* Shift to new, hotter climate could be by 2047
* Warming to affect tropics first - Nature study
* Raises question of how nature, people will adapt
By Environment Correspondent Alister Doyle
OSLO, Oct 9 (Reuters) - Billions of people could be living in regions where temperatures are hotter than their historical ranges by mid-century, creating a "new normal" that could force profound changes on nature and society, scientists said on Wednesday.
Temperatures in an average year would be hotter by 2047, give or take 14 years, than those in the warmest year from 1860-2005 if the greenhouse gas emissions continue to rise, with the tropics the first affected area, a new index indicated.
"The results shocked us. Regardless of the scenario, changes will be coming soon," lead author Camilo Mora of the University of Hawaii said. "Within my generation whatever climate we were used to will be a thing of the past."
The data suggested the cities to be hit earliest included Manokwari in Indonesia, which could shift to a new climate from 2020 and Kingston, Jamaica, from 2023 under the fastest scenario of change.
At the other extreme, Moscow would depart from historical variability only in 2063 and Anchorage in 2071.
In all, the scientists found that between 1 and 5 billion people would be living in regions outside such limits of historical variability, underscoring the impact already under way from a build-up of man-made greenhouse gases.
"Unprecedented climates will occur earliest in the tropics and among low-income countries," according to the study in the journal Nature that urged cuts in greenhouse gases to limit damage to human society and wildlife.
The tropics are most vulnerable to shifting to a new state because the climate was naturally in a narrow band, they said. The Arctic is now suffering the fastest absolute temperature rises, but temperatures have naturally swung widely in history.
However, commentators noted the study did not fully address how people may become better adapted at dealing with the warming, potentially negating some of its effects.
Sceptics who question the need for urgent action have become emboldened by the fact that temperatures rose more slowly over the past 15 years despite increasing greenhouse gas emissions, especially in emerging nations led by China.
However, leading climate scientists said last month they were more convinced than ever that humans were the main culprits for global warming, and predicted the impact from greenhouse gas emissions could linger for centuries.
HOW NATURE ADAPTS
The Intergovernmental Panel on Climate Change said the hiatus in warming was a natural variation that would not last, and the Earth was set for more floods, droughts and rising sea levels from melting ice sheets that could swamp low-lying islands.
Efforts to curb emissions could delay the average expected date for the shift to a new normal climate to 2069, according to the scientists based in Hawaii and Japan.
Other experts welcomed the study as a novel perspective on climate change - most past studies examine the climate at a fixed date such as 2050 or 2100 rather than predict the timing of a shift to a new state.
"This shows the point at which what is now an extreme year becomes the norm," Chris Huntingford of the Centre for Ecology and Hydrology in England told Reuters of a commentary in Nature he wrote with Lina Mercado of Exeter University.
Huntingford noted that the study focused on a shift in mean temperatures - meaning that freak cold years in future could still be chillier than the hottest years in the historical record examined, led by 2005 and 1998.
Huntingford said the study did not fully examine the possibility that people and nature may be better at adapting to warming than expected. "It remains one of big open questions," he said.
A heatwave in 2003 in Europe, the hottest in 500 years, killed up to 70,000 people but better preparations now would reduce the toll if it were to happen again, many scientists say.
The study examined temperatures back to 1860, for which reliable records are available. The U.N. panel of climate scientists said in its report that the period 1983-2012 was likely to have been the warmest in the past 1,400 years.
For the Nature study, click here:
(Editing by Alison Williams)