Monday, September 26, 2016

Mortality From AGW Warming: A Brief Analysis

In a recent FaceBook exchange, the question came up of why I object to the omission from most talk about AGW of the benefits from milder winters. I ended up sketching an approach to the question of whether increased mortality from hotter summers was more or less than decreased mortality from milder winters. It was a longer piece than I usually bother with on FaceBook, which suggested that it would be more appropriate here, so here, with some editing, it is.

I begin with a Lancet article that found that, globally, "cold-related deaths outnumbered heat-related deaths by a factor of nearly 20, overall." To figure out how summed deaths from heat and cold were changed by AGW so far, or estimate how they will be changed in the future, I need two more pieces of information.

1. As temperature rises, what is the relative rise in temperature of winters relative to the rise in temperature of summers. I don't have data on that and it surely varies from place to place, but the physics of greenhouse gas warming implies that it will tend to be larger in cold times than in hot, so I would expect that, on average, winters will get milder by more than summers get hotter. I picked up that point from something Freeman Dyson mentioned in The Scientist as Rebel, and so far as I can tell it is correct.

2. What is the marginal effect on mortality of changes in both highs and lows? What is the percentage increase in mortality due to heat if highs go up by one degree? What is the percentage decrease in mortality due to cold if lows go up by one degree? How do they compare?

So far as I know, data on that do not exist. So I start with the simplest assumption—that the marginal effects are the same. If so, raising both the high and the low by one degree will decrease mortality due to cold by almost twenty times as much as it increases mortality due to heat, as per the Lancet numbers. Since, as per my point 1 above, AGW will on average increase lows by more than it increases highs, a given increase in global temperature due to AGW should decrease mortality due to cold by more than twenty times the amount it increases mortality due to heat.

If the question is whether the net effect is an increase or decrease in total mortality, the argument implies that for the net to be an increase in mortality the marginal effect of higher highs has to be more than twenty times as great as of higher lows. I suppose that's possible but I can not see any reason to expect it. Perhaps someone reading this can point me at data on the marginal effect that would let me replace my guess with something better.

In my experience, news stories on global warming routinely cite figures on increased deaths from hotter summers, current or projected, but give no such figures for decreased deaths from milder winters.

P.S. The Physics

CO2 is a greenhouse gas. So is water vapor. The more of one greenhouse gas in the atmosphere, the less the effect of adding another–the combined effect, after all, cannot be to block more than a hundred percent of the long wave radiation coming up from the Earth. The warmer it is, ceteris paribus, the more water vapor is in the air. Hence the greenhouse effect tends to be less in warm times and places than in cold.

Saturday, September 17, 2016

A Little More on Climate and the Food Supply

A recent post discussed the effect of climate change on the food supply. I now have a little more information.

I start with the table from Lobell et. al. 2011 which I showed in my previous post:

The issue was the effect on food supply, so it matters how much of each crop is used for food.
Of the 440 million metric tons (MMT) of polished rice produced in the world in 2010 ( Table 1), 85% went into direct human food supply ( 5 ) . By contrast, 70% of wheat and only 15% of maize production was directly consumed by humans. (Major Cereal Grains Production and Use Around the World)
 Googling around, it looks as though about 6% of soybean production is used directly as human food, 75% as animal feed, some of the rest as soy oil consumed by humans.  I can't find a figure for the total fraction used to feed humans, so am guessing 10%. We then have:

Production and yield are from Table 1 above. The bottom right cell shows a net increase in the amount of the four crops used as human food of about a million metric tons. For a more precise calculation I should have converted tons of each crop into calories. I am assuming that the ratio is not very different for the different crops, but readers are welcome to check that.

In the course of the same conversation, one of the participants insisted that all studies of the future effect of climate change on the food supply showed it to be negative. I don't generally like getting into the game of dueling citations, for reasons I will probably discuss in another post. But I was referred to the latest IPCC report so looked at it, and found a table, Figure 7.5 in Chapter 7, that showed the distribution of predictions of the effect of climate change on mean crop yield over the 21st century. For both temperate and tropical regions, the median prediction was for a negative effect but more than 25% of the studies predicted a positive effect. Looking at the estimates that included the effect of adaptation, farmers changing what they did in response to changing circumstances, the median prediction was for a reduction in yield of less than half a percent per decade.

