New paper finds significant cooling effect of irrigation; implies water vapor feedback is negative

A new paper published in Environmental Research Letters finds a significant cooling effect of agricultural irrigation in Northwest China during the period from 1959-2006. The authors find an average cooling of May-September temperatures of −0.15 ° C to −0.10 °C per decade in areas with extensive agricultural irrigation. The data implies increased water vapor acts as a negative feedback on temperature, the opposite of claims by climate alarmists. The paper adds to prior work demonstrating that the true sign of water vapor feedback is negative, and demonstrates that the assumption of positive water vapor feedback in climate models is false.

Cooling effect of agricultural irrigation over Xinjiang, Northwest China from 1959 to 2006

Songjun Han and Zhiyong Yang

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Abstract:

The influences of agricultural irrigation on trends in surface air temperature from 1959 to 2006 over Xinjiang, Northwest China are evaluated using data from 90 meteorological stations. The 90 stations are located in landscapes with markedly different cultivated land uses. The increasing trends in daily average temperature (Ta), maximum temperature (Tmax), and minimum temperature (Tmin) for May–September (the main growing season) are negatively correlated with cultivated land proportions within 4 km of the meteorological stations, as indicated by year 2000 land use data. The correlations between the trends in Tmax and cultivated land proportions are the most significant. The trends in Ta, Tmax, and Tmin for May–September are expected to decrease by −0.018, −0.014, and −0.016 ° C per decade, respectively, along with a 10% increase in cultivated land proportion. As irrigated cultivated land occupies over 90% of total cultivated land, the dependence of temperature trends on cultivated area is attributed to irrigation. The cooling effects on stations with cultivated land proportion larger than 50% are compared to temperature trends in a reference group with cultivated land proportion smaller than 10%. The irrigation expansion from 1959 to 2006 over Xinjiang is found to be associated with cooling of May–September Ta, Tmax, and Tmin by around −0.15 ° C to −0.10 ° C/decade in the station group with extensive irrigation. Short periods of rapid irrigation expansion co-occurred with the significant cooling of the May–September temperature.