Last year's record shrinkage of Arctic sea ice and a spell of catastrophic droughts, floods and storms highlight the risk to the planet from climate change, the UN Environment Programme (UNEP) said on Monday.
In an annual review of the world's environment coinciding with ministerial-level talks in Nairobi, UNEP also warned of an alarming surge in elephant and rhino poaching.
In 2012, summer sea ice in the Arctic covered a record low area of 3.4 million square kilometres (1.31 million square miles), which was 18 percent below the previous recorded minimum in 2007, and 50 percent below the average in the 1980s and 1990s, UNEP said.
Land ice in Greenland also showed signs of melting and permafrost in high latitudes was in retreat, it said.
"Changing environmental conditions in the Arctic, often considered a bellwether for global climate change, have been an issue of concern for some time, but as of yet this awareness has not translated into urgent action," UNEP Executive Director Achim Steiner said.
He pointed to a rush to extract the oil and gas in the Arctic's seabed as the ice retreats and cautioned that the outcome could be even greater emissions of greenhouse gases.
"What we are seeing is that the melting of ice is prompting a rush for exactly the fossil-fuel resources that fuelled the melt in the first place," said Steiner.
"The rush to exploit these vast untapped reserves has consequences that must be carefully thought through by countries everywhere, given the global impacts and issues at stake."
The report -- the UNEP 2013 Year Book -- also noted a string of weather disasters, of which the United States bore the brunt, including its worst drought in decades as well as Hurricane Sandy, which flooded parts of New York City and the coastline of New Jersey.
"Extreme weather events... in 2012 draw attention to the need to prepare for and make efforts to prevent heavy losses" as a result of climate change, it said.
It quoted previously published estimates that Sandy will cost the insurance business between $20 and $25 billion, and building new flood defences to protect the city could cost $20 billion or more.
But these figures could rise, UNEP said.
The report also sounded the alarm over poaching.
"The number of elephants killed in 2011 likely ran into the tens of thousands, and early indications suggest that the same happened again in 2012," it said.
"A record 688 rhinos were poached in South Africa in that year, driving this species further towards extinction."
The Nairobi meeting, running until Friday, is the first since the UN's Rio+20 summit last June that called for "strengthening and upgrading" UNEP and boosting efforts in the green economy.