By Nina Chestney
LONDON (Reuters) - An internationally agreed target to limit rises in global average temperatures to within 2 degrees Celsius is around double the threshold that would avoid catastrophic climate change, a study by 18 eminent scientists said.
Governments decided in 2009 that such temperature increases needed to be no more than 2 degrees C (3.6 Fahrenheit) above pre-industrial levels to avoid effects such as more extreme weather, higher sea levels and ocean acidification.
They aim to agree by 2015 on a global deal to cut the greenhouse gas emissions blamed for climate change, but the reductions will not come into force until after 2020.
Last month, a United Nations conference in Warsaw kept alive hopes for the 2015 deal but nations made little progress on committing to ambitious emission cuts to keep the world on track towards the 2 degree target.
A study published in U.S.-based scientific journal PLOS One on Tuesday said the 2 degree limit was too high and a more appropriate target was around 1 degree C.
"Some climate extremes are already increasing in response to warming of several tenths of a degree in recent decades; these extremes would likely be much enhanced with warming of 2 degrees C or more," the report's authors said in a statement.
The scientists involved in the study are James Hansen and Jeffrey Sachs of the Earth Institute at Columbia University, Pushker Kharecha of the NASA Goddard Institute for Space Studies, and 15 other climate experts from universities and institutes across the world.
"An appropriate target is to keep global temperature within or close to the temperature range in the Holocene - the interglacial period in which civilization developed," they said.
The Holocene is the current geological epoch that started around 11,700 years ago and has experienced relatively stable temperatures.
The world cooled slowly in the last half of the Holocene but warming of 0.8 degree C over the past 100 years has brought the global temperature back to near the epoch's maximum, the study said.
Warming could be held to around 1 degree C if emissions from burning fossil fuels were cut by 6 percent a year from 2013 and by reforestation, which would result in 500 billion tonnes of cumulative carbon in the atmosphere near the end of the century, the study said.
However, if emissions continued to grow until 2020, they would then have to be reduced by 15 percent a year to reach 500 billion tonnes.
"The huge fossil fuel energy infrastructure now in place makes it practically certain that the 500 (billion tonne) limit will be exceeded," the study said.
The United Nations' panel of climate experts has said the world needs to stay within a 1 trillion tonne "carbon budget" to meet the 2 degree target.
However, this level would spur slower climate effects such as ice melt and ocean acidification and result in warming of 3-4 degrees C, the PLOS One study said.
The full study is available at: www.plosone.org/
(Editing by Dale Hudson)