The capacity of the world's forests to soak up greenhouse gases has dropped by 40 per cent in the past 20 years, according to a new report.
Friends of the Earth International says climate change is severely disrupting forest eco-systems, with ''forests as a whole'' predicted to lose their ability to absorb atmospheric carbon if climate changes forces a 2-degree rise in average temperatures.
It warns the world's forests are being cleared at a rate of 7.3million hectares a year, with loss of tropical forests increasing at a rate of more than 25 per cent.
The organisation, which is the world's biggest environmental network with 6000 member groups, has called for an end to logging of all the world's old growth forests, given their critical role in regulating the Earth's climate.
''The destruction of forests is likely to cause significant changes to weather and climate, both regionally and globally, and thus to ecosystems and food production,'' it says.
The report comes as anti-logging activists in Tasmania clashed with police yesterday over state Government plans to log 169ha of old growth temperate rainforest in the upper Florentine valley in southern Tasmania.
Australian Greens leader Bob Brown said the global report was ''yet another huge warning'' to Prime Minister Kevin Rudd that protecting old growth forests was ''pivotal to any serious plan to stem climate change''.
Senator Brown said despite Federal Government claims that ''land clearing was under control'', it remained a significant contributor to Australia's greenhouse emissions.
He said data showed the volume of forests logged and burned across eastern Australia had increased in recent years.
Meanwhile, the Friends of the Earth Australia spokesman Cam Walker said recently Australia's land clearing rates had only dropped by just 3 per cent in the past 10 years.
''There was an assumption that if Queensland got its clearing rates under control that would solve the issue, but other states have since emerged as major problems mainly Tasmania, Victoria, NSW and the Northern Territory. There are still huge problems with illegal land clearing, '' Mr Walker said.
Before the 2007 election, Labor had promised to introduce national standards and guidelines for land clearing, and to improve compliance and monitoring of clearing, he said.
''But we have seen little movement in this direction from the Rudd Government.''
A spokesman for Federal Environment Minister Peter Garrett said ''issues associated with land clearing'' would be considered in a number of government reviews of environmental laws and policies, including reviews of the Environment Protection and Biodiversity Act, the national biodiversity strategy and native vegetation policies.
The executive director of the Australian National University's climate change institute, Professor Will Steffen, said it was critical for Australia ''to retain as many old growth forests as possible''.
''If you map the value of ecosystem services across Australia, you'd find old growth forests were among the most valuable.''
The Friends of the Earth report, ''Forests In A Changing Climate'', warns hotter temperatures are causing widespread forest dieback, with recent studies estimating 55 per cent of the Amazon rainforest could die off within 20 years, releasing up to 26 billion tonnes of carbon into the atmosphere. The US could also lose 11 per cent of its forests.
The report calls for the United Nations to exclude timber plantations such as palm oil from global emissions trading schemes, arguing they are a major driver of global forest loss.
''Replacing old growth forests with plantations is not an option. At best, tree plantations store just 20per cent of the carbon that old-growth forests lock away,'' the report says.