Climate Change in Queensland's Grazing Lands:
Approaches and Climatic Trends

G.M. McKeon¹, W.B. Hall¹, S.J. Crimp¹, S.M. Howden², R.C. Stone³ and D.A. Jones⁴

Abstract

Analysis of daily climate surfaces for QueenslandÇs pastoral/cropping zone shows high variability in annual rainfall, which is influenced by the El Niño-Southern Oscillation (ENSO) phenomenon. However the relationship between ENSO and QueenslandÇs rainfall has not been consistent throughout this century with the 1930-40s being a period of low correlation. The temporal behaviour of other climatic variables has also changed. This is particularly true of minimum temperatures, showing a significant (P<0.01) increase over time especially in May. Over the 40 years since 1957, annual minimum temperatures have increased by 1.0^oC for the pastoral/cropping zone and coastal sub-zone, winter minimum temperatures by 1.2^oC for the pastoral/cropping zone (1.3^oC for the coastal sub-zone), summer minimum temperatures by 0.7^oC for the pastoral/cropping zone and coastal sub-zone, and May minimum temperatures by 2.8^oC for the pastoral/cropping zone (3.0^oC for the coastal sub-zone). Consistent significant trends in vapour pressure (increasing, P<0.001) and solar radiation (decreasing, P<0.05) also occurred in May.

¹Climate Impacts and Grazing Systems, Queensland Department of Natural Resources, 80 Meiers Rd Indooroopilly, Brisbane, Qld 4068

²CSIRO Wildlife and Ecology, GPO Box 284, Canberra, ACT 2601.

³Queensland Centre for Climate Applications, Queensland Department of Primary Industries, 203 Tor St Toowoomba, Qld 4350.

⁴Bureau of Meteorology Research Centre, GPO Box 1289K, Melbourne, Victoria 3001.

KEYWORDS: Climate Change, Queensland, Pastoral/Cropping Zone, Trends

Introduction

Climate change has not been explicitly perceived as a priority issue by the grazing industry, although the potential impacts are fairly widespread through effects on animal production (including property carrying capacity), economic viability and ecological sustainability. In order to begin formulating policy for the mitigation of climate change impacts on Queensland, especially the extensive primary industries, attempts must be made to accurately document current climatic trends, describe the uncertainties associated with climate change and highlight methods for evaluating climate change impacts. As much of QueenslandÇs southern rainfall is produced by both tropical and temperate atmospheric circulation patterns, a difficulty exists in generalising the impact of climatic trends or change. With this reservation an approach has been adopted of averaging climatic elements across large areas of the state to analyse general trends.

Study Description

The pastoral/cropping zone (approximately 1,095,000 km²), delimited as the area east of 19^oS 140^oE to 29^oS 144^oE and west of 152^oE, contains over 70% of QueenslandÇs sheep and cattle (expressed as beef equivalents). The coastal sub-zone (approximately 439,000 km²), delimited from 19^oS 144^oE to 29^oS 150^oE was considered a separate zone within the above area. Differing area averaged rainfall climatologies were interpolated for the pastoral/cropping zone and the coastal sub-zone from daily climate surfaces generated from the Bureau of Meteorology (Hutchinson 1991, Carter et al. 1996). Rainfall surfaces were available from 1890 to 1996; and for other climatic variables from 1957 to 1996.

Results

Rainfall trends were examined using linear regression and cube root transformation. The analysis of rainfall for these two defined regions showed that a high year-to-year variability exists even when averaged across such large areas. For the pastoral/cropping zone and the coastal sub-zone only the month of June had a statistically significant (P<0.05) trend (marginal decline) in rainfall. September rainfall also had a significant negative trend (P<0.05) for the coastal sub-zone. The analyses of temperature trends were conducted for both zones using simple linear regression. From the regression analysis of the 1957 to 1996 period, annual minimum temperatures have increased by 1.0^oC for the pastoral/cropping zone and coastal sub-zone (P<0.001). Winter minimum temperatures have increased by 1.2^oC for the pastoral/cropping zone (1.3^oC for the coastal sub-zone, P<0.01). Summer minimum temperatures increased by 0.7^oC for the pastoral/cropping zone and coastal sub-zone (P<0.01), and May minimum temperatures by 2.8^oC for the pastoral/cropping zone (Fig. 1a) (3.0^oC for the coastal sub-zone, P<0.001).

Vapour pressure, vapour pressure deficit (VPD) and solar radiation from the climate surfaces were analysed as for minimum and maximum temperature. Significant increasing trends in vapour pressure for both zones were found for May only (P<0.001) (Fig. 1c). For solar radiation in the coastal sub-zone there was an increasing trend in March (P<0.05) but decreasing trends in April and May (P<0.05), and an overall decreasing trend in winter (P<0.05). For solar radiation in the pastoral cropping zone only, May and the winter season showed significant declining trends (P<0.05) (Fig. 1d).

Discussion

The above analyses showed that significant trends have occurred in May climatic variables of (a) Minimum Temperature (⁰C), (b) Maximum Temperature (⁰C), (c) Vapour Pressure (hPa), (d) Solar Radiation (MJ/m²) for the months of May and June (Fig. 1).

Figure 1: May climate variables (a)-(d) for the Queensland pastoral/cropping zone

The increase in May minimum temperature alone accounts for approximately 25% of the annual increase and 40% of the winter increase. The increases in May minimum temperature and vapour pressure indicate that the warm humid season is extending later into autumn but few effects are carried over to June. Although there are no trends in June minimum temperature or vapour pressure, nevertheless, these variables have the same expected relationship between temperature and saturated vapour pressure. Solar radiation in May has been declining, whereas mean May maximum temperature has increased slightly, suggesting that the slightly increasing May maximum temperatures were due to the fact that the increase in minimum temperature more than compensated for the decline in solar radiation.

A mechanistic understanding of the larger increases in May temperature and vapour pressure compared to other months is provided by analysis of independent atmospheric and oceanic variables in the Australasian region. From 1950 to 1991, Mean Sea Level Pressure (MSLP) in May has increased in the Tasman Sea (35-40^oS) suggesting stronger anticyclonic circulation. This increase in MSLP is associated with increases in north-east low level winds over most of eastern Australia possibly explaining the anomalous source of increased humidity and associated night-time temperatures. April, and particularly June, show weaker trends in MSLP than occurred in May. It is possible that the warming in the Indian Ocean SSTs has contributed to the MSLP trends as elevated pressures in the south Tasman Sea can be associated with anomalously warm Indian Ocean SSTs (Simmonds 1990), though the evidence is not conclusive.

Although May and winter warming since the 1980s would be expected to increase the length of the growing season, variation in rainfall has proved to be a major limiting factor. If moisture and nutrients were not limiting factors the analysis shows that the increasing trend in May temperatures could potentially contribute to pasture growth particularly in C₄ species.

The high year-to-year variability across large areas highlights the potential impact of large-scale climate forcing on Queenslands primary industries.