Human Activity 'causing stronger storms'
Pastor Dale Morgan
*Human Activity 'causing stronger storms'
*
By Richard Black
Environment correspondent, BBC News website
Increases in hurricane intensity are down to humanity's greenhouse gas
emissions, according to new analysis.
Scientists calculate that two-thirds of the recent rise in sea
temperatures, thought to fuel hurricanes, is down to anthropogenic
emissions.
Research published last year found there had been a sharp rise in the
incidence of category 4 and 5 storms - the strongest - in recent decades.
But other scientists caution there may be errors in historical storm
records.
Hurricane formation is strongly linked to sea surface temperature, with
warmer waters more likely to form storms.
Temperature increases in these hurricane breeding grounds cannot be
explained by natural processes alone
Tom Wigley
Sea surface temperature and hurricane strength vary naturally, and
deciphering a clear impact of human greenhouse gas emissions has been
difficult.
However, the last two years have seen several major pieces of research
which have at least increased understanding of the issue, without
settling it conclusively.
Peak of intensity
In July last year, Kerry Emanuel, from Massachusetts Institute of
Technology, published research showing that the duration, maximum wind
speeds and energy released in tropical storms has increased markedly in
both the North Atlantic and the North Pacific oceans since the mid-1970s.
A few months later Peter Webster from the Georgia Institute of
Technology documented a rise in the incidence of category 4 and 5
storms; the 15-year period from 1975 to 1989 saw 171 severe hurricanes,
but the number rose to 269 for the subsequent 15 years.
He told the BBC News website at the time: "What I think we can say is
that the increase in intensity is probably accounted for by the increase
in sea surface temperature, and I think probably the sea surface
temperature increase is a manifestation of global warming."
Then in June this year, Kevin Trenberth of the US National Center for
Atmospheric Research analysed the exceptionally active 2005 North
Atlantic hurricane season.
Sea surface temperatures (SST) had been 0.9C above the long-term
average, he found; and by comparing the North Atlantic with other
regions of the ocean, he deduced that human greenhouse gas emissions
accounted for about half of this rise.
The latest research takes things another step further, using 22 computer
models of climate to examine a possible link between SSTs and
human-induced global warming.
These models typically deal in projections and probabilities, which is
inevitable with a huge and chaotic system such as global climate.
Benjamin Santer, Tom Wigley and colleagues conclude: "There is an 84%
chance that external forcing [human activities] explain at least 67% of
the observed SST increases" in the Pacific and Atlantic zones where
hurricanes form.
"The important conclusion is that the observed SST increases in these
hurricane breeding grounds cannot be explained by natural processes
alone," said Dr Wigley.
"The best explanation for these changes has to include a large human
influence."
Stormy times
On a political level, the debate over hurricane intensity has become a
poster child for climate sceptics.
They object to what they regard as overblown, opportunistic links made
by some commentators between the devastation of Hurricane Katrina, in
particular, and rising greenhouse gas concentrations.
Their arguments are given scientific underpinning by problems which some
researchers have identified in historical records of storms.
Historically a storm was given a certain strength, and then it's
re-analysed and it comes out as having been much stronger
Julian Heming
Satellite observations date back only about 35 years, and even then
there are issues of calibration; were early instruments measuring things
in precisely the same way as their successors? Before that, researchers
have to rely on written records from observations on land and at sea.
"I've seen examples where historically a storm was given a certain
strength, and then it's re-analysed and it comes out as having been much
stronger," commented Julian Heming, tropical prediction scientist with
the UK Meteorological Office.
"And even if we take it as read that there is an increase in the
baseline of sea surface temperature, there are complexities in the way
that cyclone formation reacts to that," he told the BBC News website.
"The hurricane now approaching Bermuda [Florence] struggled and
struggled to get to hurricane intensity; so there will be year-to-year
differences, and even storm-to-storm differences."
Sceptical observers also maintain that computer models are far from perfect.
But as Benjamin Santer and Tom Wigley point out, what other tools are
there for projecting the future?
"In the real world, we are performing an unprecedented and uncontrolled
geophysical experiment," they write. "We know, beyond a shadow of doubt,
that [human] activities have changed the chemical composition of the
Earth's atmosphere.
"In a post-Katrina world, we need to do the best job we possibly can to
understand the complex influences on hurricane intensity."