Heat, yields and prices: PPT version

Michael Roberts just posted a great agriculture presentation on his blog - take 5 minutes to flip through it. Not only is it a good synthesis of some meaningful content, it's very easy to follow (and as a consultant, a.k.a. professional PowerPointeer, I have high standards for these things).

New IFPRI modeling and report

IFPRI just published a new report entitled "Food Security, Farming and Climate Change to 2050: scenarios, results, policy options" with new outputs from their robust IMPACT partial equilibrium model. The punch line is:

Our analysis suggests that up to 2050, the challenges from climate change are “manageable,” in the sense that well-designed investments in land and water productivity enhancements might, conceivably, substantially offset the negative effects from climate change. But the challenges of dealing with the effects between 2050 and 2080 are likely to be much greater than those to 2050. Starting the process of slowing emissions growth today is critical to avoiding a calamitous post-2050 future.

The last sentence is a very important one. While attention (including my own) may be drifting toward adaptation, the fact that significant climate change will almost certainly occur is not a binary determination. It could be bad or very bad, depending on the level at which atmospheric greenhouse gases stabilize (or not), and thus in the long run mitigation still has an extremely important role to play. The challenge, of course, is that both the distraction of adaptation and the long time horizon make it very difficult to muster a critical mass of political will behind mitigation actions that impose any sort of economic pain whatsoever.

Adaptation to center stage

One of my favorite recent papers is by Michael Roberts and Wolfram Schlenker (regular readers may have noticed me regularly plugging it here, here, here, here, and here). Congrats to Michael and Wolfram on being mentioned prominently by The Economist in this week's long article on climate adaptation.

The entire article is pretty good and worth a read, particularly for its dogged effort to parse out climate impact and adaptation, and subsequent honesty where this proves impossible. For example:

Melissa Dell of the Massachusetts Institute of Technology and her colleagues argue that in developing countries GDP growth has been lower in hotter years than in cooler ones. This may carry over into longer-term increases in temperature. The mechanism is obscure: it may simply be that overheated people work less hard. That can be seen either as adaptation or as a worrying impact, slowing down the economic growth which is the surest foundation for other, more positive adaptations.

Not only is adaptation hard to distinguish from impact, investments in response are harder to classify:

Whereas investments in mitigation are fairly easy to understand—build windmills not coal-fired power stations, and so on—those in adaptation are harder to grasp. Action on climate bleeds into more general development measures.

The article overall is pessimistic

The fight to limit global warming to easily tolerated levels is thus over.

and while I wish I disagreed, and hold out for better-than-expected outcomes from Cancun this week, my inner skeptic and pragmatist are pushing me hard to shift focus - professional and personal - toward adaptation.

Another cut in crop forecasts

U.S. crop production forecasts were slashed again this week:

The agriculture department on Tuesday cut estimates of US corn yields for a third successive month, forecast record soyabean exports to China and warned of the slimmest cotton stocks since 1925.

“The combined production shortfalls and dramatic potential stock drawdowns mean a much tighter supply picture than just a few months ago,” the agency said in a separate grains report.

Benchmark Chicago corn futures soared above $6 a bushel for the first time since August 2008, before ending lower.

I'm looking breathlessly to Michael Roberts for the more granular data analysis he promised; this year could well be a preview of agriculture in a warmer world of the future.

In other unfortunate news, most of the $20 billion of food aid that was pledged last year has failed to materialize, and the probability of inspiring U.S. leadership on the issue has fallen substantially after the recent mid-term elections.

Heat and crop yields in 2010

This may be premature, but I'm very tempted to award a gold star for clairvoyance to Michael Roberts, whose research on the effect of heat on crop yields I've blogged here and here. Here he is on August 12:

[Current temperatures] are still rising fast. If this keeps up for a few more days I'd say yields will get hammered.

