From the NASA website :If Earth has warmed and cooled throughout...

From the NASA website :

If Earth has warmed and cooled throughout history, what makes scientists think that humans are causing global warming now?

The main reason that scientists think humans caused warming since 1950 is that none of the natural processes that influence Earth’s climate have changed enough during that time period to explain the warming.

Over the past thousand years, temperatures have been preserved in natural records like tree rings, ice cores, and coral reefs. Many independent estimates of temperatures from these sources show that while global average surface temperatures varied, at no time were they warmer or did they climb more quickly than during the latter half of the 20th century. Three things can alter global temperatures over this short period: changes in the Sun’s activity, volcanic eruptions, and human emissions of greenhouse gases and aerosols.

Even more telling is the way in which temperatures are rising. If the warming were caused by a more active Sun, then scientists would expect to see warmer temperatures in all layers of the atmosphere. Instead they have observed a warming at the surface and in the lower parts of the atmosphere and a cooling in the upper atmosphere. Something is trapping heat in the lower atmosphere, and that something is greenhouse gases.

Finally, scientists are almost certain that warming during the last 50 years was caused by human activity because models can’t reproduce the observed temperature trend without including a rise in greenhouse gases.



http://earthobservatory.nasa.gov/Features/GlobalWarmingQandA/#01