Some people who acknowledge the planet is warming insist there's no proof that humans are the cause. Instead, they discuss natural heating and cooling cycles in Earth's history, and how this period happens to be one of them.
That would be a comforting notion, except that it's incomplete. Over the past century or so, the climate has heated up at a faster rate than at any time during the previous 11,300 years, which is about as far back as scientists can calculate reliably. And our planet's actual temperature is warmer than it ever was during most of that longer period [source: Plait].
It's hard to escape the suspicion that our modern industrial civilization, which has raised levels of carbon dioxide and methane in the atmosphere to the highest levels in the past 650,000 years, might have something to do with it [source: NASA].
"While natural variations have altered the climate significantly in the past, it is very unlikely that the changes in climate observed since the mid-20th century can be explained by natural processes alone," the climate change primer on NASA's Web site explains.