(to give credit where credit is due, the market may have seen the same thing)

Anyway, fast forward to last week, the October forecasts for wheat, corn and soy are all well below the September forecasts (although USDA puts a bizarrely positive spin on it). As Michael's chart below shows, the September forecast is almost always very good, and revisions are generally upward, not downward, so something funny happened this year, and late heat seems to be a very strong hypothesis.
Michael is investigating the more detailed data and I hope will post any findings as they arise. As I mentioned before, if it was the heat, it will be very valuable to prove and communicate this to stakeholders in agriculture, some of whom have taken very skeptical views toward climate change and its impact on them specifically.

Do food prices cause "food riots"?

The usually outstanding Chris Blattman has posted some spotty analysis of the recent journalistic coverage of food riots in Mozambique, which he finds "shallow and alarmist." I would have commented, but many have already done so, so rather than pile on I'll just articulate my own take here. Like commenter @Jonomist, I think his punchline that we should be more careful attributing causality is probably defensible, but some of his arguments are not.

Here’s what a closer look at your economics and political science can tell you.

Expect price volatility to fall over time. Globalization and growth should reduce price spikes in future. More countries are producing crops. Climate shocks in Argentina are not that tied to climate shocks in Russia or China, and so price volatility from supply shocks should be going down. Falling transport costs also mean that more substitutes are available, further reducing price volatility. So things should be getting better over time, not worse, especially if trade allows countries to diversify their diet. Envision a future of diminishing instability.

This is a dangerously superficial argument, reminiscent of arguments that a more integrated global financial system would mitigate systemic risk - remember how that turned out? It's what a textbook analysis would say, but lacks grounding in the realities of these markets.

• Empirical data (most recently the price spikes of 2008 and now 2010) don't bear this out, at least in retrospect to date.


• Many agricultural stakeholders - including private companies both producing and buying food, as well as multi-laterals such as the FAO and the World Bank - are deeply concerned with price volatility in the future. (My own view is that we can't yet tell if average volatility will increase to a higher permanent plateau, but most players believe this is much more likely than the opposite.)


• Growth in both population and income is driving strong growth in agricultural demand that is unlikely to abate soon, and pushing the world toward the edge of its current supply capacity. Production can grow too (Ehrlich and Malthus were wrong) through increasing yields through research and bringing more land under cultivation, but the former is longer-term and the latter runs into seriously diminishing marginal returns. My personal belief is that we will be able to feed 9 billion people a more modern diet by 2050, but there will be bumps along the way, and any time demand nears supply capacity, the potential for price volatility increases significantly.


• Climate may become more unstable as climate change progresses, and climate change could also adversely affect baseline agricultural productivity, as Michael Roberts' excellent paper has shown and the market seems to believe.


• As several commenters noted, national policies play a substantial role in agricultural prices (notice how the wheat market responded to the Russian wheat export ban), and this intervention seems unlikely to diminish (especially in light of the above factors) - if anything it is likely to exacerbate any increases in underlying volatility caused by long-term demand growth and short-term supply shocks.

A few smaller nits:

Look to local policy, not global markets, for the real instability. Bread prices climbed 30% in Maputo, apparently due to Russian wildfires. But global wheat prices have only risen 5%. Why the disproportionate effect? I wish I knew, except I haven’t a single report from the ground that points out the disparity, let alone one that searches for an answer.

Yes, global wheat prices have risen 5% "during August" - and more than 50% since June, as commenter Bernhard Brummer points out. Some of the blame goes to the NYT writer for not picking a better baseline, but it's not a hard thing to double-check.

Chris: For riots, look to poor policing, not poverty.
@Jonomist: Are fires caused by insufficient fire marshaling?

Exactly.

Chris' track record is too strong for him to need to redeem himself, but he does so anyway (I'm not sure if intentionally) through his next post on the quality of his commenters (who came through rapidly here).

Market believes heat hurts crop yields

The market believes what Michael Roberts' popular paper showed about crop yields - higher temperatures really hurt.

Corn futures rose the most in almost two weeks and soybeans gained on speculation that the recent Midwest heat wave will mean smaller production than the record crops predicted today by the government.

August has gotten off to the second-warmest start since 1960, T-Storm Weather LLC said today in a report. Another forecaster, Commodity Weather Group LLC, said about 25 percent of the U.S. soybean-growing area won’t get enough rain for proper plant development over the next two weeks, and that the dryness could harm a third of the Midwest should rain miss sections of Illinois this weekend, as expected.

