National county-level dataset summary
National and regional maps and data are available as online resources supplemental to this article and at http://swclimatehub.info/data.
Precipitation
For most of the northern US, total precipitation is projected to increase and shift seasonally (OR II and III). Conversely, total precipitation is projected to decline across parts of the southern US. Declines in precipitation are most evident during the summer months across the nation.
In 92% of counties of the nation, the annual precipitation SD was less than 10 mm, indicating that the change in midcentury precipitation varies by less than 10 mm across each county (OR IV). Counties with annual and seasonal precipitation SD larger than 10 mm were generally located in mountainous regions of the USA, particularly the Rocky Mountain and Appalachian regions. Nearly all eastern US counties had annual and seasonal precipitation SD less than 10 mm. Based upon SD, seasonal and annual precipitation change for each county appears fairly homogenous for most of the USA suggesting projected changes in county precipitation values from the dataset could likely adequately represent mean county-level changes in most parts of the country. Projections of precipitation change in some counties of the Northwest, Northern Plains and Southwest Regions should be evaluated prior to applying changes uniformly across the counties.
Temperature
Nationally, annual and seasonal mid-century Tmax (Fig. 2) and Tmin increase in all counties. The magnitude of the increase varies slightly by region and season. Annual Tmax increases are slightly higher for the Midwest and Northern Plains regions, and slightly lower in the Southeast region, with all regional increases ranging from 2.4 to 3.9 °C. Southeast, Southern Plains, and Southwest county summer median Tmax is near or higher than the thermal tolerance for some crops in certain counties (Hatfield et al. 2011; Luck et al. 2011). Increased Tmax in these regions may further limit production of certain crops. Future winter Tmin will increase most in the coldest counties (+5 °C) and less for warmer counties (+2 °C). Increases in winter Tmin can adversely impact perennial crops with winter chilling requirements (Baldocchi and Wong 2008; Lobell and Field 2011) and snowpack accumulation. Conversely, warming of winter Tmin may allow for expansion of cold intolerant perennials (Parker and Abatzoglou 2016). Mean annual and seasonal anticipated changes provide a platform for evaluating and planning for both positive and negative potential impacts on specific commodities and ecosystems.
Tmax SD is less than 0.3 °C for all counties of the nation, indicating that the projected change in mean Tmax across counties is fairly spatially homogeneous. Most eastern counties have a lower annual and seasonal Tmax SD. The larger western counties located along the Pacific Coast have a wider ranging Tmax SD, with the largest Tmax SD occurring in summer. Tmin SD is also typically less than 0.3 °C. Most eastern counties have a low annual and seasonal Tmin SD, with the exception of some larger counties in Maine in winter. Winter Tmin SD was largest for some inland western counties whereas summer Tmin SD was largest for counties along the Pacific Coast. Given the low Tmax and Tmin SD (<0.3 °C) for mean annual and seasonal temperature changes in each county of the nation, compared to the daily variation in temperature at one location, the projected mean changes in temperature can be readily applied to counties nationwide. Data tables can be used (OR II) to estimate the 95% range of pixels comprising county values as the mean ± 2SD.
Regional summaries
Midwestern region
In the Midwest (MW), corn and soybean production occur on 75% of the arable land. Precipitation is projected to increase in fall, winter, and spring and decrease in summer. The increase in spring precipitation may reduce the number of workable field days and disrupt planting operations in the region (Hatfield et al. 2015). The median annual increase in county precipitation in the MW is projected to be 64 mm, mostly occurring in the winter and spring. The maximum annual ppt change SD of all the counties in the MW is 15.2 mm, suggesting relatively homogenous county-level precipitation change.
The average annual county-level increase in midcentury Tmax in the MW is 3.5 °C, with the highest average increase (4.0 °C) in summer. The average annual county-level increase in Tmin is 3.4 °C, with the maximum increase in Tmin (5.3 °C) occurring in winter. The largest county-level Tmax or Tmin SD is 0.07 °C, indicating small temperature variation in projected changes. Warmer temperatures in the MW create the potential for more overwintering of insects (Hatfield et al. 2015) and the advancement in the timing of spring greenup with warming may increase the potential for spring freeze events (Allstadt et al. 2015).
Northeastern region
Despite the densely populated area, the Northeast (NE) region contains significant timberland and farmland. County-level annual and seasonal precipitation increase in nearly all counties of the NE. Wet springs are already delaying planting and harvesting dates and reducing crop yields (Tobin et al. 2015). The annual average increase in county precipitation in the NE is 103 mm, mostly falling in winter (39 mm) and spring (33 mm) and expected to further challenge regional crop production. The largest SD in county-wide annual precipitation change across the NE is 21.4 mm.
The average annual increase in Tmax and Tmin in NE counties is 3.2 °C, with the largest increase in Tmax in summer (3.7 °C) and the largest increase in Tmin in winter (4.9 °C). The maximum annual and seasonal county SD for Tmax (< 0.08 °C) and Tmin (< 0.20 °C) indicate small variation in temperature change projections across NE counties.
