21 september 2012

[4C note: We post this long but important document with the kind permission of the author. For the original graphics accompanying this article, which our web format does not allow us to reproduce, please go to the original source]

Arctic Sea Ice: What, Why, And What Next (Part I)

By Ramez Naam, Climate Guest Blogger, Scientific American, September 21, 2012

On September 19th, NSIDC, the National Snow and Ice Data Center, announced that Arctic sea ice has shrunk as far as it will shrink this summer, and that the ice is beginning to reform, expanding the floating ice cap that covers the North Pole and the seas around it. The Arctic Sea Ice extent this September was far smaller than the previous record set in 2007. At 3.4 million square kilometers of ice coverage, this year’s Arctic minimum was 800,000 square kilometers smaller than the 2007 record. That difference between the previous record and this year’s is larger than the entire state of Texas. An ice-free summer in the Arctic, once projected to be more than a century away, now looks possible decades from now. Some say that it looks likely in just the next few years.

What’s happening in the Arctic? Why is it happening? And does it matter for the bulk of us who live thousands of miles away from it?

Faster and Faster

Conditions in the Arctic change dramatically through the seasons. In the depths of winter, the Earth’s tilt puts the Arctic in 24 hour-a-day darkness. Temperatures, cold year round, plunge even lower. The sea surface freezes over. At the height of summer, the opposite tilt puts the Arctic in 24 hour-a-day sunlight. While it’s a cold cold place even at these times, the constant sunshine, warmer air, and influx of warm waters from further south serve to melt the ice. The ice cap usually starts shrinking in March, and then reaches its smallest area in mid-September, before cooling temperatures and shorter days start the water freezing and the ice cap growing once again.

When scientists and reporters talk about an ice-free Arctic, they’re usually speaking of the Arctic in summer, and especially in September, when ice coverage reaches its minimum.

The amount of ice left at that minimum has indeed been plunging. In 1980, the ice shrank down to just under 8 million square kilometers before rebounding in the fall. This year’s minimum extent of 3.4 million kilometers is less than half of what we saw in 1980. Strikingly, two thirds of the loss of ice has happened in the 12 years since 2000. The ice is receding, and the process, if anything, appears to be accelerating.

As recently as a few years ago, most models of the Arctic ice anticipated that summers would remain icy until the end of the 21st century, and well into the 22nd century. But the trend line above makes that look unlikely. The amount of ice remaining, this year, is about the same as the ice lost between the mid-1990s and today. If ice loss continued at that pace, we’d see an ice free summer sometime around 2030, give or take several years.

Is that plausible? Opinions differ substantially, even among climate scientists.

At one end of the spectrum are those who see the ice lasting in summer for another 20 or 30 years, or perhaps even a bit longer.

For example, Lars-Otto Reierson, who leads the Arctic Monitoring and Assessment Programme told Reuters that most models predict the summer ice disappearing by 2030 or 2040.

Similarly, a paper published this year in Geophysical Research Letters by multiple scientists, including several from the National Snow and Ice Data Center, found that an ice-free summer in the Arctic in the “next few decades” was a “distinct possibility.”

A recent assessment from Muyin Wang at the University of Washington and James Overland at the National Oceanographic and Atmospheric Administration, using the most up to date Arctic ice models and data, projected a nearly ice free Arctic around 2030.

And Ceclia Bitz, a professor of Atmospheric Sciences at the University of Washington at part of the Polar Science Center sees a 50/50 chance that the Arctic will be ice free in summer in the next few decades.
On the other end of the spectrum are those who think the melt could happen much sooner.

Peter Wadhams, who leads the Polar Ocean Physics Group at the University of Cambridge, has predicted since 2008 that the Arctic ice could be gone in summer by 2015. He now believes there’s a chance that it could happen even sooner.

Similarly, Mark Drinkwater, the European Space Agency’s senior advisor on polar regions and a mission scientist for the CryoStat satellite that measures arctic ice, believes that the Arctic could be ice free in September by the end of this decade.

When will the ice melt? While the range of possibilities is wide today, it’s shrunk dramatically from just a few years ago, when most climate scientists expected the ice to survive through the 21st century. Now the question is whether it will be gone in decades – or in mere years.
Why is the Ice Melting?

Why is the Arctic ice cap growing so much smaller in summer? And why this summer in particular?

Some of the effect is seasonal variation. Like anything in climate, there is a fair bit of noise. This year, in particular, an immensely powerful high latitude storm in August stirred up waters, bringing warmer water from further south up to the Arctic, and accelerating the melt of the ice.