“The crops are going downhill rapidly in parts of the Midwest and South,” said Mark Schultz, the chief analyst for Northstar Commodity Investment Co. in Minneapolis. “Our farmers are already preparing for corn yields that may fall 5 percent to as much as 10 percent from earlier field samples.”

Looking for the silver lining, might this influence the farm lobby's stance on climate change?

Via Michael at Greed, Green and Grains.


Update: I'm struggling a bit to square this with USDA predictions of record U.S. corn production... I wonder if those forecasts were based on outdated inputs (like, perhaps, "earlier field samples").

Reminders of the crop yield treadmill

Recent journalistic pieces on wheat rust and brown streak (a virus which infects cassava) remind us that agriculture is a constant battle against evolving pests of many kinds. So when we think about declining funding for agricultural research and try to assess induced innovation, we need to keep in mind that a certain level of research and progress will be required to maintain yields at their current level, let alone improve them. We are on a treadmill, not standing still.

Weather as an instrument, etc.

Michael Roberts explains in a nutshell the econometrics of his fascinating and widely-referenced paper on temperature's worrying effect on corn yields. If you’ve ever wondered why more agricultural economists don’t use weather as an instrument or worry about the fundamental endogeneity of futures prices, this post is for you.

Davos, world hunger and Felix Salmon

After a conversation with Dan Barber of Blue Hill Farm (a phenomenal restaurant, by the way), Felix Salmon is convinced that the Davos approach to solving world hunger is broken:

No one talked about creating relatively small and self-sufficient agricultural communities: the model is still very much that you sell your one crop for money, and then use that money to buy whatever other food you might need.

Here’s my comment in response:

Felix, I love that you posted on this, but I don’t think where you’re coming out is specific enough to the rural smallholder farming households who make up most of the world’s hungry. They are food-insecure because they can’t produce enough to feed their families and/or they don’t make enough money from selling their surplus crops to buy enough food to close the deficit. “Creating relatively small and self-sufficient agricultural communities” is a nice goal, but we can’t get there in poor countries without figuring out the “how” of improving smallholder productivity.

Staple crop monocultures for these smallholders aren’t necessarily as dumb as they might seem from afar. Which staple crop often varies according to local growing conditions (e.g., in Ethiopia, some regions grow mainly maize, others mainly wheat, teff, or sorghum). And if you’re struggling to grow enough to feed your family on your single hectare of land, focusing on the most productive staple crop – perhaps with a small plot of a cash crop like tomatoes or onions on the side – makes sense.

Agricultural innovation (like, say, flood-resistant rice or drought-tolerant corn, or yield- and sustainability-enhancing crop rotation techniques for that matter) is critical not so the developed world can produce more to ship to the developing world, but rather so poor farmers in developing world can produce enough food to feed themselves.

Monsanto's patent expiration dance

Via Parke Wilde at U.S. Food Policy, an interesting NYT article on the dynamics of patent expiration on Monsanto's widely-used Roundup Ready 1 soybeans, which occurs in 2014.

In letters to seed companies and farm groups this week, Monsanto said that it would allow farmers to continue to grow its hugely popular Roundup Ready 1 soybeans even after the patent protecting the technology expires in 2014.

The letter countered a widespread impression in the agriculture business that Monsanto planned to force farmers and seed companies to migrate to a successor product called Roundup Ready 2 Yield, which will remain under patent and is more expensive.

Monsanto is trying to position this as a clarification rather than a retreat:

Gerald A. Steiner, executive vice president for corporate affairs at Monsanto, said Thursday that Monsanto was not changing its policy on how it would handle the soybean patent expiration, but was merely clarifying its intentions.

“What’s different,” he said, “is we have made a very comprehensive communication of what we are going to do.”

But the widespread impression in the seed business was that Monsanto was backing away from a previous policy.

