Tuesday, May 25, 2010

Unprecedented Warming in East Africa's Lake Tanganyika; Lake's surface waters are warmest in 1,500 years

Unprecedented Warming in East Africa's Lake Tanganyika

Lake's surface waters are warmest on record
Local fishermen troll the waters of Lake Tanganyika catching sardines.
Local fishermen troll the waters of Lake Tanganyika, catching sardines--for now.
National Science Foundation, May 16, 2010

Lake Tanganyika, the second-oldest and second-deepest lake in the world, could be in for some rough waters.

Geologists have determined that the East African rift lake has experienced unprecedented warming during the last century; its surface waters are the warmest on record.

That finding is important, the scientists state in this week's on-line issue of the journal Nature Geoscience, because the warm surface waters likely will affect fish stocks upon which millions of people in the region depend.

"This result is in addition to those from other African lakes showing that changes in regional climate have a significant impact on the lakes, and on the human populations that depend on the lakes' resources," said Paul Filmer, program director in the National Science Foundation (NSF)'s Division of Earth Sciences, which funded the research.

The scientists took core samples from the lakebed that laid out a 1,500-year history of the lake's surface temperature.

The resulting data showed that the lake's surface temperature, 26 °C (78.8 °F), last measured in 2003, is the warmest the lake has been for a millennium and a half.

The team also documented that Lake Tanganyika experienced its largest temperature change in the 20th century. The change has affected its unique ecosystem, which relies upon nutrients from the depths to jumpstart the food chain on which fish survive.

"Our data show a consistent relationship between lake surface temperature and productivity such as that of fish stocks," said Jessica Tierney of Brown University, the paper's lead author. "As the lake gets warmer, we expect productivity to decline, and we expect that it will affect the fishing industry."

Cores were taken in 2001 by Andrew Cohen, a geologist at the University of Arizona, and in 2004 by James Russell, a geologist at Brown University.

Lake Tanganyika is bordered by Burundi, the Democratic Republic of Congo, Tanzania, and Zambia--four of the poorest countries in the world.

An estimated 10 million people live near the lake, and depend on it for drinking water and for food.

Fishing is a crucial component of their diets and livelihoods: up to 200,000 tons of sardines and four other fish species are harvested annually from Lake Tanganyika.

The lake, one of the richest freshwater ecosystems in the world, is divided into two levels. Most of the animal species live in the upper 100 meters, including valuable sardines.

Below that, the lake holds less and less oxygen, and at certain depths, it has no oxygen.

The lake depends on wind to churn its waters and send nutrients from the depths toward the surface. These nutrients are food for algae, which supports the lake's entire food web.

But as Lake Tanganyika warms, the mixing of waters is lessened; fewer nutrients are funneled from the depths to the surface.

More warming at the surface magnifies the difference between the two lake levels; even more wind is needed to churn the waters enough to ferry nutrients toward the upper layer.

The researchers' data show that during the last 1,500 years, intervals of prolonged warming and cooling are linked with low and high algal productivity, respectively, indicating a clear link between past temperature changes and biological productivity in the lake.

"People throughout south-central Africa depend on the fish from Lake Tanganyika as a crucial source of protein," Cohen said. "This resource is likely threatened by the lake's unprecedented warming and the associated loss of lake productivity."

Climate change models show a general warming trend in the region, which would cause even greater warming of Lake Tanganyika's surface waters.

Some researchers believe that the declining fish stocks in Lake Tanganyika can be attributed mainly to overfishing, and Tierney and Russell say that may be a reason.

But they note that the warming in the lake, and the lessened mixing of critical nutrients, is exacerbating the fish stocks' decline, if not causing it.

"It's almost impossible for it not to be," Russell said.

Media Contacts Cheryl Dybas, NSF (703) 292-7734 cdybas@nsf.gov
Richard Lewis, Brown University (401) 863-3766 richard_lewis@brown.edu

The National Science Foundation (NSF) is an independent federal agency that supports fundamental research and education across all fields of science and engineering. In fiscal year (FY) 2010, its budget is about $6.9 billion. NSF funds reach all 50 states through grants to nearly 2,000 universities and institutions. Each year, NSF receives over 45,000 competitive requests for funding, and makes over 11,500 new funding awards. NSF also awards over $400 million in professional and service contracts yearly.

