Lester R. Brown

Chapter 7. Stabilizing Climate: Rising Temperatures, Falling Yields

Within just the last few years, crop ecologists in several countries have been focusing on the precise relationship between temperature and crop yields. In an age of rising temperatures, their findings are disturbing. One of the most comprehensive of these studies was the one just cited, which focused on rice yields. This study was conducted at the International Rice Research Institute (IRRI) in the Philippines, the world’s premier rice research organization. The IRRI team of eminent crop scientists noted that from 1979 to 2003, the annual mean temperature at the research site rose by roughly 0.75 degrees Celsius. 4

Using crop yield data from the experimental field plots for irrigated rice under optimal management practices for the years 1992–2003, the team’s finding confirmed the rule of thumb emerging among crop ecologists—that a 1-degree-Celsius rise in temperature lowers wheat, rice, and corn yields by 10 percent. The IRRI finding was consistent with those of other recent research projects. They concluded that “temperature increases due to global warming will make it increasingly difficult to feed Earth’s growing population.” 5

While this study analyzing rice yields was under way, an empirical historical analysis of the effect of temperature on corn and soybean yields was being conducted in the United States. It concluded that higher temperatures had an even greater effect on yields of these crops. Using data for 1982–98 from 618 counties for corn and 444 counties for soybeans, David Lobell and Gregory Asner concluded that for each 1-degree Celsius rise in temperature, yields declined by 17 percent. Given the projected temperature increases in the U.S. Corn Belt, where a large share of the world’s corn and soybeans are produced, these findings should be of grave concern to those responsible for world food security. 6

The most vulnerable part of the crop cycle is the pollination period that immediately precedes seed formation. One of the IRRI projects, for example, showed that at 34 degrees Celsius (93 degrees Fahrenheit), nearly 100 percent of the tiny flowers on a rice head turn into kernels of rice. But at 40 degrees Celsius (104 degrees Fahrenheit), only a few kernels develop, leading to crop failure. 7

Wheat and corn are similarly vulnerable. Earlier research showed that higher carbon dioxide (CO2) levels in the atmosphere led to higher grain yields, assuming that there are no constraints imposed by soil moisture, nutrient availability, or other limiting factors. What the new research shows is that the negative effect of higher temperature on crop yields overrides the positive effect of higher CO 2 levels. Indeed, if pollination fails and there is no seed formation, then the CO 2 effect on grain yield is lost entirely. 8

Abnormally high temperatures directly affect yields by stressing crops. Anyone who has been in a cornfield in mid-summer with temperatures above 35 degrees Celsius has seen how tightly the leaves curl in order to reduce moisture loss. But this also reduces photosynthesis, often to the point where the corn plant is merely maintaining itself. Under conditions of intense heat, plant growth ceases entirely. 9

As temperatures rise, crop-withering heat waves are becoming more and more common. On August 12, 2003, when the U.S. Department of Agriculture released its monthly estimate of the world grain harvest, it reported a 32-million-ton drop from the July estimate. This drop, equal to half the U.S. wheat harvest, was concentrated in Europe, where record-high temperatures had withered crops in virtually every country in the region. 10

The heat wave in Europe began in early summer 2003, when Switzerland experienced the hottest June since recordkeeping began 140 years ago. In July, the heat engulfed nearly the whole continent. In late summer, soaring temperatures were rewriting the European record book. On August 10th, the temperature in London reached 38 degrees Celsius (100 degrees Fahrenheit). France had 11 consecutive days in August with temperatures above 35 degrees Celsius. In Italy, temperatures reached 41 degrees Celsius. 11

Crops suffered the most in Eastern Europe, which harvested its smallest wheat crop in 30 years. The wheat crop in the Ukraine, already severely damaged by winterkill, was reduced further by the heat, plummeting from 21 million tons the year before to a mere 5 million tons. As a result, the Ukraine—a leading wheat exporter in 2002—was forced to import wheat in late 2003 and early 2004 as bread prices threatened to spiral out of control. Romania, which was particularly hard hit by heat and drought, harvested the smallest wheat crop on record. And the Czech Republic had its poorest grain harvest in 25 years. 12

During this life-threatening heat wave, Europeans may have felt that the temperature could not rise much more. But the increases projected for the decades ahead mean that such events will become more frequent and more intense. Just as Europeans could not have imagined the severity of the heat wave in the summer of 2003 that claimed 35,000 lives and shrank grain harvests in virtually every country, so too we have difficulty visualizing the extreme heat waves yet to come. 13