Achim Steiner said that more damaging weather extremes were in line with forecasts by the UN Climate Panel. He urged governments to stick to a timetable meant to end in December 2009 with a new UN pact to fight global warming.
"These natural disasters do reflect a pattern of change that is in line with projections" by experts on the Intergovernmental Panel on Climate Change (IPCC), he said in a telephone interview from Geneva.
"As you watch the hurricane season in the Atlantic, as we watch the cyclones and the flood events in India, clearly we have more reason than ever to be concerned about the unfolding of patterns that the IPCC has forecast," he said.
He said it was impossible to link individual weather events, such as Hurricane Gustav battering the US Gulf Coast on Monday, to climate change stoked by human activities led by use of fossil fuels.
But they match patterns forecast by the IPCC, which shared the 2007 Nobel Peace Prize with former US Vice President Al Gore. The IPCC is marking its 20th anniversary in Geneva this week.
Gustav slammed ashore on the US Gulf Coast just west of New Orleans on Monday, a new blow to the city devastated by Hurricane Katrina in 2005. Gustav weakened to a category 1, the lowest on a five-point scale.
In India, three million people have been displaced from their homes and at least 90 killed by floods in India's eastern state of Bihar, officials say, after the Kosi river burst a dam in Nepal. The floods are the worst in Bihar in 50 years.
In addition to the human suffering "we have an economic escalation from damage from natural disasters," Steiner said.
Insurers Munich Re said that first-half losses from natural catastrophes totalled about $50 billion - many linked to a rising number of extreme weather events.
The main exception was $20 billion from China's Sichuan earthquake that killed at least 70,000 people. For all of 2007, losses totalled $82 billion, it said in a July report.
"Growing populations and infrastructure means that we are going to face more and more events of this nature," Steiner said.
Katrina was the costliest hurricane in US history, killing some 1,500 people and causing over $80 billion in damage.
"Natural disasters are increasingly becoming a major risk to our economies," Steiner said. "Our societies cannot afford this, our insurance industry cannot afford an escalation of risks."
Linking 2008 floods of the Kosi to Climate Change is scandalous. The breach took place at a discharge of only 144,000 cusecs while the embankments were designed for a discharge of 950,000 cusecs. Had this discharge exceeded 950,000 cusecs or had occurred in the month of March when it normally remains at 8 to 9000 cusecs, one could link it to something unususl, climate change included. Nothing of that sort had happened and this was a clear cut case of negligence on the part of those who were responsible for the maintenance of the structures. Please do not confuse the issue.