Tuesday, December 28, 2010

The New York Times has a great piece on why global warming gives us cold weather by Judah Cohen. It's a good idea because you're always hearing people chuckling about "wot global warming?" or seriously assuming the cold winters we've been having is some kind of evidence against climate change.

Unfortunately, it's gone behind a paywall, but I think you should read it anyway, so I'm just going to quote the whole damned thing. (Sorry NYT)

THE earth continues to get warmer, yet it’s feeling a lot colder outside. Over the past few weeks, subzero temperatures in Poland claimed 66 lives; snow arrived in Seattle well before the winter solstice, and fell heavily enough in Minneapolis to make the roof of the Metrodome collapse; and last week blizzards closed Europe’s busiest airports in London and Frankfurt for days, stranding holiday travelers. The snow and record cold have invaded the Eastern United States, with more bad weather predicted.

All of this cold was met with perfect comic timing by the release of a World Meteorological Organization report showing that 2010 will probably be among the three warmest years on record, and 2001 through 2010 the warmest decade on record.

How can we reconcile this? The not-so-obvious short answer is that the overall warming of the atmosphere is actually creating cold-weather extremes. Last winter, too, was exceptionally snowy and cold across the Eastern United States and Eurasia, as were seven of the previous nine winters.

For a more detailed explanation, we must turn our attention to the snow in Siberia.

Annual cycles like El Niño/Southern Oscillation, solar variability and global ocean currents cannot account for recent winter cooling. And though it is well documented that the earth’s frozen areas are in retreat, evidence of thinning Arctic sea ice does not explain why the world’s major cities are having colder winters.

But one phenomenon that may be significant is the way in which seasonal snow cover has continued to increase even as other frozen areas are shrinking. In the past two decades, snow cover has expanded across the high latitudes of the Northern Hemisphere, especially in Siberia, just north of a series of exceptionally high mountain ranges, including the Himalayas, the Tien Shan and the Altai.

The high topography of Asia influences the atmosphere in profound ways. The jet stream, a river of fast-flowing air five to seven miles above sea level, bends around Asia’s mountains in a wavelike pattern, much as water in a stream flows around a rock or boulder. The energy from these atmospheric waves, like the energy from a sound wave, propagates both horizontally and vertically.

As global temperatures have warmed and as Arctic sea ice has melted over the past two and a half decades, more moisture has become available to fall as snow over the continents. So the snow cover across Siberia in the fall has steadily increased.

The sun’s energy reflects off the bright white snow and escapes back out to space. As a result, the temperature cools. When snow cover is more abundant in Siberia, it creates an unusually large dome of cold air next to the mountains, and this amplifies the standing waves in the atmosphere, just as a bigger rock in a stream increases the size of the waves of water flowing by.

The increased wave energy in the air spreads both horizontally, around the Northern Hemisphere, and vertically, up into the stratosphere and down toward the earth’s surface. In response, the jet stream, instead of flowing predominantly west to east as usual, meanders more north and south. In winter, this change in flow sends warm air north from the subtropical oceans into Alaska and Greenland, but it also pushes cold air south from the Arctic on the east side of the Rockies. Meanwhile, across Eurasia, cold air from Siberia spills south into East Asia and even southwestward into Europe.

That is why the Eastern United States, Northern Europe and East Asia have experienced extraordinarily snowy and cold winters since the turn of this century. Most forecasts have failed to predict these colder winters, however, because the primary drivers in their models are the oceans, which have been warming even as winters have grown chillier. They have ignored the snow in Siberia.

Last week, the British government asked its chief science adviser for an explanation. My advice to him is to look to the east.

It’s all a snow job by nature. The reality is, we’re freezing not in spite of climate change but because of it.

Judah Cohen is the director of seasonal forecasting at an atmospheric and environmental research firm.


Oli said...

The really interesting point about all this is surely not whether or not colder winters in some parts of the world are consistent with global warming.

The interesting point is that, as Judah Cohen says:

Most forecasts have failed to predict these colder winters, however, because the primary drivers in their models are the oceans, which have been warming even as winters have grown chillier.

