MEDIA mogul Rupert Murdoch was right to say the world's forests are growing faster and thicker because there is more carbon dioxide in the atmosphere, a leading climate scientist says. But it does not mean the world is better off for rising greenhouse gas emissions.
Mr Murdoch tweeted to 388,000 followers on Monday: ''World growing greener with increased carbon. Thirty years of satellite evidence. Forests growing faster and thicker.''
Mr Murdoch then referred followers to an article on the topic in The Wall Street Journal last week by Matt Ridley, a prominent climate change sceptic.
CSIRO's Pep Canadell, also executive director of the Global Carbon Project, said it was correct to say that increased concentration of carbon dioxide in the atmosphere meant vegetation is becoming greener.
''It is a fundamental physiological response to elevated CO2; we call it the CO2 fertilisation effect,'' Dr Canadell said. ''That is, plants in a greenhouse grow happier because there is more CO2. It is a trick that has been used for hundreds of years to grow more tomatoes.''
But Dr Canadell said it did not mean the proportion of total human greenhouse gases being absorbed by forests and other terrestrial carbon sinks had increased as forests had become greener. Studies by the Global Carbon Project have found the accumulation of carbon dioxide in the atmosphere is faster than the growth of terrestrial carbon sinks such as forests.
Sixty years ago only about 50 per cent of all global warming-causing human emission was absorbed by terrestrial sinks. Today the proportion of emissions absorbed remains about the same as greenhouse gas emissions increase.
There are other problems. Dr Canadell said while forests grew greener - mainly in high latitude areas - emissions are rising from the soil as frozen organic matter, such as permafrost, thaws.
''It is a catch-22. You are getting one positive and you get the exactly same negative. You are not better off,'' he said.
Forests can also only absorb so much carbon dioxide, and will reach a saturation point. That saturation point could be reached within the century, he said.
Nor does the increased ''greening'' of forests due to higher carbon dioxide emissions mean the loss of global forests has stopped.
A 2010 study by the UN Food and Agriculture Organisation found that between 2000 and 2010 5.2 million hectares of forest a year was lost globally, an area about the size of Costa Rica.
That rate was down from the 8.3 million hectares annually lost between 1990 and 2000.
The Wall Street Journal article referenced by Mr Murdoch in his tweets refers to soon-to-be-released satellite records by researchers at Boston University finding more forest has greened than browned over the last 30 years.
