The Implications of Snowcover on Climate: A Conversation with Stefan Sobolowski (Part 1)

by Lakis Polycarpou | June 15, 2010

Stefan Sobolowski says he has always had a passion for water, weather and climate—a passion he attributes to lifetime of skiing, hiking, snowboarding, and playing in oceans. Even so, he says, he never intended to become a climate researcher. When he first went back to school to get his Master’s in environmental science, his goal was get certified to teach high school. “All I really wanted to do is teach kids that, you know, recycling is good!” he says.

But when the program he enrolled in wouldn’t accept his undergraduate science credits, it started a “chain reaction” that led to a master’s degree in physical geography and, eventually, a doctorate from Columbia University’s department of Earth and Environmental Engineering.

Stefan recently defended his dissertation and will be soon be moving on to a post-doctoral position in climate science at the University of North Carolina.

For the first part of our conversation, Stefan discusses his research on the effects of continental snowcover on climate and why one cold winter in the United States doesn’t mean that there is no such thing as global warming.

You are a hydroclimatologist. What is a hydroclimatology?

We had to come up with something! So we took hydro, we took climate and we kind of stuck them together. In brief a hydroclimatologist is interested in the relationship between the water-cycle and the climate system. You could hem and haw about the semantics, and actually you could say that the climate system really is the hydrological system. It’s an incredibly broad definition, which is how I like it. It gives me a big umbrella that I get to play under. But in general that’s it: we focus on the water side of climate and climate change, the functioning of the climate system.

What are you working on now?

My dissertation research was focused on the effects that large-scale snow cover has on the overlying atmosphere, in North America in particular. Lots of folks have focused on Eurasia, because it’s this giant continent, so it has a lot of inertia to it when you cover it with snow. North America not so much. And so basically I did a lot of modeling experiments where I slapped a lot of snow over North America, and another set of experiments where I took most of it away, and then evaluated what happens in the atmosphere, and how the circulation changes.

In particular what happens, and we saw anecdotal evidence for this this past winter, is that when you have really extreme, high snow-cover events or winter in North America, it effects the orientation of what we call the storm tracks. The storm tracks can just be thought of as a region that stretches from the East Coast of North America across the North Atlantic, where storms tend to become concentrated. Think of it sort of like as a funnel. This pathway varies depending on conditions. Sometimes its horizontal, sometimes it tilts from Florida to England. It moves from north to south.

What we found with the really extreme snow events is that storm tracks are intensified. You get more intense storms, and they extend further across the North Atlantic. And we saw something very similar this winter, actually. We had a series of really intense, powerful storms that roared through the southeast of the Unites States, and barreled on up the coast and hit us pretty hard, and those storms continued up across the North Atlantic, and Europe got clobbered all winter long as well.

So, the more snow cover there is, the stronger the storms? It’s almost like a snowball effect.

Yes, that’s exactly what happens. The reason is that the snow creates a cooling over the land surface, and it intensifies this cooling and it extends further south. The effect is that the difference between the cold land surface and the relatively warm ocean, is intensified. You steepen the gradient between the two. And once you do that, it’s like giving oxygen to a fire. You’re basically enhancing the conditions for storm genesis.

Then there’s the potential for a positive feedback as well. Often, along that snow line – so the line for last winter was pretty far south for us, it was probably around Virginia, dipping down as you go into the Midwest, and then goes back up toward Washington and Oregon in the West– along that snow line it can be really iffy because the temperatures are real close to freezing. But further inland, in places that are cold to begin with and always going to be cold, it does have a positive feedback effect.

Last winter a lot of commentators were saying “what global warming?”

Right, basically the reason those conversations get started is that either there is a communication breakdown and/or the public is conflating weather with climate, and not understanding that they’re two very different manifestations of the earth system.

What’s the difference between weather and climate?

The weather is what happens on short timescales, days to weeks to months. Climate is a long-term average of the system. Thirty years or more. So in global warming context, you will have cold years, you will have warm years, you will have some years that are much colder than the average. You will have years that are much warmer than the average. But over a long period of time, that average is going up.

This winter was a good example of the difference between the two. People in the Untied States, for example, were saying “what global warming?” while people in Western Canada were like “ha ha, what are you talking about?” because they had one of the warmest winters on record there, as the Olympics showed in glaring detail. They pulled snow from pretty much every peak they could find there.

