Climate change has been the most important and complex issue on my plate in 15 years as a science and technology correspondent for The Canberra Times. So an appropriate topic for a farewell commentary for this newspaper is an emerging scientific debate with the potential to complicate the already difficult relationship between scientists and politicians on this issue.
The effect of the sun's activity on global temperatures has loomed large in arguments from climate change sceptics over the years. Several Russian scientists have argued that the current period of global warming is entirely due to a cycle of increased solar activity.
NSW Treasurer Michael Costa is understood to be among a small group of Australian politicians and other opinion-shapers to embrace this notion.
It is wise to be sceptical of many Russian scientists and all politicians, so I have given this ''solar forcing'' explanation of global warming little credence until I attended a forum at the Academy of Science earlier this year and heard it from a scientist of undoubted integrity and expertise in this area. A former head of CSIRO's division of space science, Dr Ken McCracken was awarded the Australia Prize the precursor of the Prime Minister's Science Prize in 1995. Now in his 80s, officially retired and raising cattle in the ACT hinterland, he is still very active in his research field of solar physics.
McCracken is adamantly not a climate change sceptic, agreeing that rising fossil-fuel emissions will be a long-term cause of rising global temperatures.
But his analysis of the sun's cyclical activity and global climate records has led him to the view that we are entering a period of up to two decades in which reduced solar activity may either flatten the upward trend of global temperatures or even cause a slight and temporary cooling. In a paper given in 2005 to a ''soiree'' hosted by then president of the Academy of Science, Professor Jim Peacock, McCracken said the sun was the most active it had been over 1000 years of scientific observation. This made it inevitable that its activity would decrease over the next two decades in line with historically observed solar cycles.
''The reduced 'forcing' might compensate, or over-compensate, for the effects of the increasing concentration of greenhouse gases,'' he said. ''It is likely that there will be a cessation of around 20 years in the increase in world temperature, or possibly a decrease by 0.1 [degrees] or more.''
I put this to Dr David Jones, head of climate analysis for the Bureau of Meteorology's National Climate Centre, whose overarching judgment is that the warming effect of fossil fuel emissions is an increasingly dominant factor on global temperature to the extent that it will not be slowed by lower solar activity.
After an email conversation, Jones said he and McCracken are in general agreement but differ on emphasis and one key judgment. ''Natural solar variability is potentially important, but the climate history and physics tell us that the probability of this factor sufficiently cooling the planet to offset the enhanced greenhouse effect is distinctly remote,'' Jones wrote.
The main point of disagreement was McCracken's view that the rate of global warming could be eased or reduced by a fall in solar activity. ''I have never seen a credible paper published using a climate model that shows this,'' Jones wrote.
He points to recent data which indicates that global temperatures are probably rising faster than previously thought, raising the urgency of calls from climate scientists for political action to reduce emissions. Yet any uncertainty over the sun's influence creates a lever that climate sceptics and developing nations will seize upon to stall such action.
If McCracken is wrong and temperatures continue to climb during a decade or two of low solar activity, the need for emissions reductions will be dramatically reinforced.
However, if temperatures do not rise over this period, steeling the political will for such action by all nations will be much more difficult.
The dilemma for the science sector is a classic: how to communicate uncertainty.
As McCracken rightly observed in 2005, a lull in temperature rises would provide a wonderful opportunity for political and technological effort to gain the initiative in the fight against climate change by turning global emissions around and thus hopefully avoid worst-case warming scenarios when the sun's fires stoke up again mid-century.
But he also noted the risk that mainstream climate science, led by the Intergovernmental Panel on Climate Change, would be seen by its critics and others to have been ill-informed at best or misleading at worst, diminishing its credibility and eroding political commitment to emission reductions.
McCracken believes science should be upfront. ''I believe that we must state firmly that a cooling is possible in the near future, but that the warming would then resume 10-20 years hence,'' he said via email. ''It will be very hard to argue for public trust if we say nothing about the possibility, and then try to argue our way out after it happens. Using an Aussie rules analogy, that would be like giving the climate sceptics a free kick 10m in front of goal.''
Australia is definitely entering a footy finals period, and the Earth may be entering a period where human-induced global warming slows temporarily. Many scientists will not be comfortable to consider this possibility, and even less comfortable that journalists canvas it, because in good faith they want nothing to deflect efforts to combat global warming.
However, I have always aimed to tell readers what they deserve to know, not what they may want to hear or what governments, scientists or interest groups would prefer they were told. This has earned me brickbats and bouquets over the years, as it should do, and as I expect it will on this occasion.
Simon Grose is Canberra correspondent for Science Media.
www.sciencemedia.com.au