Soil temperature increase in eastern Australia for the past 50 years

Abstract

Traditionally, soil temperatures at various depths were measured daily at agricultural research stations around the world. Short-term fluctuations are damped at depth, acting as a natural low pass filter on soil surface temperature time series. In the last 20years, a number of northern hemisphere soil temperature data sets have been analysed for long term trends, but our analysis at four sites in eastern Australian is the first in the southern hemisphere. Soil temperature data at Cowra in New South Wales, Australia was recorded daily at 09.00 at a depth of 1.8m from 1942 to 2010, together with rainfall and maximum and minimum air temperature data. From 1972 temperatures at some shallower depths were also recorded. We found significant average annual temperature rises in soil and air temperature data, with greater rises in the soil temperatures over this period. We also analysed soil and air temperature data recorded from 1970 at Gunnedah, Inverell and Brigalow in eastern Australia. To standardise for soil temperature measurements taken at various depths, we proposed a model that estimated the rise of the ground surface temperature accounting for the attenuation and delay of the surface temperature signal with depth. We estimated the rise of the soil surface temperature Cowra from about 1970 as 0.06Kyear−1. We found the rate of rise of the average of the daily maximum and minimum air temperatures to be 0.022Kyear−1. For Gunnedah, Inverell, and Brigalow, from 1970 the rates of rise of soil temperatures at 1m and air temperatures were at around 0.02Kyear−1. These soil temperature data show a clear warming trend overall since about 1970.