Jennifer Marohasy’s article in the Weekend Australian newspaper (23/8/2008) is yet another installment in the ongoing saga of Australian non-greenhouse theorists (‘sceptics’) attempting to discredit, at least in the eyes of the general public, the conclusions of Australian climate science. But one wonders why Institute for Public Affairs supporters choose to give their imprimatur to such risky advice.
In contrast to Dr Marohasy’s piece, a robust discussion of rainfall trends their relationship to the anthropogenically driven component of global warming (AGW) must involve some aspect of risk and uncertainty. Such uncertainty can be amplified (either way) to suit supporters or detractors of the evidence underpinning global warming – and hence influence actions required to avoid dangerous climate change. However a fair and robust case for risks involved in the AGW case should be put explicitly, instead of simply pretending that such detailed advice did not exist.
The basic problem with the Australian temperature and rainfall graphics used in the Marohasy article is the issue of averages and the practice of ‘smearing’ the statistics. For instance, if we examined the time series of Australian rainfall, the immediate conclusion might be that there is no trend, or even a slight upward trend, over the entire time span. However, this continental-wide data averages over both a wetter north-west and a drier eastern, south-eastern and south-western Australia. Such an average can blur very significant and environmentally important regional trends.
Indeed if things are “so average” and of little concern, why has the Australian media been full of drought stories for the last 5 years? So, on to the basics…
A quick regression will show the wet 1950s and 1970s and an overall slight trend upwards. Equally, any analysis since 1950 will bias the trend downwards, relative to the whole series. The presence of the longer-term effects on Australian rainfall from oceanic shifts, such as the Pacific Decadal Oscillation (PDO) and Interdecadal Pacific Oscillation (IPO), make the analysis of this situation, and the teasing out of the AGW influence, a complex matter.
However consider these three sequences (here, here and here). Note that any broad-scale analysis of eastern Australian rainfall since 1900 is comparing a wetting trend for NSW with a drying trend for QLD. As such, a wet and dry pattern is confounded the time series graphs given in the hyperlinks above. But any trend in the spatial analyses since 1950 show an increasingly severe drying trend in eastern Australia and the Murray Darling Basin.
Now, the Murray Darling Basin itself covers a vast area – so what has been happening in recent years in the Basin itself? An excellent summary can be had here. Six year decile analysis from Figure 1 and eleven year decile analysis from Figure 3 of the hyperlinked PDF show the major areas of the Murray headwaters to be in the lowest decile – and for some areas, it is the worst on record.
The multi-year analyses are most important for analysing water resources with respect to ‘drought for irrigation’ (as compared to agricultural drought of rain-fed pastures and crops). Antecedent conditions are very important. Because droughts progress over years, it takes a prolonged period of rainfall or a massive rainfall event to break them. Dry catchments need to soak up large volumes of water before substantial runoff occurs, and some soils can be become hydrophobic, resisting infiltration of rainfall.
The report also quotes:
“Maximum temperatures for the Murray-Darling Basin (and indeed for the whole of Australia) over the past six years were also at record levels, averaging 1.3°C above the long-term (1961-1990) average.”
Warmer temperatures can directly increase evaporation rates, and also affect the water vapour transport within soils themselves, further adding to the evaporative demand.
All this comes to a head in Wendy Craik’s report to Outlook 2008 of the situation within the Basin Slides 4, 5 and 6 vividly illustrate the severity of the situation. Modelled inflows for the current drought sequence are worse than the famous Federation drought. This is further emphasised in the latest July 2008 drought update. This chronic lack of irrigation water to cotton growers, rice growers and the diverse Murray River irrigated agriculture is well known by the industries involved. Across the Basin cereal growers have had bad seasons and good seasons in the same period. Receiving a planting opportunity from adequate and timely rainfall is the key factor.
The Brisbane River catchment which supplies the Queensland capital with water has shown similar problems, with the City only recently coming off level 6 water restrictions. The Brisbane region has seen massive expenditure on water recycling, water grids and desalination, with new dams being planned for construction.
The following analyses show worst-on-record inflows for the Brisbane River catchments above Wivenhoe dam. So south-east Queensland has also experienced a parallel situation to that of the Murray Darling Basin.
As for Marohasy’s claim of a less-than-dire salinity situation in her newspaper piece – this issue was comprehensively debated when she raised it previously. So one must ask, if she was seeking an honest and open debate, why has she chosen not to drawn attention to this much more complex picture?
Hard nosed observers may also speculate that the record-breaking droughts described above may not in fact be unprecedented over even the last few hundred years. Without reliable rainfall and run-off measurements, it is impossible to be sure. Indeed, the Burdekin seemed to have experienced very low flows for the 1760s to 1780s, based on proxy records from coral skeletons.
No scientist disputes that the Earth system is variable, and that the further back in time one looks, the greater amount of the full range of natural variability will be experienced. The point is not whether there are many influences involved with climate change – there indisputably are. The question of interest with the current drought is whether the fingerprint of AGW can be detected.
Australian climate science is documenting significant changes in the El Niño Southern Oscillation (ENSO), the Indian Ocean, southern hemisphere circulation and Tasman Sea temperatures. Although have seen a trend towards El Niño dominance and a weakening of the Walker circulation, there is still controversy as to the effect of global warming on ENSO processes. However Power and Smith now suggest that global warming needs to be taken into account in the formulation of ENSO indices.
