The precipitous loss of Arctic sea ice has been well documented — and well publicized. Now, an increasing number of scientists are turning their attention to a vital question: Once the Arctic Ocean’s summer sea ice disappears — which many scientists say could happen in roughly 20 years — what comes next?
Julienne Stroeve and her colleagues at the National Snow and Ice Data Center are just beginning to study the ripple effects of the rapid shrinking and eventual seasonal disappearance of what Stroeve calls “the air conditioner of the Northern Hemisphere.” What they have already discovered has shown that the changes will be profound. In a recent study, published in the journal The Cryosphere, Stroeve and her colleagues showed that in areas where summer sea ice already has disappeared, autumn air temperatures have been more than 5 degrees F warmer than the long-term average, in large part because the exposed ocean absorbs far more heat than sea ice.
When contemplating the effects of global warming, scientists have always been concerned about so-called “positive feedbacks”: Rising temperatures cause sea ice to melt, the dark water absorbs much more heat than the white surface of the ice, and that in turn warms up the ocean and the air even more, causing more ice to melt — all of it creating a tightening spiral of increased thawing and warming. Stroeve and others call this “Arctic amplification.” No matter what you call it, scientists concerned about climate instability don’t see much positive in “positive feedbacks” — it is a vicious cycle that is hard to break.
In an interview with Yale Environment 360 senior editor Fen Montaigne, Stroeve discussed what may lie ahead if the Arctic’s summer sea ice melts away, from further warming of Arctic land masses to the unpredictable impact on the climate of the Northern Hemisphere.
Yale Environment 360: You and your colleagues have done some very important work about how the loss of sea ice in the Arctic is beginning to have some significant impacts on regional climate. I wondered if you could give us a little background on the extent of Arctic sea ice loss, and what you have found through your research about how this is beginning to have an amplifying effect on the Arctic?
Julienne Stroeve: Well, what we have been observing, at least over the last 30 years — and probably over the last 50 years if we look at some earlier records that aren’t based on satellite data — is that the Arctic has been losing its sea ice cover basically in all the calendar months. But the ice losses are largest in summertime, particularly at the end of the melt season when you reach your minimum extent in September. Basically, over the last few years, ever since 2002, we’ve had one pronounced record minimum after another, and it has caused a pretty strong acceleration of the downward trend in the Arctic sea ice cover in September. The trend is almost -12 percent per decade right now.
What we have been noticing is that because we now have these large expanses of open water areas in September, that when the sun starts going down and the air temperatures get cool enough to start causing sea ice to re-form again for the winter, before the ocean can re-freeze it has to release all that heat back into the atmosphere. This is heat that the ocean absorbed because now instead of being covered by ice it is being exposed to the sun during the summertime. That is what we term Arctic amplification, the idea that the warming in the Arctic is going to be greater than anywhere else on the planet, largely because of this feedback effect, because when you melt away the snow and the ice you expose these darker surfaces — like the oceans or the land — which then absorb that heat, and that heat gets re-released.
We did a study where we looked at changes in atmospheric temperature in the autumn season, related to these recent extreme ice losses that we’ve been seeing in September. What we’re finding is that there is a very strong warming signal with temperatures over the Arctic Ocean, about 3 degrees C (5.4 F), on average, warmer than normal, during these last 5 years compared to the longer term record, which started in 1979. This warming is centralized over the Arctic Ocean where you’re losing the ice, but it can also spread toward the land areas by atmospheric circulation, and then cause more warming of land surface temperatures as well.
e360: And that was from 2003 to 2007, when you were detecting these much higher temperatures than the long-term average?
Stroeve: That’s right. And we’ve known that this was going to happen, looking at climate model simulations because they all show this kind of amplified warming in the Arctic. But it’s just happening sooner (than predicted) right now.
e360: I want to jump back and ask one question about the loss. You said it’s about 12 percent per decade. I take it from both the satellite records, which go back 30 years, and previous records, that a rough estimate of the overall extent of summer Arctic sea ice is that it is now roughly at 40 percent to 50 percent of what it was 50 or 60 years ago?
Stroeve: Right. We have about 40 percent to 50 percent less of the Arctic Ocean surface area covered by ice now than what we used to have 40 or 50 years ago. That’s a significant drop.
e360: Do you have any data about how much Arctic Ocean water temperatures seem to be going up in the most recent decade or two?
Stroeve: There’s a combination of a few different effects there. One is more inflow of warm waters into the Arctic, through the Bering Strait, for example, which seems to be then circulating around the Arctic, and resulting in some warmer temperatures and perhaps more melting of the ice from below. And then there’s also the effect that you get just from exposing the ocean now to the sun and letting it absorb all that solar energy, because the albedo — or the amount of solar energy that gets reflected back by the water — is very small. It absorbs almost 93 percent of all the incoming solar radiation, whereas the ice, if it is snow covered, will reflect about 80 percent of that incoming solar energy back out to space. The presence of ice definitely helps keeps things much cooler.
We’re basically about 30 years ahead right now of where the models say we should be, in terms of how quickly the ice is declining.
e360: Can you say what the earlier projections were for loss of summer sea ice cover in the Arctic, and when you might see a generally ice-free Arctic Ocean?
Stroeve: We did a study a couple of years ago where we looked at a comparison between the observed record that is coming from the satellite data and then the climate models that were used in the latest IPCC [Intergovernmental Panel on Climate Change] report. Basically, all of those models are consistent in that the ice cover has been going down over the whole period of observations. But even so, when you run these models out into the future and you do business-as-usual scenarios of greenhouse gas emissions, basically these models will show the ice disappearing entirely in the summertime sometime between 2050 to sometime beyond 2100. We’re basically about 30 years ahead right now of where the models say we should be, in terms of how quickly the ice is declining.
