climate change & science

[Paul Paryski]

FYI  Paul      
Science 10 August 2007:
Vol. 317. no. 5839,  pp. 746 - 747
DOI: 10.1126/science.317.5839.746  

News Focus
 
CLIMATE CHANGE:
Humans and  Nature Duel Over the Next Decade's Climate
Richard A. Kerr  
Rising greenhouse gases are changing global climate, but during the next few  
decades natural climate variations will have a say as well, so researchers
are  scrambling to factor them in

 (http://www.sciencemag.org/cgi/content/full/317/5839/746/F1)  CREDIT: JASON
EDWARDS/NATIONAL  GEOGRAPHIC
For a century or more, meteorologists have  known the secret to weather
forecasting: To glimpse tomorrow's weather, one must  know today's. And lately they
have realized that the same precept applies to  predicting climate years or
decades ahead. Stirrings in the North Atlantic Ocean  today that have nothing
to do with the strengthening greenhouse--just natural  jostlings of the climate
system--could lead to drought in Africa's Sahel in a  decade or two, they
recognized. Ignore today's ocean conditions, and your 2020  global-warming
forecast could be a bust. And such natural variability can be  far-reaching. In a
recent study, researchers found that when the Atlantic Ocean  swung from one
state to another, it apparently helped trigger a decade-long  climate shift in
the late 1960s that sprang from the Atlantic and reached as far  as Australia.  
But until now, climate forecasters who worry about what greenhouse gases  
could be doing to climate have ignored what's happening naturally. Most looked  
100 years ahead, far enough so that they could safely ignore what's happening  
now. No more. In this week's issue, researchers take their first stab at  
forecasting climate a decade ahead with current conditions in mind. The result  is
a bit disquieting. Natural climate variability driven by the ocean appears to
 have held greenhouse warming at bay the past few years, but the warming,  
according to the forecast, should come roaring back before the end of the  
decade.  
"This is a very valuable step forward," says meteorologist Rowan Sutton of  
the University of Reading, U.K. "It's precisely on the decadal time scale and
on  regional scales that natural variability and anthropogenic effects have  
comparable magnitudes." So improved climate forecasting of the next few decades  
could help decision-makers focus on where and when the most severe climate  
change will be happening. Or, conversely, they could recognize when the looming
 threat of global warming will be masked--temporarily--by natural
variability.  
Jiggly climate
No one ever said Earth's atmosphere was a  boring place. Air is in
continually shifting motion, from the wafting of  innumerable summer breezes to a few
roaring jet streams. But forecasters have  long recognized that certain parts of
the chaotic atmosphere are better behaved  than others. Over the North
Atlantic, for example, atmospheric pressure over  Iceland and Portugal tends to
"seesaw" over the weeks and months, rising at one  site while it falls at another.
This North Atlantic Oscillation (NAO) in turn  switches winds to and fro
across the Atlantic, guiding storms into or away from  western Europe. Other modes
of natural variability--atmospheric jigglings that  lack an external cause
such as added greenhouse gases--tend to cause atmospheric  reorganizations over
the North Pacific and the high latitudes of both  hemispheres. The tropical
warmings and coolings of the El Ni?o-La Ni?a cycle can  also hold sway in
various regions around the globe.  

 (http://www.sciencemag.org/cgi/content/full/317/5839/746/F2) Better. A model
starting from  current conditions (white) came closer to reality (black) than
one  without (blue).  
SOURCE: D. M. SMITH ET  AL./SCIENCE

Once  meteorologists recognized that natural variability offered hope of
predicting  out a few months, climate researchers began to see that the same or
similar  modes might improve forecasting a decade or more ahead. On a regional
scale, the  NAO seesaws over the decades as well. Its dramatic strengthening in
winter  between the 1960s and 1990s pumped extra heat into Northern Europe on
top of  greenhouse warming, according to a new analysis in press at the
Journal of  Geophysical Research by climate researcher David Parker of the Hadley  
Centre for Climate Prediction and Research in Exeter, U.K., and his
colleagues.  On a broader scale, natural variability over decades is clearly rooted in
the  oceans. A warm-cool cycle that spans the Pacific, both North and South,
has  lately swung back and forth on a time scale of 30 to 50 years. By Parker
and his  colleagues' data and model analysis, this so-called Interdecadal
Pacific  Oscillation seems to be driven by interactions between the tropical ocean
and  atmosphere much like those that drive El Ni?o; the IPO could be the
multidecadal  expression of the El Ni?o cycle, they say.  
Over in the Atlantic, there's the Atlantic Multidecadal Oscillation (AMO) of  
sea surface temperature. It is apparently driven by the acceleration and
slowing  of the great ocean conveyor that carries warm surface water into the
northern  North Atlantic (Science, 1 July 2005, p. _41_
(http://www.sciencemag.org/cgi/content/full/309/5731/41) ). The AMO's vacillations have been linked to  
everything from triggering drought in the Sahel and the central United States
to  alternately suppressing and--in the past decade--firing up hurricanes  
(Science, 10 November 2006, p. _910_
(http://www.sciencemag.org/cgi/content/full/314/5801/910) ).  
A global reach
Lately, researchers are finding that the  AMO may have a stronger influence
and a longer reach than they once thought.  They knew that the oscillation
affected climate around the Atlantic, but some  suspected it had also caused a
mid-century warming of the Northern Hemisphere or  even the globe.  
This past January in Geophysical Research Letters, climate modeler  Rong
Zhang and colleagues at the Geophysical Fluid Dynamics Laboratory in  Princeton,
New Jersey, showed how the AMO might have warmed at least the one  hemisphere.
They varied the warmth of the North Atlantic in their model to mimic  the way
the temperature of the real North Atlantic varied under the AMO during  the
20th century. In the model, the Northern Hemisphere warmed to midcentury and  
then cooled slightly through the 1950s and 1960s, as it did in the real world.  

