People sometimes assert that the theory that humans are warming the climate is based entirely on the knowledge that we are emitting large amounts of CO2 and the observation of thermometers that show the planet to be warming. If that were the case, we could not be as confident as we are that humans are affecting the climate in dangerous ways. After all, there wouldn’t necessarily be a causal link between the gases and the warming, which could be caused by something else.
In reality, however, we have other lines of evidence that flesh out and support the theory of dangerous human-induced global warming. A recent post on Joseph Romm’s blog demonstrates eight of these with simple graphics. Because of istopic ratios, it is possible to identify fossil fuels as the source of most of the new carbon in the air and in corals. Also, the pattern of temperature changes observed is consistent with the theory that greenhouse gases we emit block outgoing infrared radiation and warm the planet. Evidence for this includes how the upper atmosphere is cooling while the tropopause rises.
Understanding the causes and significance of all these pieces of evidence requires quite a lot of scientific explanation. Without going to all that length, however, it is possible to express an important point. Scientists haven’t just cobbled together a few observations with a theory and declared that humanity is warming the planet dangerously. Rather, it is a conclusion that has been reached on the basis of extensive investigation and cross-checking. Contrast that approach with that of some climate change deniers who simply assert an alternative cause for warming (or deny that warming is even happening) and then tell everybody not to worry and to keep burning fossil fuels without a care.