Federal Report Definitively Links Human Actions to Climate Change

Significant Consequences for Agriculture

The latest federal climate science report, despite being drafted months ago, awaits approval by the Trump administration. Prepared by 13 federal agencies as part of the congressionally mandated National Climate Assessment, the report was first made public via a nonprofit internet archive in January. Only recently has the report been brought to the attention of the general public, thanks largely to a recent New York Times article.

The 673-page report is based on thousands of studies, conducted by tens of thousands of scientists, that document climate changes across the U.S. and link them to human activity. The report definitively concludes that Americans are already experiencing the effects of climate change, and that it possible to attribute some specific weather events to climate change.

Without making any policy recommendations, the report provides scientific analysis of the recent Paris accords, holding that the projected curb of greenhouse gas emissions is a critical step to keeping global warming at manageable levels. Nearly 200 countries have agreed to limit and cut their fossil fuel emissions, with the United States a recent exception to that list since President Trump withdrew from the accord.

California produces more than half of the nation’s fruits and vegetables and more than one-fifth of the nation’s milk. The following is a summary of climate change effects most likely to directly impact the state’s agricultural production.

Longer growing seasons

The report cites that a general warming trend (projected increases of 5.0 to 7.5 degrees Fahrenheit by the late century) has and will continue to lengthen the growing season. The repercussions of this are vast and complex. In some places and under certain conditions, multiple harvests of the same crop could be made due to a longer season. In addition, shifting cold hardiness zones across the United States could expand the regions that are suitable to cultivate cold-intolerant, perennial crops. However, the report cautions, “Longer growing seasons can also limit the types of crops that can be grown, encourage invasive species encroachment or weed growth, or increase demand for irrigation, possibly beyond the limits of water availability.”

Loss of chill hours

With “very high” confidence, the report cites a reduction in the number and severity of cool nights since the 1960s, including a trending increase—in some regions amounting to a week or more—of frost-free days. These warming temperatures or, more precisely, loss of cooler temperatures, significantly threaten the viability of some of the most lucrative sectors of California’s agricultural economy: fruit and nut tree crops. To successfully set fruit, these crops depend on a certain number of “chill hours,” or number of hours below 45 degrees Fahrenheit. Each tree variety has a specific chill hour requirement required for normal flowering and food production, and farmers select specific varieties to maximize yields in their climate zone. Orchard trees typically live for decades, so shifting to new varieties to adapt to changing climate conditions is often unaffordable and infeasible for many producers.

Pollinator incompatibility

Much of agriculture production is dependent on insect pollination. According to the report, changing temperatures cause the timing of plant development to shift, which could cause a mismatch in plant growth and pollinator activity. For example, due to changing temperatures, plants may flower earlier and if the life cycle of pollinators does not shift accordingly, the result could mean a failure of the crop to be pollinated. Many farmers are already noticing changes in flowering times and fruit maturation (as one farmer noted on a recent CalCAN tour). If the life cycles and activity of pollinators get out of synch with changes in plant life cycles, yield losses are inevitable.

Droughts and water stress

The report states that though there remains debate in the scientific community around precisely how attributable California’s most recent drought has been to climate change, some aspects of the changing precipitation cycles are very clear. Warmer California winters in 2013-2014 and 2014-2015 exacerbated the soil moisture deficit and the associated stress on irrigation systems. “Precipitation extremes put pressure on agricultural soil and water assets and lead to increased irrigation, shrinking aquifers, and ground subsidence.” These effects are already acutely observable in parts of California’s Central Valley where the ground level has dropped by several feet over in recent years, causing major infrastructure damage.

Changes to larger precipitation patterns pose great risk to dependable water supplies in California. Due to the state’s highly engineered water systems that relies heavily on Sierra Nevada snowpack melting over time, warming temperatures present a significant threat to a reliable supply of water for much of the state, including our agricultural communities. Warmerconditions increase agricultural demands for water, due to both longer growing seasons and drier soil conditions, that further exacerbate issues of water scarcity. Additionally, shifts in the location of precipitation cause a lack of dependability on California’s engineered system to deliver water across the state.

Unpredictability and extreme weather events

The report’s final chapter, “Potential Surprises: Compound Extremes and Tipping Elements” underscores the element of unpredictability in both climatic changes and the occurrence of extreme weather events. While the report states that the American West (not including the Pacific Northwest) will generally experience hotter and drier conditions that will continue trends of aridification, it also explains that the specifics of California’s climate are much more difficult to predict. Extreme weather events such as torrential rains and heat waves are predicted in our future, and their unpredictability is high, making planning and preparation challenging.

“Humanity is conducting an unprecedented experiment with the Earth system through the large scale combustion of fossil fuels” and release of greenhouse gases into the atmosphere, along with widespread changes to the Earth’s landscape, according to the report. The authors cite the limited capacity of even the most sophisticated current climate models to predict with precision what the extent of future climatic changes will be. “There is significant potential for our planetary experiment to result in unanticipated surprises and a broad consensus that the further and faster the Earth system is pushed towards warming, the greater the risk of such surprises.” Unpredictable climatic conditions and more frequent extreme weather events will have devastating effects on California’s agricultural sector, and may threaten the food security of so many who depend upon it.

This report is a comprehensive and stark reminder of the importance of aggressive global action to curb greenhouse gas emissions. Even as California moves forward with its efforts to mitigate climate change, the report also reminds us of the importance of developing adaptation strategies for our farmers and ranchers who are on the frontlines of climate impacts.

The full report is available here.

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