Heatwaves, drought and floods that have struck the northern hemisphere for the third summer running are narrowing doubts that man-made warming is disrupting Earth's climate system, say some scientists.

Climate experts as a group are reluctant to ascribe a single extreme event or season to global warming.

Weather, they argue, has to be assessed over far longer periods to confirm a shift in the climate and whether natural factors or fossil-fuel emissions are the cause.

But for some, such caution is easing.

A lengthening string of brutal weather events is going hand in hand with record-breaking rises in temperatures and greenhouse-gas levels, an association so stark that it can no longer be dismissed as statistical coincidence, they say.

"We prefer to look at average annual temperatures on a global scale, rather than extreme temperatures," said Jean Jouzel, vice chairman of the UN's Nobel-winning scientists, the Intergovernmental Panel on Climate Change (IPCC).

Even so, according to computer models, "over the medium and long term, one of the clearest signs of climate change is a rise in the frequency of heatwaves", he said.

"Over the last 50 years, we have seen that as warming progresses, heatwaves are becoming more and more frequent," Jouzel said.

"If we don't do anything, the risk of a heatwave occurring will be 10 times higher by 2100 compared with the start of the century."

The past three months have seen some extraordinary weather in the United States, Europe and East and Southeast Asia.

The worst drought in more than 50 years hit the US Midwest breadbasket while forest fires stoked by fierce heat and dry undergrowth erupted in California, France, Greece, Italy, Croatia and Spain.

Heavy rains flooded Manila and Beijing and China's eastern coast was hit by an unprecedented three typhoons in a week.

Last month was the warmest ever recorded for land in the northern hemisphere and a record high for the contiguous United States, according to the US National Oceanic and Atmospheric Administration (NOAA).

Globally, the temperature in July was the fourth highest since records began in 1880, it said.

James Hansen, arguably the world's most famous climate scientist (and a bogeyman to climate skeptics), contends the link between extreme heat events and global warming is now all but irrefutable.

The evidence, he says, comes not from computer simulations but from weather observations themselves.

In a study published this month in the peer-reviewed US journal Proceedings of the National Academy of Sciences (PNAS), Hansen and colleagues compared temperatures over the past three decades to a baseline of 1951-80, a period of relative stability.

Over the last 30 years, there was 0.5-0.6 C (0.9-1.0 F) of warming, a rise that seems small but "is already having important effects", said Hansen, director of NASA's Goddard Institute for Space Studies.

During the baseline period, cold summers occurred about a third of the time, but this fell to about 10 percent in the 30-year period that followed.

Hot summers which during the baseline period occurred 33 percent of the time, rose to about 75 percent in the three decades that followed.

Even more remarkable, though, was the geographical expansion of heatwaves.

During the baseline period, an unusually hot summer would yield a heatwave that would cover just a few tenths of one percent of the world's land area.

Today, though, an above-the-norm summer causes heatwaves that in total affect about 10 percent of the land surface.

"The extreme summer climate anomalies in Texas in 2011, in Moscow in 2010 and in France in 2003 almost certainly would not have occurred in the absence of global warming with its resulting shift of the anomaly situation," says the paper.

In March, an IPCC special report said there was mounting evidence of a shift in patterns of extreme events in some regions, including more intense and longer droughts and rainfall. But it saw no increases in the frequency, length or severity of tropical storms.