The chances of northern Europe facing a new ice age, or of catastrophic sea-level rises of almost four metres that swamp the planet over the next century, have been ruled out by leading scientists.

But the risk of tropical forests succumbing to drought brought on by climate change as well as the acceleration of methane emissions from melting permafrost, is greater, according to the Met Office Hadley Centre, in its latest climate change review.

The government-run climatology centre also suggests that, by the latter half of this century, the Arctic could become largely ice-free in summer, given new evidence of a slightly faster rate of decline.

The study examined international peer-reviewed science over the past three years, and involved remodelling data on a more powerful computer. The research will feed into the Intergovernmental Panel on Climate Change's assessment in 2013.

"The evidence of the dangerous impact of climate change is clearer than ever," said Vicky Pope, head of Hadley's climate predictions programme. "New understanding of the science suggests the overall impact will be about the same [but] in some cases, like the risk of methane release from wetlands and permafrost melting, [we] now conclude that the risks are greater."

The evidence downplayed chances of the Atlantic conveyor ocean current, which warms northern Europe, from slowing, forcing temperatures down in the region. "The risks are not as great as we thought before." said Pope.

The report identified increased loss of Greenland and west Antarctic ice sheets, and new research suggesting that once gone that ice might not be able to recover. But icesheet collapse is unlikely to be catastrophic, with losses occurring at different speeds.

The worst case – of a four-metre rise in sea level – is now all but ruled out in the next century, but 20cm to 60cm rises are likely. The rise will not be the same everywhere and there is a lack of understanding about the potential regional effects.

Among other assessments, old-growth forests, which were thought to be carbon neutral, are now known to still absorb CO 2 ; there is new evidence of their susceptibility to drought, and that tropical deforestation can accelerate climate change.

There is also evidence emerging of increasing emissions of methane, a powerful, but short-lived, gas from wetlands.