The fox, the hedgehog and the spherical elephant...
Roger Pielke Sr. has a thing for land use. He believes that it is a much more important forcing than greenhouse gases. Frankly, he also believes that anything else is a much more important forcing, but, dear readers, that need not concern us here. Eli constantly interjects that pushing land use to the fore requires one to ignore a number of facts among which are that land use CHANGES contribute both positive and negative forcings. To first order it is the sum the International Panel on Coney Cuteness is seeking, plus which there isn't that much land to start with, and the amount is decreasing as sea levels rise. There are many ways that land use can affect climate, and it certainly is a huge driver of local climate, but greenhouse gases increases drive the global climate uniformly in one direction.
This difference reappears continually, both in science and art. Physicists are hedgehogs, everything is spherical, including elephants and there are but a few basic "truths", aka theories. Some physicists believe there is only one true theory that binds them all. For biologists, at least until recently, every one of a zillion things is different and must be sniffed. Cladisitics, the naming and description of things is basic. Eli trained as a physicist and wandered into chemistry as a young and clueless postdoc.
This dichotomy, most famously explored by Isaiah Berlin (pronounced eye-Zie-uh) and posthumously by Stephen J. Gould, is described in a great and entertaining speech/essay by John Kihlstrom
Archiolus: "The fox knows many things, but the hedgehog knows one big thing". At one level, this aphorism reflects the difference between the fox, with lots of resources at its command, and the hedgehog, with only a single, but highly effective, defense against them. But there is probably more to it than that, as Berlin made clear in his essay:Adopting a simile first propounded by the drunkard Christopher Hitchens in Slate, there are those like Chilingar and Khilyuk who are neither foxes nor hedgehogs but simply as dumb as stumps, but here Roger is a fox, and Eli is a hedgehog. Still, in contrast to Isaiah Berlin (pronouned eye-Zie-uh), the noble hare would consider himself a hedgehog with fox like tendencies (RTFR), a somewhat isolating predilection for a bunny.Taken figuratively, the words can be made to yield a sense in which they mark one of the deepest differences which divide writers and thinkers, and, it may be, human beings in general. For there exists a great chasm between those, on one side, who relate everything to a single central vision, one system, less or more coherent or articulate, in terms of which they understand, think and feel – a single, universal, organising principle in terms of which alone all that they are and say has significance – and, on the other side, those who pursue many ends, often unrelated and even contradictory, connected, if at all, only in some de facto way, for some psychological or physiological cause, related to no moral or aesthetic principle. These last lead lives, perform acts and entertain ideas that are centrifugal rather than centripetal; their thought is scattered or diffused, moving on many levels, seizing upon the essence of a vast variety of experiences and objects for what they are in themselves, without, consciously or unconsciously, seeking to fit them into, or exclude them from, any one unchanging, all-embracing, sometimes self-contradictory and incomplete, at times fanatical, unitary inner vision (p. 436-437.
BOE is back of the envelope, something we used to do on old Publisher's Clearinghouse mailings by the embers of the fire back in the burrow. Let us take a first cut at calculating the intensity of forcing from land use changes necessary to match that from greenhouse gas (GHG) forcing. this is going to be a very rough first cut, which will be improved with time, however, there is a benefit in putting down the rough version first and then polishing it.
We can get the GHG forcing from the IPCC TAR, 2.43 W/m^2[10% uncertainty] with 1.46 W/m^2 from CO2. The same source puts land use change forcing at -0.20 W/m^2 with the vast majority of the change in the northern hemisphere, but this is very rough, including mostly albedo changes, but not a host of other potential causes. We will return to this, or rather I expect Roger to do so here or there.
To do the calculation, we first need the total surface area of the earth. According to the physicist's spherical elephant code, as a rough estimate, one could take the radius, and use 4 pi r^2. Using a radius of 6371 km, this gives a surface area of 5.098 x108 km2 . We would then estimate 2/3 of this is ocean, leaving a land area of 1.699x108 km2 .
A biologist would name each square meter and add up all the Tom, Dicks and Rogers. But we have the Internet, and a bit of googling finds that the land area of the earth is 148.847 x 106 km2 and the sea area is 361.254x106 km2. Turns out that comes from the Chemical Rubber Company Handbook of Chemistry and Physics, something no Rabett hole should be without.
The total surface area is 5.10101 x108 km2. Since 1 km = 1000 m, a km2 has 1 x106 m2 that is 5.10101 x 1014 m2 and the total greenhouse gas forcing is Area x forcing = 1.240 x 1015 W. The surface area of the oceans is about 71% of the total surface area of the earth. Even if land use had changed over every square meter of the earth, the forcing due to land use would have to be 3.4 times that due to greenhouse gases or 8.33 W/m^2, but this is no where near the case.
True, urban areas have grown. For example in the US, urban areas went from 25,500,000 acres in 1960 to 59,587,000 in 2002. But the total land area in the US is 2,263,962,000, so even in 2002, the fraction of land in urban areas is 2.5% . Cropland changed from 341 million acres to 337 in 2005. Moreover all the changes are not in the same direction. There is lots of land in the world's deserts. While some deserts have grown, others have not, and dryland like the Sahel cycles between wet and dry. Let us be generous and assume there is a 20% change in the use of land over the past 50 years and that all of the changes associated with land use were positive. In that case, EACH m^2 of the land would have to be associated with a forcing of over 40 W/m^2. Hold your hand on a 40 W electric bulb (Mom Rabett won't use anything over 20. Here at Rabetts end we are safe, but blind).
We can improve this with improved land use data. Eli will look for it. He can find information on the US, but oom (order of magnitude. Oooom aids meditation, oooom pah pah is satisfying.), while land use is an important driver of climate, it does not look reasonable to call it the driver.
Remember, the IPCC TAR estimates TOTAL land use forcing as -0.20 W/m^2 with about the same uncertainty. Guess why.
UPDATE: The Pig wants to see the forcings: