A: Scientists have spent decades figuring out what is causing global warming. 

B: They've looked at the natural cycles and events that are known to influence climate. 

A:  But the amount and pattern of warming that's been measured can't be explained by these factors alone. The only way to explain the pattern is to include the effect of greenhouse gases (GHGs) emitted by humans.

B: The IPCC meets every few years to review the latest scientific findings and write a report summarizing all that is known about global warming.

A: Each report represents a consensus, or agreement, among hundreds of leading scientists.

B: One of the first things scientists learned is that there are several greenhouse gases responsible for warming, and humans emit them in a variety of ways. 

A: Most come from the combustion of fossil fuels in cars, factories and electricity production.

B: Different greenhouse gases have very different heat-trapping abilities. Some of them can even trap more heat than CO2. 

A: In order to understand the effects of all the gases together, scientists tend to talk about all greenhouse gases in terms of the equivalent amount of CO2.