Jay SjervenWASHINGTON — The Fourth National Climate Assessment, a product of 13 federal government departments and agencies that was released Nov. 23, warned that urgent steps are required to slow the pace of climate change and to ensure Americans, including farmers and ranchers, have the tools and technologies required to enable them to successfully adapt to the changes that are occurring now and those that will occur in the future.

The comprehensive report addresses climate change as it affects all aspects of American life and economy.

The section on agriculture and rural communities is structured around four key messages. The first concerns reduced agricultural productivity.

“Food and forage production will decline in regions experiencing increased frequency and duration of drought,” the report said. “Shifting precipitation patterns, when associated with high temperatures, will intensify wildfires that reduce forage on rangelands, accelerate the depletion of water supplies for irrigation, and expand the distribution and incidence of pests and diseases for crops and livestock.”

The report pointed to climate projections to the year 2100 that suggest continued increases are expected in the incidence of drought and elevated growing season temperatures. The report noted average crop yields decline beyond certain maximum temperature thresholds, “and thus long-term temperature increases may reduce future yields under both irrigated and dryland production.”

The report acknowledged while some regions (such as the Northern Great Plains) may see conditions conducive to expanded or alternative crop productivity over the next few decades, overall, yields of the major field crops are expected to decline because of increases in temperatures and possibly changes in water availability, soil erosion and disease and pest outbreaks.

“Increases in temperatures during the growing season in the Midwest are projected to be the largest contributing factor to declines in the productivity of U.S. agriculture,” the report said.

Suggestions that major crops may see declining yields is jolting in view of long-held assumptions, based on experience, that barring a bad year here or there, crop yields are expected to increase year-over-year along a rising trend line reflecting use of improved cultivars and farm management practices.

The report challenged those assumptions while pointing to strategies that may mitigate the impact of climate change on crop yields. The authors pointed to the need for accelerated progress in developing new cultivars through more efficient use of crop genetic resources, greater investment by both the private and public sectors and addressing societal concerns over certain crop breeding technologies, asserting “current assessments of genetically engineered crops have shown economic benefits for producers, with no substantial evidence of animal or human health or environmental impacts.”

The report noted irrigated agriculture is a major consumer of water supplies in the United States. Irrigation for use in crop production is common across most of the western United States and since 2002 has expanded into the northern Midwest and Southeast.

“Under long-term climate change, irrigated acreage is expected to decrease due to a combination of declining water resources and a diminishing relative profitability of irrigated production,” the report said.

Continuing or expanding existing levels of irrigation will be limited by the availability of water in many areas, the authors said.

“Surface water supplies are particularly vulnerable to shifts in precipitation and demand from nonagricultural sectors,” the report said. “Groundwater supplies also are in decline across major irrigated regions of the United States.”

Climate change also is expected to lead to large-scale shifts in the availability and prices of many agricultural products across the world, with corresponding impacts on U.S. agricultural producers and the U.S. economy, the report said.

The authors emphasized there are numerous adaptation strategies to cope with adverse effects of climate variability and change on agricultural production.

“These include altering what is produced, modifying the inputs used for production, adopting new technologies, and adjusting management strategies,” the report said. “However, these strategies have limits under severe climate change impacts and would require sufficient long- and short-term investment in changing practices.”

The chapter’s second key message relates to soil and water resources.

“The degradation of critical soil and water resources will expand as extreme precipitation events increase across our agricultural landscape,” the report said. “Sustainable crop production is threatened by excessive runoff, leaching, and flooding, which results in soil erosion, degraded water quality in lakes and streams, and damage to rural community infrastructure. Management practices to restore soil structure and the hydrologic function of landscapes are essential for improving resilience to these challenges.”

The third key message relates to health of rural populations and livestock.

“Extreme heat conditions contribute to heat exhaustion, heatstroke and heart attacks in humans,” the report said. “Heat stress in livestock results in large economic losses for producers. Expanded health services in rural areas, heat-tolerant livestock and improved design of confined animal housing are all important advances to minimize these challenges.”

The fourth message relates to the vulnerability and adaptive capacity of rural communities.

“Communication, transportation, water and sanitary infrastructure are vulnerable to disruption from climate stressors,” the report said. “Achieving social resilience to these challenges would require increases in local capacity to make adaptive improvements in shared community resources.”