And so, maybe an ice-free Arctic Ocean might be realized as early as 2030. There are some people who think it may happen even sooner, that the ocean is contributing a lot more to the melting of the ice than than we have been realizing, and that the volume of the ice in the Arctic is actually at an all-time record low.
e360: When you talk about the volume, of course, you are talking about both extent and thickness. Could you just briefly explain how open water absorbing more heat has an impact on how hard or thick ice is then going to re-freeze? I would think the ice just keeps getting thinner and thinner.
Stroeve: Well, certainly, by delaying the melt season, if it takes longer for the ocean to re-freeze, then you have a shorter growing period for the ice to thicken. But natural variability has also been playing a big role in helping to reduce the ice thickness in the Arctic — in the mid-1990s there was a very strong Arctic oscillation weather pattern that helped to get rid of some of that older, thicker ice, leaving behind much younger, and therefore much thinner, ice in the Arctic. And so, basically we don’t seem to have these very thick stores of ice like we used to that help stabilize the ice cover.
Say you had a really warm summer and you had conditions very favorable for melting. If you had ice conditions like you had in the 1970s, where you had much thicker ice, and a larger fraction of the Arctic Ocean was covered by thick ice, you may have a lot of melt happening and you may have a large volume loss in the total ice cover, but you wouldn’t really see that reflected in extent changes because the ice is so thick. But you can imagine as you get down to a much thinner ice cover, now we are at this point where the Arctic Ocean is covered by much thinner ice, so then when you have an usually warm summer, all of a sudden you melt out large areas of the ice cover and you have these big open water areas forming.
e360: And, I take it that the ocean can hold an awful lot of heat, obviously, compared to an ice-covered ocean. I would think that the ocean must release a tremendous amount of heat back into the atmosphere in the autumn and the winter.
Stroeve: That’s true. And, there was another study that came out not too long ago that showed that where you have these periods of rapid ice loss, the warming that you get in the autumn can spread out to the adjacent land cover, and can cause temperatures over land to warm much quicker than they would under periods of moderate or no ice loss. And that’s a concern, of course, because then you are starting to affect permafrost temperatures, and there have been trends, of course, of the permafrost starting to thaw a lot more and getting warmer and if you just give it extra feedback, or now even warmer autumn temperatures, it’s a bit of a concern because the amount of carbon in the permafrost is estimated to be around 950 gigatons. So that’s a huge feedback signal looming there.
Everything is connected, so when you change one component of the planet, the rest of the system is going to have to respond.
e360: And that would be released, that carbon, in the form, primarily, of methane?
Stroeve: In the form of methane.
e360: And when you mention that in the autumn you’re seeing — at least in the recent four or five years — temperature increases of 3 degrees C more, was that over land or over ocean?
Stroeve: We were just looking at it over ocean.
e360: You described the Arctic and its once year-round blanket of sea ice as “the air conditioner of the Northern Hemisphere.” What did you mean by that, and if you lose that ice-covered ocean, that air conditioner, could you see an impact on hemispheric weather patterns?
Stroeve: Basically it’s what keeps our planet cool, by having the presence of both snow and ice in both poles. So if you take that away you just start warming up the planet even more. And, we certainly would expect that to have an effect on atmospheric circulation around the planet, but exactly how that is going to manifest still remains quite unclear. The research on that is still very much in its infancy. But certainly, everything is connected, so when you change one component of the planet, the rest of the system is going to have to respond.
e360: Right, and in fact, at its simplest, isn’t global weather the transfer of heat away from the equator and the tropics towards cold regions at the poles?
Stroeve: Right, and so you change the temperature gradient, basically, between the poles and the equator, which will definitely impact your atmospheric circulation.
e360: You mentioned that this is all happening so fast, and that this research is in its infancy. Is there now a quickened pace of research about this whole issue of loss of Arctic sea ice and its impact on climate?
Stroeve: I think there is. I mean, we have a couple of grants right now that are focused on looking at impacts. One is on the temperatures, and one is on the snow cover. I think we’ve come to realize that the Arctic sea ice is going down, we understand why it’s happening, and we understand that it’s probably going to a state where it’s seasonally ice free in the near future. Now we really need to move into the realm of looking at impact studies.
e360: You think that in two to three decades, if these trends continue, there will be no Arctic sea ice in summer?
Stroeve: I do believe that — in summer. It would still get cold enough to freeze in the wintertime.
e360: What is your reaction to this unexpectedly rapid loss of ice and all these other feedback changes that follow from that?
Stroeve: I think when I first started out studying sea ice, or even just climate in general in the Arctic, I didn’t really think that we were in the midst of this global warming phenomena yet. But then, these last few years when we just continued to see these record ice losses, I started to change my way of thinking and realize that we are having a huge impact on our climate and we’re actually causing the ice cover to pretty much disappear now. And, yeah, it’s been alarming. Because we don’t really fully understand the implications of this, and I think that’s the biggest fear, is that we really don’t know what we are doing. It’s like we are playing with the dials on our climate and we don’t really know the outcome of it yet.
e360: Right, and I know some climate scientists talk about how sometimes the climate isn’t so much a dial as a switch, that things can change rather suddenly?
Stroeve: Yes, if you look at the geological record there is definitely evidence of very abrupt climate changes that have happened in the planet’s history. So, can our actions actually lead to something similar happening? That’s a scary thought.