[dmek25]

"Very likely," the IPCC said in a February 2007 report.

The report, based on the work of some 2,500 scientists in more than 130 countries, concluded that humans have caused all or most of the current planetary warming. Human-caused global warming is often called anthropogenic climate change.


• Industrialization, deforestation, and pollution have greatly increased atmospheric concentrations of water vapor, carbon dioxide, methane, and nitrous oxide, all greenhouse gases that help trap heat near Earth's surface. (See an interactive feature on how global warming works.)

• Humans are pouring carbon dioxide into the atmosphere much faster than plants and oceans can absorb it.

• These gases persist in the atmosphere for years, meaning that even if such emissions were eliminated today, it would not immediately stop global warming.

• Some experts point out that natural cycles in Earth's orbit can alter the planet's exposure to sunlight, which may explain the current trend. Earth has indeed experienced warming and cooling cycles roughly every hundred thousand years due to these orbital shifts, but such changes have occurred over the span of several centuries. Today's changes have taken place over the past hundred years or less.

• Other recent research has suggested that the effects of variations in the sun's output are "negligible" as a factor in warming, but other, more complicated solar mechanisms could possibly play a role.