In a new report released today, the agencies estimate that carbon emissions " equal to half the annual emissions of the global transport sector " are being captured and stored by marine ecosystems such as mangroves, salt marshes and seagrasses.
They add that a combination of reducing deforestation on land as well as restoring the coverage and health of these marine ecosystems could deliver up to 25 per cent of the emissions reductions needed to avoid "dangerous" climate change.
The report comes fewer than 60 days before the crucial UN climate change convention meeting in Copenhagen where governments are aiming to "seal the deal" an ambitious new agreement to curb greenhouse gas emissions.
"We already know that marine ecosystems are multi-trillion dollar assets linked to sectors such as tourism, coastal defense, fisheries and water purification services " now it is emerging that they are natural allies against climate change," said Achim Steiner, Executive Director of the UN Environment Programme (UNEP).
"Indeed this report estimates that halting losses and catalyzing the recovery of marine ecosystems might contribute to offsetting up to seven per cent of current fossil fuel emissions and at a fraction of the costs of technologies to capture and store carbon at power stations," he added.
At the same time, the report warns that far from maintaining and enhancing these natural carbon sinks, humanity is damaging and degrading them at an accelerating rate. It estimates that up to seven per cent of these "blue carbon sinks" are being lost annually, or seven times the rate of loss of 50 years ago.
"If more action is not taken to sustain these vital ecosystems, most may be lost within two decades," states the report, launched by UNEP, the Food and Agriculture Organization (FAO) and the Intergovernmental Oceanographic Commission of the UN Educational, Scientific and Cultural Organization (UNESCO).
Luciano Fonseca of the UNESCO Commission explained that the ocean"s absorption of the planet"s excess heat "is like a glass of whisky with ice. As long as the ice is there the whisky stays cool. The energy that is going into the glass, from your hand and room temperature, is working to convert the ice to liquid. As soon as the ice melts the whisky turns warm."
Meanwhile, Ichiro Nomura, Assistant Director-General for Fisheries and Aquaculture at FAO, noted that fishing and aquaculture communities will be heavily impacted by climate change and have a key role to play in maintaining healthy ocean ecosystems.
"An ecosystem approach to the management of ocean and coastal ecosystems cannot only enhance their natural carbon sink capacity, but also offers a way to safeguard and strengthen food and livelihood security for fisheries-dependent communities," he added.
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