Monsanto appears to have strong legal protections in their existing contracts, but it doesn't look like those will stand up to the weight of public criticism:

Some seed industry executives and academic soybean specialists say that Monsanto was not planning to renew licenses for that Roundup Ready 1 trait that expired before 2014, so that seed companies would have no choice but to move to Roundup Ready 2.

But in its letters this week, Monsanto said it would now extend all contracts for Roundup Ready 1 until the patent’s expiration date. It also said it would not enforce language in some contracts that would have required seed companies to destroy or return Roundup Ready seed when the patent expired.

But Monsanto may have another out a few years down the road:

Still, it is uncertain how long Roundup Ready 1 would survive in generic form. Some nations require licenses for the import of genetically engineered crops to be periodically renewed. Monsanto said it would maintain those licenses through 2017. But if they expired after that, American farmers would not be permitted to export the Roundup Ready 1 generic soybeans to certain countries, which would discourage them from growing those crops.

One final thought - Monsanto is much-demonized as a monopolist and worst, but it is hard to get textbook industry structure and competition in industries which require massive R&D investments. Those investments simply don't get made privately without patent protection, which looks suspiciously like a monopoly after the fact.

Heat and ag yield, cont.

Michael Roberts' great paper with Wolfram Schlenker on the effect of detrimental increased heat on crop yields (which I first excitedly blogged here) is getting a lot of (well-deserved) attention.

First, Ezra Klein cites the work in the Washington Post, and, despite a few minor inaccuracies by Klein, Michael is deservedly proud of having cracked into the mainstream media.

Second, the paper was noticed as far away as Brazil, whence came a letter to the editor challenging that Brazil (being warmer and high-yielding) refutes the Schlenker/Roberts conclusion, or at least casts into doubt its generalizability. But their quick-and-dirty analysis seems to corroborate their original findings:

First, they cherry picked the state and the year from Brazil. Mato Grasso is the highest yielding state in Brazil and 2008 was a remarkably good year for them, due to unusually good weather. Other states in Brazil have average yields that are about half those of Mato Grasso.

Second, Mato Grasso yields were higher than average yields in the U.S. as a whole, but not higher than the best yielding states in the U.S.

Third, if we narrow our comparison by looking a particular state—Illinois, the number two yielding soybean state in the U.S.—and also look more closely at the data, Mato Grasso doesn’t look much warmer. In fact, the southern half of Illinois, which has average yields comparable to those in Mato Grasso, also has comparable exposure to extreme heat. (Northern Illinois is cooler and higher yielding than both Mato Grasso and Illinois). This can be seen from careful inspection of the maps below (click the figure for more detail). It turns out that there aren’t any soybeans grown in the hottest part of Mato Grasso. It’s not clear whether the commenters took into account the locations in Mato Grasso where soybeans are actually grown.

Look for more back-and-forth discussion on this interesting area, and the Schlenker/Roberts contribution in particular.

Perennializing crops

An academic recently forwarded me this hypothetical "50-year farm bill." It claims that perennializing crops will provide substantial benefits, and we should therefore devote substantial research attention to developing perennial versions of crops like wheat and corn.

"Perennialization of the 70 percent of cropland now growing grains has potential to extend the productive life of our soils from the current tens or hundreds of years to thousands or tens of thousands. New perennial crops, like their wild relatives, seem certain to be more resilient to climate change. Without a doubt, they will increase sequestration of carbon. They will reduce the land runoff that is creating coastal dead zones and affecting fisheries, as well as saving and maintaining the quality of scarce surface and ground water. U.S. food security will improve.

Social stability and ecological sustainability resulting from secure food supplies will buy time as we are forced to confront the intersecting issues of climate, population, water and biodiversity."

I don't really understand why perennial crops carry these benefits - does anyone else have familiarity with this particular issue? FYI, here's what Wikipedia has to say:

"Although most of humanity is fed by seeds from annual grain crops, perennial crops provide numerous benefits. Perennial plants often have deep, extensive root systems which can hold soil to prevent erosion, capture dissolved nitrogen before it can contaminate ground and surface water, outcompete weeds (reducing the need for herbicides), and help to mitigate global warming by carbon sequestration. These potential benefits of perennials have resulted in new attempts to increase the seed yield of perennial species, which could result in the creation of new perennial grain crops."

