Huge amounts of heat – equivalent to the power of 150 billion electric kettles – are being continuously absorbed by the deep ocean, which could explain why global warming has “paused” over the past 10 to 15 years, scientists have concluded in a series of reports to explain why the Earth’s rate of warming has slowed down.
Global average temperatures are higher now than they have ever been since modern records began. However, after a period of rapid temperature increases during the 1980s and 1990s there has been a significant slow-down since the turn of the century, leading some sceptics to claim that global warming has stopped.
A scientific assessment of the planet’s heat balance has found that the most likely explanation for the recent hiatus in global warming is the continual absorption of thermal energy by the huge “heat sink” of the deep ocean many hundreds of metres below the sea surface, according to scientists from the Met Office.
Senior climate scientists said that they had always expected periods when the rate of increase in temperatures would level off for a few years and emphasised that the last decade was still warmer than any previous decade, with 12 of the 14 hottest years on record occurring since 2000.
Professor Rowan Sutton, a climate scientist at Reading University, said the temperatures have levelled off in the past, the latest example being in the 1940s and 1950s when sulphate pollutants from the post-war boom in industrial production may have acted as a shield against incoming solar radiation.
“Some people call it a slow-down, some call it a hiatus, some people call it a pause. The global average surface temperature has not increased substantially over the last 10 to 15 years,” Professor Sutton said.
“Climate scientists absolutely expect variations in the rate at which surface temperature will rise….but that is not to say we understand all the details of the last 10 to 15 years,” Professor Sutton said.
The problem for the Met Office is to explain why the rate of increase in global temperatures has declined in recent years while concentrations of carbon dioxide in the atmosphere have continued to accelerate. Sceptics claim that this shows there is not a strong link between the two, whereas climate scientists insist that rising carbon dioxide concentrations are largely responsible for the rise in global temperatures.
Professor Stephen Belcher, head of the Met Office Hadley Centre, said that a pause in the rate of increase in global temperatures lasting this long is unusual but not exceptional, with similar pauses of about 10 years expected on average twice every century.
The most likely explanation for the current pause is that excess heat trapped by carbon dioxide in the atmosphere is being transferred from the atmosphere to the oceans where it is being transported down to deeper layers that cannot be monitored by satellites, Professor Belcher said.
“It looks like the Earth is continuing to accumulate energy but it looks like it is being re-arranged and hidden from view,” he said.
However, measurements from hundreds of ocean floats released over the last decade, which descend and drift to depths of up to 2,000 metres, show that huge amounts of heat from the sea surface is now being transferred to the deep ocean, with unknown consequences for the environment, the scientists said.
“In summary, observations of ocean heat content and of sea-level rise suggest that the Earth system has continued to absorb heat energy over the past 15 years, and that this additional heat has been absorbed in the ocean,” says the Met Office report.
The pause, however, is unlikely to change the predictions over the future course of global warming. Temperature increases expected by 2015 will only be delayed by a further five or ten years, the scientists said. Average surface temperatures are still on course to increase by 2C this century, with further rises expected by the end of the century if nothing is done to curb carbon dioxide emissions, they said.
It is not possible to account for the recent lack of surface warming solely by looking at the difference between amount of heat being received from the Sun and the amount of thermal energy being lost from the planet – calculations show that extra heat continues to be absorbed by the Earth at a rate of 0.6 Watts per square metre, equivalent to about 300bn 1KW electric heaters or 150bn 2KW kettles distributed across the planet.
“Observations of ocean heat content and of sea-level rise suggest that the additional heat from the continued rise in atmospheric carbon dioxide concentrations has been absorbed in the ocean and has not been manifest as a rise in surface temperature,” the Met Office says in one of its three reports into the global warming pause.
“Radiated forcing by greenhouse gases has continued unabated; that heat is being held in the system but is not manifest as a rise in global mean surface temperature. Observations of ocean heat content and of sea-level rise suggest that this additional heat has been absorbed by the ocean,” it says.