Finally, finally, NASA explains in clear, simple terms why they think humans are behind global warming:
Based on a combination of paleoclimate data and models, scientists estimate that when ice ages have ended in the past, it has taken about 5,000 years for the planet to warm between 4 and 7 degrees Celsius. In the past century alone, the temperature has climbed about 0.7 degrees Celsius, which is roughly eight times faster than previous warming.
The second reason that scientists think the current warming is not from natural influences is that, over the past century, scientists from all over the world have been collecting data on natural factors that influence climate—things like changes in the Sun’s brightness, major volcanic eruptions, and cycles such as El Niño and the Pacific Decadal Oscillation. These observations have failed to show any long-term changes that could fully account for the recent, rapid warming of Earth’s temperature.
More words and a few handy charts at the link.