Behavioral challenges in ag development

Robin Hanson passes along this illuminating and frustrating anecdote:

There are higher-yielding varieties of groundnut than those that farmers in Malawi tend to plant, but getting them to switch is tough. Better seed is pricey, increasing their risk. So researchers from the World Bank ran an experiment. With local NGOs, they offered the farmers loans. Some loans even came with a crop-insurance policy: if the season was dry and the yield a dud, the debt would be forgiven. The farmers’ risk was lowered. Of farmers offered conventional loans, 33% signed up. With the added incentive of insurance, 18% did. The researchers were puzzled.

It’s been more than 30 years since microfinance began its fantastic rise, spreading billions of dollars in credit to hundreds of millions of overlooked borrowers around the world. Insurance is the next big promise of financial services for the poor. But there aren’t many takers. That’s not from lack of interest on the part of suppliers. The Gates Foundation has plowed millions of dollars into microinsurance initiatives...

Hanson speculates that people (obviously not rational actors) buy insurance more for signaling purposes than to improve their actual risk profile:

My best guess is that most insurance is bought not to reduce risk but instead to signal prudence and caring. The first life insurance companies had a terrible time selling “bets on their death,” and only succeded when they framed insurance as what a caring and prudent husband and father did to take care of his family. While simple adverse-selection theories predict that high risk folks buy the most insurance, in fact low risk folks buy more insurance. And we don’t want to insure our big life risks because such a desire would signal a lack of confidence in our prospects.

Poor folks want loans because they want the money, but to want insurance they’ll have to want to signal prudence and caring, above and beyond the signals they now use. Seems a hard sell to me.

Sounds like a tough nut to crack, and illustrative of the huge challenge of improving pitiful agricultural yields in poor countries, compared to their agroecological potential.

RIP Norman Borlaug

A great man with a controversial legacy, but one could argue that he did more for human well-being than anyone else in the history of the world.

In 1960, before his techniques were widely adopted, the world produced 692 million tons of grain for 2.2 billion people. By 1992, largely as a result of Borlaug's pioneering techniques, it was producing 1.9 billion tons for 5.6 billion people -- using only 1% more land. India and Pakistan are now agriculturally self-sufficient as a result of his intervention.

That's from this LA Times obituary, the first of many I am sure. It has an engaging discussion of his quest to improve crop yields through breeding.

Critics (particularly from the organic agriculture world) see him as the forefather of monoculture-dominated industrial agriculture and the myriad environmental and nutritional damages it has wrought. The jury is still out, but in my mind the burden of proof still lies on those who hold that organic agriculture can scale up to feed the world without bringing massive tracts of new land under cultivation.

Update: Here's a great story from later in Borlaug's life:

Borlaug formally retired from the International Wheat and Maize Project in 1979, becoming a professor at Texas A&M University. But in 1984, he got a call from Japanese industrialist Ryoichi Sasakawa, who offered Borlaug funding for five years of work to aid agriculture in Africa.

Speaking through an interpreter, Borlaug said, "I'm 71. I'm too old to start again." Sasakawa called back the next morning and said, "I'm 15 years older than you, so I guess we should have started yesterday. Let's start tomorrow."

Update 2: World-Grain.com has a good quote from Borlaug's Nobel Prize acceptance speech, showing awareness of the wider world beyond his discipline:

"We must recognize the fact that adequate food is only the first requisite for life. For a decent and humane life we must also provide an opportunity for good education, remunerative employment, comfortable housing, good clothing and effective and compassionate medical care."

It also has the interesting tidbit that Borlaug was a Hall of Fame wrestler at the University of Minnesota.

Update 3: Chris Clayton:

In the U.S., we are blessed so much that debate over agriculture focuses on the quality of food, where it comes from, how it's grown and how it's cooked. With the passing of Dr. Borlaug, perhaps people will be inspired to reexamine the higher calling of agriculture and those throughout the world who still have yet to be touched by the green revolution.

