Scientists are able to present a sort of atmospheric history of the last 400,000 years of the Earth through ice cores taken from the arctic. This information is then corroborated with other evidence from trees, coral growth, and radioactive carbon dating. What scientists have found is a correlation between the amount of atmospheric greenhouse gasses and average surface temperature. As the chart below indicates, the more greenhouse gasses in the atmosphere, the higher the temperature of the earth.
The sudden spike of the red line at the end of the chart is the industrial era, the period when we began using fossil fuels for energy. The burning of fossil fuels results in high greenhouse gas emissions. The chart also demonstrates a cycle of warming and cooling for the Earth which has caused some to suggest that the current warming is the result of a natural trend. However, our current warming has exceeded precedent (over the past 50 years the average global temperature has increased at the fastest rate history) and the current increase in temperature can only be explained when we consider human factors in addition to natural factors.
The chart above breaks down the computer modeling for natural factors (blue) vs. natural and human factors (red). The actual recorded temperature is in black. Only when we take into account human activity do we find modeling that matches the actual observed temperatures.