Wednesday, November 13, 2013

Global Precipitation Linked to Global Warming

Nov. 11, 2013 — The rain in Spain may lie mainly on the plain, but the location and intensity of that rain is changing not only in Spain but around the globe.

A new study by Lawrence Livermore National Laboratory scientists shows that observed changes in global (ocean and land) precipitation are directly affected by human activities and cannot be explained by natural variability alone. The research appears in the Nov. 11 online edition of the Proceedings of the National Academy of Sciences.
Emissions of heat-trapping and ozone-depleting gases affect the distribution of precipitation through two mechanisms. Increasing temperatures are expected to make wet regions wetter and dry regions drier (thermodynamic changes); and changes in atmospheric circulation patterns will push storm tracks and subtropical dry zones toward the poles.
"Both these changes are occurring simultaneously in global precipitation and this behavior cannot be explained by natural variability alone," said LLNL's lead author Kate Marvel. "External influences such as the increase in greenhouse gases are responsible for the changes."
The team compared climate model predications with the Global Precipitation Climatology Project's global observations, which span from 1979-2012, and found that natural variability (such as El Niños and La Niñas) does not account for the changes in global precipitation patterns. While natural fluctuations in climate can lead to either intensification or poleward shifts in precipitation, it is very rare for the two effects to occur together naturally.

"In combination, manmade increases in greenhouse gases and stratospheric ozone depletion are expected to lead to both an intensification and redistribution of global precipitation," said Céline Bonfils, the other LLNL author. "The fact that we see both of these effects simultaneously in the observations is strong evidence that humans are affecting global precipitation."
Marvel and Bonfils identified a fingerprint pattern that characterizes the simultaneous response of precipitation location and intensity to external forcing.
"Most previous work has focused on either thermodynamic or dynamic changes in isolation. By looking at both, we were able to identify a pattern of precipitation change that fits with what is expected from human-caused climate change," Marvel said.
By focusing on the underlying mechanisms that drive changes in global precipitation and by restricting the analysis to the large scales where there is confidence in the models' ability to reproduce the current climate, "we have shown that the changes observed in the satellite era are externally forced and likely to be from man," Bonfils said.
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The above story is based on materials provided byDOE/Lawrence Livermore National Laboratory.
Note: Materials may be edited for content and length. For further information, please contact the source cited above.

Sunday, November 10, 2013

Carbon storage recovers faster than plant biodiversity in re-growing tropical forests

Carbon storage recovers faster than plant biodiversity in re-growing tropical forests

Nov. 5, 2013 — A new study of re-growing tropical forests has concluded that plant biodiversity takes longer to recover than carbon storage following major disturbances such as clearance for farming.

The findings, published in the scientific journal Proceedings of the Royal Society B, have important implications for conservation since there are now many re-growing forests in South and Central America. The new study is the first large-scale analysis of the recovery of both plant biodiversity and carbon pools in re-growing forests.
Over half of all tropical forests have already been converted for agriculture, logged or burnt in the recent past. Re-growing forests could help both to soak up carbon emissions produced by human activities and to reduce species’ extinctions.
The scientists, from the Centre for Ecology & Hydrology and Bournemouth University, concluded that although carbon recovered most quickly, even after 80 years re-growing forests tended to have less carbon than old-growth forests. This is probably because these forests are often dominated by small, fast growing trees. It may take centuries for larger trees which hold more carbon to become established.
In contrast, although the number of tree species recovered relatively rapidly, many species characteristic of old-growth forests were rare in re-growing forests. This is worrying because these species are probably those most vulnerable to extinction.
The research team conducted a synthesis of data collected from more than 600 secondary forest sites from 74 previous studies, describing carbon pools and plant biodiversity. Each site had comparable data for a nearby site that was relatively free of human disturbance.

Lead author Phil Martin, a PhD student at the Centre for Ecology & Hydrology, said, “We think plant species normally found in old-growth forests are failing to colonise re-growing forests because their seeds never get there. These recovering forests are often far from old-growth forests and surrounded by farmland. This means forest animals cannot move seeds between the two forests.”
Phil Martin added, “We suggest that when conservationists aim to restore tropical forests they should help dispersal of seeds from undisturbed to re-growing areas by planting trees throughout the wider landscape.”
In the study the researchers point out that these results show that forests that are re-growing following agricultural use may be more valuable for the carbon they store than for their biodiversity for the first 100 years. Policies such as Reducing Emission from Deforestation and Degradation (REDD) often assume that carbon and biodiversity are interchangeable. This work shows this is not the case.
Co-author Professor James Bullock from the Centre for Ecology & Hydrology said, “Our results clearly indicate that preservation of old-growth forests is vital for the conservation of specialist species. While the re-growth of forests following clearance is valuable in soaking up carbon, the biodiversity benefits will take a very long time to emerge.”
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Amazon Deforestation Could Mean Droughts for Western U.S.

