David Shukman / BBC World News – 2012-11-05 11:00:03
(November 2, 2012) — The floodwaters whipped up by Hurricane Sandy have not yet receded but the temperature is rising on one of the toughest questions in modern science: whether we’re getting more extreme weather because of global warming.
Radical film-maker Michael Moore put it with characteristic bluntness. In a Tweet, he wrote: “Stop w/ the disaster porn and tell the America people the bitter truth: We have f***** up the environment & we are now paying the price.”
The governor of New York state, Andrew Cuomo, expressed it more politely: “Anyone who thinks there isn’t a change in weather patterns is denying reality.”
And to widespread surprise, New York Mayor Michael Bloomberg also made a link between Hurricane Sandy and global warming, though more guardedly.
“Our climate is changing,” he said in a statement last night, remarkable in itself in the context of a presidential election year in which the word “climate” did not get a mention in any of the contenders’ debates.
Mr Bloomberg did not seek to pin any direct blame on climate change — in fact what he said actually reflects the current of the science rather accurately.
He said the “increase in extreme weather we have experienced in New York City and around the world may or may not be the result of [climate change].”
But “the risk that it might be — given this week’s devastation — should compel all elected leaders to take immediate action.”
The question is one of risk, not of certainty – the risk that the continuing rise in greenhouse gases from human activities may exacerbate extreme weather.
To go further, as many environmental campaigners would like to — to suggest that the violence of Hurricane Sandy is the result of global warming — is to strain what scientists themselves are able to conclude.
At face value it looks obvious: the basic ingredient for a tropical storm is a sea surface temperature above 26C (79F) and, with the oceans known to be warming, that essential condition may occur more often.
But many other factors come into play with the development of tropical storms — foremost among them is a phenomenon known as “wind shear”, which can kill off storms before they become threatening.
To say that more warming means more storms is to oversimplify a highly complex situation — and attract a barrage of criticism for unjustified green “alarmism”.
The perspective of the UK Met Office – which prides itself on tropical storm forecasts — is instructive for the degree of its caution.
For a start, the view is that the most accurate record of hurricanes – essential for any comparison — only stretches back to the start of the satellite era in the late 1970s.
Before then, there is no way of knowing whether storms which developed at sea then stayed out at sea and grew or died unseen and unrecorded. So the exact frequency and power of ALL tropical storms is only known for 30 years or so — too short a period, say Met Office scientists, to form a proper judgment.
What matters they say are the strength, frequency and duration of storms, which they measure with an Accumulated Cyclone Energy Index. And so far no trends are discernible, apparently.
Degrees of Uncertainty
Second, although there are new techniques for attributing the role of climate change in weather events, this is an emerging area of science fraught with uncertainty.
Recent studies have run computer models of particular weather events — with and without the factor of man-made greenhouse gases — and concluded that they were made more-or-less likely as a result.
But these have tended to be events involving either heavy rainfall or high temperatures, which – in meteorological terms — are relatively straightforward compared to the complicated swirl of components in a tropical storm.
Sandy’s growth and journey up the Atlantic; the storm’s sudden turn West to the coast after encountering an Arctic high-pressure zone, the collision with a cold weather system — all this is extremely challenging to unpick, and its doubtful whether the science, as it stands, could tackle it rapidly.
Third, the models used to look ahead to climate change throw up a confusing set of results when it comes to hurricanes.
The posters promoting Al Gore’s move “An Inconvenient Truth” showed smoke from an industrial chimney rising into the spiral of a hurricane. But the latest research does not really support that image.
The most recent study into extreme weather, by the Intergovernmental Panel on Climate Change (IPCC), suggested that the most violent storms might become even stronger but that the overall number of hurricanes might actually diminish.
As a Met Office spokesman put it: “This is very long way of saying ‘we don’t know’.”
According to one prominent expert on disasters, Roger Pielke junior from the University of Colorado, the lesson from Hurricane Sandy is less one about climate change and more one of the need for proper preparedness.
“There are more people and more wealth in harm’s way,” he wrote in a newspaper article, and that is “mostly to the simple fact that people like being on the coast and near rivers.”
By his calculations, if Hurricane Sandy ends up costing US $20bn (Â£12bn), it would rank only 17th out of 242 storms to hit the US since 1990.
And he says the last storm to hit the US rated as a category three hurricane or higher — Sandy was just below a category one when it hit the East Coast – was back in 2005.
Dr Pielke acknowledges that “humans do affect the climate system” but he argues that there is no evidence that this can yet be blamed for recent disasters. More important, he says, is to focus on land use, protection and forecasting.
Others will point to counter-arguments:
* rising sea-levels gradually increases the risk of coastal flooding — true but over a timescale of decades;
*the record melt of sea-ice in the Arctic during this summer possibly changed the path of the jetstream and therefore the weather patterns – but the science on this is in its infancy;
*and that the warming of the atmosphere allows it to hold more moisture and therefore deliver more rain – though in the case of Hurricane Sandy, the impact was through wind and the storm surge rather than precipitation.
As the battered communities of the US East Coast try to rebuild their lives, the scholarly arguments about the cause of their misery is not likely to be uppermost in their minds.
But it’s brought into much sharper focus one of the hardest questions about climate change: what can the science reliably tell us about what global warming really means for each of us, not in the future, but here and now?
David Shukman is the BBC’s Science editor.
Posted in accordance with Title 17, Section 107, US Code, for noncommercial, educational purposes.