How dramatically is global warming really? NASA researchers have shown that the temperature rise has taken a break for 15 years. There are plenty of plausible explanations for why global warming has stalled. However, the number of guesses also shows how little the climate is understood.
Flowers are blooming earlier, sea level is rising – no doubt the climate is changing. The last year, as reported by NASA, was the ninth warmest since measurements began 132 years ago. The past decade was the warmest in this period.
But it has become common knowledge for some time that the climate has recently developed differently than predicted. The warming has stalled for 15 years; the upward trend in the average global temperature has not continued since 1998 (sic). “The standstill has led to the suggestion that global warming has stopped,” NASA admit.
The British Met Office has recently forecast that the warming standstill could continue until the end of 2017 – despite the rapid increase in greenhouse gas emissions. Then global warming would have stalled for 20 years. How many years – goes a now common question – has the temperature standstill to last until climate scientists start to reconsider their forecasts of future warming?
Scientists previously thought 14 years without further warming could be brought into line with their forecasts – but not “15 years or more,” as NASA scientists stated four years ago in the journal “Bulletin of the American Meteorological Society”. In an email to colleagues a renowned scientist wrote on 7 May 2009, at a time when the warming standstill had already lasted for eleven years: “the ‘no upward trend’ has to continue for a total of 15 years before we get worried.”
Now, 15 years without warming has happened. The warming standstill of the global surface temperature shows that the uncertainties of climate predictions are surprisingly large. The interested public anxiously awaits whether the IPCC’s new Assessment Report, which is due in September, will address the warming pause – the discussions are ongoing in Australia’s Hobart. The researchers are discussing several cogent reasons that might have slowed the upward trend of temperatures.
SPIEGEL ONLINE is documenting the scientists’ explanations for the unexpected development of the climate – and their forecasts:
Since the end of the so-called Little Ice Age in the middle of the 19th Century, the temperature curve of the earth has risen by 0.8 degrees Celsius. The strongest thrust of 0.5 degrees happened from the mid-1970s to the turn of the millennium. Since 1998 (sic), the global average temperature remains at a high level: every year during this period is among the warmest on record since the start of the systematic recording of temperatures a hundred years ago.
The past decade was probably the warmest since at least 400 years, the beginning of the Little Ice Age. Taken together, the past 15 years have, on one hand, climbed a warming record – on the other hand, the temperature curve has stagnated during this period.
Possible causes of the stagnation temperature
“The exact causes of the temperature standstill are not yet understood,” says climate researcher Doug Smith from the Met Office. Several causes are possible:
• Oceans absorb heat
Oceans are the largest heat storage: In their top three meters, they keep as much heat as the entire atmosphere of the Earth. Most of the energy that greenhouse gases retain in the atmosphere enters into the oceans; they should absorb, according to physical calculations, about 90 percent of the energy. The warming of the oceans would therefore provide the best indicator of global warming.
But there is uncertainty about the development of ocean temperatures. For a long time it appeared that the oceans too had not warmed since 2003. However, recent calculations by a group led by Norman Loeb of NASA now show a temperature rise. NASA also documented a warming of the oceans in the upper water layers.
The results, however, have not convinced others. “The uncertainty of the data is too large”, writes Kevin Trenberth of the National Center for Atmospheric Research in the United States (NOAA), a renowned expert in the field. “We need to improve our measurements.”
The problem is to monitor systematically the oceans’ large amount of water of an estimated 700 trillion litres. Just over 3,000 buoys have been floating around for almost ten years; they dive down to 2,000 meters and measure the temperature.
Until 2002 there was only little data; since 1992, satellites were used for measurements of surface temperature and so-called disposable thermographs were distributed in the water from ships. On their way to the ground, the devices registered the temperature. The data are considered defective because the depth of the sea could be determined during the fall only inaccurate – the probes are falling at different speeds.
Below 2,000 metres water depth, there are still hardly any measurements. However, experts suspect temperature changes thus there: computer simulations by a group around Gerald Meehl of the National Center for Atmospheric Research suggested that atmospheric warming can stop for a decade because the deep sea stores more and more heat.
That the warming of the oceans could explain the stop of atmospheric warming was “difficult to confirm” due to lack of data, said Doug Smith of the Met Office. “To prove this computer models would also have to correctly show climate changes since the turn of the millennium”, adds Jochem Marotzke, Director at the Max Planck Institute for Meteorology. This has not yet happened.
