Science is all about establishing and understanding cause and effect. Unfortunately, there are few examples in science where causation can be easily established, since the physical world involves myriad variables interacting in different ways.
I would argue that this is why there is a “scientific method” at all. Because it is so easy to fool ourselves regarding what causes what to happen in the physical world.
Laboratory experiments are powerful because, if you can control the factors you think are operating, you can isolate a specific effect and more reliably trace it to its cause.
But many problems are not amenable to laboratory investigation. Global warming is one of them. There is only a single subject, or ‘patient’ if you will (the Earth), it apparently has a low-grade fever, and we are trying to determine the cause of the fever.
It’s not that there isn’t any laboratory evidence supporting global warming theory. There have been many laboratory (spectroscopic) investigations where it has been convincingly established that carbon dioxide absorbs infrared radiation (the fundamental starting point for global warming theory). We even see evidence from satellites that greenhouse gases reduce the Earth’s ability to cool to space.
But to extend those observations to the conclusion that adding more CO2 to the atmosphere will cause substantial global warming is another matter entirely.
If we had hundreds of Earth-like planets nearby that we could visit with satellites and probes, we might randomly split them into two equal groups, inject one group with more carbon dioxide, then monitor them to see whether that group of planets’ climate systems warmed relative to the others. This would allow us to more reliably determine how much warming (if any) was likely due to adding CO2 versus natural, internal quasi-chaotic climate variations, which are always occurring. This is one of the more rigorous methods of research in epidemiology, but one which is not often performed due to expense and ethical issues.
Needless to say, we don’t have hundreds of Earth-like planets to do experiments on. Instead, we have only one subject to study, the Earth. Establishing causation in such a situation is dicey, at best.
Contrary to what you have probably been told, there are no ‘fingerprints’ of human-induced warming which distinguish it from other, natural radiatively induced sources of warming. The warming is indeed (as the IPCC so artfully claims) “consistent” with increasing CO2, but it would also be consistent with other potential causes. Maybe not from direct effects of solar irradiance changes, or from changes in ozone, but I could list many more possibilities which we don’t have good enough data for enough years to thoroughly investigate.
Cargo Cult Science
I am on a mailing list of a career MD/JD who claims much of what passes as policy-relevant science these days (global warming, air pollution epidemiological studies) is what physicist Richard Feynman in 1974 called “cargo cult science“.
The story goes that after primitive South Pacific tribes were exposed to the modernized world with transport planes bringing supplies, they later tried to build mock-airstrips and planes which they believe would ’cause’ the real cargo planes to reappear.
Of course, the villagers were confused about causation. In this case, the need of advanced societies to deliver cargo is what causes airports to be built, not the other way around.
Humans are endlessly ingenious at devising explanations for physical phenomena, while typically there is only one explanation. You can believe that global warming is mostly caused by increasing CO2, changing sunspots, natural climate cycles (my personal favorite), the moon, the planets, HAARP experiments in Alaska, or whatever you can dream up. But to actually prove any of those is impossible, and to even convincingly establish a connection is more a matter of how easy it is to convince people, rather than how good the evidence is.
The Earth has warmed…but there is also abundant proxy evidence that it warmed (and cooled) in the past. So, did increasing CO2 in the last half century really cause the most recent period of warming? We might never know.
The courts are increasingly deferring such matters of causation to the “expertise” of government agencies, such as the EPA. The Circuit Court of DC recently heard challenges to EPA’s 2010 endangerment finding (or ruling) that increasing CO2 is a threat to human health and welfare, and thus must be regulated under the Clean Air Act.
Yet the judges sitting on that court did not want to hear any challenges to the science(!) If the endangerment finding was based upon the science, how the hell can a court hear challenges to the Finding if it does not want to hear about the science? I’m not an attorney, but it seems like lawyers are so busy arguing procedural and obscure legal issues, they are not willing to go after the fundamental premise: that more CO2 in the atmosphere is bad for you.
Yeah, science is hard. It can make your head hurt. But if you are going to base policy on what some scientists claim, you’d better be prepared to address challenges to that science.