PAST: rising temperatures and droughts in Tulare County

4/15/2017

Mean annual temperatures in California since 1895. Source: NOAA State Annual and Seasonal Time Series.

Climate changes and the local environment:

Average annual temperatures across California have been rising since the nineteenth century, a trend that has sharply accelerated since around 1970. Average temperatures across all seasons in Tulare County in south central California have mirrored that statewide pattern. As the county warmed, it suffered a series of severe droughts, which are usually defined as long-lasting moisture deficits that have environmental and social consequences. Droughts in the county involve three sources of water: surface water at low elevations, snow at high elevations, and groundwater buried below the surface.

Precipitation in much of California has long varied by as much as 50% from year to year. Hotter temperatures, however, are increasing the rate of evaporation across California, including in Tulare County. That has affected both mountain snow and lowland liquid water, and has made local droughts even longer and drier than they once were. One of the most severe droughts in the county's history recently began in 2014 and lasted until early 2017. It was part of a broader Californian drought that started in coastal areas several years earlier, and was up to 27% more likely to occur in our warmer climate.

Drought severity in Tulare from 2000 until 2017, showing the landmark drought of 2014-2017. Darker colors represent worse droughts; higher values mean more of the county was in drought. Source: United States Drought Monitor.

How we know:

These reconstructions rely on several easy to use tools that are available online: the National Oceanic and Atmospheric Administration (NOAA) State Annual and Seasonal Time Series, the NOAA Weekly Palmer Drought Indices, and the National Drought Mitigation Center's United States Drought Monitor.

NOAA's State Annual and Seasonal Time Series relies largely on weather station measurements. NOAA's Palmer Drought Indices use a standard way of measuring drought that considers both water supply and demand in a soil moisture model. Negative values represent droughts, and positive values represent wet conditions. The United States Drought Monitor uses a number of drought indicators, including the Palmer Drought Indices, that consider both environmental and social manifestations of water shortages.

The above reconstructions also draw on an important study in Geophysical Research Letters, which estimates the possible contribution of global warming to the 2012-2014 drought in California.

Human consequences:

Severe droughts since the 1970s have badly hurt Californian farmers. In 2015 alone, farmers could not grow crops on nearly 500,000 acres of farmland, which accounted for a large share of the drought's annual economic cost to the state of nearly $3 billion.

Tulare county was among the hardest-hit communities. The county has been one of the most productive agricultural counties in the United States. Its economy depends on the dairy industry, as well as crops such as oranges and grapes, which all require abundant water. Not only did these industries suffer particular hardships during the drought, but the drought also lasted longer in Tulare than it did in most other Californian counties.

In 2014, the Tulare county Board of Supervisors declared a state of emergency and requested water transfers into the county from other parts of California. Yet hundreds of local wells remained dry into 2017. Many of the county's farms resorted to over-exploiting groundwater that may never fully recover, leaving them more vulnerable to future droughts that a changing climate will make longer and drier. While the recent drought has now ended, it may have long-lasting consequences for local agriculture.