Climate Change- In Search Of A Signal

Written by
22 November 2013

This article from the November 2013 edition of the IPA Review is written by Stewart Franks.

Perhaps the most frustrating aspect of the science of climate change is the lack of any real substance in attempts to justify the hypothesis. Often the occurrence of a drought or flood is sufficient to generate a whole range of expert speculation from those that should know better. Often the claims are couched with the disclaimer ‘of course, no single event is attributable to carbon dioxide and climate change, however this is exactly the sort of event we would expect to see.’ Such statements are meaningless as these are also the sorts of events that we do expect to see, irrespective of climate change.

What dismays me the most is that the worst examples of speculative claims often come from the scientists themselves.

Commentators from the Bureau of Meteorology and the CSIRO are among the worst for making statements that are simply incorrect.

One stated, ‘of course, the drought has not been helped by rising temperatures, which have increased losses through evaporation,’ and ‘it is very difficult to make a case that this is just simply a run of bad luck driven by a natural cycle and that a return to more normal rainfall is inevitable, as some would hope.’

In an interview with the Sydney Morning Herald, he mused that ‘perhaps we should call it our new climate.’

A similar line was adopted by another Bureau climatologist, ‘in the minds of a lot of people, the rainfall we had in the 1950s, 1960s and 1970s was a benchmark … But we are just not going to have that sort of good rain again as long as the system is warming up.’

Immediately following the 2002-3 El Niño and Murray Darling Basin drought, a report under the auspices of the WWF-Australia noted that whilst rainfall had been low, the air temperatures had been particularly elevated. This led the authors to claim that:

The higher temperatures caused a marked increase in evaporation rates, which sped up the loss of soil moisture and the drying of vegetation and watercourses. This is the first drought in Australia where the impact of humaninduced global warming can be clearly observed.

While this may sound intuitively correct, it is wrong. It completely ignores the known science of evapotranspiration and boundary layer meteorology. That is, when soil contains high moisture content, much of the sun’s energy is used in evaporation and consequently there is limited heating of the surface. However, during drought, soil moisture content is low and consequently nearly all of the incoming radiation is converted into heating the surface. The result is that air temperatures rise significantly.

The report confused cause and effect with regard to the fundamental basics of evaporation and boundary layer meteorology. Reduced evaporation causes higher air temperatures. Above all, there was no expertise evident in these claims.

La Niña eventually returned and with it we saw flooding across eastern Australia, the likes of which we had not seen since 1974 (also La Niña). Whilst the floods and cyclone were devastating in their impact, again there was nothing to suggest that their occurrence was anything but normal given the known climatology of eastern Australia. But once again the experts lined up to proclaim that, no doubt, climate change was involved.

Dr Kevin Trenberth, who leads the IPCC science, provided all the inspiration required in a recent paper entitled ‘Framing the climate debate.’ His essential message was that to ask ‘to what degree climate change contributed to an event’ is actually to ask the wrong question. His message is that every event is influenced by climate change.

The implication of this is that because the science cannot answer the question, it doesn’t have to—scientists should just claim everything is a sign of climate change. None too surprisingly, the former Australian government Climate Commission heeded this call which was most clearly advocated in its recent ‘Angry Summer’ report.

It concluded that everything that happened that summer was due in part to climate change. This is the kind of science many might prefer to the real thing—a science where one doesn’t actually have to do anything to justify one’s claims. In reality, Trenberth’s framing of the climate science debate has little to do with science—it is merely advocacy for a catastrophic future outlook.

One very unfortunate event followed just a week after the widespread floods of 2010-11— a paper was published in the journal Nature which claimed to have linked increases in rainfall to anthropogenic climate change. This was heralded across the ABC as a significant result. Expert commentators were sought to evaluate its meaning. To paraphrase, one climate scientist announced that ‘it was published in Nature, so it must be right’, another claimed that ‘we already knew this, so it only confirms what we already thought’.

Such comments could have been as easily made without even bothering to read the paper. No critical analysis was ever provided. The unfortunate timing, coming so soon after the floods, meant that inevitably many scientists were emboldened in linking our floods to increased temperatures and consequently, climate change.

The paper has gone on to enjoy great academic success having been cited more than 300 times in its short period of existence. There is however one rather major problem with the paper—the study never did compare the calculated rainfall probabilities against the corresponding temperature. If they had, they would have noted that there was no correlation at all between the two.

The figure above shows the five year average one-day rainfall probabilities (dark blue line) from 1951 onwards. There is a spike at the end, but no substantive evidence for a consistent trend.

The light blue line shows northern hemisphere temperature anomalies for the same periods. Note that between 1951 and 1980, temperature anomalies were declining, whilst half of the apparent increase in rainfall occurred. There is no correlation between temperature and rainfall intensities. Nor should there be—rainfall processes are far more complicated than being driven simply by temperature.

A more appropriate conclusion of the paper could have been that (i) rainfall intensities are highly variable in time, and (ii) temperature appears not to significantly influence the risk of intense rainfalls. The paper would not have been published in Nature, nor received any of the attention that it got (and still gets).

The reality is that climate is a hugely variable and hopelessly complicated thing, that it is actually meaningless to attempt to identify anthropogenic climate change through climate events like floods or droughts. So if this is the case, how does one test climate change?

The key to testing claims of catastrophic anthropogenic climate change lies in the simplicity of the mechanism itself—atmospheric carbon dioxide concentrations are rising, carbon dioxide is a greenhouse gas absorbing and re-radiating longwave radiation downwards. The radiation is trapped (or at least delayed) meaning that there is more of it around and hence temperatures go up. So the simplest way to test the hypothesis is to measure the planet’s energy balance, in particular downward longwave radiation.

In search of the signal of climate change, a recent NASA study has attempted to do just this. The method itself is rather complex involving multiple satellite remote sensing platforms, radiative modelling, and a whole raft of assumptions and uncertainties. That said, the results are viewed as the most meaningful way to estimate the role and context of carbon dioxide in the atmosphere.

What is immediately apparent from the 24 year record of longwave radiation is that there are large year to year variations. Major positive variations are linked to particularly strong El Niño events. Underlying this variability, one can perceive a relatively minor general increase in longwave flux, however it is small relative to the variability within the series. It is worthwhile to note at this point that atmospheric carbon dioxide rose by approximately 25 per cent over this period—the longwave response to this (the actual mechanism of climate change) appears rather muted if not entirely underwhelming.

Of even greater significance is the shortwave radiation. This is the amount of solar radiation (sunshine) received at the surface and so is primarily influenced by global cloudiness but also dust and aerosols (there is a two to three year decline in shortwave following the 1991 eruption of Mt. Pinatubo).

The fervour with which some speculate following natural climate disasters is in stark contrast to the ability to attribute these events to atmospheric carbon dioxide concentrations. In their frustration, many now claim all events are influenced—we just don’t know by how much.

A more scientific approach is to directly test the hypothesis by directly measuring the proposed mechanism, specifically, the energy balance of the planet. The results to date, whilst only suggestive not definitive, do point to a much more conservative role of carbon dioxide in influencing the energy balance than the climate models predict. Consequently, predictions of catastrophic climate change (above and beyond natural catastrophic climate variability) seem somewhat premature, if not grossly exaggerated. Above all, there is no insight gained by speculating on individual floods, droughts or even their shortterm trends.

I do recall one study some years back that claimed that a species of sheep was becoming smaller as local temperatures rose. One author of the study ruefully commented ‘if climate change continues like this, then one day you’ll be able to fit them in your pocket’. I assume that at least this scientist could see the absurdity of such speculation.

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