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 user 2006-09-13 at 11:58:00 am Views: 66
  • #16436

    Humans ‘causing stronger storms’

    Increases in hurricane intensity are down to humanity’s greenhouse gas emissions, according to a new analysis.

    Scientists calculate that two-thirds of the recent rise in sea
    temperatures, thought to fuel hurricanes, is down to anthropogenic

    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

    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 the 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

    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