Let’s begin by putting a careful name to what we are talking about. We are discussing the hypothesis of “catastrophic man-made global warming theory.” We are not just talking about warming but warming that is somehow man-made. And we are not talking about a little bit of warming, but enough that the effects are catastrophic and thus justify immediate and likely expensive government action. In discussing this theory, we’ll use the UN’s Intergovernmental Panel on Climate Change (IPCC) as our main source. After reading through most of the IPCC’s last two reports, I think it is fair to boil the logic behind the theory to this picture:
As you can see, the theory is actually a chain of at least three steps: 1. CO2, via the greenhouse effect, causes some warming.
2. A series of processes in the climate multiply this warming by several times, such that most of the projected warming in various IPCC and other forecasts come from this feedback, rather than directly from the greenhouse gas effect of CO2. 3. Warming only matters if it is harmful, so there are a variety of theories about how warming might increase hazardous weather (e.g. hurricanes, tornadoes, floods, droughts), raise sea levels, or affect biological processes. In parallel with this theoretical work, scientists are looking for confirmation of the theory in observations. They have a variety of ways to measure the temperature of the Earth, all of which have shown warming over the past century. With this warming in hand, they then attempt to demonstrate how much of this warming is from CO2. The IPCC believes that much of past warming was from CO2, and recent work by IPCC authors argues that only exogenous effects prevented CO2-driven warming from being even higher. This is just a summary. We will walk through each step in turn. CO2 as a Greenhouse Gas
The first step in the theory is the basic greenhouse gas theory — that CO2 will raise the temperature of the Earth as its concentration increases (through a process of absorption and re-radiation that we will not get into). Its probably irresponsible to call anything in a science so young as climate “settled,” but the fact that increased atmospheric CO2 will warm the Earth by some amount is pretty close to being universally accepted. More debatable is how much warming will occur. We have measurements of warming from laboratory experiments, but these are hard to translate...