Thursday, February 7, 2013

A Climate of Scepticism - Part 1

I recently had a piece in Anthony Watt's  Watts Up With That blog.  It attracted a few hundred comments, which was gratifying.  It was written in ordinary language, so used some inexactitudes because I felt that communication was more important, in this case, than precision. Some suggested it should be republished elsewhere - so this is the first part of a long story.

The world has been getting slightly warmer.  Of that there is little doubt.  The measurements by which we know that it is warming are poor.  The figures are not accessible, and keep on changing[i].  Many points at which temperature is measured are badly sited, and bound to give misleading results[ii].  Nevertheless, almost everyone agrees that the world is warmer today than it was 150 years ago.

There are some fairly clear signals of a warmer world.  The Arctic ice is less than it was[iii]. Many glaciers are retreating[iv].  Some glaciers – for instance, those on Kilimanjaro – are shrinking because the long-term precipitation is less than it was 150 years ago, not because it is warmer[v]. Others seem to be shrinking from a warmer climate. There are, however, little data on this[vi].

Where the sceptic differs from many other scientists is in ascribing the warming to human activities – specifically, the burning of fossil fuels and the concomitant rise in the carbon dioxide concentration in the atmosphere.  The hypothesis is that the carbon dioxide traps infra-red radiation that would otherwise escape to space.  This means that some of the energy received from the sun is not lost, and the trapped energy leads to a warming of the globe. 

The physics of how carbon dioxide traps infra-red radiation is well known[vii].  But there are other molecules in the atmosphere that also trap infra-red radiation.  Water vapour is the predominant “greenhouse gas”[viii]. What is not so clear is the extent to which the trapping of energy causes heating.  There are wonderful mathematical models that claim to show how heating occurs.  Unfortunately, all the models suffer from identifiable flaws, a point considered later.

A prime difficulty with the anthropogenic warming thesis is that it is not known how much of the warming is natural and how much might be caused by carbon dioxide.  It is simple to illustrate this. Figure 1 shows the global temperature record as kept by the Climate Research Unit at the University of East Anglia[ix].
Figure 1 Global temperatures, relative to 1950-1990 average

The global temperature dropped from 1850 to 1860; rose until 1880; dropped until 1910; rose until 1945; dropped until 1980; rose until 2000; and has dropped slightly since then.

Figure 2 shows the carbon dioxide record. Careful measurements have been made at Mauna Loa on Hawaii since 1958[x].  The pre-industrial level of CO2 in the atmosphere is generally accepted to have been about 280ppm[xi]. Figure 2 shows a reasonable extrapolation of the data back to about 280ppm in 1800.

Figure 2 Atmospheric CO2 concentrations, measured and estimated.

It is a reasonable assumption that the measured rise is the result of fossil fuel consumption.  Figure 3 shows annual CO2 emissions over time[xii].  It only exceeded 5 billion tons per annum in the later 1940’s.  Thereafter it grew rapidly, passing 10 billion tons in 1963, 15 billion in 1971, 20 billion in 1986 and 30 billion in 2006.
Figure 3 Annual CO2 emissions from fossil fuel consumption.
Comparison of Figures 2 and 3 makes it clear that the rise in atmospheric carbon dioxide is very likely directly related to the emissions from fossil fuels.  However, the low levels of emissions up until about 1945 make it clear that the impact of the fossil fuel combustion prior to 1945 must have been very small if not negligible.  Therefore the changes in global temperatures prior to 1945, shown in Figure 1, were largely natural. The additional carbon dioxide from human activities cannot have played a significant part in the changes prior to 1945.

If most of the temperature changes before 1945 were largely natural, then there is great difficulty in determining how much of the temperature change after 1945 is natural and how much might be driven by increasing carbon dioxide.  This raises the question of what the natural variation in temperature might be.

To answer this question, consider the Vostok ice core record over the past 9000 years[xiii].  The core was sampled every metre of depth, which represented ~20 years of accumulation in the upper layers and ~50 years in the lower levels.  The temperature was estimated from differences in the oxygen isotope ratios.  While a point measurement such as this cannot give a good measure of the average global temperature, it is a reasonable measure of changes in global temperature, and it is primarily temperature changes that are of interest.

The data are shown in Figure 4. There has been a slight cooling over the past 9 millennia, as shown by the least-squares line.  The data were therefore detrended before further analysis – the mean temperature at any one date was added to the reported relative temperature.  The detrended temperatures were what is known as “normally distributed”, i.e. there was nothing abnormal or skewed about them. Then the rate of change between each detrended temperature and the temperature approximately 100±20 years earlier was calculated and expressed as a rate per century.  The results were also normally distributed, with a standard deviation of 0.94oC per century.

Figure 4. Relative temperatures over the past 9000 years.

Thus there is about a 2:1 chance that the temperature may vary by up to 1oC per century from natural causes, but only about a 1 in 10 chance that it will vary by more than 1.9oC naturally. Between 1900 and 2000, it varied by about 0.9oC, which is therefore within the range of natural variation.

And that, in simple terms, is why there is scepticism about the thesis that carbon dioxide is causing global warming – there is no clear signal of any such warming effect.

[iv] Paul, F., Kääb, A. and Haeberli, W. Recent glacier changes in the Alps observed by satellite: Consequences for future monitoring strategies, Global and Planetary Change, Volume 56, Issues 1–2, March 2007, Pages 111-122, ISSN 0921-8181, 10.1016/j.gloplacha.2006.07.007.
[v] Mölg, T., and D. R. Hardy (2004), Ablation and associated energy balance of a horizontal glacier surface on Kilimanjaro, J. Geophys. Res., 109, D16104, doi:10.1029/2003JD004338.
[vi] UNEP Global glacier changes: facts and figures. World Glacier Monitoring Service, 2008

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