Causes of Global Warming
There's a good case that the warming of the last 30 years (about 1°F) is human-caused. Here are the three steps.
Human emissions of CO2 predict the increased atmospheric CO2 almost perfectly for the last 250 years.
Is the earth really getting warmer?Yes, and there is complete agreement among several differnent temperature records that in the last 30 years it has warmed dramatically. (See the oldest record of measured temperatures.)
Did the CO2 cause the global warming?We can't be absolutely sure. Here's the best graph of CO2 and global temperature. The only other known contender is the sun, so check that next.
Is the sun causing it? We now have 27 years of accurate solar temperature data from NASA satellites. Although the sun has changed climate in the past, it looks like it is not the culprit this time.