A team led by Dominick Spracklen and Catherine Scott, atmospheric chemists at the University of Leeds, UK, ran a model that included how aerosols from forests can seed clouds, which reflect sunlight. They concluded that the net effect of VOCs from forests is to cool the global climate 4. Unger, who is now at the University of Exeter, UK, and Spracklen are discussing using a common experimental design to try to resolve their differences.
They and other researchers say that such studies are hamstrung by sparse data sets on forest emissions. The latest findings are piling on even more complexity. Ecologist Sunitha Pangala at Lancaster University, UK, spent much of and in the Amazon rainforest, where she placed gas-measuring chambers around the trunks of more than 2, trees. Researchers had previously assumed that methane leaked into the air directly from the soil, where it is produced by microbes.
The new work suggests that trees could be another conduit for that microbial methane, potentially explaining why more methane has been detected above tropical wetlands than has been measured emanating from soil alone. In a study first published last October, Gauci and other colleagues added another wrinkle when they found both methane and nitrous oxide, also a greenhouse gas, leaking from trees in upland forests 6.
The global significance of these findings is still unclear. Pangala and Gauci both estimate that the cooling effect of trees taking up carbon greatly outstrips the warming from tree emissions of methane and nitrous oxide. But Kristofer Covey, an environmental scientist at Skidmore College in Saratoga Springs, New York, has found methane leaking from non-wetland trees in temperate forests 7 , and argues that such emissions could, in some places, diminish the climate benefits of trees more than researchers and environmentalists realize.
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The recent explosion of results underscores the need for a full account of the impacts of forests, says Unger. Scientists who champion forests say that although more research is always good, existing results are mature enough to support the use of forests to fight climate change, especially given the urgency of the problem. Researchers are now turning to sophisticated computer models and using larger and more-comprehensive data sets to nail down exactly what forests in different places do to the climate.
In some cases, the results have been sobering. Last October, a team led by ecologist Sebastiaan Luyssaert at the Free University of Amsterdam modelled a variety of European forest-management scenarios 8. The researchers concluded that none of the scenarios would yield a significant global climate impact, because the effects of surface darkening and cloud-cover changes from any added forests would roughly eliminate their carbon-storage benefits. To estimate the climate impact of planting forests in different parts of the United States, ecologist Christopher Williams at Clark University in Worcester, Massachusetts, is combining global satellite data collected over more than a decade with carbon-sequestration figures based on data from the US Forest Service.
He has found in preliminary work that adding trees to the US west coast and to regions east of the Mississippi River makes sense, climatically speaking. But albedo changes make forest planting in the Rockies and the southwestern United States a bad deal for the climate in most cases, because the conifers that thrive in those regions are dark and absorb more sunlight than do underlying soils or snow. Getting planners to adopt such methods could prove challenging, however. Williams has found that some resist considering albedo effects, including representatives of companies hoping to sell carbon credits for forest projects.
More data about the climate impacts of forests could come from long-term studies that track the gases and chemicals that trees emit and absorb. Researchers are using a metre tower in the Amazon to monitor carbon, water and other chemical fluxes over a roughly square-kilometre area of intact rainforest northeast of Manaus in Brazil. A companion tower in Siberia does the same. Teams have erected smaller research towers to collect similar samples at hundreds of sites around the globe amid different types of forest; a tower in Norway, for example, will soon be the first in that country to start taking data in a forest.
But many important areas have not yet been covered. Scientists who debate the climate impacts of forests are eager to get their hands on these data. And even those who are firmly convinced that forest projects can fight climate change welcome the added rigour of more-comprehensive studies. Ellis, for one, acknowledges that the analysis he co-authored 1 considered albedo effects only crudely; the team did not consider VOCs and methane emissions from trees.
Griscom, B. Natl Acad.
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USA , — Pan, Y. Science , — Unger, N. Nature Clim. Change 4 , — Scott, C. Nature Commun. Pangala, S. Welch, B. Change Biol. Covey, K. II, Lee, X.
Luyssaert, S. Download references. An essential round-up of science news, opinion and analysis, delivered to your inbox every weekday. Advanced search. Search for this author in: Pub Med Nature. PDF version. References 1. PubMed Article Google Scholar 2.
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Article Google Scholar 7. Article Google Scholar 8. This measure would reportedly open up an area larger than Iran for extractive industries. A growing wave of speculative land claims are being registered inside reserves, which is putting more pressure on the boundaries. Other factors might have contributed to the increase. The first few months of this year were cloudy and rainy, which made satellite monitoring more difficult, so some areas might have been missed by earlier sweeps.
The bad weather could also have prompted loggers and farmers to delay land clearance until May. The economy, which is often a driver of deforestation during period of high beef and soy prices, has also been in the doldrums, though Bolsonaro has indicated that agribusiness can help to lift Brazil towards positive growth. In this regard, he is echoing and amplifying the message of his predecessors as president, Dilma Rousseff and Michel Temer, both of whom presided over periods of accelerated forest clearance as they became reliant on the rural lobby and commodity exports to China and Europe.
Another factor is an expansion of infrastructure projects, including roads and hydroelectric plants.
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