Friday, February 24th 2012, 8:15 AM EST
The recent advice given to the UK Government by the Parliamentary Office of Science and Technology (POST) in a report called “Climate Variability and Weather,” highlighted an interesting dilemma facing anyone trying to predict climate change in the future, and attribute causes to climate change observed in the recent past.
The summary of the report was: “Short-term differences from long-term climate, or ‘climate variability’, can increase the risk of damaging extreme weather events. This POSTnote examines the causes of climate variability and the use of global climate models to understand, and predict, these variations.”
I have an expectation that in the absence of the global annual average temperature increasing very much, if at all, in the past 10-15 years, the new talisman behind which many will rally will be the increased prevalence, supposedly, of extreme weather events due to “increased energy in the Earth’s climate system.” Such thinking is apparent in recent reports about the Russian heatwave. See here, here and here.
The dubious logic behind the conclusion, when trying to explain a single outlier event, that man-made global warming made the Russian heatwave three times more “likely” even though it was within natural limits of variability, will be something to return to in the future.
The POST report says that for the UK, natural climate variability will be dominant over the next few decades, although the influence of greenhouse gasses will increase “as the century progresses.” The report is also a plea for the UK Met Office to get bigger computers.
When will the man-made global warming signal emerge on regional scales? How can it be apparent in global climate statistics if it is swamped by natural climate variability over the Earth’s component regions?
The IPCC says that it is unequivocal that, when viewed globally, the man-made signal has emerged, although in this typical report details about past attribution are skimpy, everything is about model projections for the future.
A Foreign Country
From here the logic starts to unravel. One comment on the 2007 AR4 report was, "We can be very confident that the net effect of human activity since 1750 has been one of warming.” Reports also said that the IPCC had concluded that it was at least 90% certain that human emissions of greenhouse gases rather than natural variations have warmed our planet's surface over the past 50 years, and that CO2 is the dominant climatic influence.
However, elsewhere the IPCC says that climate variations seen prior to 1960-80 (0.5 deg C warming) are natural, probably due to the Sun and volcanoes, whereas the 0.4 deg C warming observed between 1980 - 2000 is almost entirely (better than 90%) due to man-made global warming.
So how is it that when the regions of the Earth are considered the man-made global warming signal has yet to emerge from the “noise” of climatic variability, (even the Russian heatwave is explainable due to natural variability) yet on a global scale it is obvious at more than 90% probability?
The UK Met Office says; “Over the next decade, changes in climate are expected to be due to a combination of anthropogenic changes in atmospheric greenhouse-gas and aerosol concentrations; natural variations in volcanic and solar activity, and natural, unforced internal variability.”
The important point is that natural variability does not increase in time, whereas the man-made global warming component does rendering natural variability smaller and smaller as time goes by. The IPCC says:
“Clearly, there is a range of model results for each year, but over time this range due to internal variability becomes smaller as a fraction of the mean warming. The range is somewhat smaller than the range of warming at the end of the 21st century for the A2 scenario.”
How is it that predicting the future is laced with such qualifications, yet explaining the past 30 years is given such overwhelming certainty? One wonders why it is so obvious globally yet well down in the noise regionally, especially since most of the world is currently not warming, with the exception of the northern arctic regions and the Antarctic peninsula, which may not be due to anthropogenic influences.
The reality of a changing climate means that decision makers are seeking quantitative predictions of regional and local climate. The problem is that model uncertainty is much larger than internal climatic variability, and we don’t know much about internal variability on decadal timescales, and we have different scientific standards when looking back from those looking forward. The past, as they say, is a foreign country: they do things differently there.