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.

In the last 100 years, the Earth's average surface temperature has risen by about 1 degree Fahrenheit (0.6 degrees Celsius), and it may climb 2.5 to 10 degrees Fahrenheit (1.5 to 6 degrees Celsius) higher by 2100, the Intergovernmental Panel on Climate Change (IPCC) predicts.

Glaciers are melting, sea levels are rising, cloud forests are drying, and wildlife is scrambling to keep pace. It's becoming clear that humans have caused most of the past century's warming by releasing heat-trapping gases as we power our modern lives. Called greenhouse gases, their levels are higher now than in the last 650,000 years.

The planet is warming, from North Pole to South Pole, and everywhere in between. Globally, the mercury is already up more than 1 degree Fahrenheit (0.8 degree Celsius), and even more in sensitive polar regions. And the effects of rising temperatures aren't waiting for some far-flung future. They're happening right now.