Three different recent studies from the Potsdam Institute for Climate Impact Research (PIK) show how difficult it is to reliably predict the consequences of climate change. Many of the effects appear contradictory and their interdependency is sketchy.
For example, in recent decades, global warming could have been responsible for a cooling of the sea along Europe's northern coastline. A team of researchers around Stefan Rahmstorf comes to that conclusion in a study published in "Nature Climate-Change" on March 23.
Cooler water in the northern Atlantic
The researchers looked into the changes of ocean temperatures by studying sediment and ice cores as well as coralls and tree rings. The result: A part of the northern Atlantic – specifically south of Greenland – has cooled despite global warming.
The PIK researchers believe that this observation, which seems paradox, derives from a weakening of the Gulf Stream – a warm ocean current from the Caribbean to the coasts of central and northern Europe. It is part of the Atlantic Meridional Overturning Circulation (AMOC).
The warm water from the gulf stream cools down around Greenland and sinks to the bottom of the ocean. There it flows south all the way to Antarctica. Rahmstorf believes that the slowing of the Gulf Stream originates around Greenland: Global warming triggers the melting of ice, which then flows into the ocean as freshwater. This, he argues, changes the density of the ocean-water and the entire flow of the AMOC.
Less storms – more heat
Another group of scientists used data from satellites and weather stations to conclude that air currents are also likely to slow down. The team around geophysicist Dim Coumou published a study in "Science" on March 12, arguing that the number of storms over the northern hemisphere is likely to decline during the summer months.
Overall, the weather conditions are likely to become more stable, they argue. During the summer, storms transport humid air from the oceans to the continents. This brings rain and cooler temperatures. But if this does not happen, we are likely to see more extended dry periods, like in Russia in 2010.
The scientists believe that the following mechanism is at work here: The polar region takes in more heat than the rest of the northern hemisphere. Therefore, the temperature gap between those regions declines. But this gap is what is driving the jet stream. The result: Less difference in temperature, fewer storms.
More snow – less ice
This seems to clash with a study that suggests considerably more precipitation as a result of global warming. A working group around the mathematician Katja Frieler discovered a double paradox. On 16 March, she published a study in "Nature Climate Change" arguing that there is likely to be more snowfall in Antarctica as a result of global warming.
The scientist calculated that every degree Centigrade of global warming is likely to trigger about five percent more snowfall in Antarctica. The reason is easy to understand: Due to the higher temperatures, more water will vaporize over the oceans and form clouds. Huge amounts of water then fall as snow over Antarctica.
The PIK-scientists calculated that the additional snow layer over the south pole would be sufficient to reduce the sea level by four centimeters over a period of 100 years. But they see no reason to celebrate: This effect will be more than compensated, they argue, by the dynamics of the glaciers.
The additional load of snow is going to put enough pressure on the glaciers to push them even faster than today outside towards the ice shelf. There, they argue, the ice will then melt even faster, because of warmer undercurrents coming from the ocean. At the end, the rise of the sea-level could even be accelerated, they suggest.