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.
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.
''Australia is a significant contributor to the global pattern, and the findings are consistent to what we have seen here,'' said a senior CSIRO researcher and director of the Global Carbon Project, Dr Josep Canadell.
''There has been a measurable decline in the leaf area of plants this decade, though we don't have all the data for Australia yet. What we have seen is strongly consistent with projected patterns of climate change.''
The Science study, Drought-induced Reduction in Global Terrestrial Net Primary Production from 2000 through 2009, used data from a NASA satellite that orbits Earth every 15 days to build up a global map of changing leaf density and forest cover. It estimated net primary production, a measurement of how much CO2 is taken in by plants and stored as part of their biomass.
The study found that in some areas of the world, higher temperatures had driven more plant growth. But these gains have been cancelled out by drier conditions in rainforests, leading to the overall decline in total amount of CO2 the forests are soaking up.
The findings reinforce work being done at the Australian Bureau of Rural Sciences, which is researching how much carbon can be stored on a long-term basis in the landscape.
Scientists say that a sustained decline in the amount of carbon being stored in forests risks locking in a vicious cycle, in which trees absorb less carbon because the world is warmer and drier, while the rising carbon levels in the atmosphere continue to trap heat.
''There is no single silver bullet answer to this, but one of the partial solutions is the protection of old-growth forests, which store a lot of CO2, and the replanting of those that have been removed,'' said Professor Andy Pitman, the co-director of the Climate Change Research Centre at the University of NSW.
''This doesn't actually get to the heart of the problem though, which is rising CO2 emissions from human activity.''
Rainfall patterns in Australia are expected to alter significantly over the next few decades as average temperatures increase, with more rain likely to fall in the north and north-west and less precipitation likely in southern Australia. This means that many of Australia's existing old-growth forests, which are located in NSW, Victoria and Tasmania, can be expected to become less efficient carbon sinks.