Marginal Utility

by Tom Bozzo

Orley Ashenfelter writes, in regard to this post:

Sorry, but global warming does mean more solar radiation. The usual
model involves a prediction of heat, but could as easily have been a
prediction of solar radiation increase. The normal argument is that CO2
will alter the atmosphere permitting more solar radiation to effectively
hit the earth--needless to say this is complicated.

True that it's complicated, though Ashenfelter's is not the best explanation around. To my mind it would be more accurate to say that a lot of radiation reaches Earth via sunlight ("solar radiation"), heating it to habitability, most of which is relatively promptly re-radiated into space. "Greenhouse" gases in the atmosphere absorb some of the re-radiated energy and send it back to the surface at different wavelengths. Indeed, it took me about two minutes with Google to come across this science news item from the Beeb from last fall, remarking on a paper recently published in Geophysical Research Letters that used this factoid to identify the source of increased energy reaching the earth. The main result:

"We observed that between 1995 and 2002, the amount of longwave radiation coming downwards to the Earth in Europe increased significantly [the main culprit, reportedly, being increased water vapor], whereas solar radiation did not," said study leader Rolf Philipona, from the World Radiation Center in Davos, Switzerland. [emphasis added]

Meanwhile, Ashenfelter continues:

I would suggest you read our paper, as we make it clear that the theory of solar radiation works great when there is no atmosphere (indeed, we assume that for
purposes of making some calculations, and it works well for our prediction of vineyard prices because, of course, what atmosphere there is in such a small area is the same for each vineyard--so held constant for our purposes). So your critique misses the point--solar radiationis how you get heat--and indeed, your tomatoes will get riper if it stays warm at night, even if it is dark out--check with a gardener (or botanist)! The same is true for grapes or any other plant.

A not-republished disclaimer that he doesn't read blogs notwithstanding, Prof. Ashenfelter might have read my critique more closely. For one thing, the earlier post was sprinked with hints that I did, in fact, read his paper, even though I only quoted the abstract. I also never suggested that solar radiation isn't how you get heat.

The point of that portion of my critique is that heat and light are imperfect substitutes for the purposes of growing plants — pace the tomato example (and the heat wave was, indeed, good for mine) stick one plant in a hot closet and another plant in a cool closet, and I guarantee identical results after a few days.

My impression from Ashenfelter and Storchmann's paper is that variations in the vineyard-specific solar energy input used to explain vineyard quality are largely due to variations in solar radiation (e.g., due to orientation), while there's evidence out there (as noted above) that Europe is warming due to non-solar radiation. This is not to say that more warmth needn't help the grapes, other things equal — that's why I mentioned another paper co-authored by Storchmann where the warming effect appeared to be transmitted via degree-days in the growing season as at least being clearer in the mechanisms. Maybe the solar radiation and re-re-radiation heating effects are sufficiently strongly correlated that this doesn't matter, but Ashenfelter and Storchmann haven't (to my reading) shown it.

Now we get to the bottom line.

On the other hand, though this paper shows a location (in Germany) that will unambiguously benefit from global warming there are, of course, many places that will be harmed. Providing a really credible measure for even one area (helped or harmed) is extremely difficult--which is a key point of the paper. (As of this time, by the way, there are really no estimates other than simple correlations between climate and agriculatural prices.) Ideally, the same method we use could be used in other places and for other plants to provide evidence of help or harm.

Here's where Ashenfelter remains in big trouble with what was my main critique from before, which is that he's on shakier ground than he seems to think in suggesting that Mosel valley vineyard owners will "unambiguously benefit" from global warming. There are really two potential layers to the failure of the other-things-equal analysis. As mentioned previously, nature might not cooperate in the sense of providing the potentially beneficial (to them) temperature increase along with other changes (precipitation patterns, seasonal temperature extremes) that need not benefit the grapes. Second, there's no guarantee that global warming will arrive (or proceed) without demand-side effects — as customers near oceans find themselves substituting flood protection for all other expenditures. Then there's the prospect of really big changes like disruption of ocean currents massively changing European climates, possibly even in the face of a net warming.

So, as I said before, and at the risk of being pre-declined for membership in the wine economics association, Mosel valley vintners shouldn't be taking global warming to the bank.