The big climate shifts occurred before industrialisation and so have natural causes. Even so, they could play a mighty role in man-made global warming by amplifying or easing the peril, the study says.

The northern part of the planet experienced two warm peaks at around the year 1000 to 1100, it says. Then, in the 16th and 17th centuries, there was a significant cooling.

Overall, the temperature swing between these two events was in the region of 0.65-0.9 C (1.17-1.62 F), which is huge in terms of the potential impact on the climate, according to the study, led by meteorologist Anders Moberg at Stockholm University in Sweden.

Previous evidence has already suggested Earth's climate suddenly warmed and abruptly cooled at these times. But the best bet was a far smaller temperature range of 0.5 C (0.9 F).

The suspected causes for such natural climate shifts are tiny fluctuations in the planet's orbit, as well as "wobbles" in the spin of its axis. Minute changes such as this can greatly change the exposure of part of the world to heat from the Sun.

Another potential source of climate change are large volcanic eruptions, which disgorge carbon dioxide (CO2) -- the "greenhouse" gas which traps solar heat and forces up the surface temperature.

The new research is important because it shows how natural events have the ability either to worsen man-made global warming or alternatively to brake it.

But so far, no one can predict when the next big climate swing will occur and whether it will make things hotter or colder.

Already, Earth's surface temperature has warmed by 0.7-0.8 Csince 1900, and the 1990s was the hottest decade on record.

If fossil-fuel greenhouse gases -- released by burning oil, gas and coal -- are allowed to grow unchecked, the stage could be set for dramatic climate change just a few decades from now, scientists say.

Moberg's study, which appears on Thursday in the weekly British science journal Nature, took a new approach to reconstructing climate change.

It applied a new mathematical model, called wavelet analysis, to evidence gained from sedimentary cores, stalagmites and tree rings.

In sedimentary cores, levels of preserved pollen, shells and algae called diatoms give indicators as to the temperatures that prevailed at that point in history.

Like stalagmites -- which grow faster when it is warming -- cores give evidence over centuries about climate change.

However, this data is broad, and lacks fine resolution about climate change on a scale of decades.

For that, tree ring evidence is useful. These rings show fast trees grew in response to higher or lower levels of CO2, a gas needed for photosynthesis.

In another climate study, likewise published in Nature, US and Canadian scientists cast doubt on hopes that plants can adapt quickly if, as is gloomily expected, CO2 levels surge this century.

They studied colonies of a fungus over six years, exposing different groups to varying levels of CO2. Fungus communities that were exposed to quickly rising levels of CO2 were the worst affected -- they were thinner and less rich in species diversity.

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