I think that supports my view of the effect of climate change both on the food supply and more generally–that there are both positive and negative effects, both are quite uncertain, and the sum might turn out to be negative or positive, might make us worse off or better off.

Saturday, September 10, 2016

Future Climate and the Food Supply

I recently had an experience both rare and pleasant, a civil and informative argument about climate on FaceBook. It was started by
One of the commenters, although not prepared to defend the hysterical tone of the posted piece, was willing to argue that climate change was making the global food situation worse and threatened to make it much worse in the future. In defense of that claim, he cited "one recent study showing four major global crops declining (relative to no climate change)." The article, "Climate Trends and Global Crop Production Since 1980"  (Lobell et. al. 2011), was an attempt to separate out the effects on four major crops of different environmental changes–temperature, precipitation, and CO2 concentration–occurring from 1980 to 2008.

Reading it, I noticed that what the authors defined as the effect of climate change included temperature and precipitation but not CO2; its (positive) effect was listed separately. Including it changed the conclusion from four crops down to two down, two up. The commenter who offered the article as evidence had apparently missed that fact.

I also noticed that while they found a significant warming trend over the period, the trend in precipitation was statistically insignificant – consistent with random change. Redoing the calculation using only the two effects we knew were associated with AGW, warming and increased CO2 concentration, made the percentage increase in rice equal to the decrease in maize, the increase in soybeans larger than the decrease in wheat. The figures are shown in Table 1 from the article.

The table showed no effect of increased CO2 on the yield of Maize. Maize, as the gentleman I was arguing with pointed out, is a C4 crop, the other crops C3, the difference being in the details of the mechanism for photosynthesis. The effect of CO2 fertilization on C4 crops is substantially less than on C3 crops but not zero. Looking at another article that had been linked in the discussion, this one from the EPA, I found:
The yields for some crops, like wheat and soybeans, could increase by 30% or more under a doubling of CO2 concentrations. The yields for other crops, such as corn, exhibit a much smaller response (less than 10% increase).
That suggests that the effect is less than a third as large as the effect on the C3 crops but still substantial. Including it on Table 1 makes the negative net effect on maize smaller than the positive effect on rice.

Looking at the EPA article I noticed that the increase in  yield due to CO2 fertilization was presented as a fact, various things that might decrease yields as possibilities.
"if temperature exceeds a crop's optimal level or if sufficient water and nutrients are not available, yield increases may be reduced or reversed." 
"Extreme events, especially floods and droughts, can harm crops and reduce yields."
 No evidence was offered that any of those things would happen or how large the effects would be if they did. It looked as though the authors wanted to give the impression that climate change would reduce agricultural yields but prudently stopped short of saying so.

I also noticed:
"Overall, climate change could make it more difficult to grow crops, raise animals, and catch fish in the same ways and same places as we have done in the past."
As conditions change, people change what they do in response. If temperatures rise, farmers will shift to crop varieties suited to a warmer environment. If rainfall increases or decreases, they will adjust crop varieties, irrigation, other details accordingly. What would happen if farmers ignored environmental changes in deciding how to farm tells us very little about what will happen in the real world. Whether or not we have global warming, it is quite unlikely that, a century from now, people will grow crops, raise animals, and catch fish in the same ways and the same places as they do now.

The same issue is relevant to the other article. The authors estimated the effect of increased temperature on yield by looking at how yields had varied with temperature, year by year, in the past. Those estimates were  used to calculate the effect of the overall increase in temperature over the period and suggest possible effects of future increases.

To see the problem with that approach, consider a farmer at planting time. He does not know how hot the year will be, how much rainfall there will be. Decisions such as when to plant and what varieties to plant can only be based on the expected value of those variables.

A farmer in 2100 knows what changes in climate have occurred over the previous century so  can take account of those changes in how he farms. It follows that models based on observations of year to year variation will show a more negative effect of climate change than can be expected from gradual change over a long period of time. The authors of the article noted that problem along with other limitations to their analysis.

Most of the time, all I learn from arguing climate with people on FaceBook is how unreasonable most people engaged in the argument, on both sides, are. This was a pleasant change. 

I will have to wait to see whether my opponent has become less confident that climate change threatens the global food supply now that he knows that the article that he thought supported that claim is, if anything, mild evidence against it.