How green was my Amazon Basin model?
Weatherlawyer
article:
For several years after Terra, launched in 1999, an ecologist had been
worrying over the data processing and mapping technique he and his team
had proposed. For nearly two decades, National Oceanic and Atmospheric
Administration had been mapping global vegetation:
Huete had to deal with a new kind of satellite method for producing the
vegetation maps."When you see something you are not expecting, you
have to ask: 'What are the possibilities for it going wrong?'"
Aerosols and clouds, which can reduce signal obtained and flooding
during the wet season,etc.
Each time they made a change, they wondered if the dry-season green-up
would disappear. But with each refinement, it stayed.
At a meeting of the American Geophysical Union, he saw a poster by
ecologist Scott Saleska, showing results of field studies at a location
in the Tapajos National Forest in Brazil.
Among the most important measurements collected was the uptake and
release of carbon dioxide by the forest. The observations would reveal
whether, overall, the area was a sink or a source of carbon.
With two years of data, Saleska realized photosynthesis was greater
during the dry season. Results from a second tower showed exactly the
same pattern, boosting Saleska's confidence. Saleska and his
colleagues combined the results from the towers and presented them on a
poster for the meeting.
"For several years, we had been seeing this dry-season green up [in
the satellite data] and wondering whether it was real or not," says
Huete. "But when I saw Scott's results, that the ground data at the
tower sites showed the same thing as the satellite data, it really
changed everything. I realized we could stop focusing on 'what's
wrong' and focus on how to demonstrate that what we were seeing was
real."
In 2003, ecologist Rama Nemani and others published research that
linked global changes in vegetation productivity between 1982 and 1999
to precipitation, sunlight, and temperature. Using 20 years of climate
data combined with satellite-based vegetation maps, the team developed
a model that predicted which of the three factors most influenced the
vegetation in different places on Earth.
The Amazon had experienced a large increase in productivity, apparently
because of decreased cloud cover and increased sunlight, the Amazon was
light-limited, rather than water-limited.
The forest is so big that satellites are the only way to make
observations of the entire forest. But measurements collected from
hundreds of kilometers above can sometimes be hard to tie to specific
biological processes on the ground.
On the other hand, a handful of ground stations scattered throughout
7.5 million square kilometers of forest can't tell the forest's
entire story, either. To make a convincing case for an Amazon-wide,
dry-season green-up, Huete knew that he would need both perspectives:
space-based and ground-based.
His team compiled 5 years of satellite vegetation data based on the
relative amounts of red and near-infrared light that the sensor detects
over a location on Earth.
Chlorophyll in vegetation absorbs red light, while "scaffolding"
(like cell walls) in the plants' leaves reflect near-infrared. An
area that reflects very little red light but a lot of near-infrared
light back to space is likely covered in vegetation; scientists call
this signal "greenness."
Greenness is an optical (light-based) way to measure forest
productivity.
The Amazon is very cloudy in the rainy season. In the dry season, it
can be pretty smoky from agricultural fires. The high humidity can also
interfere. To make sure their vegetation maps were free of clouds and
other data contamination, Huete and his colleagues selected only the
best-quality data from 2000-2005 and averaged them into a single
"typical" Amazon year.
Looking at seasonal changes in greenness at three different scales: the
whole Amazon, regional slices and small areas surrounding research
towers like the one where Saleska worked.
They subtracted wet season and dry season greenness values to identify
seasonal patterns.
At tower locations in undisturbed forests and sites that had been
converted to pasture, they made week-by-week comparisons of satellite
greenness and ground-based measurements of carbon dioxide uptake.
At each scale, they saw the same pattern: undisturbed rainforests
became "greener" and increased their photosynthesis throughout the
dry season.
In the regional transects, the scientists discovered that the longer
the area's dry season was, the greater the greening effect was.
Even though it might seem like months with little or no rain ought to
slow down the forests' ability to photosynthesize, Huete says the
reverse appears true. "The dry season, with less clouds and higher
sunlight, is actually the 'good' season." in tune with research
about the soil-water-tapping potential of mature rainforest trees.
Since the early 1990s, field studies and soil-moisture modeling
research have been accumulating evidence that in the undisturbed
rainforest, roots extend as far as 20 meters (more than 60 feet) into
the soil, where the wet-season rains are stored.
Some current ecological models have the Amazon getting browner during
the dry season.
If the seasonal cycle of green-up and photosynthesis in a model is
wrong, the ability to predict uptake and release of carbon dioxide,
water availability, and fire risk would probably be off as well.
"When the modelers couple their climate models to ecosystem models of
the Amazon and run them out over the next century, some models predict
that Earth's warming climate will cause the whole Amazon ecosystem to
collapse, to become a savanna."
The Amazon region may be neutral with respect to carbon losses and
gains. But if savanna replaced the rainforest, "the whole area would
switch from being close to neutral on average to being a big source of
carbon."
"But if these models are getting the seasonality wrong, then the
impacts [of climate change] may not be what we expect". Predictions
of ecosystem collapse are based on the idea that the dry season is a
time of stress and declining greenness.
If that isn't true, then perhaps the Amazon will be more resilient
than the models predict. On the other hand, a typical dry season
isn't the same as a lengthy El Niño-induced drought.
With forest disturbance on the rise and predictions by some climate
models that El Niño events may increase as climate warms, the fate of
the Amazon is unclear. Observations collected during the next strong El
Nino event could provide the next key piece of the puzzle of how the
Amazon responds to large-scale climate variation and change.
http://earthobservatory.nasa.gov/Newsroom/NewImages/images.php3?img_id=17323
So now expect a rash of posts from emotionally disturbed people rushing
to appologise for all their foolish thoughts and foolish lack of
thought about glowballing.