Jet Stream Trouble
This is not how it was supposed to happen. The standard climate change predictions said that people in the tropics and the sub-tropics would be badly hurt by global warming long before people living in the temperate zones (farther away from the equator) were feeling much pain at all.
That was unfair, because it was the people of the rich countries in the temperate zone — North America, Europe and Japan, mainly — who industrialized early and started burning large amounts of fossil fuel as long as two centuries ago. That’s how they got rich. Their carbon dioxide emissions over the years account for 80 per cent of the greenhouse gases of human origin that are now in the atmosphere, causing the warming — yet they get hurt least and last.
Well, what did you expect? The gods of climate are almost certainly sky gods, and sky gods are never fair. But they have always liked jokes, especially cruel ones, and they have come up with a great one this time. The people of the temperate zones are going to get hurt early after all, but not by gradual warming. Their weather is just going to get more and more extreme: heat waves, blizzards and flooding on an unprecedented scale.
“In 2012 we had the second wettest winter on record and this winter is a one-in-250-years event,” British opposition leader Ed Milliband told The Observer newspaper last Friday. “If you keep throwing the dice and you keep getting sixes, then the dice are loaded. Something is going on.”
The “something” is abrupt climate change. In Britain, it’s an unprecedented series of great storms blowing in off the North Atlantic, dropping enormous amounts of rain and causing disastrous floods. In the United States and Canada, it’s huge blizzards, ice-storms and record low temperatures that last much longer and reach much further south than normal. Welcome to the “temperate” zone of the northern hemisphere.
There have been extremes in the “temperate” parts of the southern hemisphere, too. Australia has just had the hottest year ever, with record-breaking heat waves and severe bush fires. Argentina had one of its worst-ever heat waves in December, and parts of Brazil had record rainfall, floods and landslides. But that is probably just the result of gradual, relentless warming. The abrupt changes seem to be mainly in the northern hemisphere.
Geography may explain the differences. There isn’t all that much land in the southern temperate zone, and the vast expanses of ocean that surround it moderate the land temperatures. Moreover, the polar jet stream in the southern hemisphere simply circles the Antarctic continent, and does not operate over land — whereas the northern polar jet stream flows right across North America and Europe.
And it’s the jet stream that matters.
The extreme weather trend in North America and Europe is less than five years old, so the science that might explain exactly what is happening is still quite tentative. The first hypothesis that sounded plausible, published in 2012 in Geophysical Letters, blamed a slowing of the northern hemisphere’s polar jet stream.
The paper, entitled “Evidence Linking Arctic Amplification to Extreme Weather in Mid-Latitudes,” was written by Jennifer Francis of Rutgers University and Stephen Vavrus of the University of Wisconsin-Madison. The authors’ methodology has been challenged by other climate scientists, but I think that in the end Francis and Vavrus will turn out to be largely right. That is not good news.
They start with the fact that the Arctic has been warming faster than anywhere else on Earth, so the difference in temperature between the Arctic air mass and the air over the temperate zone has been shrinking. Since that difference in temperature is what drives the jet stream that flows along the boundary between the two air masses, a lower difference means a slower jet stream.
Now, a fast jet stream travels in a pretty straight line around the planet from west to east, just like a mountain stream goes pretty straight downhill. A slower jet stream, however, meanders like a river crossing a flood plain — and the big loops it makes extend much further south and north than when it was moving fast.
In a big southerly loop, you will have Arctic air much further south than usual, while there will be relatively warm air from the temperate air mass in a northerly loop that extends up into the Arctic. Moreover, the slower-moving jet stream tends to get “stuck,” so that a given kind of weather — snow, or rain, or heat — will stay longer over the same area.
Hence the “polar-vortex” winter in North America this year, the record snowfalls in Japan in 2012 and again this winter, the lethal heat waves in the eastern U.S. in 2012 — and the floods in Britain this winter.
“They’ve been pummeled by storm after storm this winter [in Britain],” said Jennifer Francis at the American Association for the Advancement of Science conference in Chicago last week. “It’s been amazing what’s going on, and it’s because the pattern this winter has been stuck in one place ever since early December.”
There’s no particular reason to think that it will move on soon, either.