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Climate Variability


  Last month saw the publication of Hansen et al's paper, "Global Surface Temperature Change" in Reviews of Geophysics. Yesterday, we received James Hansen's comments on his team's paper. It's available for download here. Those comments are intended to explain to lay people why we are experiencing extremes of weather such as the Moscow heatwave and the Texas/Oklahoma drought and what that means for the future.

Whether he succeeds in his aim to bring the science home to the ordinary person is questionable. So here we try to explain the crux of the problem he sees.

This graph is from Fig. 4 in his notes and describes the temperature anomalies for the whole world for the months of June, July and August (JJA). By anomaly he means the difference between the average temperature for those months compared with those of same months in the 30 years 1981 to 2010. Those years were chosen because they coincide with satellite (more accurate) measuring of temperature. Temperatures higher than normal have positive values, those below normal have negative values.

They first plot a statistical device known as a 'normal distribution', often called a 'bell curve', for the JJA temperatures in 1981-2010. This is the black plot that you see on the graph and it represents a useful and common statistical method of measuring probablities. For example, if you threw a dice 100 times and added up the values you got, then repeated the test (say) ten thousand times, you would get a range of values between 100 (100x1) and 600 (100x6). As you can imagine, the chance of throwing a one 100 times in succession is pretty slim. The same is true for 100 sixes. If you then plotted the number of times a particular total occured on the vertical axis and the value itself (100 to 600) on the horizontal axis, you would end up with a 'normal distribution' plotted on your graph. The peak would occur at the most common value which will be the average - 350.

By arithmetic, they derived from their data a value for what is called the standard deviation (σ), which is a measure of the variability in the data. The measure is used in the graph along the horizontal axis. 2 means 2xσ above normal; -3 means 3xσ below normal.

Now they plot the temperature anomalies for each deacde (the coloured plots). The earlier decades show taller plots. That means that they contained less variablity than 1981-2010. The later ones show increasing variability decade by decade. So global temperatures are showing wider fluctuations in these decades. Also, you can see that the peaks are moving to the right. That shows that global warming is occurring. Finally, you can see that a 3σ event is most improbable (nearly zero chance) in the early years, but is increasingly likely as time moves on. The Moscow heatwave was assessed as a 3σ event (once in 1000 years). Science, derived purely from measuring temperatures, is showing us that the likelihood of extreme temperature events is increasing and will continue to increase as the century unfolds.

If global warming approaches 3°C by the end of the century, it is estimated that 21-52% of species will be committed to extinction. We are on track for 6°C. Scientists are increasingly worried that their message is being ignored. Here at fractual we believe that we must forego our dependence on fossil fuels as quickly as we can. Natural gas is another fossil fuel and fracking for it probably makes it as dirty in GHG terms as coal.

Ian Perrin. 6.1.2012
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