperatures have increased 0.74°C over the past 100 years. Eleven of the last 12 years (1995-2006) are among the warmest years recorded since 1850. The snow cover in the Northern Hemisphere and floating ice in the Arctic Ocean have decreased. Globally, sea level has risen 4-10 inches over the past century. Worldwide precipitation over land has increased by about one percent. The frequency of extreme rainfall events has increased throughout much of the United States.

Increasing concentrations of greenhouse gases are likely to accelerate the rate of climate change. Scientists expect that the average global surface temperature could rise 1.6-6.3°F by 2100, with significant regional variation. Evaporation will increase as the climate warms, which will increase average global precipitation. Soil moisture is likely to decline in many regions, and intense rainstorms are likely to become more frequent. Sea level is likely to rise two feet along most of the U.S. coast.

Calculations of climate change for specific areas are much less reliable than global ones, and it is unclear whether regional climate will become more variable.

Some research indicates that there may be levels of GHG emissions that, when reached, cause sudden and irreversible changes in the Earth's climate. For more information of the issue of Tipping Points, please see Climate Tipping Points: Current Perspectives and State of Knowledge.

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