Wetland plants and forests act as carbon sinks, locking away substantial amounts of carbon absorbed from the atmosphere. The ocean absorbs some 25% of the world's carbon-dioxide emissions, and in its watery depths are acres of seagrass meadows that use about 15% of the dissolved carbon to grow. Mangroves and salt-marsh vegetation similarly accumulate carbon, and when they decompose their carbon is locked away in watery, peaty sediments for millennia.
Yet the world's coastal wetlands have been in continuous decline over the past century and now cover just 2% of the seabed1. Between 1980 and 2005, nearly 35,000 square kilometres of mangroves were cleared so that coastal land could be used for agriculture, aquaculture and beach resorts. When coastal wetlands are drained, the soil is oxidized and carbon dioxide is released into the air, contributing to climate change.
The 'blue carbon' concept aims to protect some of the most endangered wetlands by assigning credits to their stored carbon2. The credits can then be traded on a carbon market, explains Emily Pidgeon, director of the marine climate change programme at Conservation International, the environmental group in Washington DC that has been promoting the concept alongside the International Union for Conservation of Nature in Gland, Switzerland, and the Intergovernmental Oceanographic Commission based in Paris.
The effort faces some sizeable scientific hurdles. For starters, no one knows how much carbon is stored by wetlands around the world – largely because no one knows exactly how many seagrass beds and salt marshes exist. Some regions, such as North America, are well audited; others, such as Africa, remain a mystery. That means there is no good estimate of how much wetlands destruction contributes to global emissions.
Stephen Crooks, climate change programme manager at the environmental consultancy ESA PWA in San Francisco, California, estimates that emissions from drained mangroves and salt marshes total half a billion tonnes of carbon dioxide a year. That is 1–2% of global emissions. By comparison, deforestation contributes 10–25% of emissions. But Crooks acknowledges that his estimate is based on limited data and many assumptions. "The error bars are 100%," he says.
The water muddies even more when it comes to seagrasses. Earth-observing satellites lack the ability to look into turbid waters, and cannot tell the difference between the desirable grasses that store carbon, sea lettuce that stores little carbon and algae attached to rocks.
Data from Landsat satellites revealed the true extent of mangroves only last year. The survey found that in 2000, mangroves covered 137,760 square kilometres in tropical and subtropical regions3, some 12% smaller than the previous estimate from the United Nations.
Some scientists argue that despite their ecological importance, protecting wetlands may not make a huge difference to anthropogenic greenhouse-gas emissions. Oceanographer Christoph Heinze at the University of Bergen, Norway, points out that the carbon sequestration abilities of wetland plants are, quite literally, a drop in the ocean compared with Earth's other carbon sinks.
But Crooks points out that marine carbon circulation models have tended to consider wetlands' current carbon sequestration abilities, yet ignore the impact of releasing thousands of years of stored carbon when the lands are dried out.
Pidgeon acknowledges that a financial system such as blue carbon credits is at least a decade down the line. But pilot carbon credit projects will begin this year, she says, in parallel with further efforts to quantify the scale of Earth's wetlands and how much carbon they hold.
- Duarte, C. M., Middelburg, J. J. & Caraco, N. Biogeosciences 2, 1-8 (2005). | Article | ISI | ChemPort |
- Laffoley, D. & Grimsditch, G. (eds). The management of natural coastal carbon sinks. (IUCN, 2009); available at http://go.nature.com/vbJCD3
- Giri, C. et al. Glob. Ecol. Biogeogr. 20, 154-159 (2010). | Article | ISI
Source : http://www.nature.com/news/2011/110221/full/news.2011.112.htm