In recent years, global warming has been the subject of a great deal of political controversy. As scientific knowledge has grown, this debate is moving away from whether humans are causing warming and toward questions of how best to respond.

Signs that the Earth is warming are recorded all over the globe. The easiest way to see increasing temperatures is through the thermometer records kept over the past century and a half. Around the world, the Earth's average temperature has risen more than 1 degree Fahrenheit (0.8 degrees Celsius) over the last century, and about twice that in parts of the Arctic.

This doesn’t mean that temperatures haven't fluctuated among regions of the globe or between seasons and times of day. But if you average out the temperature all over the world over the course of a year, you see that temperatures have been creeping upward.

Although we can't look at thermometers going back thousands of years, we do have some records that help us figure out what temperatures and concentrations were like in the distant past. For example, trees store information about the climate in the place where they live. Each year, trees grow thicker and form new rings. In warmer and wetter years, the rings are thicker. Old trees and wood can tell us about conditions hundreds or even several thousands of years ago.

Keys to the past are also buried under lakes and oceans. Pollen, creatures, and particles fall to the bottom of oceans and lakes each year, forming sediments. Sediments preserve all these bits and pieces, which contain a wealth of information about what was in the air and water when they fell. Scientists reveal this record by inserting hollow tubes into the mud to collect sediment layers going back millions of years.

For a direct look at the atmosphere of the past, scientists drill cores through the Earth's polar ice sheets. Tiny bubbles trapped in the gas are actually pieces of the Earth's past atmosphere, frozen in time. That's how we know that the concentrations of greenhouse gases since the industrial revolution are higher than they've been for hundreds of thousands of years.

Computer models help scientists to understand the Earth's climate, or long-term weather patterns. Models also allow scientists to make predictions about the future climate. Basically, models simulate how the atmosphere and oceans absorb energy from the sun and transport it around the globe. Factors that affect the amount of the sun's energy reaching Earth's surface are what drive the climate in these models, as in real life. These include things like greenhouse gases, particles in the atmosphere (such as from volcanoes), and changes in energy coming from the sun itself.