LESS rainfall and rising global temperatures are damaging one of the world's best guardians against climate change: trees.

A global study, published in the journal Science, shows that the amount of carbon dioxide being soaked up by the world's forests in the past decade has declined, reversing a 20-year trend.

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It diminishes hopes that global warming can be seriously slowed down by the mass planting of trees in carbon sinks. Although plants generally grow bigger as a result of absorbing carbon-enriched air, they need more water and nutrients to do so, and they have been getting less.

A fierce drought that dried out vast areas of the Amazon Basin in 2005 is seen as a key to the global decline in carbon sinks in the past decade, but Australia is not immune.