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Science: How Climate Change Impacts Agriculture

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William Cline
Senior Fellow, Center for Global Development and the Peterson Institute
Monday, November 19, 2007; 11:00 AM

William Cline, a senior fellow at the Center for Global Development and the Peterson Institute for International Economnics, was online Monday, Nov. 19 at 11 a.m. ET to answer questions about the impact of climate change on global food demand.

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Read more about this topic in Monday's Science Page story: Facing a Threat to Farming and Food Supply (By Rick Weiss, The Post)

This discussion is part of The Washington Post's series: In The Greenhouse: Confronting a Changing Climate.


Buffalo, N.Y.: It seems to me that for some major parts of the U.S., the combination of shorter duration but more intense rainfall (which means more runoff and therefore less infiltration) combined with increased evapotranspiration and increased irrigation will lead to groundwater depletion. Is this sort of secondary effect being incorporated into the models attempting to assess costs of climate change?

William Cline: What is not included is increased incidence of droughts. Otherwise, to the extent that the observed variation in today's climate across different regions already reflects shorter, more intense rainfall in the warmer areas, at least one of the two sets of agricultural impact models I use -- the "Ricardian" statistical models -- should already incorporate this effect. On the other hand, in my book I do point out that the Ricardian models may not have fully separated out the influence of irrigation. In at least one case, India, where July rainfall increases sharply from already high levels in the climate projections, the model does seem to be picking up a negative impact. The broad answer, however, is that I suspect both sets of models (the other one is "crop models") may not fully capture possible effects of increased rainfall intensity and shorter duration.


Washington, D.C.: While agriculture may be threatened, do you think our farming technology is improving at a fast enough rate to supplement what Mother Nature can't provide?

William Cline: The last chapter in my book considers whether we should be relaxed because technical change will solve the problem. I was surprised to find that, using FAO data, the green revolution has slowed down considerably. In the 1960s and 1970s annual global yield increases for grains rose at about 2.5 percent per year. For the subsequent 25 years this pace slowed down to 1.5 percent per year. The Washington Post article focused on new varieties resistant to heat, drought, and flooding. But if you think about it, focusing research efforts on new varieties that stave off damage means less net gain than we had in the past when new varieties mean outright improvements in total production potential. Basically I think such technical change will be needed and should be encouraged, but I do not think we should fail to take action reducing carbon emissions because of the expectation that improved varieties will solve the problem.


Alexandria, Va.: Is there a great threat to the U.S. or other strong economies, or is the fear only in developing countries? And as a follow-up: Which country/region is the MOST at risk, and which nations may be the least at risk. I suspect this has more to do with the economic/technological set-up of a country rather than what the actual weather will do, correct?

William Cline: The threat is mainly to developing countries, because they tend to be closer to the equator, and the damage is greater closer to the equator where temperatures are already high. But there will be losses in the Southern United States, Southern Europe, and especially Australia, given the same phenomenon. Least at risk would be say Scandinavia. Yes, the economy's ability to adapt matters, but the Australia example suggests that climate and location cannot fully be offset by having more resources for adaptation.


South Ryegate, Vt.: In the late 80s Congress directed the EPA to produce a document identifying two things. First, an analysis of the potential impacts to the U.S. from global climate change, sector by sector. Second, to identify effective adaptive strategies. In 1988 EPA produced a thirteen volume analysis accomplishing the first, anticipating much that is reflected in current thinking. However, the second charge as not executed.

Who, if anyone, in any sector of government or business in this country is focused on this issue, particularly as it relates to not just commercial advantage or opportunities, but also the welfare of the commons and promotion of the public good? Thank You

William Cline: It is my understanding that the World Bank and the Rockefeller Foundation are increasing their efforts on adaptation for climate change in Africa in particular. My impression is that most of US government research funding on climate change has concerned new energy sources, such as coal gasification with carbon sequestration in underground caverns. It would certainly seem to me that the Department of Agriculture should be conducting research on farm adaptation and new seed varieties, and no doubt it is doing some of this already, but others would have more details on this than I do.


Harrisburg, Pa.: What is global warming doing on our supply of water? Are warmer temperatures part of the explanation why water resources seem to be dwindling?

William Cline: There has been a relatively high incidence of drought in the United States and elsewhere in the past year. Like Katrina, it is ambiguous whether we should attribute this to the effects already being felt from global warming, but the answer is probably "yes to some extent." Basically global warming intensifies evaporation and rainfall. The problem is that the increase in the rainfall tends to be more concentrated in areas other than the mid-continents. Meanwhile evaporation rises rapidly with temperature, and it is the difference between evaporation and rainfall that determines drying. In short, yes, I think the warming already is having some effect on water availability, but the impacts are likely to be much more severe with the kind of warming possible by late this century.


