REFILE-FACTBOX-Climate change sceptics say humans not to blame

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April 2 (Reuters) - A group of scientists is fighting a rearguard action to challenge mainstream evidence that humans are to blame for climate change.

They point to natural shifts in the sun's heat, a cooling of the planet in the mid-20th century and an apparent slowdown of temperature rises in the past decade.

The U.N. Intergovernmental Panel on Climate Change (IPCC) said in February that it was "very likely" -- more than 90 percent -- that human activities, namely fossil fuel burning, explained most of an "unequivocal" warming in the past 50 years.

The panel said temperatures will likely rise by between 1.8 and 4.0 Celsius (3.2 and 7.8 Fahrenheit) this century. The IPCC, made up of about 2,500 scientists, is endorsed by governments.

"There is always a bit of room for doubt...it's in the nature of science," said Rajendra Pachauri, head of the U.N. climate panel. "But I cannot think of any tangible reasons for doubt."

The "sceptics" who doubt some IPCC claims include meteorology professor Richard Lindzen of the Massachussets Institutue of Technology, Professor Paul Reiter from the medical entomology department at the the Pasteur Institute in Paris and author Michael Crichton.

Many scientists also say U.S. President George W. Bush has exaggerated the uncertainties about scientific findings to appease powerful business and oil lobbies.



SCEPTIC VIEW

Here are some of the arguments of those who cast doubt on mankind's responsibility for climate change, and beneath each a response by the Hadley Centre of Britain's Meteorological Office, its official centre for climate change research.



1. Temperatures dropped for several decades after 1945, despite rising carbon dioxide emissions

*** Along with carbon dioxide, fossil fuels also release particles called aerosols, which cool the climate by reflecting sunlight. Aerosols dominated the warming effect of CO2 prior to clean-air acts of the 1960s and 1970s.



2. Carbon dioxide concentrations in the atmosphere lag temperature rises in an ice core record dating back 600,000 years.

*** Over the past several hundred thousand years, changes in the earth's orbit around the sun led to temperature changes, which in turn affected CO2 levels.

Concentrations of C02 are higher than they had been during the past 600,000 years. The counter-effect is that human-induced increases of C02, such as factory emissions, have enhanced the greenhouse effect and led to warming.



3. Changes in solar activity also produce good correlations with temperature change.

*** There are many factors which may contribute to climate change. Satellite measurements showed no big change in solar heating in the last three decades of the 20th century. CO2 has been shown to have caused most warming in the past 50 years.



4. Rising temperatures in the second half of the last century have plateaued in the past 10 years.

*** 1998 was extremely warm due to the weather anomaly El Nino warming the Pacific Ocean, and subsequent years were colder. Ten years is too short a period to see long-term trends. While the World Meteorological Organization says 1998 was the warmest year since records began 150 years ago, NASA says 2005 was warmer.



See related factbox for changes in temperatures, CO2 emissions - [ID:nL14484051]