It's been a tough winter in North America. Many regions in the North­east experienced record amounts of snow. Some southern states endured some of their harshest winter storms. On January 11, every state in the union except Florida had snow on the ground. All this winter weather prompted some people to ask the familiar question: so where are the signs of global warming?

Actually, meteorologists say this kind of weather is what can be expected in North America if global temperatures are rising. Higher temperatures put more moisture into the air, increasing the chances of storms that release large amounts of snow or rain. As climate scientists have been telling us, global warming means more volatile weather all around—droughts in some places, floods or heavy snowstorms elsewhere.

We can't, of course, extrapolate from a few events to a conclusion about global warming. That's why we need the scientists who have been monitoring data over decades and tracing it over the centuries. That's why we need to do more than talk about the weather. We need to attend to the unusually rapid increase in the Earth's temperature over the past 50 years and to the strong evidence that the Arctic ice cap is melting, the glaciers are disappearing, the sea level is rising, the oceans are becoming more acidic, and plants and animals are altering their behavior in response to shifting temperatures. And we need to pay attention to the overwhelming number of scientists who say that the global warming is caused at least in part by human behavior in the form of carbon emissions, which are concentrating greenhouse gases in the atmosphere and warming up the Earth for decades or centuries to come—perhaps permanently.