Ice volume around the Arctic region hit the lowest level ever recorded this year as climate extremes brought death and devastation to many parts of the world, according to the UN weather agency World Meteorological Organisation.
Although the world's average temperature this year was, at 14.3°C, by a fraction of a degree the coolest so far this century, the direction towards a warmer climate remained steady, it reported.
"What is happening in the Arctic is one of the key indicators of global warming," said Michel Jarraud, secretary general of the WMO. "The overall trend is still upwards."
A report presented by Mr Jarraud at a news conference showed Arctic ice cover dropping to its second lowest extent during this year's melt season since satellite measuring began in 1979.
However, the Geneva-based agency said, "because ice was thinner this year, overall ice volume was less than in any other year." It added: "The season strongly reinforced the 30-year downward trend in the extent of Arctic Sea ice."
The dramatic collapse of a quarter of ancient ice shelves on Canada's Ellesmere Island in the north of the Arctic Ocean added to earlier meltdowns, reducing cover in the region from 9,000 square kilometres a century ago to just 1,000 square kilometres.
The WMO said the slight slowdown in warming this year, an increase of 0.31°C over the 14°C of the base period 1961-90, against an average 0.43°C for 2001-2007, was due to a moderate-to-strong La Nina in the Pacific in late last year.
"This decade is almost 0.2°C warmer compared to the previous decade. We have to look at it in that way, comparing decades not years," Peter Stott, a climate scientist at Britain's Hadley Centre, which provided data for the WMO report said.
La Nina is a periodic weather pattern that develops when Pacific sea water cools. It alternates irregularly with the related El Nino - when the Pacific warms up - and both affect the climate all round the world.
The WMO report was based on statistics and analyses compiled by weather services among its 188 member countries and specialist research institutions, including government-backed bodies in the US and Britain.
"Climate extremes, including devastating floods, severe and persistent droughts, snow storms, heat waves and cold waves were recorded in many parts of the world," the agency said. In many of these, hundreds or even thousands of people died.
Among the disasters was Cyclone Nargis, which killed some 78,000 in Myanmar's southern delta region in early May. In the western Atlantic and Caribbean there were 16 major tropical storms, eight of which developed into hurricanes.
In an average year, there are 11 storms of which six become hurricanes and two become major hurricanes. This year, five major hurricanes developed, and for the first time on record six tropical storms in a row made landfall in the US.
The WMO says the 10 hottest years since global records were first kept in 1850 have all been since 1997, with the warmest at 14.79°C in 2005. Countries have been struggling for years to reach agreement on how to halt the trend.
This month a two-week meeting of leaders in Poznan, Poland, called to prepare a treaty for late 2009 seemed to falter amid rows between rich and poor nations and what some climate campaigners say was lack of will to get things done. This year will be the coolest since 1997 but still the tenth hottest in a temperature record dating back 150 years, the World Meteorological Organisation said.
The global mean temperature for this year was 14.3°C, climate scientists at the UK's Met Office Hadley Centre and Climate Research Unit at the University of East Anglia, who compiled data for the WMO, said.
"Human influence, particularly emission of greenhouse gases, has greatly increased the chance of having such warm years," the Met Office's Peter Stott said in a statement.
"Comparing observations with the expected response to man-made and natural drivers of climate change, it is shown that global temperature is now over 0.7°C warmer than if humans were not altering the climate."
Global temperatures vary annually according to natural cycles. For example, they are driven by shifting ocean currents, and scientists say dips do not undermine the case that man-made greenhouse gas emissions are causing long-term global warming.
The 10 warmest years measured since records began in 1850 have occurred since 1997, with global temperatures for 2000-2008 standing at almost 0.2°C above the average for the decade 1990-1999, the Met Office said.
"Globally this year would have been considered warm, even as recently as the 1970s or 1980s, but a scorcher for our Victorian ancestors," said Oxford University's Myles Allen.
"As a result of climate change, what would once have been an exceptionally unusual year has now become quite normal. Without human influence on climate change we would be more than 50 times less likely of seeing a year as warm as 2008," added Dr Stott.