Current warming and likely future impacts - Climate Change: Biological and Human Aspects - page 299

the number and proportion of the most powerful hurricanes have risen. In this precise
sense the two reports disagree: although there was agreement that climate change was
bringing more hurricane destruction, whether it was frequency and/or intensity of the
more intense events was unclear. And so the first years of the 21st century saw much
debate about the exact link between climate change and the nature and/or frequency
of hurricanes even though it was agreed that the frequency spectrum of hurricanes in
a given year was shifting in the direction of greater intensity. Things were to become
a little clearer in 2008 when Jason Elsner, James Kossin and Thomas Jagger had their
analysis of 1981-2006 Atlantic tropical hurricanes published. While previous studies
had looked at hurricanes' average strength, this new work looked at the maximum
strengths reached by hurricanes. Here they found that in 1981-2006 the maximum
strength of hurricanes was increasing and suggested that as seas warm the ocean has
more energy to convert to tropical cyclone wind. This conclusion appeared in the
2009 USGCRP report.
Both the 2001 and 2009 USGCRP reports looked at each region. Alaska, with
its ecologically important Arctic tundra and sub-Arctic environments, is likely to
see much change in its wildlife conservation and biodiversity concerns. This applies
to much of Canada too. Elsewhere in the USA the question largely becomes one
of having to manage change. However, in some places the change is not one of
movement along a broad ecotone (a gradual environmental gradient, such as a hill)
but catastrophic change on a comparatively isolated ecosystem type. For example,
the encroachment of salt water due to sea-level rise on the coastal forests around the
Gulf of Mexico has led to trees dying. This problem is particularly acute in parts of
south Florida and Louisiana, with the creation of dead or dying areas called ghost
forests. In other areas there is the problem of increased summer heat which, despite
increased rainfall, is likely to lead to greater aridity that will raise the likelihood of
wildfires. This again is likely to be of particular concern to those in the south-east
USA. The reports' regional conclusions are summarised in the following paragraphs.
The midwest, which contains Chicago, the nation's rail hub, is likely to see trans-
portation problems due to extensive flooding, which might in turn result in nutrient
enrichment (eutrophication) of aquatic systems. Ironically, despite increased rainfall
(much of which will be in seasonal bursts), it is anticipated that there will be a
reduction in river and lake levels. These in turn will result in transport impacts as
docks, locks and associated navigation structures will require re-engineering. The
reports cite the anticipated reduction of water levels in the Great Lakes and the 2009
report forecasts that by the end of the century, under IPCC Business-as-Usual (B-a-U)
scenarios, heatwaves will occur ever other year.
The west, the USGCRP reports conclude, will continue to see an increase in pre-
cipitation, as it has already done during the 20th century. In some parts river flow
may double by the end of the 21st century. However, some parts of the west are arid
and it is not clear whether increased temperature would offset any extra precipita-
tion. Droughts are likely to increase; a conclusion in line with palaeoclimatological
evidence.
The Pacific north-west is likely to see far greater temperature changes, at 4-4.5 ◦ C
by the end of the 21st century, than the 1.5 ◦ C rise experienced in the 20th century.
Rainfall across the region is likely to increase, although decreases in some areas