SCIENTISTS are struggling to explain a slowdown in climate change that has exposed gaps in their understanding and defies a rise in global greenhouse gas emissions.
Often focused on century-long trends, most climate models failed to predict that the temperature rise would slow, starting around 2000. Scientists are now intent on figuring out the causes and determining whether the respite will be brief or a more lasting phenomenon.
Getting this right is essential for the short and long-term planning of governments and businesses ranging from energy to construction, from agriculture to insurance. Many scientists say they expect a revival of warming in coming years.
Theories for the pause include that deep oceans have taken up more heat with the result that the surface is cooler than expected, that industrial pollution in Asia or clouds are blocking the sun, or that greenhouse gases trap less heat than previously believed.
The change may be a result of an observed decline in heat-trapping water vapour in the high atmosphere, for unknown reasons. It could be a combination of factors or some as yet unknown natural variations, scientists say.
Weak economic growth and the pause in warming is undermining governments' willingness to make a rapid billion-dollar shift from fossil fuels. Almost 200 governments have agreed to work out a plan by the end of 2015 to combat global warming.
"The climate system is not quite so simple as people thought," said Bjorn Lomborg, a Danish statistician and author of "The Skeptical Environmentalist" who estimates that moderate warming will be beneficial for crop growth and human health.
Some experts say their trust in climate science has declined because of the many uncertainties. The UN's Intergovernmental Panel on Climate Change (IPCC) had to correct a 2007 report that exaggerated the pace of melt of the Himalayan glaciers and wrongly said they could all vanish by 2035.
"My own confidence in the data has gone down in the past five years," said Richard Tol, an expert in climate change and professor of economics at the University of Sussex in England.
Swedish chemist Svante Arrhenius first showed in the 1890s how man-made carbon dioxide, from coal for instance, traps heat in the atmosphere. Many of the exact effects are still unknown.
Greenhouse gas emissions have hit repeated record highs with annual growth of about 3 percent in most of the decade to 2010, partly powered by rises in China and India. World emissions were 75 percent higher in 2010 than in 1970, UN data show.
UN PANEL SEEKS EXPLANATION
A rapid rise in global temperatures in the 1980s and 1990s - when clean air laws in developed nations cut pollution and made sunshine stronger at the earth's surface - made for a compelling argument that human emissions were to blame.
The IPCC will seek to explain the current pause in a report to be released in three parts from late 2013 as the main scientific roadmap for governments in shifting from fossil fuels towards renewable energies such as solar or wind power, the panel's chairman Rajendra Pachauri said.
According to Pachauri, temperature records since 1850 "show there are fluctuations. They are 10, 15 years in duration. But the trend is unmistakable."
The IPCC has consistently said that fluctuations in the weather, perhaps caused by variations in sunspots or a La Nina cooling of the Pacific, can mask any warming trend and the panel has never predicted a year-by-year rise in temperatures.
Experts say short-term climate forecasts are vital to help governments, insurers and energy companies to plan.
Governments will find little point in reinforcing road bridges over rivers, for instance, if a prediction of more floods by 2100 doesn't apply to the 2020s.
A section of a draft IPCC report, looking at short-term trends, says temperatures are likely to be 0.4 to 1.0 degree Celsius (0.7-1.8F) warmer from 2016-35 than in the two decades to 2005. Rain and snow may increase in areas that already have high precipitation and decline in areas with scarcity, it says.
EXCEPTIONS AND CHALLENGES
Pachauri said climate change can have counter-intuitive effects, like more snowfall in winter that some people find hard to accept as side-effects of a warming trend. An IPCC report last year said warmer air can absorb more moisture, leading to heavier snowfall in some areas.
A study by Dutch experts this month sought to explain why there is now more sea ice in winter. It concluded melted ice from Antarctica was refreezing on the ocean surface - this fresh water freezes more easily than dense salt water.
Some experts challenged the findings.
"The hypothesis is plausible I just don't believe the study proves it to be true," said Paul Holland, an ice expert at the British Antarctic Survey.
Concern about climate change is rising in some nations, however, opinion polls show. Extreme events, such as Superstorm Sandy that hit the U.S. east coast last year, may be the cause. A record heatwave in Australia this summer forced weather forecasters to add a new dark magenta colour to the map for temperatures up to 54 degrees Celsius (129F).