By Alister Doyle, Environment Correspondent

OSLO, Aug 6 (Reuters) - Global warming has been going on for so long that most people were not even born the last time the Earth was cooler than average in 1985 in a shift that is altering perceptions of a "normal" climate, scientists said.

Decades of climate change bring risks that people will accept higher temperatures, with more heatwaves, downpours and droughts, as normal and complicate government plans to do more to cut emissions of greenhouse gas emissions.

"Because the last three decades have seen such a significant rise in global and regional temperatures, most people under the age of 30 have not lived in a world without global warming," Michel Jarraud, Secretary-General of the U.N.'s World Meteorological Organization (WMO), told Reuters.

"On human timescales the changes in our climate can seem gradual, so we will increasingly need to remind the public about just how rapid and unprecedented the changes truly are," Jarraud said.

February 1985 was the last month when global temperatures were below the 20th century average, according to the U.S. National Oceanic and Atmospheric Administration (NOAA), a leading source of global temperature data.

Meanwhile, the estimated median age of the world population in 2014 is 29.4 years, meaning half are older and half younger, Francois Pelletier of the U.N. Population Division told Reuters.

Taken together, the NOAA and U.N. yardsticks mean the world's 7.2 billion population has shifted in recent weeks for the first time to a majority born since the last cool month.

"People have to get used to continuous change in the climate," said Thomas Peterson, principal scientist at NOAA's National Climatic Data Center and president of the WMO Commission for Climatology.

Some other weather agencies, using differing methods and baselines, estimate later dates for the most recent cold month than NOAA. The WMO, which compiles annual data, says 1985 was the last colder-than-average year.

Global averages go largely unnoticed because individuals experience weather and climate locally - this past winter was bitterly cold in parts of North America, for instance. But the overall warming trend is clear.

DROUGHTS, FLOODS

Peter Thorne, a climate researcher at the Nansen Environmental and Remote Sensing Center in Bergen, west Norway, said people are more likely to remember extreme weather events than to notice any fractional rise in temperatures.

"Heatwaves, droughts and extreme floods are more likely to trigger associations with climate change," he said. And more extremes could in turn put pressure on governments to act.

Almost 200 governments have agreed to work out a deal to slow global warming at a summit in Paris in late 2015, mainly by cutting emissions of greenhouse gases from cars, power plants and factories.

Governments have promised to limit warming to below 2 degrees (3.6F) above pre-industrial times - average temperatures have already risen by about 0.8C (1.4F).

Peterson said historical records of average temperatures, used by everyone from farmers planning crops to companies deciding how much insulation to install in new buildings, were no longer a reliable guide to the future.

His WMO commission said last month that the concept of "normal" weather should to be updated more frequently to take better account of warming.

Currently, the WMO period for normal weather is 1961-1990 and is due to be replaced by 1991-2020 in 2021. The Commission wants rolling updates every decade, meaning the current period would be 1981-2010 and become 1991-2020 in 2021. (Reporting By Alister Doyle; Editing by Toby Chopra and Raissa Kasolowsky)