Earth's climate constantly changes. About 15,000 years ago, we were in a major "ice age." On the other hand, snowy Norway was a grape-growing region a thousand years ago. Scientists still debate the cause of climate changes. Glaciers expand and contract in natural 20,000-year cycles. Spots on the sun occur in 11-year cycles. Both affect Earth's climate. Nonetheless, climatologists believe that in large part, human actions, not natural cycles, caused the rapid temperature increases of the past century. Burning fossil fuels (coal, oil, and gas) for energy has meant that greenhouse gases are more common in our atmosphere. So it traps more heat.

No visit to the doctor seems complete without getting your temperature taken, even if you've only jammed your thumb or twisted your ankle. That's because a spike or dip in your temperature can alert the doctor to problems—infection or shock—that might otherwise go unnoticed.

click to enlarge image

The black line on this graph shows a clear trend for global temperatures —upward. Source: IPCC

Earth's thermometer is reflecting a big change. As the graph shows, in the past century, the planet's global surface temperature has risen a full degree, according to the familiar Fahrenheit scale. (In Celsius, which uses larger degrees, the increase comes to 60 percent or 0.6 of a degree.)

One degree may not sound like much. But that's a big jump for a single century. In fact, this temperature increase happened four to five times faster than any other climate change in the past millennium.

DEFINITION OF GLOBAL WARMING

These numbers add up to global warming or climate change, terms that you've heard in the news and learned in school. Global warming means Earth is gradually heating up.

The United Nations' Intergovernmental Panel on Climate Change (IPCC) defines global warming more formally: "An increasing body of observations gives a collective picture of a warming world and other changes in the climate system." The IPCC's Summary for Policymakers provides a good overview of the whole issue.

Scientists expect Earth's average temperature to rise about 1.5 to 6 degrees Celsius, or maybe higher, by 2100. Source: IPCC

This warming trend raises two big questions: What's going on? And why does it matter?

Source: Environmental Protection Agency

The burning of fossil fuels has made greenhouse gases more common in Earth's atmosphere. Those gases trap heat from the sun. The more heat our atmosphere captures, the warmer our planet gets.

Even small increases in temperature can have huge impacts. Global warming may disrupt climate patterns around the world and could lead to outbreaks of disease. Some symptoms are already appearing: rising tides, shrinking glaciers, melting permafrost, and shifts in plant and animal habitats. Global climate change may mean that some areas will actually become colder. Melting glaciers, for example, could cool the Gulf Stream, which would create colder weather in much of western Europe.

click to enlarge image

Source: United Nations Environment Programme (UNEP)

What is the "greenhouse effect"?