The surface of the planet is now more than 0.6°C warmer than it was in 1951. The pace of warming has varied during the period since then. It started accelerating in the 1980’s, hit 0.28°C per decade in the 1990’s but fell to 0.09°C in the 2000s. So the planet is not warming as fast as it was. But the details vary greatly according to how you calculate the trend. Pick the very hot year of 1998 as your starting point and the slowdown appears dramatic; the rate of warming from 1998 to 2012 was ‘just’ 0.04°C per decade. If this rate continued, the planet’s surface would be just 1°C hotter in 2100 that in 1950 – which is below the 2°C target agreed by the scientific community as the level that man-made warming must stay below.
But these figures are based on the global surface temperature record compiled the UK’s Hadley Centre and this does not include the fastest-warming region on Earth, the Arctic, as there are so few observations there. According to NASA’s record, which guestimates Arctic temper-atures based on the nearest weather stations, the warming rate was 0.07°C per decade from 1998 to 2012. And according to a study published in November 2013, by Kevin Cowtan of the University of York, extrapolated Arctic temperatures from satellite data showed that the 1998 to 2012 rate was 0.12°C per decade. This would mean warming has only slowed from 0.18°C to 0.12°C in the 2000s – which would result in warming beyond the 2°C accepted as the outer limit by 2100.
But whatever the precise figures, it is clear that temperatures over the coming decades will rise. The past decade has been the hottest since records began. In terms of heat, there are three possible reasons why Earth’s surface temperature hasn’t increased as predicted.
The first is that the sun has been getting dimmer. The sun’s heat output rises and falls in an 11 year cycle. The spacecraft SOHO has provided data that confirms the heat has dipped recently.
The second reason could be that more heat than usual has been escaping from the top of the atmosphere. Sulphur aerosols don’t prevent the sun’s rays entering the atmosphere, but they do reflect more of the sun’s heat back into space. Sulphur aerosols are produced by coal burning and volcanic eruptions. There have been lots of small volcanic eruptions during this period as well coal burning (aerosol emissions from China are particularly significant) in this respect.
Thirdly, it is possible that the planet has still been gaining heat, but it hasn’t stayed in the lower atmosphere, but rather in the oceans which cover more than 70% of the planet. Around 4 times as much energy is needed to warm any volume of water by 1°C as is needed to warm the same volume of air.
It is unhelpful to focus solely on the temperature of the thin layer of air that we live in. This is only one of the indicators of warming, the oceans are another. 94% Of the heat energy gained by the planet since 1991 has ended up in the oceans, with another 4% absorbed by the land and ice.
During the last 60 years the depths of the Pacific have warmed 15 times as fast as at any time in the previous 10,000 years. Further more the oceans may have been soaking up the heat faster still over the past few years. Heat constantly sloshes back and forth between the oceans and the atmosphere; a main cause of natural variability. What happens in the vast Pacific Ocean matters most.
During the phenomenon called El Nino, when easterly winds spread hot water across the top of much of the tropical Pacific, so much heat flows into the air that the entire surface of the planet warms. There was an especially strong El Nino in 1998, which is why it was such a warm year.
During the opposite event, called La Nina, when westerly winds spread up-welling cold water across the sea surface, the tropical Pacific soaks up so much heat that it cools the planet’s surface. Lately there have been lots of La Ninas. There has not been a major El Nino for the past 15 years;, all this helps explain why the atmosphere isn’t warming as fast.
The IPCC reports that the mainstream view is that about half of the surface temperature slowdown is due to the oceans and the other half is due to the sun dip and extra volcanic activities. However, warming oceans generate specific impacts. Typhoon Haiyan, thought to be the fourth strongest storm in history and known as Typhoon Yolanda in the Philippines, was a category 5 -Super Typhoon that devastated areas of South-east Asia, particularly the Philippines, in November 2013, leaving behind more than 5,000 immediate fatalities and hundreds of thousands of displaced and homeless people. Such storms usually stir up cooler waters that limit their strength; Haiyan, by contrast, kept gaining energy from the surrounding ocean. Warming oceans pump more moisture into the air, which is then dumped, often very rapidly.
The unprecedented rainfall in Colorado USA in September 2013 is a prescient example of what is in store. The equivalent of a month’s rainfall fell in less than 36hrs, without warning, generating unprecedented chaos. In 2013 Australia registered the warmest September–March on record, the hottest summer on record, the hottest month on record, the hottest day on record, 49.6°C, and the longest heatwave, along with the highest recorded sea-surface temperatures in the region.
Despite decades of research nobody is really sure how sensitive the climate is to CO2.
Our best guess, according to the most recent IPCC reports, is that doubling the CO2 concentration in the atmosphere would raise global temperatures by between 1.4 and 4.5°C. There is no consensus on this climate sensitivity.
To estimate how much climate sensitivity varies. Gary Russell of the NASA Goddard Institute for Space Studies in New York and colleagues ran a climate model repeatedly. Each run of the model kicked off with a different concentration of CO2 in the atmosphere. In each case they ran the model for 100 years to see how much the world warmed as CO2 levels increased. The model had a fairly high climate sensitivity to begin with, predicting a warming of 5.15°C. As Russell kept doubling the CO2, the model responded even more. For each doubling of CO2 from
8 times to 32 times the initial concentration, the temperature rose by 8OC, as clouds moved higher into the sky, trapping ever more heat. “We think climate sensitivity will be greater in 2100 than it was in 2000,” says Russell. “At the moment we’re on a path to a doubling (of CO2), and over the next 50-150 years we could see a quadrupling,” says Chris Forest of Penn State University in University Park. That means if we keep burning fossil fuels, the warming could start accelerating.
Intergovernmental Panel on Climate Change