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Nov. 7, 2013 — In research meant to highlight how the destruction of the Amazon rainforest could affect climate elsewhere, Princeton University-led researchers report that the total deforestation of the Amazon may significantly reduce rain and snowfall in the western United States, resulting in water and food shortages, and a greater risk of forest fires.

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The researchers report in the Journal of Climate that an Amazon stripped bare could mean 20 percent less rain for the coastal Northwest and a 50 percent reduction in the Sierra Nevada snowpack, a crucial source of water for cities and farms in California. Previous research has shown that deforestation will likely produce dry air over the Amazon. Using high-resolution climate simulations, the researchers are the first to find that the atmosphere's normal weather-moving mechanics would create a ripple effect that would move that dry air directly over the western United States from December to February.
Specifically, a denuded Amazon would develop a weather cycle consisting of abnormally dry air in the sun-scorched northern Amazon around the equator weighted by wetter air in the cooler south. Research has speculated that this pattern would be similar to the warm-water climate pattern El Niño, which during the winter months brings heavy precipitation to southern California and the Sierra Nevada region while drying out the Pacific Northwest.
The Princeton-led researchers found that the Amazon pattern would be subject to the same meandering high-altitude winds known as Rossby waves that distribute the El Niño system worldwide from its source over the Pacific Ocean. Rossby waves are instrumental forces in Earth's weather that move east or west across the planet, often capturing the weather of one region -- such as chill Arctic air -- and transporting it to another. Because the Amazon pattern forms several thousand miles to the southeast from El Niño, the researchers report, the Rossby waves that put the rainy side of El Niño over southern California would instead subject that region to the dry end of the Amazon pattern. The pattern's rainy portion would be over the Pacific Ocean south of Mexico.
First author David Medvigy, an assistant professor of geosciences at Princeton, explained that the findings stand as one possible outcome of Amazon deforestation in regions outside of South America -- consequences that scientists are working to understand. The rainforest influences various aspects of the surrounding climate, including cloud coverage, heat absorption and rainfall.
"The big point is that Amazon deforestation will not only affect the Amazon -- it will not be contained. It will hit the atmosphere and the atmosphere will carry those responses," Medvigy said.
"It just so happens that one of the locations feeling that response will be one we care about most agriculturally," he said. "If you change the snowpack in the Sierra Nevada, where most of the irrigation for California's Central Valley comes from, then by this study deforestation of the Amazon could have serious consequences for the food supply of the United States."
Because the exact result of Amazon deforestation is impossible to know currently, the behavior and impact of El Niño provides one of the best ideas of how the loss of the Amazon could play out, Medvigy said. Studies have suggested since 1993 that an Amazon without trees will develop an El Niño-like pattern, the researchers reported. The researchers then focused on the northwestern United States because the region is typically sensitive to El Niño.
"We don't know what the world will be like without the Amazon. We know exactly what happens with El Niño -- it's been studied extensively," Medvigy said. "Our intention with this paper was to identify an analogy between El Niño and Amazon deforestation. There's good reason to believe there will be strong climatic similarities between the two. Research like this will give us a handle on what to expect from Amazon deforestation."
Medvigy worked with second author Robert Walko, a senior scientist in the division of meteorology and physical oceanography at the University of Miami; Martin Otte, a postdoctoral fellow at the U.S. Environmental Protection Agency's Atmospheric Modeling and Analysis Division; and Roni Avissar, a University of Miami professor of meteorology and physical oceanography and dean of the Rosenstiel School of Marine and Atmospheric Science.
The high resolution of the researchers' climate model allowed them to see the otherwise subtle pull of the Rossby waves, Medvigy said. The typical model buries finer atmospheric features under a scale of about 200 kilometers -- twice the width of the Andes Mountains. Medvigy and his co-authors spotted the intricacies of the Amazon's future weather pattern using a resolution as fine as 25 kilometers, he said.
The researchers based their simulation on the Amazon's complete removal, an exaggerated level of destruction needed to produce a noticeable effect, Medvigy said. Nonetheless, clear-cutting of the Amazon marches on, although conservation efforts have significantly slowed deforestation in countries such as Brazil since the mid-2000s. In addition, research has shown that climate change, especially a spike in the global temperature, could wipe out as much as 85 percent of the forest.
The Amazon's fragility and vulnerability -- combined with its outsized sway over the climate -- add an urgency to better understanding how the forest's disappearance will affect the larger climate, particularly for agriculturally important areas such as California, Medvigy said.
"We know the Amazon is being deforested, but we don't know for sure what's going to happen because of it," Medvigy said. "Other scientists need to do these simulations and see if they get the same results. If they do, then policymakers will have to take notice."
This work was supported by awards from the National Science Foundation (grant numbers 1151102 and 0902197).