Massive floods, blistering heatwaves and bizarre cold snaps since the start of the year may not be the result of climate change, but extreme weather has become more frequent, some scientists say.

The United Nation's World Meteorological Organisation has reported "there is an increasing trend in extreme events observed during the last 50 years.  It adds that "weather and climate are marked by record extremes in many regions across the world since January 2007".

Examples are not hard to find.  The death toll from the worst monsoon floods to hit South Asia in decades has passed 2,000, while Britain's recent floods were the country's worst for 60 years.  Southern Europe has dealt with record temperatures this northern summer in a brutal heatwave.  South Africa has seen unusually heavy snowfalls and the Argentinean Capital Buenos Aires got snow for the first time in 89 years.  Cyclone Gonu, the first documented tropical cyclone in the Arabian Sea, hit Oman and Iran in June, causing 50 deaths.

But establishing a link between climate change and extreme weather is controversial.  The UN's weather agency says its Intergovernmental Panel on Climate Change has found that "the warming of the climate is unequivocal".  Preliminary observations indicated global land surface temperatures in January and April reached the highest levels ever recorded for those months, it said.  "Climate change projections indicate it to be very likely that hot extremes, heatwaves and heavy precipitation events will continue to become more frequent," it said.

A study by researchers from the US National Centre for Atmospheric Research and the Georgina Institute of Technology says about twice as many Atlantic hurricanes form each year on average than a century ago.  It blamed warmer sea surface temperatures and altered wind patterns associated with global climate change for "fuelling much of the increase", the centre said.

But scientists caution there is not enough evidence to blame global warming for recent extreme weather, and there are those who say there is no proof that extreme weather events are becoming more frequent.  Barry Gromett, of Britain's Met Office weather service, said much of the extreme weather was down to variability in the climate, which was affected by greenhouse gases but also other factors such as El Nino.

El Nino events are when drastic changes in sea temperatures in tropical areas affect atmospheric pressure in the Pacific Ocean region, having a knock-on effect on rainfall.  "There's a danger in taking isolated incidents in any given year and attributing this to something like climate change," Mr Gromett said.  "It's really important to look for trends over a longer period of time.  More heat equals more moisture equals probably higher rains, so in that respect some of it ties in quite nicely [with climate change].  But there are many different facets that appear to contradict each other."

A British Met Office study issued on Thursday found natural weather variations helped offset the effects of global warming in the past couple of years, but temperatures are set to rise to record levels beginning in 2009.

Intergovernmental Panel on Climate Change climatologist Jean Jouzel said several more years were needed to establish a link between these extremes and global warming.