Yep, it's the biggest problem we've ever faced. But you can get the basics in 600 words.
1. Global warming isn't happening slowly, and it's not a problem (just) for the future. We've already burned enough coal and gas and oil to increase the temperature more than a degree, and this has already caused enormous changes across the planet: Arctic sea ice is melting very fast, along with almost every other frozen thing on the planet. Because warm air holds more water vapor than cold, we're seeing far more deluge and flood. People are dying already, and species are going extinct. That one degree will become four or five degrees before the century is out -- unless we quickly take the steps to stop burning that coal and gas and oil.
2. Scientists are not unsure or confused. Scientists are scared. Their professional societies and national academies have called on governments to take action. Here's Lonnie Thompson, perhaps the earth's greatest glaciologist, speaking of his fellow researchers: "Virtually all of us are now convinced that global warming poses a clear and present danger to civilization."
3. Just like the scientists, engineers have done their job too. We have myriad new technologies that can provide us with clean kinds of energy. Wind power, for instance, is the fastest-growing source of electric generation around the planet -- but it starts from such a small base that it isn't increasing fast enough to head off global warming. If we made fossil fuels foot the bill for the damage they're doing to the atmosphere, we could move much more quickly.