Graphs at the National Climatic Data Center's website (www.ncdc.noaa.gov/indicators) vividly display trends from the late 1800s to the present in atmospheric, surface and ocean temperatures, as well as changes in sea level, snow cover and glacier volume. All point to a warming climate, something many former skeptics now accept as fact.
But is the warming human-caused? The complex interactions of air, sea, ice cover, vegetation and astronomical cycles make projections difficult. Earth's climate has undergone many natural changes in its long history, but no known natural cause explains the current warming trend.
Human causes can. Noncontroversial facts point to human causation: