As I write this (February 2010), Michigan is cold and snowy as usual, the East Coast has been clobbered with record-setting feet of snow, and skeptics are crowing that global warming looks pretty unlikely now.
This reflects a sweet, naive picture of global warming, predicting that everything is going to stay pretty much the same, only two, five, or eight degrees warmer. Unfortunately, this picture is seriously wrong. Here's a better one.
Stretch out your hand, point your finger, and draw a circle in the air a foot or so in diameter. Keep your finger moving around and around that circle.
This is the yearly weather cycle. It's warm during the summer, cooling off through the autumn, cold during the winter, then warming up in the spring, to reach summer again. (The actual weather cycle is a lot more complicated, but this simple cycle gives some reasonable intuitions.)
What happens when we pump energy into this system? The one-foot circle you were drawing in the air doesn't just move upwards by a few degrees. Keep moving around the cycle, but expand its diameter to a foot-and-a-half or even two feet.
The highs get higher. The lows get lower. And the winds blow harder.
Cold arctic winds blow harder and travel farther, bringing freezing temperatures farther south. Other winds pick up more moisture than before, and bring it to new regions before they drop it, as rain or snow.
The real signs of global warming are not warmer temperatures, but more intense, higher-energy weather. Local climates will change, becoming warmer or colder, wetter or dryer, because new weather is blowing in from farther away, and the expected weather is blowing onward, to happen somewhere else.
You can argue about exactly why this is happening, and what to do about it. But don't let unusual blizzards in the winter fool you into thinking that global warming is not happening.