Before the Industrial Revolution, the amount of carbon dioxide and other greenhouse gases released into the atmosphere was in a rough balance with what could be stored on Earth.
Since the mid-1700s, humans have been emitting additional large amounts of green house gases from burning more fossil fuels to run our cars, trucks, factories, planes and power plants.
The result is that the globe has heated up by about one degree Fahrenheit over the past century -- and it has heated up more intensely over the past two decades. If one degree doesn't sound like a lot, consider the difference in global average temperatures between modern times and the last ice age was only about 9 degrees Fahrenheit.
Already, people have increased the amount of carbon dioxide, the chief global warming pollutant, in the atmosphere to 31 percent above pre-industrial levels. There is more carbon dioxide in the atmosphere now than at any time in the last 650,000 years.
Scientists expect that in the absence of effective policies to reduce greenhouse gas pollution, the global average temperature will increase another 2 degrees by 2100.
Even if the temperature change is at the small end of the predictions, the alterations to the climate are expected to be serious: more intense storms, more pronounced droughts, coastal erosion and increased distribution of infectious diseases. At the high end of the predication, the world could face abrupt, catastrophic and irreversible consequences.