In other words, many people are losing faith in the skill of the models. Even Judah is suggesting that there's something important missing from the models. On that basis why should we be confident in their predictions for 100 years time?

Of course seasonal predictions are possibly the most difficult weather/climate predictions to get right, but the Met Office predictions (for example) appear to be regularly wrong. OK, I haven't been keeping track of this, but the impression is that they are almost doing worse than chance at seasonal prediction.

They are certainly doing a very poor PR job. Their story should be something like: "Sure we got this one wrong, but the last 8 out of 10 seasons we predicted correctly, which is completely in line with our statistical confidence in our model." ..... or whatever.

Instead, I believe they are essentially giving up saying anything publicly about seasonal prediction!

The other broader point is that the scientists and campaigners tried to 'win' the debate over the last number of years by using rhetoric that stretched the science. They shouldn't be so surprised when others use simple rhetoric 'against' their position.

That's why, IMHO, the scientists should not try to 'dumb down' the science and pretend that you can remove the error bars from the story. Rather, there should be a push to try to improve the general public's basic ability to understand statistical results.

Then the coldness of the winter would not be the story. The story should be how well the last number of seasons have fit the statistical predictions of the models. Now that's a story I'd be interested to read.

Composing said...

Well, I'd say given the naivety that some people seem to display about climate (read the comment columns in the newspapers) or complex systems in general, it genuinely *is* news to them that colder winters in England and the US could be symptoms of global warming, rather than evidence against it.

But I agree it is also interesting that Cohen points out that ocean oriented models missed this detail.

I'd like to understand that better.

My hunch would have been that as the climate warmed we'd get more volatility and so prediction about details would be harder as we were in a more "chaotic" region of weather-space. But I agree that isn't the explanation he seems to be giving for the prediction failures.

Is it that they're literally missing the jet-stream as a component of the model? Is it that they don't model actual geography so details like "this bit of land next to that current" aren't part of their output?

As for people losing faith in the skill of the models, I suggest that this *is* much a wider political issue.

Do we think that the models are less well founded or that climate scientists are stupider or more venal than any other scientists working with complex systems? All scientific models are simplifications and all scientific knowledge is provisional.

The difference between climate and neuro, genetic or birth-of-the-universe science is that the knowledge and recommendations of climate scientists challenge the way that almost everybody on earth who has any wealth and power wants to keep living, and so there is a huge desire for these scientists to be wrong and huge vested interests in not listening to them. (Interesting )

This is no more the fault of the scientists' lack of PR skills than being mugged is the fault of a man's lack of kung fu skills.

Oli said...

Part of the problem comes from the pressure that "extraordinary claims require extraordinary evidence".

The claim that CO2 is a greenhouse gas is neither extraordinary, nor does it lack in very compelling evidence. This is not the issue.

The extraordinary claim (to exaggerate) is that: "if we don't fundamentally change the economic order of our society now this planet is toast". The problem is that the evidence for this prediction comes mostly from almost untestable computer models.

It's this mismatch of evidential weight compared to the extraordinary call to action that sits at the heart of the dispute.

The case for extraordinary action would be seriously boosted if the predictions of organisations like the Met Office were consistently right. That would build the necessary confidence that their predictions for 100 years henceforth are also right.

As it is, I've just read a story about how the latest 'new improved' climate model from NASA predicts less warming than the previous generation of models.

The reality is that this area of science is still relatively new and the uncertainties for model predictions pretty large.

It's therefore not so surprising that many people are sceptical of the extraordinary call to action, especially when, as you say (almost), this call to action goes against our vested interests in the luxurious lifestyles many of us live in the west.

As you know, there are plenty of other reasons why we should act to diversify our energy sources and improve energy efficiency.

Unfortunately though, as this article on electric cars amply demonstrates, it's still the case that nothing beats the remarkably useful characteristics of liquid hydrocarbon fuels. We've just got find sustainable ways to create them. Simple :)