A group of scientists from New Jersey has studied how the jet stream affects our weather patterns. The jet stream is the strong winds at high altitudes in the atmosphere that moves in a wave-like pattern around the Earth where the wind speed can reach 200 meters per second.

The jet stream is closely linked to other weather systems in the atmosphere and therefore affects the weather continuously. Its strong winds come from the large temperature difference between the equator and the poles. However, if the temperature difference between the equator and the poles decreases, there is a risk that the jet stream is like a river that comes out on flat ground.

- It then becomes weaker and its wave pattern changed, said Jennifer Francis, a professor at Rutgers University in New Jersey.

The winds have subsided in strength
The difference in temperature has already decreased because the ice around the Arctic has declined rapidly during the recent decades, and right now the spread of sea ice in the Arctic is one of the lowest ever for this time of year. The ice in the Arctic helps to create a colder climate there. But when the ice disappears, the surroundings are heated up and the large-scale wind circulation, which the jet stream is part of, decreases in strength.

In the Arctic, temperatures have increased 2-3 degrees more than the rest of the world; making the jetsteam, and thus the weather, remain in the same place for a long time. According to the Met Office's chief scientist Julia Slingo the strongest winds from west to east has, on average, declined in strength over the past two decades.

- This is something that we can measure, said Julia Slingo on one of the AAAS meeting in February this year.

The weather stays
In Sweden, it does not necessarily mean that we will experience stormy weather every year. Next year, it may instead be extremely cold and windy for an extended period. What becomes more common, is that the weather conditions persist for a longer time.

There are examples of this kind of pattern occuring around the world. This winter, Britain recieved many low pressure systems with high winds and heavy precipitation. At the same time, Alaska experienced high temperatures with rain instead of snow for the first time in December in the Alaskan Mountains. In California, there was a record drought for a very long time.

A line with research
This is in line with our hypothesis about what will happen when the Arctic is heated up very quickly. The observations coincide with the models' predictions about what might happen with the jet stream, said Jennifer Francis.

She also points out that there are other factors that might affect the jet stream, but so far it seems that the only factor that has drastically changed, is the melting of the Arctic.

- At the moment, I can not say that it is the most likely explanation. This is still a new area of ​​research. But I trully believe it is one of the most plausible explanations. People notice that the weather patterns change and are no longer what they used to be, says Jennifer Francis.

Lively debate among researchers
Another climate scientist in England, Mat Collins at the Met Office, has criticized Jennifer Francis' research by saying that there is no evidence that the change of the jet stream has affected this years' winter weather.

But Michael Tjernström, professor at MISU, is opposed to the concept of evidence and says instead that more research is needed.

- The concept of evidence has nothing to do with science. Evidence can be presented in a court maybe, but scientists do not prove things; we test hypotheses, and this is a hypothesis that is being tested very vibrant right now and there is a lively discussion going on in the research community, says Michael Tjernström.

More research is needed
Tjernström says that Jennifer's theory is not non-physical or wrong in any way, but simply that more extreme events are needed to be able to draw more conclusions.

- Anyone who has played Ludo knows that you can get 3 sixes in a row, and it's perfectly natural. But if you get twenty sixes in a row, somehing is probably wrong with the dice. And it's the same thing here.

We have now had two, maybe three winters when you have had this type of extreme events and then one can begin to speculate on what causes it. But to do a statistical link between climate change and extreme weather like this, requires more events, because there is so much more that affects the weather, says Michael.

More research is thus needed in the field and SMHI has already started a research project called GREENICE to find out more about the interplay between changes in climate and changes in sea ice and snow cover.

Sweden is not as vulnerable
When asked if we should worry about climate change in the future Michael Tjernström says:

- Yes, I definitely think that, but not because of this reason. On the whole, Sweden, Scandinavia and northern Europe is likely to be doing quite well, even in a changing climate, but other parts of the world are more vulnerable.

IPCC's second working group submitted its report recently, and they point out a number of areas where we really should be concerned about. It's about access to fresh water and food security, thus generating enough food for all people. These issues are much more important than if Sweden gets a cold winter or a rainy summer, concludes Michael Tjernström.

Fritt översatt från Joel Mellin och svt.se.

See the video from SVT here (in Swedish).