There was not much good news was in last week's IPCC assessment of the consequences of climate change. The negative impact of climate change is already discernible "on all continents and across the oceans," and it turns out that a few potential benefits mentioned in the previous IPCC report were false hopes. That report, issued in 2007, suggested that higher levels of carbon dioxide in the atmosphere might make crops grow better. But research conducted since then shows that any beneficial effects of more carbon dioxide on crops are likely to be wiped out by the negative effects of a warmer world. The implications are clear: We have to adapt our agriculture. How easy will this be? Not very. One way to see the challenges we face is to look at our 10,000-year relationship with wheat.
The recent paleo-diet craze notwithstanding, wheat provides about 20 percent of the world's food calories. Humans have cultivated wheat for nearly 10,000 years, and it is now grown on more of the world's arable land than any other crop. But wheat's success as a staple crop is not simply a matter of planting more land with wheat. Wheat plays a major role in the world's diet because, over the millennia, we've made continual upgrades that yield more calories per acre.
Stunning improvements in agriculture have made it possible for our food production capacity to keep pace with a growing population. But now climate change is beginning to push back.
It may seem surprising that, like wheeled transportation, today's wheat varieties are the result of thousands of years of R&D. On the surface, growing a crop like wheat seems straightforward. You plant your seeds, provide them with enough water, sunlight, and fertilizer, keep the pests away, and in a few months you harvest. Next year you repeat the process. But if we replaced today's major wheat varieties with those that were grown only a century ago, we'd plunge much of the world into famine. Without sustained improvements to wheat and other staple crops, there wouldn't be enough land area in the world to grow the calories needed to feed seven billion people.
THE FIRST MAJOR IMPROVEMENT to wheat occurred without human intervention nearly half a million years ago, when a chance encounter between wild wheat and goat grass resulted in a new hybrid form, wild emmer wheat. Wild emmer wheat was gathered as early as 19,000 years ago by people who lived near the Sea of Galilee. Ten thousand years later people began cultivating first wild emmer, and then, following another goat grass hybridization, spelt, the direct ancestor of modern bread wheat. Early farmers in the Fertile Crescent selected wheat plants that grew well, had larger grains, and were easy to thresh. Over the millennia, new spring and winter wheat varieties that could be planted in new environments and in new seasons were created: dwarf wheats with short stalks that that didn't collapse under the weight of large grain heads and wheats that resist rust and other disease. Major improvements in wheat cultivation were made during the mid-20th century Green Revolution. New varieties introduced by Norman Borlaug and his colleagues led to the quadrupling of wheat yields in Mexico between 1950 and 1985. Over the past 50 years, worldwide production of wheat and other grains has tripled (PDF), despite only a nine-percent increase in arable land.
These stunning improvements in agriculture have made it possible for our food production capacity to keep pace with a growing population. But now climate change is beginning to push back. As last week's report describes it, while the previous assessment "concluded with medium confidence that in mid- to high-latitude regions moderate warming will raise crop yields, new knowledge suggests that temperate wheat yield decreases are about as likely as not for moderate warming." Some of that new knowledge is summarized in a study in this month's issue of Nature Climate Change. The researchers, led by Andrew Challinor at the University of Leeds, looked at the results of 1,700 studies of crop yields, and found that yields of wheat, maize, and rice are expected to fall as local temperatures go up. By making some improvements in planting time, irrigation methods, and the varieties planted, farmers can mitigate some of the negative effects of climate change over the next few decades. But the authors project that, as the century progresses and the world heats up, "more systemic or transformational adaptations may be needed to avoid the risk of significant reductions in mean yield."
And even if we can find ways to stave off a decline in crop yields, what we harvest may not deliver the calories we depend on. A study published this week by a team of scientists at the University of California-Davis provides new evidence that high levels of carbon dioxide in the atmosphere causes wheat plants to make less protein. "Consequently," the authors write, "protein available for human consumption may diminish by about 3% as atmospheric CO2 reaches the levels anticipated during the next few decades."
As the world population grows to nine billion in the next few decades, we should be thinking about the next crop upgrades that will grow our food supply—a task that is challenging enough on its own. But instead of growing our capacity to feed the world, climate change is forcing us to worry about how to prevent our food supply from shrinking.