No doubt many of you have seen stories during the last few years about the repeated episodes of cold and snow in Western Europe-- and particularly Britain. Last December, some called it the snowiest winter in 25 years and the coldest in over 100 years.
On average, Britain’s winter climate is mild and has been so for many decades, probably since the end of the “Little Ice Age” in the 1800s. So what’s going on? Is the Gulf Stream changing? Or is it something else?
Despite its overall mild climate, Britain’s winters are also extremely variable, occasionally resembling those usually seen in Eastern Europe, but with less intense cold.
What has caught the eye of British climatologists is that last winter was the third in a row with unusual cold and snow for at least part of the winter. In an article entitled “Do freezing temperatures across Europe indicate another ice age is imminent?”, the author says that “scientists will be studying facts and figures to determine if ……the cold weather conditions are a temporary blip or an indication of something more serious such as the return to an ice-age period.”
All of this may have prompted one of our blog commenters last year to make reference to a Discover Magazine article of May 22, 2004: “A New Ice Age: The Day After Tomorrow?” (The article, in turn was apparently prompted by a movie with a similar title.)
In the story, Brad Lemley discusses the theory proposed by climate scientist William Curry and others at the Woods Hole Institution in Massachusetts, that in the face of global warming, changes in North Atlantic ocean currents will result in a mini ice age for both western Europe and the eastern United States for perhaps as long as the last one—400-500 years!
The commenter asked for the Capital Weather Gang’s thoughts on the above theory. Speaking for myself, I was intrigued not so much by the Woods Hole concept (because I had heard of it before—and disregarded it), but by the opposing theory of Richard Seager, a climate scientist at Columbia University’s Lamont-Doherty Earth Observatory.
In Seager’s study, entitled “Is the Gulf Stream responsible for Europe’s mild winters?”, Seager and his team maintain that the Gulf Stream has little to do with Britain’s relatively mild winters, that even a total Gulf Stream shut-down would have little impact on the British climate and contribute to only a modest cooling. Seager believes the idea was started by M.F. Maury in his 1855 book The Physical Geography of the Sea and its Meteorology, and has been perpetuated ever since.
All of which brings me back to my earliest science classes, when we were told that the only reason London, England, for example, which is even further north than Winnipeg, Canada, is so much milder is because of the Gulf Stream. The current, it was (and continues to be) said, carries vast quantities of heat across the North Atlantic, much of which is released into the atmosphere of western Europe by the prevailing westerlies.
The underlying concept of the above theory seems plausible enough. But I always thought that a huge mistake, as Seager points out, was that without the Gulf Stream, the prevailing winds would still be picking up and carrying some warmth—albeit minimal--and moisture to the British Isles, which would still have a primarily maritime, rather than a continental, climate. After all, these winds would not have passed over snow-covered prairies.
On the contrary, we know that the eastern United States is downwind, more or less, from the huge North American land mass. Under the right conditions, arctic high pressure systems can race southeastward over snow covered terrain, with relatively little modification from their source region.
In order for anything close to the above process to occur in Britain, either (1) a polar continental air mass must expand westward and southward from Russia and through Scandinavia; or (2) what’s called (in Britain) an Arctic maritime air mass must dive due south. Both of these scenarios occur relatively infrequently because they are contrary to the more typical west-to-east circulation pattern.
But during the “Little Ice Age,” which lasted roughly 500 years until the mid-nineteenth century, conditions were much different, as Brian Fagan points out in his book, The Little Ice Age, (Basic Books, Copyright 2000). Water temperatures were significantly colder, sub-polar ice floes extended much further south, and winter harbor ice in some years was quite extensive off the coasts of eastern England, France, and Holland, sometimes closing down shipping lanes in the North Sea. As a result, frigid air masses could approach Britain with less modification.
However, even if the Gulf Stream showed signs of shutting down completely, prompting some experts to call for another mini-ice age, the “thousand-pound gorilla,” as Seager puts it, would be the NAO (North Atlantic Oscillation).
A negative NAO strongly correlates with cold western European and eastern U.S. winters. But in Seager’s opinion, with or without the Gulf Stream, the NAO is apt to frequently go positive in the future, providing an extended period of balmy winters. The culprit, he believes: increasing atmospheric greenhouse gases, (Seager modestly declines to be thought of as an expert on the future gyrations of the NAO.)
Brian Fagan seems to bolster Seager’s research, with convincing evidence of a persistent negative NAO during that frigid period’s 500 year reign.
In summary, I do think Seager’s basic hypothesis deserves more debate by meteorologists, climatologists, and lay people alike. What do you, our valued readers, think about Seager’s theory? We’ve set up a poll. Give us your opinion.
If you vote no, give us your thoughts on the primary driver of Britain’s climate...