Overall, over the globe, this winter was the fifth warmest winter on record, despite the fact that the United States–and also, actually, much of Europe–was colder than normal. So it’s really important that people understand the difference between weather and climate – weather is the short-term, highly unpredictable, chaotic, phenomena that operates on short time scales, and that climate is a much longer-term, typically much more stable phenomena.

The changes that we’re seeing now in climate are pretty much unprecedented in the records that we have. And I think that’s what makes scientists so nervous about it. Because the speed at which change is occurring, really there are very few if any historical analogues for us to look back at and say, “oh, well, something similar to this happened in 2,000 BC somewhere” and we can sort of build a scenario from that. We really don’t have those historical reference points. So that’s disturbing.

Also, it’s important to realize that in the discussion of global warming the importance of that word, global. We’re talking about a global effect. The globe being a pretty large place, when you take an average over the whole thing, it wipes out all of the variation you have in different regions. So you have a cold winter in the United States, super warm winter in Western Canada. It’s the same thing happening all over the world. So that’s an important distinction that needs to be communicated.

So what are the climate change implications of your research on snowcover?

Well, the conclusions are from the opposite side. It’s not any increase in snow cover that’s having an impact, it’s a decrease—a decrease that’s going to combine with other changes in the atmospheric circulation.

The implication is that aside from the internal areas of the continents–Northern Canada, Saskatchewan, Siberia, places like that, which even in a warming climate are still going to be cold in the winter and where the addition of more water into the atmosphere is going to create deeper snow conditions–the rest of the Northern Hemisphere snow cover has been in retreat and is likely to continue retreating. The snow season–November, through March, basically–is getting shorter and shorter, and the spatial extent of the snow cover is geographically shrinking.

And the implication is also that the circulation will change–it will be more horizontal. So if you look on a weather map, any forecast, in the winter time they often show the jet stream. And the jet stream is usually shown as a big wave, almost an S-curve. That’s a sign of a really active atmosphere. So when you see that you have low pressure and high pressure dotted around, and after one of the big downward curves there’ll be a storm usually, and they’ll say “this is the thing that’s going to ruin your entire week!” That’s a sign of an active atmosphere, and it’s typically what we get in the winter time.

The decrease in the snow cover is going to contribute to stretching that big S out and making it less pronounced. What that means is that you’ll have less variable weather, fewer storms, and the storms aren’t as strong. It changes the way moisture flows through the atmosphere. Those storm tracks I was talking about will shift north, so we won’t have the same frequency of storms going across the southern part of the United States, probably. And those all have implications for water availability.

What it can do is set up conditions prior to the summer dry season where you start off the season already in a fairly dry state. Think of it as loading the dice. It’s very hard to ascribe direct impacts. All of these changes in circulation, and changes of available water in the atmosphere, say, they’re all sort of like die-loading. So then you roll those dice and the probability of a certain event happening, like a drought or an extreme flood, increases.

So that’s really what the decreasing snow cover contributes to. The changes in the circulation—we’re fairly confident that those are starting to happen, and should be noticeable within the next 15 to 20 years. So it’s a much less active atmosphere, especially in the winter time. In the summer time it’s already not so very active in the northern hemisphere.

Are there any positive feedback loops, where, the less snow cover you have, the less light and heat is reflected and the warmer it gets, for example?

Yes, absolutely. That’s a snow albedo feedback, and that’s a positive feedback that will have the most effect near the snow line—near those mid-latitude regions, where whether it snows or not isn’t really determined by precipitation, but by the temperature–whether it’s freezing, a little above freezing or a little below freezing.

Take a big wide band of the Northern Hemisphere and we call them the mid-latitudes—and that’s right around where we’re located, around 40 degrees—and that’s where the effects of snow albedo feedback will be felt. Those regions tend to also have the largest populations of humans as well.

And also places where people are really reliant on snow as a storage reservoir, like the Sierra-Nevada out in California. If you’re not now getting snow in those mountains, but are getting a little snow and then rain, you’re losing the storage which is released in the springtime during the big snowmelt. So you have potentially devastating effects for farmers, to say nothing of municipal water supplies downstream.

To be continued

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