Expert reaction to paper on floods and climate warming

UK/CANADA SMC: Expert reaction to Nature Letters ‘Human contribution to more-intense precipitation extremes’, Seung-Ki Min et al., and ‘Anthropogenic greenhouse gas contribution to flood risk in England and Wales in autumn 2000’, Pardeep Pall et al.

Some initial media coverage of the paper:

Guardian: Climate change doubled likelihood of devastating UK floods of 2000

BBC: Flood risk: ‘higher with warming’

From the UK SMC

Dr Richard Allan, Department of Meteorology, University of Reading, said:

“Observations and theory show that a warming atmosphere carries greater quantities of gaseous water vapour which fuels the most intense rainfall events. So basic physics tells us that rainfall is likely to be more intense in a warmer world and this is backed up by observational evidence and detailed simulations.

“The two studies demonstrate through careful use of observations and detailed simulations of weather patterns, rainfall distributions and river flow, that heavy rainfall events are becoming more intense and liable to cause serious damage through human-induced emissions of greenhouse gases and the associated global warming.

“Exact local changes in rainfall extremes are difficult to predict with certainty since they depend upon small shifts in atmospheric circulation systems.

“However, the two studies demonstrate that a human impact upon the intensification of rainfall and associated flooding is already detectable.”

Professor Roger Pielke Jr. of Environmental Studies at the University of Colorado, said:

“It is exciting to see the application of innovative approaches to connecting the dots between greenhouse gas emissions and damage from extreme events. Pall et al. seek to quantify the increased risk due to greenhouse gas emissions for a particular flood event in England and Wales in 2000. Their methodology extends an approach first applied in the context of the European heat wave of 2003. Wide acceptance of such a methodology will most likely have to await the ability to demonstrate skill in seasonal forecasting 9of the future) that improves upon methods that do not consider the influence of greenhouse gases. This is particularly the case in situations such as flooding in Wales and England, where the authors observe that the region has seen no long-term trends in either flooding or precipitation. Such important research is in its infancy.”

Professor Mark Maslin, Co-Director of the Environment Institute at UCL, said:

“Since we first recognised the threat of global warming scientists have consistently stated that no single extreme weather event can be definitively link to anthropogenic climate change. This is because there has always been extreme weather events and attributing cause of individual events is very difficult. The paper by Pall et al. in Nature fundamentally changes this. By running thousands of model runs they demonstrate that in human induced climate change did have a significant influence on the occurrence of the destructive floods in the UK in year 2000. These floods damaged nearly 10,000 properties and caused an estimated insured damages are over £1.3 billion. But this is small scale compared to the immense damage and loss of life caused by the recent floods in Pakistan, Brazil and Australia. In the same issue of Nature Min et al. has been shown that over two thirds of the extreme rainfall events over the last 50 years were made more intense by human-induced increases in greenhouse gases. These papers taken together provide a very strong scientific message to politicians that the occurrence and magnitude of huge floods around the world have been made worse by anthropogenic climate change, and there we were thinking all the effects of climate change would be in the far future.”

Professor Tim Palmer, Royal Society Research Professor in Climate Physics, Oxford University, said:

“The papers both suggest that observed patterns of increase in extreme precipitation are broadly what we would expect, based on existing climate models forced with increased GHGs. However, there remains considerably uncertainty about the magnitude of future climate change, both regionally and globally, and these results should not be interpreted as implying that the current generation of climate models is good enough. Refining our models in order that we can simulate climate extremes with more fidelity, must be a priority for the future. A lot of this is linked with supercomputing capability and capacity.”

Professor Sir Brian Hoskins, Royal Society Research Professor at the Grantham Institute for Climate Change, and member of the new Committee on Climate Change, said:

“Increases in intense precipitation have been observed in many places, and according to basic physical ideas this would be expected to accompany global warming. For any event or period, such as Autumn 2000 in England and Wales, we have always had to make a rather vague statement that such events are more likely because of global warming but that we cannot say this particular event is because of global warming. These two papers are notable in their inventive use of climate model experiments with the aim of quantifying the link with our greenhouse gas emissions. One paper is able to ascribe much of the general increase in heavy rainfall events to human activity. The other is able to give a firm idea of the increased likelihood of our Autumn floods. However, both studies depend heavily on the accuracy of their computer models. We need to understand better the actual physics of different flooding events and make sure that the models are able to capture this. Model based studies like these should be repeated as models continue to improve.”

From the Canadian SMC:

Our greenhouse gases worsen floods

In less than 50 years, from 1951 to 1999, the intensity of extreme rains and floods increased by 7% in the whole northern hemisphere, according to two Canadian scientists who are trying for the first time to measure the impact of climate warming on the intensity of extreme precipitation.

The surprising increase they have measured comes from observations at over 6000 weather stations and is more than twice the increase predicted by climate modeling. The study will be published on Wednesday in the journal Nature.

Two of the researchers, Francis Zwiers of the University of Victoria and Xuebin Zhang of Environment Canada, show that our emissions of greenhouse gasses already have a worsening effect on flooding and extreme rains. Even though they do not have equivalent weather data for the southern hemisphere, they believe that the same increase in precipitation affects the entire planet.

In North America, says Dr. Zwiers, “precipitation extremes correspond to the El Nino effect in pretty characteristic ways, where some regions get heavy rainfall while others receive less extreme precipitation. But we don’t see these spatial variations in our study and our models don’t generate that kind of spatial structure either. The evidence is leading us in another direction, to a phenomena that influences precipitations in a global scale… and the only thing we can think of is the changing composition of the atmosphere: warmer air contains more moisture and leads to more extreme precipitation.”

“The eleven climate models we used show a significant average increase of extreme precipitation in response to greenhouse gases, but their prediction is much smaller than our measurements. Observations show that extreme daily rains increased by 7.1 per cent while five day maxima increased by 6.4 per cent. Our models show smaller increases of 2.9 per cent and 2.7 per cent. Models are clearly underestimating the impact of warming climate on extreme precipitation.”

In his opinion, current models don’t give an exact image of precipitation because they look at areas of about 100 km by 100 km. This doesn’t let them look at stormclouds or the relief of the local terrain. “Rain is a much more local phenomena than temperature, and we will need much more sophistication in our models to make precise predictions,” says Dr. Zwiers. A surprise is that the models that give the most divergent results are those who also try and take into account variations in solar intensity and the effects of volcanic emissions. “It may be that those models don’t give a good simulation of the consequence of a decrease in solar radiation. There may be some missing physics there.

Dr. Zwiers estimates that Quebec has been more badly hit by extreme weather than other regions during the last few years: “The Saguenay Flood and the ice storm are both examples of extreme precipitation,” he says. “Quebec is a very rich place, hydrologically, and receives a lot of moisture.”

One of the authors, Dr Xuebin Zhang, Research Scientist with Environment Canada, provides the following comments:

What did you and your co-authors find?

“Our research provides the first scientific evidence that human-induced greenhouse gas increases have contributed to the observed intensification of heavy precipitation events over large parts of the Northern Hemisphere, including North America, mid-latitude Eurasia, and India.”

Why is it significant?

“We know that weather and climate extreme events have potentially devastating effects on human society and the economy. Understanding past changes in the characteristics of these events is critical for reliable prediction of future changes. This type of information is needed to support credible science and effective adaptation decision-making.”