Long-term concerns remain | Unexpected slowdown in global warming puzzles scientists and policy makers
OSLO, Norway (Reuters) — 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.
Most climate models, which often focused on century-long trends, 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 government and business as they make short- and long-term plans for industries as diverse as energy, construction, agriculture and insurance.
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Many scientists say they expect a revival of warming in coming years.
There are several theories for the pause:
- Water at the oceans’ surface is cooler than expected because deep oceans have taken up more heat.
- Clouds and industrial pollution in Asia are blocking the sun.
- Greenhouse gases trap less heat than previously believed.
- An observed decline in heat-trapping water vapour in the high atmosphere, for unknown reasons.
- A combination of factors or some as yet unknown natural variations.
Weak economic growth and the pause in warming is undermining governments’ willingness to make a rapid billion-dollar shift from fossil fuel. 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 benefit crop growth and human health.
Some experts say their trust in climate science has declined because of the many uncertainties. The United Nations’ Intergovernmental Panel on Climate Change 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 traps heat in the atmosphere. Many of the exact effects are still unknown.
Greenhouse gas emissions have hit repeated record highs in most of the decade to 2010, with annual growth of three percent. The increase was partly powered by rises in China and India. World emissions were 75 percent higher in 2010 than in 1970, UN data show.
A rapid rise in global temperatures in the 1980s and 1990s, when clean air laws in developed nations cut pollution and strengthened sunshine 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 three-part report that will serve as the main scientific road map for governments as they shift from fossil fuels toward renewable energy such as solar or wind power.
Panel chair Rajendra Pachauri said 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 warming trends. As well, 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 plan.
For example, governments will find little point in reinforcing road bridges over rivers if a prediction of more floods by 2100 doesn’t apply to the 2020s.
A section of a draft IPCC report that looks at short-term trends said temperatures are likely to be 0.4 to 1 C warmer from 2016 to 2035 than in the two decades preceding 2005. Rain and snow may increase in areas that already have high precipitation and decline in areas with scarcity, it added.
Pachauri said climate change can have counter-intuitive effects, such as more snowfall in winter, which 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 recent study by Dutch experts sought to explain why there is now more sea ice in winter. It concluded that melted ice from Antarctica was refreezing on the ocean surface because 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.
However, opinion polls show that concern about climate change is rising in some nations. Extreme events, such as Superstorm Sandy that hit the U.S. east coast last year, may be the cause.
A record heat wave in Australia this summer forced weather forecasters to add a new dark magenta colour to the map for temperatures up to 54 C.
