The Weather and Climate Challenge of Food Security: Extreme Events in the U.S

As a major exporter of food, the U.S. has a critical interest in carefully monitoring and projecting changes in extreme events as a vital part of an effective strategy to ensure global food security. A variety of indicators have been calculated for extremes of temperature, drought, and precipitation. Many of these indicators now reveal significant increases, notably for temperature and precipitation.

Nationally, the trend in mean monthly minimum temperatures indicates that there is now a 40% chance of any given month being in the upper tenth percentile of the distribution (much above normal) compared to the early decades of the 20^th century. For the mean maximum temperature, the chance has increased to 30%.  The proportion of the U.S. affected by record-low daily and monthly temperatures has not exceeded five percent of the contiguous U.S. since the 1990s.  No decade of the 20^th century has had so few record-low temperatures, and the decreasing trend is highly significant.  Meanwhile, the number of record-high daily and monthly minimum temperatures is significantly increasing.  In contrast, depending on the indicator, record-high daily maximum temperatures are just now similar to or still below the records of 1930s, but mean monthly maximum temperatures records have exceeded the 1930s.

Nationwide, drought intensity and frequency is now no greater than what was observed in much of the 20^th century.  The 1930s drought still stands out as the most remarkable series of droughts in the instrumental record.  There are regions in the Southwest and South, however, where recent droughts have exceeded the severity of previous droughts of record.  Overall, significant national trends are not evident, but if temperatures continue to warm, as projected, then during times of dry weather, droughts will become more severe.  This was the case in 2011 and 2012.

For precipitation, there is a statistically significant increase in 1-day, 2-day, 5-day extreme events as well as consecutive rain-day total precipitation based on data back to the early 20^th century.  This increase has strongly emerged over the past several decades.  Increases are most apparent in the eastern half of the U.S., where increases in water vapor have played a major role in these extreme events.   Water vapor increases are expected with higher atmospheric temperatures.

All of these changes in extremes are expected to continue as the world warms, and they will be important considerations in adaptive agricultural practices.