* Marsh plants soak up carbon, transfer it to sediment
* Rising sea levels will choke off marshes towards 2100
By Alister Doyle
OSLO, Sept 26 (Reuters) - Salt marshes around the world's coasts will help slow climate change until about 2050 by soaking up greenhouse gases but then risk making the problem even worse as sea levels rise, a study showed on Wednesday.
Plants such as grasses and shrubs - which thrive in salt marshes found from India to the United States - absorb heat-trapping carbon from the air. Much of it then ends up buried in sediment where it no longer stokes global warming.
"The net impact of temperature warming and sea level rise is to increase carbon burial rates in the first half of the twenty-first century," researchers in the journal Nature wrote.
Beyond about 2050, rising sea levels would start water logging plants however, the study said, halting the transfer of carbon into the muddy sediment. "At some point too much flooding is bad," lead author Matthew Kirwan at the University of Virginia told Reuters.
Initially, gradually rising sea levels caused by temperature increases that will melt ice on land and make water in the oceans expand, would help wetland plants grow better. Flooding brings in more nutrients and washes out toxins.
At their peak - around mid-century - the study said salt marshes could absorb about 2 kilos (4.4 lbs) of organic matter per square metre per year, much of it carbon. Marshes could easily withstand a sea level rise of 1 mm (0.04 inch) or more a year, it added.
But absorption would stop and the marshes were likely to start emitting carbon if the rise in sea level accelerated, it said. Wind and waves could then erode the seabed sediment, releasing trapped carbon stored beneath the now-dead salt marsh.
"If the plant is submerged 100 percent of the time the roots cannot get more oxygen and the plant dies. Once the marsh dies it is not sequestering any carbon," said Kirwan.
RISING SEA LEVELS
The U.N. panel of climate scientists has projected that sea levels will rise by between 18 and 59 cms (7-23 inches) by 2100 or more if Antarctica or Greenland melt faster. Seas are now rising by about 3 mm a year (0.1 inch).
Kirwan said that the "million dollar question" was whether rising seas would simply create more marshes further inland.
"The big catch is whether there will be land available," he said. In many nations, governments seek to protect coasts - from New Orleans in the United States to Amsterdam in the Netherlands - and to prevent the formation of marshes.
He estimated that salt marshes worldwide absorbed between 10 and 130 million tonnes of carbon a year - at the top end of the range roughly equivalent to Italy or Mexico's greenhouse gas emissions.
In 2009, a "Blue Carbon" report by the U.N. Environment Programme also said that mangroves, salt marshes and sea grasses captured up to 450 million tonnes of carbon a year. It urged far greater efforts to protect such huge stores.
Kirwan said that his study - with colleague Simon Mudd who works at both the University of Edinburgh and the University of California, Santa Barbara - was the first to try to assess the effect of a rising sea level on salt marshes. (Reporting By Alister Doyle; Editing by Andrew Osborn)