presented some strong evidence for global warming--a chart showing northern latitudes temperature and north polar sea ice coverage from 1880 to 2011. Since about 1970 the temperature has been rising and the ice coverage shrinking, and these trends seem to be accelerating.
Here is some closer-to-home evidence--the "Plant Hardiness Zones" calculated by the US Department of Agriculture. They are based on the lowest winter temperature and each zone differs from the next one by 10 degrees Fahrenheit. The continental US has eight zones, with DC and most of Virginia and Maryland in Zone 7, meaning the normal lowest winter temperature is 0-10 degrees.
The attached graphic shows maps of the hardiness Zones in 1990 and 2006, with a larger map showing changes in the zones. The striking fact is that almost half of the country shifted to a warmer zone in just 16 years.
While some regions of the world would benefit from warmer temperatures, most areas would suffer consequences that range from bad to catastrophic--higher sea levels, more droughts and other extreme weather events, extinction of species, and other negative effects. CO2 is also making the oceans more acidic, endangering coral reefs and shellfish.
So what can be done? Some people see the answer in intrusive government controls, but the most cost-effective policy relies mainly on free markets.
To understand this, go back to basic economics. The free market is the best device ever created for advancing human welfare but it doesn't work in certain situations. Some of these involve what economists call "externalities". Burning of fossil fuels involves a "negative externality", because of the CO2 released, but this cost is born by society at large, not by the buyer or seller of the fuel.
The most cost-effective answer, then, is a tax on CO2 emissions--a "carbon tax"--imposed gradually over many years according to a pre-announced schedule, so that businesses and consumers can adjust gradually; other taxes would be lowered to keep the tax burden constant, or the government could simply rebate the money to citizens. ("Cap and trade" is roughly equivalent, but not quite as good.) Of course, other major nations would have to act similarly, so that the US is not put at a competitive disadvantage.
Given this financial incentive, the power of human ingenuity would be unleashed, as producers and consumers find ways to use energy more efficiently, and produce more of it from non-fossil sources. No one can predict what these changes will be, which is exactly why government should only provide a general monetary incentive--and not do stupid things such as mandating use of ethanol in gasoline.