This is the fourth in a series of reposts from gregladen.com on global warming.
“Climate Studies” is a “causal” science. Most sciences are “causal” in nature, which is why the sciences and scientists are often loathed and distrusted by people in the humanities and some of the soft sciences. There is not the time or space right now to address this issue, but I’ll just say this: People who criticize science for its interest in causality usually do not understand what scientists are talking about. I think this is partly because people in the humanities and social sciences have gone gaga over the concepts of “agency” and “intentionality,” yet often do not understand those concepts, and often confused them with “cause” and, worst of all, “explanation.” So, the idea of explaining something and the idea of blaming someone become conflated. (I’ve been ragging on the soft sciences and humanities lately: I just want to say that some of the smartest people I know, or know of, are in these areas. It’s the 96% of the rest of them that need to go away or at least become less annoying.)
If the sun got hotter, and thus more solar radiation struck the earth, that would be a “forcing effect” with respect to atmospheric temperature, and more specifically a positive forcing effect (an effect that increases the atmospheric temperature). All else being equal, the earth’s atmosphere would heat up, because the cause of heat in the atmosphere is 99.999% from solar radiation. We would explain the heating up of the atmosphere by this increase in radiation.
It is possible that all else would NOT be equal. Perhaps an increase in solar radiation causes a climatic phenomenon involving fog, fog is everywhere, so the ground does not become heated by the radiation as much (because the radiation is diffused and reflected away by the fog) and the earth cools down. In that (hypothetical) case, low-level water vapor is a forcing effect. Two opposite forcing effect can have … no big surprise … opposite effects. Ideally, a forcing model involves measuring all relevant factors such that when they are summed you get a meaningful result. Forcing is a term used for specific effects, causality is what it is all about, and by understanding the relationship between forcing and the system (in which the forcing works) we get closer to explaining what we see, and most importantly to many people, predicting what may happen in the future.
(This … prediction … can be difficult. I think it was Niels Bohr who said: “Prediction is very difficult. Especially if it’s about the future…”)
The IPCC report identifies and quantifies several “Radiative Forcing Components” related to the earth’s atmosphere. “Radiative Forcing” is the set of effects on how the sun warms the atmosphere. This is mainly the strength of the sun itself, the relative position of the sun and earth (earth’s “orbital geometry” … the subject of another post, I’m sure, in the future) and the terrestrial or atmospheric factors that affect the retention, redistribution, or reflection (away) of this energy. The IPCC provides the following graphic as part of their executive summary (click the graphic for a larger version):
The graph shows the eight most measurable, largest, and best understood anthropogenic (caused by humans) effects, and the only significant and consistent natural effect, on radiative forcing. The value that is being graphed is a measure of energy per unit area above or below a “no-effect” baseline. Bars extending to the right mean more energy retained in the atmosphere, bars to the left mean less energy. The global temperature would be constant (averaged over a year) if the bars on both sides added up to zero. The column of numbers to the right of the bar graph is simply the numbers on which the graph is based.
The column after this is the “scale” at which the effect is likely to be observed. This is a tricky one. Greenhouse gases are considered to be global because dispersal and mixing of the gases is pretty quick, and that is all they do … disperse. They don’t break down into something else ore get sequestered very quickly, and they don’t leave the planet. An effect that disappeared (rather than dissipated) could be very local under certain conditions. A given amount of additional greenhouse gases are pretty much distributed evenly over the Earth in a year or so (this is a matter of ongoing research, and I’m not sure of the current thinking … I believe mixing within a hemisphere … north vs. south … is very quick, and then it takes a little longer to mix between the hemispheres. But the mixing has been observed to be faster than people had previously thought.) Surface albedo is the reflection of solar energy by highly reflective surfaces such as snow/ice. This can have a local effect because the snow/ice is local. Look at the temperatures of cities and towns in the US Plains States during the winter. There is less reflection away from the surface in the cities, where there are buildings and dirty snow, and lots of pavement. The urban temperatures are higher than the rural/farmland temperatures, and this albedo may be the main explanation.
The last column, “LOSU” is in some ways the most important column on this graph, because it is new and central to the concept that people have to stop whining about there bing “two sides” to the issue of global warming. LOSU is “Level of Scientific Understanding.”
Within the community of scientists, there is a scale that we have been using for years for this sort of thing. It goes like this:
My best friend’s
A randomly selected colleague
My worst enemy
This is the list, in order from most certain to least certain, of who’s reputation you would bet on a particular assertion or conclusion being correct. The IPCC’s LOSU is a little but more “scientific” and it is defined statistically, but I’ll bet it would be pretty similar. If certainty is high, you’d bet your own or your best friend’s reputation, if it is low, you’d bet an enemy’s reputation, and so on. The important thing here is that the most important forcing components are the best known. Why? Because so much work has gone into their study, as they are the most important … sort of obvious but worth pointing out.
There are forcing factors that are not included in this graph for one of two reasons: The LOSU is very low, or the effects are highly episodic. The most important of these is probably aerosols from volcanic eruptions. It is probably the case that the main reason that the very rapid increases in temperature and arctic ice cap melting of the last few years have happened when they did instead of earlier is because there have not been any really massive volcanic eruptions recently. These volcanoes have been reducing the effects of global warming. A decade or two without a major volcanic eruption is kind of a scary prospect right about now… The eruption of Mt. Pinatubo in 1991 probably cooled climate for a few years. If volcanoes were on this graph, they would mainly be “Natural,” “Aerosol” and “Global,” although there are some greenhouse gases put out by volcanoes.