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The cold start into spring has made people in parts of Europe wonder if the climate is really warming. Global temperatures have not been rising in recent years. Is the earth cooling instead of warming?
Looking at the average temperature over five years during the last 15 years, global temperatures might appear to be flattening out.
"Over the last decade there has been very little new warming," says Ed Hawkins from the University of Reading in the United Kingdom. The development is not unexpected, he told DW: "We are confident that human emissions of greenhouse gases have caused a large component of the warming over the last 150 years, but at the same time we do not expect every year to be warmer than the last. There are reasons why temperatures may remain flat for a decade and continue to warm later on." Hawkins cites periods in the 1960s and 1970s when temperatures were actually cooling.
The current flattening out of the temperature curve could be due to natural fluctuation:
"The earth's system undergoes variability on decadal time scales, which causes heat to be mixed into the deeper ocean. We think that's what's going on at the moment. Some of the heat is being mixed down into the deep ocean, where we do not observe it as well, so the surface is not heating up as much as it might have done."
Another possible reason Hawkins mentions is the increased burning of coal in countries like China and India. The particulates produced help reflect solar radiation back into space, and so cool the planet. He stresses it is not yet possible to say whether the current slow-down in temperature rise is being caused by natural variability or is human-induced.
Too short a time to tell
Peter Lemke is head of Climate Science at Germany's Alfred-Wegener Institute for Polar and Marine studies (AWI). He warns against taking a short five-year average as an indicator of how the climate is developing:
"When it comes to climate processes, we always look at 30 year periods", he told DW. "And for the last 30 years there is a clear upward trend." Using a shorter time span means that one single year can distort the picture. If, for instance, 1998, an exceptionally hot year, is taken as the start or end of a five-year period, the trend would appear to be rising or falling, accordingly. Lemke says temperatures have indeed risen in the last 15 years - if you look at the overall development and not at shorter intervals within the time span. "2010 was the warmest year from a global perspective," says Lemke. "2005 was not far behind it. Since 1978, temperatures have not been within the normal range but considerably above it." "Normal" means an average measured over a 30-year period.
But certainty that global warming is happening does not mean certainty over its effect, says Lemke: "The question is: how sensitive is our climate system to the rise in the CO2 concentration? And what will it mean in terms of precipitation, temperature rise or ice melt? That is a complicated issue."
But this very issue is the crucial one for those who have to take decisions about preparing for and adapting to climate change. A whole range of different models estimate the likely temperature rise in the next 100 years as somewhere between 1.5 and 6 degrees Celsius. The next IPPC report, due to be published in 2014, is unlikely to be able to solve the problem of this wide range of estimates and methods, says Lemke. Nevertheless, he says the current slow-down of the warming rate is no reason to stop worrying about climate change.
British scientist Hawkins thinks there may be a "slight downwards nudge" in the IPCC estimates of climate sensitivity. Nevertheless, he says, "we are still predicting a large warming over coming decades. It may be that we have slightly more time than we did before, which is obviously a good thing, but the impacts are still projected to occur. "
Climate warming = colder winters?
The long winter and cold spring which might give people in western Europe the impression the earth is cooling rather than warming have been caused by a stable area of high pressure over Scandinavia - a weather phenomenon rather than climate change. AWI climate chief Lemke stresses that seen from a global point of view, the winter was warmer than usual.
"The interesting thing is that some climate studies indicate that a stable area of high pressure over Scandinavia like this one could occur more often when the pack ice in the Arctic has been significantly reduced in the previous summer and autumn", Lemke explains. This lack of ice means the ocean warms up and releases the warmth into the atmosphere in the autumn. That disturbs the usual circulation patterns, so that we get the area of high pressure, and that it is more stable than usual." In 2012, Arctic sea ice coverage reached a record low. The long period of cold spring weather would seem to confirm the theory expressed by Vladimir Petoukhov from the Potsdam Institute for Climate Impact Research. When he published his paper in late 2010, his comment was "hard winters do not contradict our image of global warming. On the contrary, they complete the picture. "