Global Warming and Agriculture

• Author: William R. Cline
• Position: Senior Fellow at the Peterson Institute for International Economics and the Center for Global Development
• Pages: 23-27

Page 23

If steps are not taken to curb carbon emissions, agricultural productivity could fall dramatically, especially in developing countries

JOHN STEINBECK'S The Grapes of Wrath provides a verbal mural depicting America's experience in the Dust Bowl of the 1930s, with its migration of "Okies" from ruined farmlands in Oklahoma and texas to a not-so-promised land in California. this historical experience and perhaps the present-day drought of biblical proportions in Australia should alert international policymakers to the risks to world agriculture of a hotter and drier world by late this century as a consequence of unarrested global warming.

In the long list of potential problems from global warming, the risks to world agriculture stand out as among the most important. Yet there has been a tendency in the climate economics literature in recent years to downplay this risk, and even to argue that a couple of degrees Celsius warming might benefit world agriculture. But such studies typically have too short a time horizon (generally out to about 2050). they also focus on overall temperature change (which includes oceans), rather than on the changes that will occur over land (which warms more easily and quickly than water)-and specifically agricultural land.

It has been widely recognized that developing countries in general stand to lose more from the effects of global warming on agriculture than do industrial countries. Most developing countries have less capacity to adapt than do their wealthier neighbors. Most are in warmer parts of the globe, where temperatures are already close to or beyond thresholds at which further warming will reduce rather than increase agricultural output. And agriculture is a larger share of developing economies than of industrial Page 24 economies. But it has been difficult to estimate just how much individual countries are likely to be affected.

For that reason, this study (Cline, 2007) was undertaken both to get a better long-term fix on overall world effects under current policies (the so-called baseline or business-as-usual scenario) and to understand the likely impact on individual countries and regions. the time frame stretched out to the average for 2070-99, what is called the "2080s." Climate model projections are available on a comparable basis for this period, which is far enough in the future to allow sizable warming and potential damage to materialize but close enough to the present to elicit public concern. the study, which is explored in this article, suggests that there is good reason not to downplay the risks to agriculture from global warming.

How climate affects agriculture

Climate change can affect agriculture in a variety of ways. Beyond a certain range of temperatures, warming tends to reduce yields because crops speed through their development, producing less grain in the process. And higher temperatures also interfere with the ability of plants to get and use moisture. Evaporation from the soil accelerates when temperatures rise and plants increase transpiration-that is, lose more moisture from their leaves. the combined effect is called "evapotranspiration." Because global warming is likely to increase rainfall, the net impact of higher temperatures on water availability is a race between higher evapotranspiration and higher precipitation. typically, that race is won by higher evapotranspiration.

But a key culprit in climate change-carbon emissions- can also help agriculture by enhancing photosynthesis in many important, so-called C3, crops (such as wheat, rice, and soybeans). the science, however, is far from certain on the benefits of carbon...