February 23, 2014

expert reaction to climate change and winter deaths

Nature Climate Change published research showing socio-economic and health-care conditions factors in England and Wales may be a better explanation for the decreasing numbers of winter deaths than milder winters due to climate change. A before the headlines analysis accompanied these comments.

Explanatory comment from the lead author of the paper:

Dr Philip Staddon, Research Fellow in Climate Change, Sustainability and Human Health, University of Exeter Medical School, said:

“To clarify the thrust of this paper, we are interested in explaining the year to year variation in excess winter deaths – not the daily variation – and we are not saying that temperature does not play a role, if it didn’t there would be no excess winter deaths. What we aim to demonstrate is that how harsh a winter is no longer predicts how many excess winter deaths there will be.

“An extensive literature attests to the fact that changes in daily temperature influence health outcomes at the local levels, and that EWDs are influenced by temperature. However, our data suggest that year to year variation in EWDs is no longer explained by the year to year variation in winter temperature: winter temperatures now contribute little to the yearly variation in excess winter mortality so that milder winters resulting from climate change are unlikely to offer a winter health dividend.

“And the reasons behind the apparent contradiction (strong daily temp link, but not very strong whole winter temp link):  First, let us reiterate the association between cold temperatures and increased deaths is not in question; that is a fact and explains the existence of the EWD phenomenon. Clearly more research must be carried to identify why this apparent contradiction is occurring. However, we hypothesise that this can occur through the following mechanisms: (1) help to the most vulnerable people is becoming more targeted (this is certainly true in many parts of the UK, e.g. Cornwall Council’s Winter Wellbeing activities); (2) it’s not low temperature per se but rather the drop in temperature (after the first day of cold, many people are prepared for the following days – this explains why cold spells in early winter often lead to greater mortality than similarly cold but later spells); (3) increased temperature volatility, making cold spells more unpredictable, catches people off guard (this is part of the explanation as to why European with warmer winters such as the UK, Portugal, Italy suffer greater EWDs than Northern Europe). These ideas are briefly discussed in the latter part of the paper.”

Commenting on the statistics:

Prof Kevin McConway, Professor of Applied Statistics, The Open University, said:

“This paper makes some very interesting points, but the research that it reports does not really support the title of the paper. If it had been called “Climate warming may not reduce winter mortality”, I’d have been happier.

“Statisticians like quoting a few mantras, and this study breaks two of them.

“First, “Correlation in not causation”. The research is entirely based on correlations – situations where one quantity (say, excess winter deaths) goes up and down roughly in step with another quantity (say, flu activity). But you can’t conclude from such data that the changes in one quantity are causing the changes in the other one. There are always other possible explanations – maybe some other quantity, that wasn’t included in the study, is independently causing changes in flu activity and excess winter deaths. A study of correlation can indicate interesting things to investigate in researching what causes what, but it can’t show what is causing what.

“The second mantra, which isn’t confined to statisticians, is “You can’t prove a negative”. The paper’s title sounds as if the researchers have proved a negative. But all they have really shown is that it’s possible that climate warning won’t decrease winter mortality. In the paper itself, for instance, the researchers point out that climate change may lead to winters where the temperatures vary much more than they did in the past. They speculate that this increased variability might increase winter deaths – but, although that’s very plausible, they do not really have data to support it. Maybe this change will work alongside others so that winter deaths increase – or maybe they will decrease, or remain unchanged. We still don’t really know which.

“It’s good that these researchers have pointed out that winter deaths are related to more than just cold temperatures. But their work has made it clear the prediction of future winter deaths is difficult, because of all the factors potentially involved, rather than establishing for certain what the effect of climate change will be.”

Prof Patrick Wolfe, Professor of Statistics, UCL, said:

“It’s important to remember that no amount of data analysis can prove a negative.  The authors of this study are attempting to isolate the effects of the number of cold winter days on additional deaths during wintertime – and they rightly point out that a number of potential factors are at play, including improvements in housing and healthcare from the 1950s onward.

“But I think it’s very difficult to extract the signal from the noise in this situation, given the data the authors are working with relative to the problem they are tackling: a long-term decline in the incidence of excess winter deaths relative to the UK population aged 65 and above, and a number of potential factors that have shifted and varied over these same long-term time scales.

“And of course, just because the authors found no evidence in the data they considered does not mean for certain that no evidence exists. In light of these facts, I’d interpret this study as evidence that the association between cold winter days and excess winter deaths is worth a closer look.  A finer granularity of data for recent years (concomitant with the declining incidence of excess winter deaths) might enable to enable a more comprehensive analysis, and help to tease out any potentially spurious associations.”

Commenting on the influenza aspect:

Dr Michael Skinner, Senior Lecturer in Virology, Imperial College London, said:

“As a virologist I cannot argue with the conclusion in the paper that “particular attention should also be paid to public health initiatives to reduce the risk of infection with flu-like illnesses”, not because of the findings of this study about winter temperatures but because of what we have long known about influenza virus variability – in fact most of the influenza virus peaks shown here correspond to pandemic shifts or major seasonal influenza drifts in antigenicity. We would expect the impact of such outbreaks on the incidence of EWDs to be far higher than any of the complicated temperature-dependent changes on normal, seasonal influenza virus transmission or susceptibility.

“The authors say “Improving [inactivated influenza vaccine] uptake in the over 65s would be very beneficial”; although I cannot disagree with the sentiment, as a virologist I cannot see that this paper makes a significant contribution to the debate about how best to protect the vulnerable from seasonal influenza, and the paper they cite on this point is not intended to be an authoritative investigation of that particular issue.”

Dr Ben Neuman, Virologist, University of Reading, said:

“Flu viruses are unwelcome and sometimes deadly winter guests. These findings show that while better housing, cheaper heating and government policy may have reduced winter deaths, flu-related deaths are the next challenge we need to meet.  Modern flu vaccines are cheaper and more effective than ever. This study reinforces the importance of vaccination programmes, especially in elderly people who are most at risk of severe disease.”

‘Climate warming will not decrease winter mortality’ by Philip L. Staddon et al. published in Nature Climate Change on Sunday 23 February 2014