Everyone who studies the ice melt agrees that natural weather variation from year to year has played a role. Both 2007 and 2012 were unusually bad years for the ice, and would have been even if the planet’s climate hadn’t changed. Looking over the data from 1979 to 2011 (before this year’s new record low), a team of scientists led by Julienne Stroeve at the National Snow and Ice Data Center found that those random variations accounted for around 40 percent of the change in ice cover to date, and that human activity accounted for around 60 percent of the change.

Other research points to an even greater human component. In 2011, a team led by Chilean scientist Christophe Kinnard published a paper in Nature that used data from 69 sites around the Arctic to reconstruct the extent of the ice over the last 1450 years – all the way back to the 6th century AD. What they found was that late summer ice coverage over that entire fourteen and a half century period stayed between 9 million and 11 million square kilometers, a little higher than it was before satellite observation started in 1979, or roughly three times the minimum that we hit this September.

Kinnard was kind enough to send me the team’s underlying data. Combining it with satellite based observations from 1979 onward, the last few decades pop out. Ice coverage fluctuates for centuries, but stays in a narrow band, until suddenly, in the last few decades, the amount of ice left in late summer plunges.

All the sea ice loss, including that before satellite observation occurred, has happened since the start of the industrial revolution, and the beginning of human emissions of CO2 and other greenhouse gases on a massive scale.

In this context, random variations look rather small. In fact, there are at least three distinctly non-random factors leading to the disintegration of the ice.

1. A Warming Planet.
The first factor is the most obvious. Ice melts faster in warm air than in cold air, and faster in warm water than in cold water. And the Arctic has warmed more rapidly than any place on Earth. In the last 40 years, the world as a whole has warmed by around 0.8 degrees Celsius. That alone would accelerate the melt of ice. But the warming in the Arctic has been twice at fast, at roughly 1.9 degrees Celsius, or 3.5 degrees Fahrenheit. That may not sound like much, but when ambient conditions are so close to freezing already, that additional heating can make a tremendous difference in the rate of melting.

2. Positive Feedback
The second factor is positive feedback. Not the “good job” kind of positive feedback, mind you. This type of feedback is more similar to what happens when a microphone comes too close to a speaker, and a random piece of noise gets amplified out of control. Any time a process can amplify itself, it’s a positive feedback loop. The melting ice is, in fact, amplifying its own destruction by helping to fuel accelerated arctic warming and thus more rapid disappearance of the ice that remains.

Here’s how: Ice and snow reflect light. Thick ice covered with snow will reflect the large majority of the sun’s energy back into space, absorbing only 10 to 20% of the sunlight as heat. Ocean water, on the other hand, reflects very little of the sun’s energy back into space, absorbing more than 90% of it as heat.

So the less ice there is, the more the sun warms the waters that remain. And 90% of the ice cap is under the surface, bathing in that water. Less ice means faster melting of the ice that remains.

The melting ice and warming atmosphere cause a second, less appreciated feedback loop. Water vapor is an important greenhouse gas. It captures heat radiating from the planet and traps it in the atmosphere, warming the planet by tens of degrees Celsius.

Cold air doesn’t hold much water, though. The air above the Arctic is dry – a supercold desert. As a result, Arctic air doesn’t trap much of the heat that radiates away from the water and ice. But as the air warms, it can hold more water – about 7% more for every degree Celsius the temperature rises. And that added humidity traps more heat in the atmosphere above the Arctic, raising local temperatures and further accelerating the melt of the ice.

All in all, there’s good reason to believe: the less ice remains, the faster the ice that’s left will go.

3. Thin Ice.
There’s one more important factor to bear in mind – the increasingly thin state of the ice. Over the last few decades, the average thickness of the ice that covers the Arctic in summer has dropped in half, from an average of more than 2 meters thick (7 feet) to roughly 1 meter thick (3 feet).

That drop in thickness has happened, in large part, because the ice that remains in the Arctic in summer is now predominantly very young ice, most of it formed just in the past winter. In the mid 1980s, only around 25% of the summer ice was this new, thin, extremely vulnerable stage, and almost two thirds of the ice was older than two years old – old enough to have accumulated more water and grown thicker and sturdier. Now the numbers have nearly reversed. Very little of the thick multi-year ice remains. The ice that’s left, being thinner, takes less heat to melt.

The reduced thickness of the ice not only bodes poorly for its survival, it tells a dramatically different story of how far from an ice free summer we might be. Most models of arctic ice coverage predict that some ice will remain – even at the September low point – for decades to come. Looking at the trend of ice coverage and doing the very simplest extrapolation, we could see the first ice free Arctic days by 2030, a little sooner than many experts predict, but still nearly a generation in the future.

