Empirical evidence for a recent slowdown in irrigation-induced cooling

Abstract

Understanding the influence of past land use changes on climate is needed to improve regional projections of future climate change and inform debates about the tradeoffs associated with land use decisions. The effects of rapid expansion of irrigated area in the 20th century has remained unclear relative to other land use changes, such as urbanization, that affected a similar total land area. Using spatial and temporal variations in temperature and irrigation extent observed in California, we show that irrigation expansion has had a large cooling effect on summertime average daily daytime temperatures (-0.14 degrees C to -0.25 degrees C per decade), which corresponds to an estimated cooling of -1.8 degrees C to -3.2 degrees C since the introduction of irrigation practices. Irrigation has negligible effects on nighttime temperatures, leading to a net cooling effect of irrigation on climate (-0.06 degrees C to -0.19 degrees C per decade). Stabilization of irrigated area has occurred in California since 1980 and is expected in the near future for many irrigated regions. The suppression of past human-induced greenhouse warming by increased irrigation is therefore likely to slow in the future, and a potential decrease in irrigation may even contribute to a more rapid warming. Changes in irrigation alone are not expected to influence broad-scale temperatures, but they may introduce large uncertainties in climate projections for irrigated agricultural regions, which provide approximately 40% of global food production.

Fig. 1.

Global map of the fraction of each 5′ × 5′ grid cell equipped for irrigation (percent, 37). Circles indicate major irrigation regions used in this study. AB, Aral Sea Basin; CA, California; CH, Eastern China; IP, Indo-Gangetic Plains of India and Pakistan; NE, Nebraska; TH, Thailand.

Fig. 2.

Observed time-series of irrigated land cover in California and June–August temperature differences between intensively irrigated lands (CIF > 50%) and a reference area (0.1–10% CIF), both located in the Central Valley region (CV: 118.25–126.25°W, 34.75–40.25°N). T_max (A) and T_min (B) time-series are estimated by using UW (red), PRISM (green), CRU2.1 (blue), and CRU2.0 (brown) data below 500 m of elevation. Unfiltered (pale dotted lines) and low-pass-filtered averages (performed with a 11-point binomial filter, bright solid lines) are shown, both with climatology subtracted for clarity. Irrigation time-series (black line, right, vertical scale is reversed) is interpolated from data collected in a total of 12 U.S. Department of Agriculture censuses (represented by black dots). r numbers show correlation coefficients between irrigation and each filtered temperature time-series using the 12 paired observations. An asterisk indicates that temperature and irrigation are highly correlated (P < 0.01).

Fig. 3.

Observed 1915–1979 trends in June–August (green) and December–February (blue). (A) T_max in regions of the Central Valley partitioned by CIF classes, relative to the trend in the reference class. Filled circles indicate that trend of d(t) is statistically significant at P < 0.01. The same analyses were conducted for T_min (B), T_ave (C), and DTR (D).

Fig. 4.

Irrigated area expressed in 10⁶ ha (A) and in percent relative to 1961 (B) for six different regions of the World.

Fig. 5.

Observed 1950–2000 June–August (JJA) and December–February (DJF) T_max trends in regions partitioned by CIF classes, relative to a reference class (0.1–10% CIF) for five irrigated regions, by using the PRISM data set for the Nebraska (NE) region and CRU2.1 for the four other regions. Filled/open circles indicate that the trend of d(t) is statistically significant at P < 0.01/0.05. TH, Thailand; IP, Indo-Gangetic Plains of India and Pakistan; AB, Aral Sea Basin; CH, Eastern China.

Fig. 6.

Observed 1979–1992 June–August (JJA) and December–February (DJF) average in aerosol optical depth (AOD, unitless) from the Total Ozone Mapping Spectrometer data set (Nimbus7-TOMS, 28) in six regions partitioned by CIF classes. CA, California, TH, Thailand; IP, Indo-Gangetic Plains of India and Pakistan; AB, Aral Sea Basin; NE, Nebraska; CH, Eastern China.