The blogosphere is full of posts by reasonably intelligent people pooh-poohing anthropogenic global warming (AGW). They cite anecdotal evidence for local cooling and sea level stasis, every bit as relevant as the anecdotal evidence for local warming and sea level rise trotted out by the other side. They look askance at the admittedly scattered plot of temperature rise vs. time (Figure 1). The messianic fervour with which the AGW propagandists push totalitarian 'solutions' to the problem pushes them to deny that AGW exists, in the same way as William Jennings Bryan was pushed into denying evolution by the way it was abused to justify Prussian militarism and robber-baron capitalism.
Figure 1: HadCRUT3 Global Temperature Data Set
Why are scientists convinced, in the main, that the AGW hypothesis is correct? It is not because of some spotty y = mx + b fit to a curve of surface temperature vs. atmospheric [CO2](Figure 2). It is because there is a very clear mechanism by which increasing atmospheric carbon dioxide concentration should increase surface temperatures, as sure as eggs are eggs. This mechanism is dependent on fundamental physical laws that are as incontrovertible as anything can be in this crazy mixed up world of ours.
Figure 2: y = mx + b ono
In many ways AGW is the converse of continental drift. For hundreds of years, anyone with eyes could see, and say: 'Hey! This bulge in Brazil fits perfectly into the Bight of Benin!' But for hundreds of years, scientists quite properly pooh-poohed the idea of continents moving around. There was no plausible mechanism for this to happen. As soon as evidence for a mechanism arrived, so did continental drift as a reputable theory. With AGW, the lump in South America might not look very much like the dint in Africa, but the mechanism is so good that any claim that it isn't happening is bound to look like clutching as straws.
Here is the mechanism:
Energy cannot be created or destroyed. Therefore, the energy in the sunlight incident on the Earth has to be balanced by the energy in the light re-radiated by the Earth, or the temperature of the Earth will increase.
The sun sends all kinds of electromagnetic radiation out in all directions, some of which impacts the Earth, as shown in Figure 3.
Figure 3: Radiation Incident on the Earth
The difference between the upper dotted line (sunlight at the top of the atmosphere) and the lower solid line (sunlight at the bottom of the atmosphere) is the first lot of energy we need to worry about. Part of it looks like it is scattered back into space (the general fact that the solid line is lower than the dotted line) and part of it goes into increasing the kinetic energy of various molecules floating around in the air (those are all the little dimples in the solid line). These molecules (mostly water) can then knock into other molecules and increase the general kinetic energy- that is, the temperature- of the air. The more scatterers there are in the air- dust, soot, water droplets, etc.- the more energy will be scattered away, and the more water vapour (mostly) there is, the more the atmosphere will be heated directly. But on average, the solid line should not change much over time.
Now, what happens to the solid line when it reaches the earth’s surface? Either it will be reflected, and zip back off into space, or it will be adsorbed. This will be very variable indeed, and will depend on where the clouds are (they count as surface), and where the snow is, etc. Nobody is at all sure how this balance between reflection and adsorption will respond to an increase in global temperature, but a reasonable guess might be that it is likely to stay about the same.
The adsorbed energy heats the Earth’s surface. But because the whole thing has to balance to keep the Earth’s temperature the same, it has to go somewhere: and where it goes is the energy radiated by a black body heated to a not-terribly-high temperature, as shown in Figure 4.
Figure 4: Heat radiated by Earth cf. Black Body curve
The heavy green line is the theoretical curve for a black body at 255 K, and the narrower green line is observational data from an area of the Pacific ocean at about 290 K. Now you can see the bending signal of carbon dioxide! This is the big dip in the middle of the Pacific ocean curve. This dip is the rational basis for being fretty about carbon dioxide. If the dip caused by carbon dioxide gets bigger, the total area of the curve has to increase to balance the average energy coming in with the energy being radiated out. Let’s say the dip increases to where it takes up an extra 10% of the total area under the curve: the surface temperature then has to increase by a factor of approximately the fourth root of 1.1, an increase of about 6 K. 10% is of course a ruinously gloom and doom eyeballing estimate by me that probably requires a quintupling of carbon dioxide concentration, so people are worried about an increase rather less than that.
Those who are concerned about the big government, anti-Third-World-economic-development prescriptions for slowing global warming should abandon the indefensible trenches and fall back to the more defensible ones. Nobody has demonstrated conclusively that a warmer Earth will be a bad thing. A warmer Earth ought to be better for biodiversity. If some regions become unviable for human settlement, they will be regions that were marginal and dangerous for human settlement anyway. Nobody ought to live on a table-flat coast where five metre storm surges are possible, or in a fragile semi-arid region where every decade brings a drought that kills all your stock. Evidence to date is that global warming is much stronger in high latitudes, where it will improve human health, reduce energy consumption, and be an enabler of economic development. Adapting to global warming is a challenge and an opportunity. Stopping global warming is an impossible dream.
[Memo to self: remember to add citations for the images...]
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