The heaviest rainfall in 200 years, floods and gales experienced in Britain this winter are a foretaste of what is certain to occur in a warming world, the head of the UN's climate science panel has told the Guardian.

"Each of the last three decades has been warmer than the last. Extreme events are on the increase. Even if what we have just had [this winter] was not caused by anthropogenic climate change, events of this nature are increasing both in intensity and frequency," said Rajendra Pachauri, chair of the Intergovernmental Panel on Climate Change.

"Two types of extreme events are going to occur more frequently – extreme precipitation and heatwaves. It is important for societies to deal with climate change if we want to avoid the impacts."

Pachauri was speaking ahead of the publication next month of a major global assessment of the impact of climate change on the world's food supplies, human health, cities and rural areas. Leaked copies seen by the Guardian warn of crop yields falling 2% a decade even as the demand from rapidly growing population increases by 14% per decade. It also warns of extreme heat stress in cities, increased precipitation and widespread flooding.

"Due to sea level rise throughout the century and beyond, coastal and low-lying areas will increasingly experience adverse impacts such as coastal erosion and flooding. Without adaptation, hundreds of millions of people will be affected by coastal flooding and will be displaced due to land loss," says the draft copy of the report, which has been prepared by the IPCC with the input of several thousand scientists.

Negotiators from 193 countries resume the UN's long-running climate talks in Bonn on Monday. UN climate chief Christiana Figueres, told the Guardian this week that the "very strange" weather experienced across the world recently was a sign that we are already experiencing climate change.

"If you take them individually you can say maybe it's a fluke. The problem is it's not a fluke and you can't take them individually," she said. "What it's doing is giving us a pattern of abnormality that's becoming the norm. These very strange extreme weather events are going to continue in their frequency and their severity … It's not that climate change is going to be here in the future, we are experiencing climate change."

"Climate change is now built into the system," said Pachauri. "We can say that there will be serious impacts on food production in every region of the world with climate change. The Mediterranean will have severe problems with water scarcity which will impact on food. Some parts of Africa could have declines of 50% [in crop yields] as soon as 2020. We must keep in mind the fact that population is going up very rapidly."

He added that the world did not have much time. "There is a very short window of opportunity to cut emissions enough to hold temperatures to an increase of 2C. Six years ago we said that emissions would have to peak by 2015 if we wanted to hold them to 2C. The cost rises the later you do it. Countries have to decide what would be the implications of inaction."

Pachauri's warnings came as a new paper in the journal Science suggests that even a 1C rise in temperatures could lead to an extra three million malaria cases in children under 15 years of age per year. The disease, which infects more than 200 million people per year and kills around 600,000, is spread by mosquitoes and will start to affect higher elevations as temperatures rise.