Global Relationships between Cropland Intensification and Summer Temperature Extremes over the Last 50 Years

Ethan E Butler,Peter Huybers,Nathaniel D Mueller,N Michele Holbrook,Andrew Rhines,Stefan Siebert,Deepak K Ray

doi:10.1175/jcli-d-17-0096.1

Abstract

Abstract Conversion of native ecosystems to cropland and the use of irrigation are considered dominant pathways through which agricultural land-use change alters regional climate. Recent research proposes that increases in cropland productivity, or intensification, also influences climate through increasing evapotranspiration. Increases in evapotranspiration are expected to have the greatest temperature influence on extremely hot summer days with high vapor pressure deficits. Here, the generalizability and importance of such relationships are assessed by examining historical land-use and climate trends in seven regions across the globe, each containing a major temperate or subtropical cropping area. Trends in summer high-temperature extremes are sequentially compared against trends in cropland area, area equipped for irrigation, precipitation, and summer cropping intensity. Trends in temperature extremes are estimated using quantile regression of weather station observations, and land-use data are from agricultural inventories and remote sensing. Intensification is the best predictor of trends in extreme temperatures among the factors that are considered and is generally associated with trends that are 0.2°–0.4°C decade−1 cooler than in adjacent regions. Neither cropland area nor precipitation trends are systematically associated with extreme temperature trends across regions, although high temperatures are suppressed over those portions of central North America and East Asia experiencing growth in irrigation. Both the temperature trends associated with intensification and increased irrigation can be understood as a consequence of increased latent cooling. These results underscore that the weather experienced by crops is not entirely external but also depends on agricultural practices.

Highlights

Climate is a central determinant of crop distribution and productivity, yet climate itself can be influenced by agricultural land use and land cover via biophysical changes to surface albedo, rates of evapotranspiration, and surface roughness (Foley et al 2003; Brovkin et al 2004; Fed48 dema et al 2005; Diffenbaugh 2009; Pielke Sr. et al 2011)
Trends in summer high temperature extremes are sequentially compared against trends in cropland area, area equipped for irrigation, precipitation, and summer cropping intensity
Summer cropping intensity trends To evaluate trends in summer cropping intensity, we develop an index of Summer Cropping Intensity (SCI) that quantifies yearly summer crop biomass production across the landscape in units of grams of Carbon per square meter produced over the summer growing season, i.e. g C m−2 summer−1

Summary

Introduction

Climate is a central determinant of crop distribution and productivity, yet climate itself can be influenced by agricultural land use and land cover via biophysical changes to surface albedo, rates of evapotranspiration, and surface roughness (Foley et al 2003; Brovkin et al 2004; Fed dema et al 2005; Diffenbaugh 2009; Pielke Sr. et al 2011). Given that the pace of cropland expansion has been relatively slow since 1950 (Ramankutty and Foley 1999), and that widespread increases in crop productivity occurred during this time period due to the adoption of “Green Revolution” technologies and management practices (Tilman et al 2002), intensification of existing croplands may be a dominant mechanism through which agricultural practices change regional climate This relationship has only been docu mented in the the US Midwest (Mueller et al 2016), an area that exhibits the most pronounced peak summer vegetation growth of anywhere on the planet (Guanter et al 2014; Mueller et al 2016). Hot extremes exhibit unique trends relative to lower percentiles of the temperature distribution (McKinnon et al 2016; Mueller et al 2016), and are sensitive to changes in evapotranspiration (Seneviratne et al 2010; Mueller and Seneviratne 2012; Huybers et al 2014; Mueller et al 2016)

Data and Methods

Results and Discussion

Conclusions