evidence: "I think that measuring with precision human activity on the climate is something very challenging to do and there’s tremendous disagreement about the degree of impact, so no, I would not agree that it’s a primary contributor to the global warming that we see. But we don’t know that yet, we need to continue the debate we need to continue the review and analysis." Fact #1: Rising levels of carbon dioxide (CO2) are the primary contributor to global warming This fact is not new. Scientists have investigated both natural and human contributions and have concluded that carbon dioxide (CO2) is the primary cause of global warming. The "apples to apples" comparison below uses the unit of radiative forcing (the effect caused by greenhouse gases in the atmosphere)—positive numbers indicate a contribution to warming and negative numbers a contribution to cooling. Carbon dioxide is the largest contributor to warming since both 1750 (see IPCC figure 8-15) and 1980 (see IPCC figure 8-20 below).