Over the past several decades scientists have been measuring the average temperature of the earth’s atmosphere and its oceans. They have determined that the earth’s average temperature is rising sharply in the 21st century, largely because of increased amounts of carbon dioxide and other greenhouse gases trapped in the atmosphere.
This increasing level of carbon is due to the burning of fossil fuels, volcanic emissions, and the fact that large areas of forest have been cleared, therefore decreasing the earth's ability to absorb carbon.
Although carbon in the atmosphere is a good thing because it keeps the earth inhabitable, if the amount of carbon exceed safe levels, it will cause massive changes in the earth, including melting glaciers, rising ocean temperatures and higher sea levels.
The Intergovernmental Panel on Climate Change (IPCC) predicts that the global temperatures may increase by between 3° and 9° Farenheit, (or 1.7–4.9°C) by the year 2100. This temperature increase could lead to a sea level rise of 25 meters (Dr. James Hansen, NASA)