The national debate over what to do, if anything, about the increasing concentration of greenhouse gases in the atmosphere has become less a debate about scientific or economic issues than an exercise in political theater. The reason is that the issue of global climate change is pregnant with far‐reaching implications for human society and the kind of world our children will live in decades from now.
Introducing nuance and clear‐headed reason to this debate is something of a struggle. As Cato Institute chairman William Niskanen has noted, for any international action to merit support, all of the following propositions must be proven true:
- A continued increase in the emission of greenhouse gases will increase global temperature.
- An increase in average temperature will generate more costs than benefits.
- Emissions controls are the most efficient means to prevent an increase in global temperature.
- Early measures to control emissions are superior to later measures.
- Emissions controls can be effectively monitored and enforced.
- Governments of the treaty countries will approve the necessary control measures.
- Controlling emissions is compatible with a modern economy.
The case for any one of those statements is surprisingly weak. The case for a global warming treaty, which depends on the accuracy of all those statements, is shockingly weak. My talk this afternoon will concentrate on a few of the most important of those propositions.
A Continued Increase in the Emission of Greenhouse Gases Will Increase Global Temperature
First off, this subject is terribly complex; the 2nd Assessment Report of the International Panel on Climate Change is 500 pages long with 75 pages of references. As Ben Santer, author of the key IPCC chapter that summarized climate change science, has noted, there are legions of qualifications in those pages about what we know and what we don’t. But, unfortunately, those qualifications get lost in the journalistic and political discourse.
I will dispense with an introductory discussion of the rudimentary elements of greenhouse theory. I’m sure you’re all familiar with it. Largely on the basis of computer models, which attempt to reflect what we know, what we assume, and what we can guess, many people believe that continued emissions of anthropogenic greenhouse gasses will increase global temperatures anywhere from 1 to 3.5 degrees Celsius.
At this point, I should note that those estimates have been coming down over time. The 1990 IPCC report predicted a little more than twice this amount of warming, and projections have been declining ever since as better models have been constructed. One wonders, at this rate, whether the models will continue to predict increasingly smaller amounts of warming until even the upper bound forecasts become so moderate as to be unimportant.
What We Know – And What We Don’t Know
Here’s what the data say, about which there is little debate; ground‐based temperatures stations indicate that the planet has warmed somewhere between .3 and .6 degrees Celsius since about 1850, with about half of this warming occurring since WWII. Moreover:
- Most of the warming occurs over land, not over water;
- Most of the warming occurs at night; and
- Most of the warming moderates of wintertime low temperatures.
But even here, we have uncertainties. Shorter sets of data collected by far more precise NASA satellites and weather balloons show a slight cooling trend over the past 19 years, the very period during which we supposedly began detecting the greenhouse signal. Those data are generally more reliable because satellite and balloons survey 99% of the earth’s surface, whereas land‐based data (1) only unevenly cover the three‐quarters of the earth’s surface covered by oceans and (2) virtually ignore polar regions.
While some of that cooling was undoubtedly a result of Mt. Pinetumbo and the increased strength of the El Nino southern oscillation, those events fail to explain why the cooling occurred both before and after those weather events were played out and why, even correcting for those events, the temperature data show no significant warming during the 19‐year period.
While it is true, as critics point out, that the satellite and weather balloons measure temperatures in the atmosphere and not on the ground
- where ground‐based measurements are most reliable – over the North American and European landmasses, the correlation coefficient between satellite and surface measurements is 0.95 – close to perfect agreement, and
- the computer models predict at least as much warming in the lower atmosphere as at the surface, so if warming were occurring, it should be detectable by the satellites and weather balloons.
Even assuming that ground‐based temperature data are more reflective of true climate patterns, that still leaves us with a mystery. When fed past emissions data, most of the computer models predict a far greater amount of warming by now than has actually occurred (the models that are reasonably capable of replicating known conditions are a tale unto themselves to which I’ll return in a moment). Notes the IPCC, “When increases in greenhouse gases only are taken into account … most climate models produce a greater mean warming than has been observed to date, unless a lower climate sensitivity is used.” Indeed, the most intensive scientific research is being done on why the amount of warming that has occurred so far is so low. After all, a .3-.6 degree Celsius warming trend over the last 150 years all but disappears within the statistical noise of natural climate variability.
There are three possibilities:
- something’s wrong with the temperature data;
- something’s masking the warming that would otherwise be observed; or
- the atmosphere is not as sensitive to anthropogenic greenhouse gases as the models assume.
Indirect Evidence of Global Temperature
Scientists who argue the first possibility cite the largely incompatible, imprecise, and incomplete nature of even recent land‐based temperature records. Those observations, of course, are absolutely correct. Instead, these scientists concentrate on indirect evidence suggesting that the planet has been warming and has been warming significantly over the relatively recent past. They typically point to precipitation trends, glacial movement, sea level increases, and increased extreme temperature variability as suggestive of a significant warming trend. Let’s take each of these issues in turn.