 (http://www.sciencemag.org/cgi/content/full/317/5839/746/F3) Doin' the
shift. All sorts of  regional climate--from African rainfall to hurricane
activity--changed  in the late 1960s, especially around the Atlantic.  
SOURCE: P. G. BAINES AND C. K.  FOLLAND/JOURNAL OF  CLIMATE

In work  accepted at the Journal of Climate, climate researchers Sergey
Kravtsov  and Christopher Spannagle of the University of Wisconsin, Milwaukee,
extract  what looks like an AMO temperature signal from not just the hemispheric
but the  global record as well. To gauge the effect of natural variations, they
took 20th  century temperature records from around the globe and subtracted
the warming due  to rising greenhouse gases, as simulated by 16 climate models.
The difference--a  strong warming over southern Greenland, a warming North
Atlantic, a cooling  South Atlantic, and a weak warming in the far North
Pacific--looks like the  pattern and timing attributed to the AMO. Kravtsov and
Spannagle conclude that  the shifting ocean circulation behind the AMO has global
effects on global  warming. The AMO may have had a hand in a more dramatic
global climate event,  according to meteorologist Peter Baines of the University
of Melbourne,  Australia, and climatologist Chris Folland of the Hadley Centre,
writing in the  15 June issue of the Journal of Climate. Their climate shift
rattled  the circum-Atlantic region over a decade starting in the early 1960s
and reached  around the globe.  
First, Baines and Folland pulled together a range of regional changes in  
temperature, rainfall, and atmospheric circulation around the Atlantic that  
could all be tied back to a cooling of the North Atlantic. The AMO presumably  
cooled the ocean--perhaps with the help of sun-shielding pollutant hazes--as the  
warm conveyor slowed. Greenland cooled, Brazilian rainfall swelled, hurricane
 activity dropped, and the Sahel dried to the most catastrophic drought in
more  than a century. These changes, which are most evident in the northern
summer,  can all be linked to the reduction and relocation of the ocean's transfer
of  heat into the atmosphere, Baines and Folland say. Those shifts, in turn,
led to  changes in atmospheric circulation and precipitation over adjacent
continents.  
Searching for the most remote limits of this climate shift, Baines and  
Folland looked out along the atmospheric circulations ultimately driven by  
tropical ocean heating in the Atlantic. There they found changes in subtropical  jet
streams in both hemispheres and poleward shifts in storm paths. In southwest  
Australia, for example, the shift reduced the rains and brought long-term  
drought. Baines and Folland's explication of a globe-girdling late-'60s climate  
shift only reinforces the view that "the AMO does affect global climate," says
 meteorologist Mojib Latif of the University of Kiel, Germany. "It's not just
 regional climate."  
Anticipating nature
Appreciating the power and reach of  natural climate variations is a major
step. To put that information to use,  however, climate forecasters must find a
way to model the future course of the  variations themselves, starting from
current conditions. Climate researchers  from the Hadley Centre, led by Douglas
Smith, are the first to try that, as they  report on page _796_
(http://www.sciencemag.org/cgi/content/full/317/5839/796) .  
The Hadley group tested the usefulness of their new prediction model by  
"hindcasting" the climate of two past decades. Starting from the observed  
distribution of ocean heat content, the model outperformed its own forecasts  that
lacked observed initial conditions. Errors in predicting global temperature  
declined by 20% or 36%, depending on the type of error. The model successfully  
predicted the warming of El Ni?o and the effect of unusually warm or cold
waters  around the world. An actual forecast starting in June 2005 correctly
predicted  that natural variability--the appearance of cooler water in the tropical
Pacific  and a resistance to warming in the Southern Ocean--would offset
greenhouse  warming until now. But beyond 2008, warming sets in with a vengeance.
"At least  half of the 5 years after 2009 are predicted to be warmer than 1998,
the warmest  year currently on record," the Hadley Centre group writes.  
"Smith et al. is an important first step in setting out the method,"  says
meteorologist Tim Palmer of the European Centre for Medium-Range Weather  
Forecasts in Reading, U.K. Now researchers need to amass more computing power,  more
past observations to test the method better, and more future observations  to
feed the models, he says. And time is of the essence. If the AMO in fact  
played a substantial role in the rapid warming and enhanced hurricane activity  
of the past decade or two, says Sutton, "there will in all probability be a  
turnaround [of the AMO], possibly in the next decade." It would be nice to know  
for sure.






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