How a persistent winter could have its roots in global warming

If it seems difficult to imagine how a warming climate can contribute to a cold, persistent winter like the one we've been experiencing, consider this: The rapid disappearance of sea ice and the overall warming of the Arctic tend to push the jet stream farther south. In turn, the jet stream tends to make weather patterns stay put.

Jennifer Francis, research professor with the Rutgers Institute of Coastal and Marine Science, is quoted in The Guardian as saying that the loss of Arctic ice "is affecting the jet stream and leading to the extreme weather we are seeing in mid-latitudes ... It allows the cold air from the Arctic to plunge much further south. The pattern can be slow to change because the [southern] wave of the jet stream is getting bigger. It's now at a near record position, so whatever weather you have now is going to stick around."

The melting ice not only results from the warming pattern but contributes to it, because the exposed ocean absorbs sunlight that ice would bounce back into space. Scientists say the annual accumulation of ice has now reached its maximum for the season and is beginning to recede. The maximum ice this season was the sixth-lowest accumulation on record.

Francis sat in for MPR News Chief Meteorologist Paul Huttner on Thursday's Climate Cast. Here's an edited transcript of the conversation:

Kerri Miller: Will you give me a sense of the scope of how fast the Arctic sea ice is actually melting?

Jennifer Francis: It's a mindboggling change that's happening up there. Just in the last 30 years, we've seen the extent of Arctic sea ice, which is the amount of real estate it covers, diminish by half. So in the summertime now there's only half as much sea ice when we look down from space with our satellites. And in addition, if you take into account the thickness of the ice and multiply it by the extent, which gives you the volume, the volume of the ice has decreased by 80 percent just in the last 30 years. It's just amazing.

Miller: Is this all about warming water temperature, or is there something else going on?

Francis: It's really all about increasing greenhouse gases in the atmosphere and the warming that's happening in a global sense. But the sea ice acts as an accelerator to that warming, so we see the atmosphere warming two to three times faster that it is, say, down here where we live, down here in Minnesota or where I am in Massachusetts.

Miller: Why is that? Is it because of the way air currents move, or where these greenhouse gases get collected?

Francis: There are several reasons why the Arctic is warming so much faster. One of them is the sea ice itself. As that sea ice retreats, it allows more of the sun's energy to be absorbed by the now-dark ocean where there used to be ice. That energy would have been reflected back to outer space if that ice had been there, and it never would have entered the climate system at all. So it's acting like a whole new source of energy for the system. The same thing is happening with loss of snow cover in the spring on high-latitude land areas. And we also see the increase in the amount of water vapor in the atmosphere as a result of the warming temperatures of the air increasing the greenhouse effect in the Arctic. There's a lot of different factors that contribute to the enhanced warming in the Arctic.

Randy in Maplewood (on the phone): I wonder if there is going to be a feedback loop, where it warms so much there's increased moisture, maybe more cloud cover that could help reverse the process or perhaps stabilize and cool us a bit? Is it possible?

Francis: It depends where those clouds actually end up forming. If clouds increase in the Arctic, which in fact we're seeing, then those clouds actually have a much stronger warming effect than they do a cooling effect. In general, the latest research suggests overall on the globe the changes in cloudiness should have a slight warming effect overall. Clouds act like greenhouse gases in a way. They help trap the energy emitted from the earth and keep us warmer than we would be otherwise. In general it looks like any changes in the clouds are probably going to slightly increase the warming, not decrease it.

Miller: You are in Massachusetts. You've had a very snowy winter where you are in Massachusetts?

Francis: It's been quite a chilly, cold, stormy winter here.

Miller: Same here and pretty different from the last couple of winters. How is it that our weather is being affected by what's happening at the Arctic?

Francis: The difference in temperature between the Arctic and areas farther south is what drives the jet stream. The jet stream is this very fast-moving river of air over our heads that generates weather patterns and also steers them. The jet stream naturally has these north-south waves in it. As the warming in the Arctic increases faster than it does in lower latitudes, this actually tends to weaken the west-east winds in the jet stream. We know that a weaker jet stream tends to be a wavier jet stream. So those north-south waves are tending to get larger and those waves are what control our weather.

So what's happening right now is we have a big southward dip over the eastern half of the United States, and that allows the cold air from the Arctic to plunge pretty far south. And as the jet stream curves northward again along the East Coast, that tends to be a stormy pattern. Just last March, a year ago, the waves that we have now were actually shifted over to the western half of the country. In both cases, the jet stream was in this very wavy pattern that we believe is connected to the enhanced warming of the Arctic, but it was just in a different location. Both extremes — last year it was warm, this year it's cold — both can be connected to the enhanced warming of the Arctic through this enhanced waviness of the jet stream.

Miller: Just to be clear, when the jet stream sags in our area, cold air basically fills in that sagging. And that's why anything above the sagging jet stream is going to be cold in the winter.

Francis: The jet stream is basically a boundary between the colder air to the north and the warmer air to the south. So if the jet stream is to the south of you, like it is right now, then we're on the cold side.

Miller: Jim in Andover says, "If we lost 80 percent of Arctic ice, why haven't we seen a subsequent increase in sea level?"

Francis: Great question. The reason is because the sea ice that's on the Arctic Ocean is already floating on the surface of the ocean. Its weight is already displacing ocean water. When it melts, like when you put ice cubes in your glass of water — you can do this test at home — and you measure the water level in your glass before and after the ice cubes melt, you'll see it's almost exactly the same. So as the sea ice melts in the Arctic, it does not have an appreciable effect on the sea levels. But as ice on land — such as on Greenland, glaciers and Antarctica — if that ice melts, then it flows directly into the ocean. It adds water to the ocean. That does have a big impact on sea levels.

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