[Grad-postdoc-assn] Fwd: News Alert: Mon Feb 2
hagan at ucar.edu
Mon Feb 2 09:31:46 MST 2009
I thought that you'd be interested in this article highlighting one
Jian Lu's recent pubs. Best regards.
p.s. Congratulations, Jian!
Begin forwarded message:
> Drought warning as the tropics expand
> New Scientist - Online
> Return to Top
> California's governor, Arnold Schwarzenegger, warned on Thursday
> that his state 'is headed toward one of the worst water crises in
> its history'.
> Now new research suggests that the three-year drought in the Golden
> State may be a consequence of the expanding tropics, which are
> gradually growing as human emissions of greenhouse gases warm the
> Climate scientists have documented a slow progression of low-
> latitude weather systems towards the poles, and this has been
> matched by rising temperatures in many temperate regions. Deciding
> whether this broadening of the tropical belt is linked to the
> greenhouse effect has been difficult, however.
> Part of the reason, explains Thomas Reichler of the University of
> Utah, is that there are many ways of defining the tropics.
> Geographically, the tropical belt is contained between the Tropics
> of Cancer and Capricorn. It is also the region on either side of the
> equator where temperatures tend to be hot and humid all year.
> Where weather forms
> But the simplest and most easily tracked characteristic of the
> tropics lies high above, at the boundary between the troposphere,
> where weather systems form, and the stratosphere above it.
> Over the tropics, the tropopause, as this boundary is known, tends
> to lie several kilometres higher up in the atmosphere. The change in
> altitude is relatively easy to measure. 'It is much more difficult
> to detect significant changes in the lower levels of the atmosphere
> and surface rainfall pattern,' says Jian Lu of the US National
> Center for Atmospheric Research in Colorado.
> Creeping outwards
> Using the tropopause, Lu and Reichler tracked the position of the
> tropical belt since the 1960s and found it has slowly been getting
> wider. 'There is a lot of natural variation from year to year,' says
> Reichler, 'but we see a slow, gradual change.' On average, the
> tropical boundaries are moving 0.7 degrees towards the poles each
> decade. This amounts to roughly 70 kilometres per decade, or 350
> kilometres in 50 years.
> The team then plugged their data into a leading climate model. If
> the model included human emissions, it matched the real data.
> Without the emissions, it didn't.
> 'Our main conclusion is that greenhouse gases and [the depletion of
> stratospheric] ozone are the culprits for the widening,' says Lu.
> 'These two work in the same direction, both pushing the boundary of
> the tropics polewards.'
> Subtropical deserts
> Reichler says that the expansion of the subtropics is more feared
> than the widening of the tropical zone itself. While the tropical
> belt is hot and humid, the subtropics suffer from severe drought.
> The Sahara and Sahel are both subtropical regions.
> The climate models quoted by the Intergovernmental Panel on Climate
> Change predict that the Mediterranean region and the south-west of
> the US are heading towards devastating droughts. Reichler says this
> latest study suggests this is a result of the poleward march of the
> Southern California is already subtropical in the summer. But with
> climate change, dry conditions could spread to areas like northern
> California, Washington and Utah, which now get far more rain and snow.
> According to a recent survey by the California Department of Water
> Resources, the snowpack on California's mountains is currently
> carrying only 61% of the water of normal years. The Sierra snowpack
> alone provides two-thirds of California's water supply, and these
> mountains have so far only received one-third of the expected annual
> snowfall, despite December and January normally being the wettest
> Journal reference: Geophysical Research Letters (DOI:
> 10.1029/2008GL036076, in press)
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