From cautiously advising that man-made, heat-trapping carbon gases would disrupt Earth's climate system, mainstream scientists are increasingly convinced that the first signs of change are already here.
Following are the main indicators, reported in the scientific press over past three years:
RISING SEAS: Sea levels have risen in tandem with global warming, according to the UN's Intergovernmental Panel on Climate Change (IPCC). The global average sea level has risen since 1961 at an average rate of 1.8mm (0.07 inches) per year, but accelerated from 1991 to 3.1mm (0.12 inches) per year. The IPCC estimated sea levels would rise 18-59 centimetres (7.2-23.2 inches) by 2100. But added runoff from melting land ice is accelerating. According to Germany's Potsdam Institute for Climate Impact Research (PIK), the global sea level is likely to rise at least twice as much as projected. If emissions are not curbed, "it may well exceed one metre (3.25 feet)."
SHRINKING GLACIERS: Mountain glaciers and snow cover in both hemispheres have widely retreated in the past few decades. One of the most closely-observed sites, the Cook glacier on the southern Indian Ocean island of Kerguelen, has shrunk by a fifth in 40 years. Around 1.3 billion people depend on the water that flows down from Himalayan glaciers, which in some places are falling back at up to 70 metres (230 feet) per year. The snows capping Mount Kilimanjaro, Africa's tallest peak, could vanish entirely in 20 years, US experts reported this month.
SHIFTING SEASONS: Some species of birds and fish are shifting habitat in response to warmer temperatures. The range of 105 bird species in France moved north, on average, 91 kilometres (56.5 miles) from 1989 to 2006. Average temperatures, however, shifted northward 273 kilometres (170 miles) over the same period, nearly three times farther. Twenty-one out of 36 species of fish in the North Sea migrated northwards between 1962 and 2001 in search of cooler waters. Anecdotal evidence from commercial fishermen says once-exotic species of fish from warmer latitudes now inhabit southern British waters.
OCEAN ACIDIFICATION: The acidity of the seas is rising as oceans absorb more carbon dioxide (CO2), with an impact on coral and micro-organisms, marine biologists say. Since the start of the Industrial Revolution, the protective calcium shell of amoeba-like organisms living in the Southern Ocean called foraminifera, a vital link in the food chain, has fallen in weight by a third. "Within decades," acidification could severely affect biodiversity and fisheries, 150 marine scientists jointly warned last January.
ARCTIC ICE: The Greenland ice sheet has lost 1,500 billion tonnes of ice since 2000, contributing 0.75 mm (0.03 inch) annually to sea levels, according to a study published this month. In 2009, the Arctic summer sea ice pack thawed to its third smallest size on record, confirming a shrinkage trend seen over the past 30 years. Some experts believe the Arctic ice cap will disappear completely in summer months within 20 to 30 years.
ANTARCTIC WARMING: The Antarctic peninsula has warmed by 2.5 degrees Celsius (4.5 degrees Fahrenheit) in the last 50 years, around six times the global average. In the past 20 years, Antarctica has lost seven ice shelves - huge floating ledges of ice, attached to the shore, that are fed by glaciers.
PERMAFROST RETREAT: Emissions of the potent greenhouse gas methane were found to be soaring at sites investigated in 2006 by University of Alaska scientists at lakes in northern Siberia. The reason is thawing of the permafrost, causing the warmed soil to release gas that had been stored for thousands of years. Billions of tonnes of methane, which comes from natural sources such as decomposing vegetation and marshland, are stored in the frozen lands of Siberia, Canada and Alaska.
CHANGED PRECIPITATION: Patterns of rainfall or snowfall increased "significantly" from 1900-2005 in eastern parts of North and South America, northern Europe and northern and central Asia but declined in the Sahel, southern Africa and parts of southern Asia, says the IPCC. "Globally, the area affected by drought has likely increased since the 1970s," it adds.
STORMS: A mooted link between climate change and extreme events has little scientific consensus. A 2008 study by the Benfield UCL Hazard Research Centre at University College London found that warmer seas accounted for 40 percent of a large increase (from six a year to eight a year) in the number of Atlantic hurricanes from 1996-2005. Other scientists say it is hard to say whether a drought, flood or cyclone is part of the longer trend which is climate change or simply just a one-off event, or series of them.
SOURCES: IPCC 4th Assessment Report (2007); Antarctic Climate and Ecosystems Cooperative Research Center (Australia); Potsdam Institute for Climate Impact Research (PIK); Nature; Science; Nature Geoscience; Laboratory for Studying Geophysics and Space Oceanography (France); French National Museum of Natural History; Pen Hadow Arctic expedition; US National Snow and Ice Data Center; British Antarctic Survey (BAS); University of Alaska at Fairbanks.
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