Today's global warming is the driving force behind nearly a fifth of heavy downpours and three quarters of extremely hot days, according to new research by scientists in Switzerland.

It also found the level of the human influence on weather increases as warming does.

In a paper published in Nature Climate Change, researchers found that if temperatures rise to 2 degrees Celsius above pre-industrial levels, the proportion of precipitation extremes caused by humans will increase to 40%.

Scientists used two metrics to evaluate what proportion of global high temperature and heavy precipitation events can be blamed on humans.

They then applied the metrics to 25 climate models spanning the last century and projecting forward into the period up to 2100.

They based their model on an assumption of a continuation of high carbon emissions.

The researchers claim that the rarest and most extreme weather events are likely to be more heavily influenced by greenhouse-gas emissions.

"We were surprised," said co-author Erich Fischer of the findings, especially by the high impact of global warming on heat extremes.

Last year was the warmest since records began in the 19th century, according to the United Nation's World Meteorological Organisation.

Heavy flooding hit countries including Serbia, Bangladesh, Pakistan, South Africa, Morocco and Brazil.

Global average temperatures have risen 0.85 degrees Celsius above pre-industrial times so far and further warming would increase risks of extremes, the Swiss study found.

A rise in temperatures to a UN ceiling of 2 degrees Celsius above pre-industrial times would raise the share of heat extremes attributable to warming to 96% and the share of extreme precipitation events to 40%, it said.