The impact of the Gulf Stream on Europe's climateIf the Gulf Stream stops, Europe freezes? It's not that simple, says Heinz Wanner, President of the Oeschger Centre for Climate Change Research.
Allianz Knowledge: What have been the most dramatic climate changes in Europe's history?
Heinz Wanner: There was a big change about 2.7 million years ago. Before that there was no ice, CO2 levels were very high, and there was a lot of volcanism. Then the Earth’s orbit changed and we started getting ice ages and interglacial periods.
In the last ice age there were two strong peaks, one about 63,000 years ago and the second about 23,000 years ago. Then about 20,000 years ago the ice started melting in the Alps and the southern margins of the European continental ice sheet, and by about 12,000 years ago it was quite warm.
But then the temperature collapsed for about 700 years—the Younger Dryas period—because melt water from the North American ice sheet weakened the Gulf Stream.
Freshwater is not as heavy as salt water and so it does not mix, it just swims over the top of the salt water and so dampens the circulation of water in the ocean.
If Greenland melts very quickly we could have a weakening of the Gulf Stream. Without the Gulf Stream we would have a climate like northern Canada.
How does the Gulf Stream keep Europe warm?
If the Gulf Stream gets stronger then more energy and heat is transported from the southern to the northern hemisphere and Europe becomes warmer.
However, it is not the only factor. And it is just one influence on the North Atlantic Oscillation, which also affects Europe’s climate.
What does the Oscillation do?
The Oscillation is the difference in atmospheric pressure between the Iceland low pressure system in the north and the Azores high pressure system in the south. It is also affected by sea ice and stratospheric pressure.
If the Gulf Stream is strong and the northern Atlantic becomes warm, the Iceland low pressure system weakens. Consequently the westerly winds weaken and so cold continental air dominates Europe.
But if the Gulf Stream weakens and the northern Atlantic becomes cold, that drives westerly winds into continental Europe so it has a warmer winter climate.
When did the Gulf Stream return to Europe?
The Gulf Stream returned about 11,700 years ago, the start of the era we are now living in—the Holocene interglacial. The Gulf Stream’s warmer waters returned because solar insolation [solar energy hitting the Earth] was very high at the time.
However, insolation dropped in the last 4000 years due to orbital changes—the track of the Earth around the sun, the angle of the earth to the sun—so the European climate became slightly cooler.
We got more Arctic sea ice and the color of the oceans and the continent changed from black to white. There was much stronger reflection of the sun’s rays and the continent lost a lot of energy. Alpine glaciers started growing again.
So orbital changes are the biggest factors affecting Europe’s climate?
If you are looking at a period of 10,000 years or more, yes. You can reconstruct orbital cycles a million years back and can calculate them for up to 200,000 years into the future.
If you are breaking it down to shorter periods the strongest changes are due to ocean dynamics and volcanic eruptions in the tropics.
But in the last 50 years the strongest influence has been anthropogenic climate change. We might still be living in the Little Ice Age— from 1350 to 1860—if it wasn’t for anthropogenic greenhouse gas emissions. If we were just considering orbital forcing, glaciers should be growing today but they are shrinking.
And if humans had not started changing the climate the next true ice age would start in about 20,000 to 30,000 years.
How did the Little Ice Age change European ecosystems?
During the Medieval Warm Period from about 950 to 1300 agriculture grew, even in northern Europe. The Arctic sea ice retreaded allowing the Vikings to travel to Iceland, Greenland, and then Canada.
Then when the Little Ice Age started the Arctic sea ice grew, and agriculture and keeping livestock became more difficult in northern Europe and mountainous areas. In the Alps, some of the highest settlements were abandoned.
What about sea levels around Europe? What does the historical record look like?
The biggest influence on sea levels is ice ages. During ice ages, one could have walked from France to England because the sea level was 130 meters lower than today.
The next biggest influence is temperature because the volume of water rises as the temperature rises and water expands.
The third major factor is melt water. There were sea level rises until about 7000 years ago because of melting ice sheets. If Greenland and west Antarctica melted, places like Holland and the UK would get sea level rises of several meters.