Climatologists use their computer models to draw temperature curves that continue well into the future. They predict that the average global temperature will increase by about three degrees Celsius (5.4 degrees Fahrenheit) by the end of the century, unless humanity manages to drastically reduce greenhouse gas emissions. However, no one really knows what exactly the world climate will look like in the not-so-distant future, that is, in 2015, 2030 or 2050.
This is because it is not just human influence but natural factors that affect the Earth's climate. For instance, currents in the world's oceans are subject to certain cycles, as is solar activity. Major volcanic eruptions can also curb rising temperatures in the medium term. The eruption of Mount Pinatubo in June 1991, for example, caused world temperatures to drop by an average of 0.5 degrees Celsius, thereby prolonging a cooler climate phase that had begun in the late 1980s.
But the Mount Pinatubo eruption happened too long ago to be related to the current slowdown in global warming. So what is behind this more recent phenomenon?
Weaker Solar Activity
The fact is that the sun is weakening slightly. Its radiation activity is currently at a minimum, as evidenced by the small number of sunspots on its surface. According to calculations performed by a group of NASA scientists led by David Rind, which were recently published in the journal Geophysical Research Letters, this reduced solar activity is the most important cause of stagnating global warming.
Latif, on the other hand, attributes the stagnation to so-called Pacific decadal oscillation (PDO). This phenomenon in the Pacific Ocean allows a larger volume of cold deep-sea water to rise to the surface at the equator. According to Latif, this has a significant cooling effect on the Earth's atmosphere.
With his team at the Leibniz Institute of Marine Sciences, Latif has been one of the first to develop a model to create medium-term prognoses for the next five to 10 years. "We are slowly starting to attempt (such models)," says Marotzke, who is also launching a major project in this area, funded by the Federal Ministry for Research and Technology.
Despite their current findings, scientists agree that temperatures will continue to rise in the long term. The big question is: When will it start getting warmer again?
If the deep waters of the Pacific are, in fact, the most important factor holding up global warming, climate change will remain at a standstill until the middle of the next decade, says Latif. But if the cooling trend is the result of reduced solar activity, things could start getting warmer again much sooner. Based on past experience, solar activity will likely increase again in the next few years.
Betting on Warmer Temperatures
The Hadley Center group expects warming to resume in the coming years. "That resumption could come as a bit of a jolt," says Hadley climatologist Adam Scaife, explaining that natural cyclical warming would then be augmented by the warming effect caused by anthropogenic greenhouse gas emissions.
While climatologists at conferences engage in passionate debates over when temperatures will start rising again, global warming's next steps have also become the subject of betting activity.
Climatologist Stefan Rahmstorf is so convinced that his predictions will be correct in the end that he is willing to back up his conviction with a 2,500 ($3,700) bet. "I will win," says Rahmstorf.
His adversary Latif turned down the bet, saying that the matter was too serious for gambling. "We are scientists, not poker players."
Translated from the German by Christopher Sultan
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