First: Global Warming is no longer the term scientists are using, because it causes too much confusion. The new, official term is Climate Change.
Second: All this snow in DC actually helps to prove that "Global Warming" is happening.
1990-2000 was the warmest decade in the past thousand years. Climate change is a slow process, not a day-to-day occurrence; therefore, an unusually cold day in DC is not really a big deal, because it's not the big picture. But also, think of it this way: when you heat up a pot of water, the hotter it gets, the more quickly the water molecules move. A boiling pot bubbles out of control. The same thing happens when we heat up our climate: it gets more intensely active. Which means bigger snow storms, hurricanes, tsunamis, etc. The heat and damage we're inflicting upon our thin atmosphere is essentially super-charging the weather. And as everyone can see, that's not such a good thing for mankind.
The dominant force powering the Earth's cycles is heat, both from the sun and the Earth's core. Damage to the atmosphere reduces our very fragile shield from the sun. The more sun [heat] allowed in, the more powerful the Earth's cycles become. Natural disasters have gotten much more regular and intense in the past years-- plain and simple, there's no denying this.
But even if you don't believe in Climate Change, there's absolutely no reason to destroy our planet's protection from the rest of the non-life supporting universe. It's like climbing up on your roof every day and tearing a shingle out just so you can drive your SUV or eat food that's imported from thousands of miles away. It doesn't make sense. But since people don't feel directly connected to the damage that's occurring, it's easy to ignore... But at this rate, it looks like the weather will become pretty hard to ignore soon.
Here's an entertaining video from the Rachel Maddow show about why the snow on the East Coast does not disprove Global Warming. Featuring everybody's favorite, Bill Nye the Science Guy (yes, he IS alive).