Earth Report

This month we’re going to ask if anthropogenic global warming is really settled science. To do that, we will divide the science of AGW into four parts: basic physics, the instrumental (thermometer) record, reconstructions of past climate from proxy data, and global climate models (GCMs). Let’s take them one by one.

The basic physics of greenhouse effect is a century old. The earth absorbs light from the sun. It then radiates the energy into space as heat. The two are in balance, so if we reduce the heat radiation, the earth will warm up, restoring the balance. The atmosphere acts like a blanket, blocking radiant heat and warming the earth. However, most of the atmosphere has no capacity to absorb radiant heat. Of the atmospheric components that can block heat, the most important are water vapor (H2O) and carbon dioxide (CO2).

Everyone, from Al Gore on down, focuses on CO2, because humankind has recently been pumping CO2 into the atmosphere by burning fossil fuels. Prior to 1800, the CO2 content of the atmosphere was about 280 parts per million (ppm). Now it’s 390 ppm and rising steadily. On the one hand, 390 ppm is not very much. On the other, it’s a 40 percent increase over 1800 levels.

Because of quantum mechanics, CO2 blocks radiant heat only in very specific and narrow energy bands. It blocks emission completely within those bands, but in the wide gaps between them, it is almost completely transparent. So if the atmosphere is a blanket, CO2 is like a fishing net. You can’t stay warm covered by a fishing net: Increasing CO2 by 30 percent blocks maybe 1 percent extra heat.

In contrast, H2O is a wonderful blanket. Although it traps heat only at specific energies, there are many more of those energies, and so much wider coverage. It’s also a much more intense absorber – think thicker blanket – and if it gets together with other molecules of water, in droplets or haze particles, its absorption smears out and covers most of the gaps. As a result, where the H2O content of the atmosphere is high – in the tropics – very little of the earth’s heat escapes.

H2O is however a royal pain for climate scientists. The H2O content of the atmosphere depends on temperature. So, when the earth warms, the blanket gets thicker. On its own, that would lead to what we call a runaway greenhouse – more H2O warms the atmosphere, which evaporates more H2O, which warms more, etc. But H2O also condenses into clouds, which reflect sunlight back into space, cooling the atmosphere.

So if the blanket dominates, H2O amplifies CO2; if reflection rules, H2O neutralizes CO2. Which effect wins? Physics doesn’t tell you. You have to do some observations.

Is the earth actually getting warmer? Probably, but you might not think so, if you just looked at temperature records for Lincoln. If you plot our average January temperatures for the last 125 years, you won’t see any trend at all. No warming, no cooling. Oh, the 1930s and the 1990s look a little warmer than average, the 1970s a little cooler, but there’s no long-term trend. The same is true for much of the earth’s surface.

The two major groups studying global temperature – NASA Goddard and theHadley Center at the University of East Anglia in England – find that a few stations, mostly in the Arctic, have warmed a great deal. Extrapolating up to 700 miles away from these thermometers, they infer from very limited data that the poles have warmed a lot. This is probably real, but it’s always troubling to a scientist when you see the biggest effect where you have the least information.

Even more troubling was the recent announcement by Hadley (after being harried for years by climate-skeptic superhero Steve McIntyre) that they have lost almost all their raw data! Trust our reconstructions, they say; we know what we’re doing. Unfortunately, in science, the motto is “trust, but verify.” If you can’t produce the raw records of your experiments – and my research students are sick of hearing this – you never did them.

What about climate reconstructions from proxy data? In short, they’re useless. The most spectacular reconstructions – the tree ring data that show the fabled “hockey-stick,” with a flat 1000-year “stick” and a “blade” upturning in the last 100 years – have been discredited. One such paper, again by the Hadley group, was recently shown to have “cherry-picked” the data – chosen only those data that showed what they wanted to see.

Other groups have mistakenly used the proxy data with the temperature scale turned upside-down – oops! – or they won’t release their raw data at all. There have been so many screw-ups in this field, it’s a full-time job to keep track of what is still considered valid. The “hockey stick,” even though it was prominently displayed in “An Inconvenient Truth” as proof of AGW, was never convincing anyway; it failed to show either the medieval warm period or the little ice age, and we’re reasonably sure both were real.

Some proxy data – based on isotopes rather than tree rings – do show those phenomena, but for that very reason we should also be skeptical. After all, if you pick and choose your proxies based on what you already expect, you’re not confirming anything but your bias. But for what it’s worth, our current climate does not seem to be significantly warmer than it was in the 12th century, and claims that the climate is currently warmer than it’s been for a thousand/two thousand/a bazillion years are bogus.

It would be nice if we could predict future climate based on pure physics, but we can’t. The global circulation models (GCMs) used to forecast future warming are complex and impressive computer programs, but to make the model agree with what we already know, they have to include literally hundreds of “tweaks.” If you tweak your model to agree with your data, you cannot use it to confirm your data.

So of these four legs on which the science of AGW stands, only one – the instrumental record – really has evidentiary value about the reality of AGW, and even it has weaknesses. It tells us the earth is warming, mostly at the poles. It doesn’t tell us how much of that warming is due to greenhouse gases.

We should be certainly concerned about AGW, but is AGW settled science? Not even close! Next month I’ll try to put the science in the context of public policy. Stay tuned.