[Grad-postdoc-assn] Fwd: News Alert: Mon Feb 2

Maura Hagan hagan at ucar.edu
Mon Feb 2 09:31:46 MST 2009

Hi all-
   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
> 02/01/2009
> 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  
> planet.
> 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  
> tropics.
> 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  
> months.
> Journal reference: Geophysical Research Letters (DOI:  
> 10.1029/2008GL036076, in press)

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