Update 4: A few people try to size how many lives Norman Borlaug saved. The Indian Minister of Agriculture says 245 million. The Liberty Papers says 1 billion. Greed, Green and Grains thinks even 1 billion might be an understatement (and with over 1 billion people currently suffering from undernourishment, I think he could be right).

Update 5: A long and interesting interview with Borlaug from 2000.

Update 6: Here is a provocative but loosely-reasoned piece on Norman Borlaug's complicated legacy. I applaud the author's attempt to explore the rippling ramifications of Borlaug's work, but, as this commenter details, there are "several inaccuracies - some tiny, some major."

Update 7: Here's a longer NYT article, including this reference to Vandana Shiva; I'm sympathetic with Borlaug's response.

In a characteristic complaint, Vandana Shiva, an Indian critic, wrote in 1991 that “in perceiving nature’s limits as constraints on productivity that had to be removed, American experts spread ecologically destructive and unsustainable practices worldwide.”

Dr. Borlaug declared that such arguments often came from “elitists” who were rich enough not to worry about where their next meal was coming from. But over time, he acknowledged the validity of some environmental concerns, and embraced more judicious use of fertilizers and pesticides. He was frustrated throughout his life that governments did not do more to tackle what he called “the population monster” by lowering birth rates.

Agricultural yield upside and population growth

This image will come out far too small on the blog, so see the full-size original connected to this short Atlantic piece titled "The Next Breadbasket?", which gets some major messages right:

Sub-Saharan Africa, despite its long history of food insecurity, is one place where yields could increase dramatically; agricultural basics such as good seed and fertilizer would go far in a region that the green revolution bypassed. “We could increase yields in sub-Saharan Africa threefold tomorrow with off-the-shelf technology,” says Kenneth Cassman, a well-regarded agronomist who researches potential yields. The problem is the continent’s long history of corruption, poor infrastructure, and lack of market access.

Agricultural investment in Africa—and in a few other high-potential places such as Ukraine and Russia—may be the world’s best bet for keeping food plentiful and cheap. This investment could bring other benefits too; the World Bank estimates that agricultural development is twice as effective at reducing poverty as other sources of growth.

However, the article's reference to Paul Ehrlich (of The Population Bomb) goes off track. Contrary the predictions of Ehrlich (and Thomas Malthus, for that matter), population growth has been slowing as fertility rates drop off with higher wealth; most experts now predict human population will stop growing some time this century. The inexorable driver of demand growth is increasingly per capita consumption, which rises with economic development (mainly calories and meat for food, but also energy, water, etc.). What is really scary is not 9 billion people, but 9 billion people with the environmental footprint of the average American.

And the map itself is a good idea, but some of its numbers seem funny to me, which makes me doubt its rigor. I buy that Sub-Saharan Africa and Eastern Europe are low, but not that Australia is equally low. I don't think India should be higher than Latin America - India's farmers are notoriously constrained for inputs and credit - and I find it very hard to believe that the U.S., whose industrial monocultures produce the best per-acre average yields in the world, is worse than Myanmar. I like the concept of the map, but the execution seems sloppy.

Research: Higher temperatures TERRIBLE for staple crops

I knew Michael Roberts had a good blog; I didn't know he was working on monumentally important and relevant research on the impact of temperature on staple crop yields:

We set out to develop a better statistical model linking weather and U.S. crop yields for corn, soybeans and cotton, the largest three crops in the U.S. in production value. Our major new finding is that (by far) the best predictor of yield is a measure of extreme heat: how much temperatures exceed about 29C (84F) during the growing season. The threshold varies somewhat by crop--29C is the threshold for corn. Below this threshold, warmer temperatures are more beneficial for yields, but the damaging effects of temperatures much above 29C are staggeringly large.

They introduce a measure I had not seen before, but makes a lot of sense - "degree days above 29C".

