A skeptic, not a denier

Warren Meyer, writing for Forbes, provides the most cogent explanation I've yet seen for why many of us are skeptical regarding anthropogenic global warming, or AGW. He outlines a very important distinction between being skeptical that any sort of climate change is occurring, and being skeptical that we are heading for some sort of global climate disaster.

It is important to begin by emphasizing that few skeptics doubt or deny that carbon dioxide (CO2) is a greenhouse gas or that it and other greenhouse gasses (water vapor being the most important) help to warm the surface of the Earth. Further, few skeptics deny that man is probably contributing to higher CO2 levels through his burning of fossil fuels, though remember we are talking about a maximum total change in atmospheric CO2 concentration due to man of about 0.01% over the last 100 years.

What skeptics deny is the catastrophe, the notion that man’s incremental contributions to CO2 levels will create catastrophic warming and wildly adverse climate changes. To understand the skeptic’s position requires understanding something about the alarmists’ case that is seldom discussed in the press: the theory of catastrophic man-made global warming is actually comprised of two separate, linked theories, of which only the first is frequently discussed in the media.

The first theory is that a doubling of atmospheric CO2 levels (approximately what we might see under the more extreme emission assumptions for the next century) will lead to about a degree Celsius of warming. Though some quibble over the number – it might be a half degree, it might be a degree and a half – most skeptics, alarmists and even the UN’s IPCC are roughly in agreement on this fact.

But one degree due to the all the CO2 emissions we might see over the next century is hardly a catastrophe. The catastrophe, then, comes from the second theory, that the climate is dominated by positive feedbacks (basically acceleration factors) that multiply the warming from CO2 many fold. Thus one degree of warming from the greenhouse gas effect of CO2 might be multiplied to five or eight or even more degrees.

Back in my college days, I took a class on systems of linear differential equations. I've long since forgotten the details but one of the practical applications was to model systems. The solution of a system of linear differential equations was an equation whose graph depended on starting conditions. For modeling systems, the horizontal coordinate was usually time and the vertical coordinate was some value such as population. Generally, these fell into two types. The first was a convergent solution. Different starting conditions would produce graphs that, as the value for time increased, would converge to a specific value.

The second was a divergent solution. If the starting conditions were altered, then the graphs would diverge away from the value. For example, if the starting condition was low enough, the graph could plunge toward zero. If it was high enough, it could skyrocket toward infinity. Let's say that the system modeled the population of a particular species. If the starting value was too low, the population wouldn't be self-sustaining and would die out. Too high, and overpopulation would occur.

The essence of the final paragraph of the excerpt above is that the second theory predicts a divergent solution; in this case the vertical coordinate is temperature. Adding CO2 artificially alters the starting condition of the graph which skyrockets to a high temperature value. Skeptics, based on historical analysis of temperatures and other factors, think it much more likely that the solution is convergent, or at least not fully divergent.