Link:  http://www.nsf.gov/news/news_summ.jsp?cntn_id=116956&WT.mc_id=USNSF_51&WT.mc_ev=click

J. E. Tierney et al., Nature Geosci., (2010), Late-twentieth-century warming in Lake Tanganyika unprecedented since AD 500

Nature Geoscience, published online 16 May 2010; doi: 10.1038/ngeo865

Late-twentieth-century warming in Lake Tanganyika unprecedented since AD 500

Jessica E. Tierney* (Brown University Department of Geological Sciences, Box #1846, Providence, RI 02912, U.S.A.), Marc T. Mayes (Brown University Department of Geological Sciences, Box #1846, Providence, RI 02912, U.S.A., and Center for Sustainability and the Global Environment, Nelson Institute for Environmental Studies, University of Wisconsin-Madison, 1710 University Ave., Madison, WI 53726, U.S.A.), Natacha Meyer (Brown University Department of Geological Sciences, Box #1846, Providence, RI 02912, U.S.A.), Christopher Johnson (Department of Geosciences, University of Arizona, 1040 E 4th St., Tucson, AZ 85721, U.S.A., and Department of Ecology and Evolutionary Biology, University of California, Los Angeles, CA 90095, U.S.A.), Peter W. Swarzenski (United States Geological Survey, 400 Natural Bridges Drive, Santa Cruz, CA 95060, U.S.A.), Andrew S. Cohen (Department of Geosciences, University of Arizona, 1040 E 4th St., Tucson, AZ 85721, U.S.A.) and James M. Russell (Brown University Department of Geological Sciences, Box #1846, Providence, RI 02912, U.S.A.)

Instrumental observations suggest that Lake Tanganyika, the largest rift lake in East Africa, has become warmer, increasingly stratified and less productive over the past 90years (refs 1,2). These trends have been attributed to anthropogenic climate change. However, it remains unclear whether the decrease in productivity is linked to the temperature rise3, 4, and whether the twentieth-century trends are anomalous within the context of longer-term variability. Here, we use the TEX86 temperature proxy, the weight per cent of biogenic silica and charcoal abundance from Lake Tanganyika sediment cores to reconstruct lake-surface temperature, productivity and regional wildfire frequency, respectively, for the past 1,500years. We detect a negative correlation between lake-surface temperature and primary productivity, and our estimates of fire frequency, and hence humidity, preclude decreased nutrient input through runoff as a cause for observed periods of low productivity. We suggest that, throughout the past 1,500years, rising lake-surface temperatures increased the stratification of the lake water column, preventing nutrient recharge from below and limiting primary productivity. Our records indicate that changes in the temperature of Lake Tanganyika in the past few decades exceed previous natural variability. We conclude that these unprecedented temperatures and a corresponding decrease in productivity can be attributed to anthropogenic global warming, with potentially important implications for the Lake Tanganyika fishery.

*Correspondence e-mail: Jessica_Tierney@brown.edu

Link:  http://www.nature.com/ngeo/journal/vaop/ncurrent/abs/ngeo865.html

Sunday, May 9, 2010

Identifying potential synergies and trade-offs for meeting food security and climate change objectives in sub-Saharan Africa

Proceedings of the National Academy of Sciences,

Identifying potential synergies and trade-offs for meeting food security and climate change objectives in sub-Saharan Africa

Cheryl A. Palm et al.


Potential interactions between food production and climate mitigation are explored for two situations in sub-Saharan Africa, where deforestation and land degradation overlap with hunger and poverty. Three agriculture intensification scenarios for supplying nitrogen to increase crop production (mineral fertilizer, herbaceous legume cover crops—green manures—and agroforestry—legume improved tree fallows) are compared to baseline food production, land requirements to meet basic caloric requirements, and greenhouse gas emissions. At low population densities and high land availability, food security and climate mitigation goals are met with all intensification scenarios, resulting in surplus crop area for reforestation. In contrast, for high population density and small farm sizes, attaining food security and reducing greenhouse gas emissions require mineral fertilizers to make land available for reforestation; green manure or improved tree fallows do not provide sufficient increases in yields to permit reforestation. Tree fallows sequester significant carbon on cropland, but green manures result in net carbon dioxide equivalent emissions because of nitrogen additions. Although these results are encouraging, agricultural intensification in sub-Saharan Africa with mineral fertilizers, green manures, or improved tree fallows will remain low without policies that address access, costs, and lack of incentives. Carbon financing for small-holder agriculture could increase the likelihood of success of Reducing Emissions from Deforestation and Forest Degradation in Developing Countries programs and climate change mitigation but also promote food security in the region.

*Correspondence e-mail: cpalm@ei.columbia.edu

Link:  http://www.pnas.org/content/early/2010/05/06/0912248107.abstract

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