Changing Climate and Changing Cropland

Why is it important to predict changes to cropping patterns?

Analyzing changes in local crop acreage in relation to local climate can be used to generate a set of projections about future cropping patterns based on how farmers have responded to past climate change. The analysis in this study uses data from 100 years of local climate history and 60 years of crop acreages in Yolo County, CA, to establish statistical relationships between climate change and crop acreage patterns (Lee and Sumner in Jackson et al., in prep.) [1].

Wheat in Yolo County

Figure 1: Wheat in Yolo County. Photo credit Susan Ellsworth.

These crop-climate relationships help us understand the general magnitude of changes within the agricultural landscape that may be caused by climate change. Though agricultural production is subject to a range of influences of which climate change is just one, this information is important for planning, resource management and the development of mitigation and adaptation strategies.

How has agriculture in Yolo County changed over the last 60 years?

According to Yolo County Crop Reports, which have been reliably published since 1950, total crop acreage in Yolo County has decreased over time. Vegetable and orchard crop acreage has increased while field crop acreage has declined (Figure 2). Higher value crops are increasingly being grown, including processing tomatoes, wine grapes, walnuts and other orchard crops.

In particular, the following trends can be seen:

  • A major shift of cropland out of barley due to low prices and increased demand for alfalfa
  • A virtual disappearance of sugar beet acreage due to lack of competitiveness with other regions
  • A rapid increase in wine grape acreage driven by increased wine consumption
  • An increase in alfalfa, wheat, and walnut acreage, and decline in apricot acreage
  • Processing tomatoes continue to dominate vegetable acreage making up more than 90 percent
  • Four crops (alfalfa, tomatoes, rice and wheat) continuing to account for major crop acreage accounting for close to 70 percent)
Historical Yolo county crop acreage

Figure 2. Historical crop acreage by crop category for selected years during 1950-2008

How has the climate in Yolo County changed over the last 100 years?

100 years of weather data, obtained from weather stations in the county, was aggregated and synthesized to generate the following trends:

  • The annual temperature has risen by an average of 0.02°F (0.01°C) per year over the past century. This increase has been driven more by warmer winters rather than by warmer summers, with three times larger percentage increases in the average temperature in January than that in July.
  • Daily minimum temperatures have risen considerably, while daily maximum temperatures have remained roughly constant.

Climate related variables deemed relevant for Yolo agriculture include growing degree days for summer crops (April through August), growing degree days for winter crops (November through May), and winter chill hours. Growing degree days are a measure of heat accumulation that allows plants to grow and mature. Winter chill hours are the cumulative number of hours below 45°F during the winter season. The following trends have been noted with respect to growing degree days and winter chill hours:

  • Growing degree days for summer and winter crops are increasing with the increase for winter crops double that for summer crops.
  • Annual winter chill hours in Yolo County have declined by about 150 hours over the last century.
Growing degree days for summer crops

Figure 3. Growing degree days for summer crops 1909-2009

Growing degree days for winter crops

Figure 4. Growing degree days for winter crops for 1912-2009

Annual chill hours

Figure 5. Annual chill hours accumulated over November through February for the period of 1912-2009

What changes in acreage have we observed from the historical data in response to climate change?

Some of significant results include:

  • The increase in growing degree days had little effect for summer crops, but had notable effects for some winter crops by reducing wheat acreage and increasing alfalfa acreage.
  • Reduced winter chill hours resulted in decreased acreage of prunes and walnuts.

What changes can be anticipated in Yolo County crop acreage in the next 40 years?

Historical relationships observed between climate and acreage are applied to two future climate scenarios to investigate how future climate change may affect acreage patterns in Yolo County between 2010 and 2050. The projection holds all relevant drivers of crop acreage constant, except for climate, which varies under the two IPCC scenarios, A2 and B1. A2 is characterized by higher population growth and land conversion for urbanization as well as economic growth and increased GHG emissions in keeping with the current trajectory. B1 is characterized by less intense population growth and urban expansion, as well as a reduction in resource intensity, growth in clean technologies, and lower GHG emissions. The following potential changes to cropping pattern are projected:

  • Under both scenarios, rice, alfalfa, and wheat remain as major field crops throughout the period to 2050, although wheat acreage declines by 2050 compared to earlier decades in response to warming.
  • Warmer winters from 2035 to 2050 cause an increase in alfalfa and tomato acreage (Figure 5). In aggregate this implies less field crop acreage and more vegetable acreage as the majority of vegetable crops in Yolo are tomatoes.
  • Projections indicate that there will be little changes in tree and vine crop acreage over the 2010 to 2050 period for both A2 and B1. Specifically, trends suggest a slight decline in prune and walnut acreage due to reduced winter chill, an increase in almonds which require less winter chill and a nearly constant grape acreage.
Yolo County alfalfa acreage

Figure 6. Alfalfa acreage in Yolo County, past and projected based on data from 1950-2008. The left half presents actual and projected acreage in solid and dotted lines, respectively, and the right half presents projected acreage for B1 and A2 scenarios for 2010-2050.

References

[1]Jackson, L.E., V.R. Haden, A. Hollander, H. Lee, M. Lubell, V. Mehta, T. O'Geen, M. Niles, J. Perlman, D. Purkey, W. Salas, D. Sumner, M. Tomuta, M. Dempsey, and S. Wheeler. Agricultural Mitigation and Adaptation to Climate Change in Yolo County, CA. In preparation for California Climate Change Center Report.