There is a substantial case for linking anthropogenic impacts of Antarctic stratospheric ozone depletion (due to CFCs) and greenhouse warming on the southern hemisphere atmospheric circulations and Australian climate. For instance:
Thompson, D.W.J. & Solomon, S. (2002) Interpretation of recent Southern Hemisphere climate change. Science 296, 895.
Shindell, D.T. &. Schmidt, G.A. (2004) Southern Hemisphere climate response to ozone changes and greenhouse gas increases. Geophys. Res. Lett. 31, L18209.
Cai, W. (2006) Antarctic ozone depletion causes an intensification of the Southern Ocean super-gyre circulation. Geophys. Res. Lett. 33, L03712.
Cai, W. & Cowan, T. (2006) SAM and regional rainfall in IPCC AR4 models: Can anthropogenic forcing account for southwest Western Australian winter rainfall reduction? Geophys. Res. Lett. 33, L24708.
The south-east QLD drought analysis also mentions the feedback effects of land clearing. Preliminary modelling studies indicate that past land-use changes in Australia, associated with major clearing, may have also suppressed rainfall over eastern Australia and contributed to hotter, more severe droughts during recent El Niño events.
The joint CSIRO – Bureau of Meteorology South-east Australian Climate Initiative attributes a role of global climate change in the drying of south-eastern Australia via specific changes to the sub-tropical ridge (STR):
“The intensity of the STR also peaked in the 1940s at the time of the previous dry decade in the south-east. By and large during the 20th century, the long-term evolution of the intensity of the STR follows the curve of the global temperature of the planet. This relationship gives a high likelihood that the current rainfall deficit is linked to the global warming of the planet, through the intensification of the STR.”
Drs Wenju Cai and Tim Cowan have further dissected the climatic reasons for the decline in Murray flows and the impacts of temperature on the water balance:
Cai, W. & Cowan, T. (2008) Evidence of impacts from rising temperature on inflows to the Murray-Darling Basin. Geophys. Res. Lett. 35, L07701.
Cai, W. & Cowan, T. (2008) Dynamics of late autumn rainfall reduction over southeastern Australia. Geophys. Res. Lett. 35, L09708.
The 2001–2007 Australian drought was the hottest on record, with inflows to Australia’s longest river system, the Murray-Darling, reaching an historical low. Here we examine the relative importance of rising temperature and decreasing rainfall over its catchment, the Murray Darling Basin (MDB). Although annual-total inflow is more sensitive to rainfall over the southern MDB, where rainfall since 2001, has been the lowest on record, this alone can not explain the observed inflow decline. A relationship exists between inflow variations and fluctuations of temperature not associated with rainfall in the austral winter and spring: a rise of 1°C leads to an approximate 15% reduction in the climatological annual inflow. Our results provide strong evidence that rising temperatures due to the enhanced greenhouse effect have a strong impact on southern Australia’s water resources, in addition to any reduction in rainfall, and project a long-term decline in inflows to this river system as the greenhouse effect continues.
The wetting trend of north-western Australia has been linked to remote Anthropogenic Asian aerosols, such as from the burning of Indonesia’s tropical forests, although there remains considerable uncertainty on this point.
Thus a case emerges of: (i) severe rainfall declines in parts of eastern Australia, (ii) record low inflows in major river systems of considerable economic, social and ecological importance; and (iii) a growing body of peer-reviewed climate science that, at the very least, implies some involvement of anthropogenic factors in the current drought – an influence likely superimposed upon a complex of natural variability.
On a non-drought related point – it’s fascinating that sceptics like to use, when it suits their purpose, the same temperature series they discredit to prove their latest “cooling” idea. Surely they can’t have it both ways. The excellent Wood-for-Trees website allows one to plot, compare and contrast, in as many ways as you can imagine, the relative differences between the two ground-based and two satellite temperature analyses. The trends are very similar, with the major differences being in how GISTEMP treats averaging of stations across the Arctic, and in the different baseline periods used to compute the ‘temperature anomaly’.
It rather strange that the Marohasy article implies some dependence of the Bureau of Meteorology on the UK East Anglia and Hadley Centres for temperature trends. Perhaps she does not realise that for the local Australian situation this is not so at all – the Bureau has its own high quality data sites which do not include capital cities – and Australian analyses (here and here) of temperature trends.
Both the differences between different climate analyses, and the management of Bureau of Meteorology station data, are explained by Andrew Watson (BOM) and myself in the Climate Change Q&A seminar 1: “Is the Earth Warming?”.
A deeper question to emerge from the above is this: exactly whose interest is being served by the deliberate attempt of various non-greenhouse theorists to blur and fog this debate, the latest example of which is the Weekend Australian piece by Dr Marohasy? Why have they not openly and honestly provided a full disclosure of the rainfall trends, inflows and statistics behind the story? Why the need for a sleight of hand, in which the data are very selectively presented and interpreted? Most importantly, why has the latest peer-reviewed scientific literature not been mentioned? A much more serious analysis on this point is needed.
[Thanks to an anonymous contributor for their very helpful contributions to the above piece]