'Climate Change Impacts' report forecasts sharp drop in corn yields

By Jon H. Harsch

© Copyright Agri-Pulse Communications, Inc.

WASHINGTON, April 26 - Three climate change experts briefed congressional staff Monday to explain how a new approach has narrowed the range of potential climate change impacts. The latest forecasts now show that for each one degree increase in global temperature, expected impacts include:

5% to 15% reductions in the yields of crops as currently grown,

► 5% to 10% changes in precipitation across many regions,

► 3% to 10% increases in the amount of rain falling during the heaviest precipitation events,

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► 5% to 10% changes in stream flow across many river basins,

► 200% to 400% increases in the areas burned by wildfire in parts of the western United States.

Two authors of the “Climate Stabilization” report from the National Research Council of the National Academy of Sciences (NAS), explained that it's up to policymakers to decide whether to take the greenhouse gas emissions control actions needed to limit temperature increases to a level such as 2, 3, 4 or more degrees. Speaking in the Environmental and Energy Study Institute (EESI) briefing, co-author Katharine Hayhoe, an Atmospheric Science professor at Texas Tech University, said that science's role is to be as precise as possible in forecasting impacts so that policymakers can make sound judgements.

Hayhoe said U.S. corn yields are forecast to drop perhaps 20% with 2 degree warming or 40% with 3 degree warming. She said it's up to policymakers to answer the question “is a 3 degree Celsius target acceptable?” Now that science has been able to pinpoint “the consequences in terms of our global food supply,” she explained, “It is not up to science to tell us what's acceptable. It's up to science to define what would happen at a given level” and it's up to the policymakers “to decide what an acceptable limit is.”

Loss of Crop Yields per Degree Warming


Global Temperature Change per degree Celsius. Source: The National Research Council.

Another report author, Economics Professor Gary Yohe of Wesleyan University, stressed that the costs of dealing with climate change impacts will continue to rise if action is delayed. He pointed out that the costs to the U.S. economy may increase even more sharply because other countries are developing new technologies to combat climate change. He said China in particular appears poised to introduce a cap and trade policy in 2014 which could benefit its economy while the U.S. falls further behind.

Mike MacCracken, Chief Scientist for Climate Change Programs at the Climate Institute, said it's clear that “there's a lot you can do . . . it wouldn't be hard to get a lot of emissions down . . . and there are tremendous benefits” - including the direct financial benefit of capturing the energy value of the methane and black carbon “wasted” by being released into the atmosphere.

To read the 298-page report, “Climate Stabilization Targets: Emissions, Concentrations, and Impacts Over Decades to Millennia,” to read a summary, or to purchase a copy, click HERE. For the EESI briefing presentations, click HERE.

The “Climate Stabilization” report includes these specific comments focused on U.S. corn yield impacts (pages 162-163):

Nearly 40% of global maize (or corn) production occurs in the United States, much of which is exported to other nations. The future yield of U.S. maize is therefore important for nearly all aspects of domestic and international agriculture. Higher temperatures speed development of maize, increase soil evaporation rates, and above 35°C can compromise pollen viability, all of which reduce final yields. High temperatures and low soil moisture during the flowering stage are especially harmful as they can inhibit successful formation of kernels. In northern states, warmer years generally improve yields as they extend the frost-free growing season and bring temperature closer to optimum levels for photosynthesis. The majority of production, however, occurs in areas where yields are favored by cooler than normal years, so that warming associated with climate change would lower average national yields. The most robust studies, based on analysis of thousands of weather station and harvest statistics for rainfed maize (>80% of U.S. production), suggest a roughly 7% yield loss per °C of local warming, which is in line with previous estimates (USCCSP, 2008b). Given the rate of local warming in the Corn Belt relative to global average, this implies an 11% yield loss per °C of global warming.

Whether these losses are realized will depend in large part on the effectiveness of adaptation strategies, which include shifts in sowing dates, switches to longer maturing varieties, and development of new seeds that can better withstand water and heat stress and better utilize elevated CO2. A wide range of maize varieties are currently sown throughout the country, customized to local factors such as latitude, growing season length, and soil, and new varieties are continually developed by private seed companies. These companies have historically focused on biotic stresses, but are now releasing the first varieties explicitly targeted for drought resistance. Heat tolerance has not received much investment outside of drought-related traits, likely because of limited economic incentives in current climate. A comparison of maize yields in northern and southern states suggests minimal historical adaptation to heat, as varieties that are more frequently exposed to temperatures above 30°C exhibit similar sensitivities to varieties grown in the North (Schlenker and Roberts, 2009). A major challenge in developing drought and heat tolerance is that traits that confer these often reduce yields in good years, and growers and seed companies have little economic incentive to accept this trade-off given current markets and insurance programs. Another persistent challenge is the decade or more lag between initial investments and seed release. In short, adaptation could offer large benefits, but only if formidable technical and institutional barriers are overcome. To put the challenge in context, global cereal demand is expected to rise by roughly 1.2% per year (FAO, 2006), so that adapting to 1°C global warming (or avoiding 11% yield loss) is equivalent to keeping pace with roughly 9 years of demand growth.The corresponding expected impact of 2°C global warming is 25%, or roughly 20 years of demand growth.”

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