Rockville, Md.: My experience with farming was when my father tried to get cotton crops to fit the short growing season north of Lubbock, Texas. I would expect a longer season to make that easier with an earlier plant date and/or a later fall frost. How is that bad?

William Cline: The effect of warming on the planting and harvesting date is already taken into account in the agricultural models I have used. One effect you don't mention is that the higher temperature speeds the crop through its cycle and leaves less time for grain-filling. But the impact also differs by crop, and although my book does not go into product detail, I would suspect cotton does better than many other crops in a world with global warming.


Munich, Germany: How would you rate likelihood of the scenarios of drought and saltwater invasion to farmland that you describe in your article? Is this close to worst case conditions?

I've read that scientists believe that mankind must undertake drastic measures by 2015 in order to prevent the worst, but no matter what we do, the temperature of the Earth will still rise a few degrees because of current CO2 levels. Will this best case condition still result in ocean level increases?

William Cline: The article's discussion of saltwater invasion drew on materials other than my book. On drought, I emphasize that my projections are probably too optimistic because they do not explicitly include increased incidence of drought. Yes, the "committed" warming will take about 3 decades to arrive because of "ocean thermal lag." So we would see more warming even if carbon emissions were cut by 50-60 percent today. Although there would be some sea level rise, it would be on the order of a few centimeters. But if we stay on our present course, then on a time scale of centuries, the sea-level rise could be in the range of 20 feet or more.


College Park, Md.: Numerous studies have shown that organic livestock production results in 25 percent to 30 percent more greenhouse gas emissions. This is due in part to organic rules forbidding the use of ionophores in beef cattle and dairy cattle which reduce methane and the use of feeding limiting amino acids in poultry and swine because the amino acids were created just like insulin is made for human medicine, by genetically modified bacteria.

What will it take to educate consumers that purchasing organic livestock products is worse for the environment than purchasing non organic products?

William Cline: This is an interesting argument, but unfortunately I am not qualified to comment on it.


washingtonpost.com: Graphic: A Prediction of Agricultural Production


Rockville, Md.: Do you have any projections that show the effect of a new ice age?

William Cline: The Intergovernmental Panel on Climate Change states that a new ice age is highly unlikely within the next 30,000 (thirty-thousand) years. Ice ages are result of changes in the earth's orbit (Milankovitch cycle).


Washington, D.C.: So the point is that we must reduce carbon, not increase food production? If we increase agricultural productivity regardless of pollution effects, then won't there be more food generally? More food, regardless of anything else, is better than less food, is it not?

William Cline: Certainly the policy implication of my book is that prospective negative impacts of global warming on agriculture are one of many reasons to move meaningfully toward a curbing of carbon dioxide emissions. If we do nothing, they will rise from 8 billion tons of carbon today to over 20 billion tons by the end of this century.


Washington, D.C.: To assess the accuracy of your estimates, can you identify some futuristic agricultural productivity estimates from 1937 (70 years ago) to compare to today's levels? Seventy years is a long time. Compared to today's levels, how much more productive will agriculture be just before the estimated 20 percent decrease?

William Cline: In the final chapter of my book I use 1.6 percent annual yield increase as the baseline. This is the average for the past 25 years. This multiplies baseline output potential by about 3-fold by 2085. But much land could be diverted from food, and food demand is also likely to rise by 3-fold or more because of higher population and incomes. So there is a tight race between supply and demand, such that a 20 percent or more reduction in the baseline because of global warming damages could be painful.


Arlington, Va.: Have you herad of "global Dimming"?


Global dimming is the gradual reduction in the amount of global direct irradiance at the Earth's surface that was observed for several decades after the start of systematic measurements in 1950s. It is thought to have been caused by an increase in particulates such as sulfur aerosols in the atmosphere due to human action.

Supposedly, this reduced irradiance has countered the increasing temperatures caused by greenhouse gases.

Is it true? Does reduced irradiance affect crop yields? If we clean out atmosphere of particulates will global warming accelerate?

William Cline: Yes, increased sulfate aerosols have partially masked global warming by acting as a screen and changing properties of clouds. As increased clean-up of local pollution occurs, this cooling effect could be unmasked. The climate models specifically incorporate these effects.


William Cline: This completes the hour for this interactive session. Thank you all who have participated. I have tried to answer the questions for which I felt I had sufficient information to make a useful reply. - Bill Cline


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