The lack of evidence for the warming of the oceans is, of course, not a proof that global warming has actually stopped. “The most plausible scenario seems to me that the temperature plateau is caused by the fact that energy is transported into the depths of the oceans”, says Marotzke. Without an extension of the data network, however, one could wait very long for evidence for this theory, researchers fear: “We need more measurements in the deep ocean,” says Norman Loeb.
• Dry stratosphere
One reason for the stagnation of the warming lies in the stratosphere, claim researchers led by Susan Solomon of the Earth System Research Laboratory in Boulder, Colorado. Since 2000, the stratosphere has become much drier. The water up there warms the Earth’s surface indirectly: water drops radiate heat. Through the unexpected dryness, the temperature near the surface has risen more slowly by a quarter.
The researchers speculate that the loss of moisture could be possibly explained by the fact that there were fewer thunderstorms in the tropics. The towering clouds act as a water dispenser for the stratosphere. Climate models predict that in the long term more water vapour would amass in the stratosphere: Increasing global warming, this is the calculation, would cause more water to evaporate. However, climate models do not represent stratospheric water vapour “very well”, admits Marotzke. The forecasts remain vague.
• Exhaust gases in Asia
Climate researchers believe a the possible cause for the lack of warming is linked to Asia: Sulphurous gases, emitted by the industrial economies of China and India, act as a sunshade, they dim the light. Susan Solomon and her colleagues calculate that the sulphur dome would reduce warming by a tenth of a watt per square metre. In this way, the CO2-driven warming would be weakened by a third. They claim that climate models have underestimated the effect of sulphur dioxide emissions from Asia: if the air were cleaner, global warming would accelerate.
• Cold water flood in the Pacific
Every few years, a flood of cool water comes to the surface in the Pacific. Freshening trade winds drive it from east to west. The so-called La Niña weather cools half the globe. La Niñas have happened three times since 1998; they have slowed the warming without doubt: years with La Niña are responsible for the biggest downward swings in global temperature.
Meteorologists interpret that 2011 and 2012 were the warmest La Niña years since records began as a sign of progressive warming. The influence of La Niña has been particularly high since 1998, says Norman Loeb.
The ocean cooling has been particularly strong because the warming opposite – El Niño – has been weaker during this period. If the effect of La Niña would be subtracted from the data, the temperature trend would also show upwards in recent years, NASA claim.
Thus there are plenty of plausible explanations for why global warming has temporarily slowed down. However, the number of guesses also shows how inexact the climate is understood. Could La Niña, for example, continue to have a cooling effect? “The jury is still out on this”, NASA explains.
The bottom line is, however, that several signs of warming remain: The sea level is rising, the summer sea ice in the Arctic has been halved, and glaciers are melting. In some places, there is evidence that extreme weather events are increasing. “There are many signs of global warming,” says Kevin Trenberth, “the surface air temperature is only one.”
Yet again and again, new data spring surprises: a new study shows that soot particles from unfiltered diesel exhaust and open fires contribute twice as much to global warming as previously expected. Because soot emissions in developing countries have been increasing for a long time, this effect should have been felt for a long time. But there are also positive surprises: recently, computer simulations have showed that the warming has made tropical cyclones less frequent. The strongest hurricanes could nevertheless gain in strength.
Other important environmental factors are insufficiently understood, as the IPCC has acknowledged in its report. These include:
• The influence of solar radiation on the formation of clouds
• The water cycle: in particular, the influence of water vapour, a greenhouse gas, in the atmosphere on the temperature.
• The effect of particles from industrial, heating and auto exhaust, from oceans, volcanoes and the soil: the particles serve as seeds for clouds. It is estimated that an increase in cloud cover by one-hundredth could offset the doubling of the CO2 content in the air.
Climate forecasts for periods of several years therefore remain particularly uncertain: “Our prediction system has let us down in this regard,” says MPI Director Marotzke. “but we are working on it.” His NASA colleague Norman Loeb points out that gaps in the data will continue to provide for surprises: “There is everywhere room for improvement in the data collection”, says the expert. Especially for short-term climatic trends there could still be surprises.
Researchers have more trust in long-term forecasts. These forecasts are essentially based on the greenhouse effect of CO2, the so-called climate sensitivity. A lot of studies have shown that a doubling in the amount of CO2 in the air due to the increased formation of water vapour will most likely lead to warming by 2 and 4.5 degrees.
Current forecasts warn of warming of five degrees if CO2 emissions continue in a “business as usual” scenario. However, it is less well known how strong natural climate influences could alter the temperature rise – this, too, has been revealed by the new NASA data.
Translation: Philipp Mueller