THE deferral of Australia's emissions trading scheme for three years allows us time for additional scientific studies that may be critical in shaping future legislation.
A touchstone in the debate on causes of global warming is the record of global temperatures of past millennia. Most who follow this debate are familiar with the cooling from the 16th to 18th centuries known as the Little Ice Age; this is generally accepted as a global phenomenon. Most are also aware of the Medieval Warm Period covering much of the 9th to 15th centuries. This has been the source of greater debate because, while it is clear in anecdotal descriptions from Europe, such as Vikings growing crops in Greenland, it is less clear whether it is a global phenomenon. The debate has high stakes because the rate of warming and temperatures attained in Europe during the MWP are of similar order to the warming of past decades. If the MWP were to be proven to be global, then the basis of present science stating that industrial-era carbon emissions are the dominant cause of today's warming would be significantly undermined.
One of the giants of global warming science, Wally Broecker of Columbia University in New York, wrote a discussion in 2001 of evidence for the MWP being a global phenomenon, concluding tentative support for its global nature. Three years later, Phil Jones, now director of the Climate Research Unit, East Anglia, co-authored a review that concluded the MWP was a regional phenomenon. The IPCC4 report of 2007 concluded similarly; curiously, Broecker's paper did not get a mention.
Proving the MWP or other historic and prehistoric European warm periods to be global is not easy because large-scale atmospheric-ocean interactions are capable of producing either or both of warming in one hemisphere matched by cooling in the other, and warming in high latitudes balanced by cooling in tropical latitudes. A statistical analysis of all the available temperature records by Michael Mann and colleagues of the Earth and Environmental Systems Institute, Pennsylvania State University, published in Science last year, also concluded the MWP was regional. However, that study was dominated by northern hemisphere records, leaving open the question of whether more global data may give a more global conclusion.
The ongoing importance of debate over the MWP is underscored by comments by Jones in a recent BBC interview, where he said the MWP was best expressed in records from the northern hemisphere, adding: "If the MWP was shown to be global in extent and as warm or warmer than today [based on an equivalent coverage over the northern and southern hemispheres] then obviously the late-20th-century warmth would not be unprecedented."
Undoubtedly the truth is contained in temperature records written in terrestrial biological records, ice sheets and rocks. Thus far, however, the process of deciphering those records has been successful at only a couple of dozen sites, distributed unevenly across the globe.
There are climate records from the southern hemisphere, from Cold Air Cave stalagmites in northern South Africa, tree rings in Tasmania and New Zealand, and ice core records in Law Dome, Antarctica, all of which show an imprint of a medieval warming.
One of these localities, the Cold Air Cave stalagmites, has been studied for more than a decade by a team led by Karin Holmgren of Stockholm University, Sweden. A reduction in temperature of about a degree is evident for the Little Ice Age. Before that we see a 700-year stretch of time corresponding to the MWP, which contains perhaps eight approximately 100-year-long cycles, of which five show temperatures similar to or greater than those of the past century. The authors postulate these centennial cycles are driven by variations attributable to the sun. But results from a single site do not prove the warming and cooling to be global.
I am not aware of any comparable published studies in Australia; it would be most instructive if evidence for a MWP and centennial climate cycles were to emerge - or be proved absent - from studies across a range of latitudes on this continent. Indeed, if the centennial cycles noted in South Africa are sun-driven, we may well ask if we have similar cycling in our own climate; the great Federation drought (1895-1902) and the present drought of southeastern Australia might be seen as part of a cyclic continuum rather than the latter being attributed mainly to anthropogenic global warming.
Another tool for documenting climate change in past centuries was announced in March in the journal Nature. William Patterson, an isotope chemist at the University of Saskatchewan, Canada, led a team in a study of oxygen isotope data in clam shells recovered from a drill hole in a bay off the coast of Iceland. Unlike tree rings (which yield at best annual temperature variations) growth lines in the clam shells yield weekly or even daily temperature records. Patterson's work affirms evidence for the MWP in Iceland.
This high-resolution method may be applied to clam or other shells in coastal geological records the world over. It has the potential to answer quantitatively the key scientific question of whether the medieval warming was a global phenomenon. If the answer were to be yes, then warming during the past century should be seen as predominantly natural climate change rather than driven by man-made carbon emissions. A legislative response would be no less important but would focus on environmental management of the consequences of change.
There is a huge opportunity for CSIRO and the Bureau of Meteorology to extend their recent climate assessment, which was based on 1960-2010 data, to incorporate fossil-shell, cave-deposit and tree-ring records from tropical to Antarctic Australia and territories. This would cost a few per cent of the $652 million allocated on Tuesday to the new Renewable Energy Future Fund. It would make Australia a leader in addressing a great scientific challenge of our time.
Michael Asten is a professorial fellow in the school of geosciences, Monash University.