Experiments that try to simulate global warming's impact on plants badly underestimate what happens in the real world, according to a study published on Wednesday in the journal Nature.

The investigation backs anecdotal evidence from farmers and gardeners, especially in the northern hemisphere, who say seasonal plants are stirring into life far earlier than in the past.

Artificial experiments into global warming usually entail encasing a plant in an open-top greenhouse-like chamber, or in a canopy that has a small heater in its roof, in order to replicate rising temperature.

These experiments have determined that flowering and leafing occur between 1.9 and 3.3 days earlier for every one degree Celsius (1.8 degrees Fahrenheit) in temperature increase.

But the study says the true figure is far higher.

Plants start to grow leaves and flowers between 2.5 and five days earlier per one degree C (1.8 F), it says.

It bases this on a comparison between warming experiments on 1,634 plant species and long-term observations of these species in the wild, carried out by some 20 institutions in North America, Japan and Australia.

"Up to now, it's been assumed that experimental systems will respond the same as natural systems respond -- but they don't," co-author Benjamin Cook of the Lamont-Doherty Earth Observatory at Columbia University, New York, said in a press release.

The experimental methods may be flawed because they reduce light, wind or soil moisture, all of which affect the plant's seasonal maturation, says the paper.

From 1906 to 2005, global surface temperatures rose by 0.74 C (1.33 F), according to the UN's Intergovernmental Panel on Climate Change (IPCC) in its 2007 Fourth Assessment Report.

On present trends of heat-trapping carbon emissions, Earth is on track for additional warming of two degrees Celsius (3.6 F) or more, according to estimates published by other sources last year.

Some experts deem those estimates conservative and note many locations are warming far faster than the global average.

"The meticulously recorded and celebrated blooming of Washington DC's cherry blossoms has advanced about a week since the 1970s," said the press release, issued by Columbia University's Earth Institute.

"If the trend continues, some recent projections say that by 2080 they will be coming out in February."

[The "blooming of Washington DC's cherry blossoms has advanced about a week since the 1970s via AFP / Nicholas Kamm]