Global emissions of carbon dioxide and other greenhouse gases rise every year, and within decades are expected to hit a worrisome atmospheric concentration threshold of 450 parts per million. At that point, there's a high probability that average global temperatures will be at least 2 degrees Celsius higher than they were in 1850 (they're already 1 C higher). Our children would live in a world of mass migrations, wars and conflicts fueled by scarce water supplies, infrastructure destruction as rising sea levels swallow coastlines, extreme weather events, wildfires and increased poverty and disease. These are not the predictions of wild-eyed liberal pundits but of thousands of climate researchers around the world, along with organizations such as the Intergovernmental Panel on Climate Change, the U.S. Global Change Research Program and the National Academies of Sciences.
It gets worse. No one really knows what would happen if average temperatures hit 5 C higher than 1850 -- a level we could easily reach within a century under a business-as-usual scenario -- but changes to the physical geography of the planet become probable: land masses would vanish; ecosystems would collapse. Human civilization would change, and not for the better.
This process can still be slowed at a moderate economic cost, but time is short -- delays make both fighting climate change and adapting to it dramatically more expensive, and eventually could make it impossible. It's foolish to say we can't afford to pass a climate bill during a recession. We can't afford not to.