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Extreme Events Mean Nothing Without Uncertainties

Dr David Whitehouse

For many years it seems to have been the desire of some scientists to look at a series of extreme events and say they are directly due to global warming. It would be an important piece of observational evidence as climate theories suggest that as the temperature increases so does the moisture carrying capacity of the atmosphere. This would increase the amount of precipitation.

However, it has always struck me as poor logic to say that while any individual extreme weather event such as a heat wave or severe precipitation cannot be attributed to the effects of global warming, when taken as a whole they demonstrate a trend that is consistent with predictions as theories say their likelihood increases in a warming world.

Stated like this, it is logical nonsense. If no single extreme event in an ensemble is due to a warming world then when taken together they cannot be taken as evidence for the contrary. Perhaps then only some of them are due to global warming and the others are what would have happened anyway? But which ones? What distinguishes the two groups? Such questions bring out the absurdity in this logic.

Weather is complex and variable and so is climate. Weather is influenced by large amplitude short-term events and has lots of ‘noise.’ But then so does climate, long-term trends, noise and so-called decadal fluctuations all influence it as well as the weaker anthropogenic signal. If anyone doubts the natural fluctuations in the climate just consider what the global temperature has been doing in the past decade, and that in a world that is ‘getting warmer’ most global temperature databases give 1998, twelve years ago, as the warmest year on record (due to a natural fluctuation, an El Nino.)

Reporting of extreme events has never been easier. Our electronic world is wired for catastrophe. Events that just a few decades ago would have been poorly reported, if at all, with sketchy information, witnessed by victims but not by scientists, are now scrutinised by flotillas of satellites, ground stations and reporters sending back instant information via the internet. Great care must be taken with such a severe selection effect. We know about the recent floods and heat waves in far more detail than we do of past events.

Two papers in Nature on the relationship between extreme events and man-made global warming have attracted a lot of attention. They have been uncritically described in some quarters as being the first time a clear link between global warming and extreme precipitation.

One of them, by Seung-Ki Min et al links rising CO2 levels to the intensification of rain in the Northern Hemisphere finding a result that has been reported as being worse than the models predicted. The researchers say that nothing can explain their results except the slow steady rise in temperatures caused by greenhouse gasses.

In the other Pall et al looks at the 2000 severe flooding event in the UK and its relationship to global warming. They use a series of simulations of the weather in 2000 with and without an increase in temperature caused by global warming and they conclude that global warming had an effect. One’s confidence is a little dented by the fact that the simulations are based on seasonal forecast simulations made by Met Office scientists – the kind that were withdrawn from circulation to the public because their either were not accurate of because the public couldn’t understand them, depending upon your point of view.

Not Proof

However, being consistent is far from proof. The idea that something is consistent with a theory is a scientific statement of very limited usefulness that many media commentators and reporters have taken at face value. Is it really a discovery to be reported with no caveats, as many media outlets did, if the majority of a series of computer simulations cannot reproduce real world data without greenhouse gas warming. Remember these are computer models with all the limitations that implies. And remember also this is the climate we are talking about, with all its unknowns an unpredictability.

Looking in detail at this research paper there are uncertainties in the modelling, the observations, their reduction and knowledge of other factors that might influence the climate in the same way.

But reading the abstract of the papers give no hint of the true nature of these uncertainties. This is Seung-Ki Min et al’s abstract’

“Extremes of weather and climate can have devastating effects on human society and the environment. Understanding past changes in the characteristics of such events, including recent increases in the intensity of heavy precipitation events over a large part of the Northern Hemisphere land area is critical for reliable projections of future changes. Given that atmospheric water-holding capacity is expected to increase roughly exponentially with temperature—and that atmospheric water content is increasing in accord with this theoretical expectation — it has been suggested that human-influenced global warming may be partly responsible for increases in heavy precipitation. Because of the limited availability of daily observations, however, most previous studies have examined only the potential detectability of changes in extreme precipitation through model–model comparisons. Here we show that human-induced increases in greenhouse gases have contributed to the observed intensification of heavy precipitation events found over approximately two-thirds of data-covered parts of Northern Hemisphere land areas. These results are based on a comparison of observed and multi-model simulated changes in extreme precipitation over the latter half of the twentieth century analysed with an optimal fingerprinting technique. Changes in extreme precipitation projected by models, and thus the impacts of future changes in extreme precipitation, may be underestimated because models seem to underestimate the observed increase in heavy precipitation with warming.”