A good measure extreme heat is degree days above 29C. This is calculated as (Degrees Above 29C x Days) summed up for all time (including fractions of days) at each temperature above 29C. The more degree days above 29C, the lower are corn yields. Historically, average degree days above 29C during the growing season were about 57. Under the slow warming scenario (we cut emissions to 50 percent of 1991 levels by 2050) this number is projected to rise to 194 by 2070-2099 and corn yields are estimated to decline by 46 percent [emphasis mine]. Under the fast-warming (business as usual) scenario, degree days above 29C are projected to increase to about 413, and estimated corn yields decline 82 percent [emphasis mine].

Their findings on adaptation are particularly troubling:

Somewhat surprisingly, we find no evidence that farmers in the warmer south have been successful in adapting to the higher frequency of temperatures above 29C. This is troublesome as we hoped to learn from warmer regions how farmers might adapt to more frequent temperature extremes.

What are the implications?

These big estimated impacts are important because the U.S. is by far the largest producer and exporter of agricultural commodities. This is especially true for corn and soybeans, which are two out of the world's four most important staples (the other two are wheat and rice). If U.S. yields go down a lot, it drives up prices of staple food commodities all around the world. Almost surely the poor in other parts of the world, particularly developing countries that import food, would suffer far more than the U.S. would.

And the caveats?

There are three major caveats: (1) CO2 fertilization may offset some of these negative effects--something still under significant debate; (2) Seed companies (Monsanto) might develop more heat tolerant crops in the future (but we find little evidence of adaptation in the past); and (3) Farmers will be able to offset some of the losses by shifting where they grow different kinds of crops.

There are more detailed findings in his post. I'll say again that this appears to be critical research that highlights the consequences of climate change for both the broad world food system and U.S. farmers in particular. In fact, it would be great if this were publicized more broadly and were able to shift the opinions of some obstinate agricultural states on the importance of addressing climate change.

Update: He answers my question on longer growing seasons:

R: Great research - love your approach and the conclusions are frightening (but better that we know, and have our eyes wide open).

One other mitigating factor I can imagine is longer growing seasons - any idea how much this could counterbalance the fall in yields? (probably not much, just curious)

Michael: R,

Thanks for your comment and good questions.

We did investigate adjusting growing seasons by moving everything back one month (see the supplement). This does offset damages a little (I recall about about 10 percentage points in the long-run worst-case scenario, but see the supplement for a precise number). We were wary about moving things much earlier since the sunlight and sun orientation would shift so much and the the model implicitly holds these fixed.

Biophysical limits of crop production

I just started reading Feeding the World by Vaclav Smil and it is excellent. He's a biophysicist by training and his pragmatic, scientifically driven, top-down approach reminds me of David Mackay's in "Sustainable Energy - Without the Hot Air" (free for download and also highly recommended, particularly for anyone with an interest in renewable energy).

Brief background - Thomas Malthus famously said that agricultural production could only grow linearly and thus could not support geometric population growth (some argue he recanted later in life, but this doesn't enter the popular perception). VERY briefly, the reason humankind has been able to refute this (up until now) has been huge improvements in agricultural productivity (per hectare of land cropped). Whether this improvement can continue is a huge topic for debate.

Smil brings the top-down biophysical perspective, and says that ag productivity per hectare basically boils down to the photosynthesis rate (how fast carbon is converted to plant matter) and the harvest index (what % of the plant matter is edible). Interestingly, centuries of cultivation - from ancient breeding to modern biotech and GM crops - has not improved the photosynthesis rate AT ALL. Recent yield improvements have been driven entirely by an increase in the harvest index - for example, traditional wheat had a harvest index of 20-30% at the beginning of the 20th century, but by the 1970s certain semidwarf strains were approaching 50%. The theoretical maximum for most cereals is 65% - you can only have so much edible "phytomass" and the rest needs to go to inedible structural components.

What does this mean? There is still room for yield improvements via higher harvest indices, but there is also an upper bound not too far off. Once we hit that asymptote, our remaining options to produce more food are:

1) figure out how to improve the photosynthetic rate
2) plant more land (obviously there is an upper bound on that too, although it too is a subject of fierce debate)
3) improve yields in poorer countries (e.g. Sub-Saharan Africa) to approach those in developed countries (e.g. the U.S.)