Northwestern region
Nearly a quarter of the land in the Northwest (NW) region is used for agricultural production, supplying half the nation’s potato crop and 70% of the nation’s apples. Total annual precipitation increases in all NW counties, whereas summer precipitation decreases in three-fourths of NW counties. Spring and fall precipitation increase in 90% of the regions’ counties. Increased rainfall in the region can impact fruits, tree nuts, and berries via increased fungal pathogens and rain cracking, characterized by splitting in the outside layer of the cuticle (Creighton et al. 2015). Conversely, increased cool season precipitation, warmer temperatures, and elevated CO2 may be beneficial for rainfed winter crops such as winter wheat that can capitalize on an earlier start to the growing season when soil water is more available (Stöckle et al. 2010). Reduced mountain snowpack and the lack of water management infrastructure may impact irrigated crop production in the NW as more runoff is projected to occur during the winter and spring and less during the summer months when irrigation is required (Fritze et al. 2011; Mote et al. 2005). The NW has a relatively large annual precipitation SD with only 36% of counties with an SD of less than 10 mm, but all counties with an SD of less than 80 mm. This indicates that the projected changes in precipitation can vary widely across NW counties, likely consistent with the present high and variable county-level precipitation. Use of NW county-level ppt projections should be evaluated on a seasonal, county, and application basis.
Annual average increase in NW Tmax and Tmin are approximately 3.0 °C. The maximum temperature change SD across NW counties is less than 0.26 °C, with the highest SD in summer.
Northern plains region
The northern plains (NP) region, containing more than one-third of the nation’s rangeland, is expected to undergo seasonal shifts in precipitation. Annual and winter precipitation is projected to increase in nearly all NP counties. In contrast, summer precipitation decreases in 88% of NP counties, influencing availability of water for producers and municipalities (Derner et al. 2015). The highest annual ppt change SD for NP counties (Glacier County, MT) is 53.1 mm. NP winter and spring ppt have counties with larger (>20 mm) SD whereas all counties have lower summer SD (<4 mm). Use of NP county-level ppt projections should be evaluated on a county basis, particularly in winter and spring where deviation of >20 mm across a county may have resource management implications.
Annual average increases in midcentury NP county-level Tmax and Tmin are approximately 3.4 °C with the largest increases in summer for Tmax (4.1 °C) and winter for Tmin (5.3 °C). Longer and warmer growing seasons in the NP can alter pest and weed pressure and reduce livestock carrying capacity by favoring non-native, invasive plant expansion (Derner et al. 2015).
Southeastern region
The Southeastern (SE) region produces much of the nation’s timber and pulp wood supplies along with cotton, peanuts, citrus, and specialty crops. Reduced regional farm and forest productivity may result from altered rainfall patterns and increasing climate variability (McNulty et al. 2015). Annual precipitation is projected to increase in 88% of SE counties, whereas summer precipitation is projected to decline in half of SE counties, mostly located in south Florida and southwestern Mississippi and Louisiana. The annual maximum ppt change SD of the 1009 counties comprising the SE region is 31 mm, with each season/county combination showing a county-wide SD of <10 mm, indicating little variance in ppt projections within SE counties. Both drought and excessive water are listed as vulnerabilities for crops common to the region including cotton, corn, rice, wheat, minor grains, and strawberries.
Midcentury annual mean county-level temperatures increase in the SE, with the largest increases occurring in the summer (Tmax 4.1 °C; Tmin 3.4 °C). Rising temperatures in the SE may increase the risk of heat stress for crops and increase production costs (McNulty et al. 2015).
Southern Plains region
The Southern Plains (SP) region contributes substantially to the nation’s wheat and beef production. Over half SP counties are projected to have a decrease in annual precipitation by midcentury. While the decline is most evident in summer, with 93% of SP counties showing a precipitation decline, county precipitation declines are also evident in other seasons. SP ppt change SD is low, with annual (<7.8 mm) and seasonal (<4.4 mm) values indicating low variability in county-level ppt change. Reduced precipitation may make the region more prone to drought thereby reducing production and economic viability, increasing reliance on groundwater and further reducing Ogallala Aquifer levels (Steiner et al. 2015).
SP annual mean county-level Tmax and Tmin are projected to increase 3.1 and 2.9 °C, respectively, with the largest increase occurring in summer. Increased temperatures in the SPmay increase vulnerability to late season frost, pest pressure and heat stress (Steiner et al. 2015).
Southwestern region
Unlike other regions of the USA, where a decline in precipitation is most evident during the summer months, precipitation declines in southwestern (SW) counties are most evident in spring with 69% of counties exhibiting a decline in projected spring precipitation. Declines in spring precipitation lead to drier conditions during planting of corn, cotton, spring barley, and other regional crops (Elias et al. 2015). The largest SW annual ppt change SD is 24.2 mm, but most annual ppt change SD in the SW is <10 mm. Winter ppt change SD is high in several SW counties, located in California and Utah, whereas fall, spring and summer SD are <10 mm in all SW counties.
Mean annual Tmax and Tmin across SW counties is projected to increase by 3.2 and 2.9 °C, respectively. The highest increase in Tmax (4.0 °C) occurs in summer, whereas the highest increase in Tmin (4.1 °C) occurs in winter. The SW contains several counties, mostly in central CA, with the largest national annual and seasonal temperature SD. While Fresno County in California consistently has the largest Tmax SD, it still represents a relatively small value (0.3 °C) indicating that projected temperature changes vary minimally across counties and are likely less important than ppt SD values.
Data access via web-based decision support tool
Annual and seasonal precipitation and temperature data for the historical period, change between historical and future, and future period are provided by county (USDA Southwest Climate Hub 2016). National and regional static and interactive maps of Tmax, Tmin, ppt, and SD by season are provided online (http://swclimatehub.info/data) and as a supplement to this submission. Agricultural advisors can access county-level data tables for Tmax, Tmin, ppt, and SD online, as well as boxplots of regional differences. To access data via the interactive map showing historic (1971–2000) and future (2040–2069) data and graphs, users select the variable and county of interest. Users can also download data for each region and export it to an external table.