The paper by Pall et al is little better;

“Interest in attributing the risk of damaging weather-related events to anthropogenic climate change is increasing. Yet climate models used to study the attribution problem typically do not resolve the weather systems associated with damaging events such as the UK floods of October and November 2000. Occurring during the wettest autumn in England and Wales since records began in 1766, these floods damaged nearly 10,000 properties across that region, disrupted services severely, and caused insured losses estimated at £1.3 billion. Although the flooding was deemed a ‘wake-up call’ to the impacts of climate change at the time, such claims are typically supported only by general thermodynamic arguments that suggest increased extreme precipitation under global warming, but fail to account fully for the complex hydrometeorology associated with flooding. Here we present a multi-step, physically based ‘probabilistic event attribution’ framework showing that it is very likely that global anthropogenic greenhouse gas emissions substantially increased the risk of flood occurrence in England and Wales in autumn 2000. Using publicly volunteered distributed computing we generate several thousand seasonal-forecast-resolution climate model simulations of autumn 2000 weather, both under realistic conditions, and under conditions as they might have been had these greenhouse gas emissions and the resulting large-scale warming never occurred. Results are fed into a precipitation-runoff model that is used to simulate severe daily river runoff events in England and Wales (proxy indicators of flood events). The precise magnitude of the anthropogenic contribution remains uncertain, but in nine out of ten cases our model results indicate that twentieth-century anthropogenic greenhouse gas emissions increased the risk of floods occurring in England and Wales in autumn 2000 by more than 20%, and in two out of three cases by more than 90%.”

The abstract is, of course, a technical shorthand summary of the paper’s findings written for a scientific audience who should be well aware of the associated caveats. Some journalists and commentators however are not. Abstracts are almost always restricted in length by the journal concerned so it could be argued that there is no space for equivocation. Besides journals want definite results and don’t want them diluted (in the abstract at least) with ifs and buts.

This is a problem because even in scientific circles when reviews of research are conducted positive statements are lifted from abstracts and passed on with none of the accompanying qualifications to be found in the depths of the paper. We have seen this in the way the IPCC gathered data and summarised it. Positive results are selected, uncertainties suppressed and then the results are amplified and simplified to produce a false certainty devoid of uncertainty.

No scientific statement based on observational data, let alone based on a series of computer ‘simulations,’ should be made without a statement of uncertainty, and this includes the abstract of research papers. If the journal in question thinks it makes the abstract to uncertain, too long, or just isn’t house style then it should change such things.

Likewise when preparing press releases based on research papers more care should be given to the uncertainties. This is Nature’s press release;

“Anthropogenic greenhouse gasses have significantly increased the probability of heavy precipitation and local flood risk, report two papers in Nature this week. The findings are among the first formal identification of human contribution to extreme hydrological events. It has previously been suggested that human-induced global warming may be partly responsible for increases in heavy precipitation. However, because of the limited availability of daily observations, most studies to date have only examined the potential detectability of changes in precipitation through model-model comparisons. Francis Zwiers and colleagues studies rainfall from 1951 – 1999 in Northern Hemisphere land areas, including North America and Eurasia (including India). They show that human-induced increases in greenhouse gasses have contributed to the observed intensification of heavy-precipitation events found in approximately two-thirds of data-covered parts of Northern Hemisphere land areas.”

The problem is that the results of scientific research enter the canon of commentators and policy makers without their associated uncertainties. In may cases such uncertainties are forgotten altogether and anyone who subsequently raises them is called a denier, rather than what they are which is being scientific.