But coverage is just the area the ice covers. Volume – the area covered multiplied by the thickness – is how much ice there actually is. Imagine a sheet of ice covering a frozen lake in early spring. The ice may cover the entire lake, but a two inch thick layer is much sturdier – and will take longer to melt – than a one inch thick layer.
When we look at volume instead of area, we don’t see that half of all the ice has disappeared since 1980. Instead, we see that almost 80% of the September ice has disappeared in that time. And most of that loss has been in the last 12 years. 70% of the ice volume we saw in 2000 has disappeared. Less than one third of that ice volume – from just 12 years ago – is what we see today.

If the Arctic sea ice loses volume at the same rate that it has over the last 12 years, then the first ice-free Arctic day in September could happen in the next 5 years. If the rate of ice volume loss continues to accelerate, as it has been, then that day could be even sooner. Ice volume tells a story much more like that of Peter Wadhams, the leader of the Polar Ocean Physics Group at Cambridge, who for years has been predicting an ice free Arctic September as early as 2015.

An Ice Free Solstice?

Here’s what the future holds. One September, perhaps as early as 2015, perhaps decades later, we’ll learn that the Arctic is ice-free. There will still be ice bergs and occasional floes of ice in the Arctic, but they’ll be scattered, surrounded by far more water than ice. Nowhere will ice constitute the majority of the Arctic.

You’ll probably hear this referred to as the first ice-free Arctic summer. But, in reality that first “ice-free Arctic” will most likely be a period of a few days or a week or two in mid-to-late September. August will still have an ice cap, if a small one. July will have a larger one than that. June larger still. And March, the month of the year when the Arctic ice cap is at its maximum, will still have seen plenty of ice.

Yet from that year on, the ice-free period will likely grow, expanding in duration to start earlier and end later year over year. Most likely, it’ll follow the same jagged, two-steps-forward one-step-back progression we see in climate in general. The first year after the first “ice-free Arctic” year, we may see some ice cap persist all the way through the summer again. For that matter, next year, 2013, it’s quite possible that we’ll see more ice than this year. Climate is bumpy that way. But, if recent history is any lesson, bit by bit, step by jagged step, the ice free period will lengthen, and the ice coverage in other months of the year will shrink.

This is particularly important because September is not a very sunny month in the Arctic. The sun never rises high above the horizon, and so its heating power is muted. Those dark waters are absorbing more of the solar energy that strikes them than ice would, but there’s simply less solar energy striking the arctic in September than there is for the spring and summer months leading up to it.

The sunniest time of year in the northern hemisphere is the summer solstice, in late June, and the weeks preceding and following it. For several weeks the sun’s rays are at their most intense and the Arctic receives 24/7 sunlight, giving it a double whammy of heating. In fact, in June, July, and the latter half of May, the Arctic receives more total solar energy per day than regions at the equator do at any time of year. The sun’s rays are never as powerful in the Arctic as they are at the equator, but the 24/7 availability of sun more than makes up for that. (If you doubt this, see NASA’s Earth Observatory page on the topic or use NASA’s monthly insolation-by-latitude calculator.)

Thus, every patch of dark ocean water revealed by melting ice in June, or May, or July, has a warming effect much larger – as much as five or six times larger – than the same change in ice coverage in September, when the ice hits its minimum today. The loss of ice in September is one thing. Loss of ice in June would have a far bigger impact on the region and the planet.

So far, the ice extent change in June has been modest. Between 1979 and 2012, NSIDC reports that the Arctic ice coverage in June shrank by around 10%, compared to the roughly 50% shrinkage that the September ice coverage has seen. And at first blush, the trend looks more or less linear. At that pace, we wouldn’t expect to see an ice-free June in the Arctic for another 300 years.

But again, while coverage matters, it’s volume of ice that really tells us how healthy the ice is, and how much there is left of it. And June ice volume in the Arctic has sunk fast – by more than half since 1980. What’s worse is that the trend is accelerating. In the decade from 1980 to 1990, June ice volume dropped by around 3.8 thousand cubic kilometers. In the last decade, between 2002 and 2012, it dropped by almost 9.5 thousand cubic kilometers, two and a half times as fast. If the pace of the last decade were to continue on this recent pace – and didn’t keep on accelerating – June ice would be gone by 2026, exposing the dark waters of the Arctic Ocean to the year’s most intense influx of solar energy, which would then be captured as heat.

Of course, if we see one thing in the loss of Arctic sea ice, it’s that the process isn’t continuing at a steady pace. It’s accelerating. And as more dark water gets exposed to the warm June sun, the lifetime of the remaining ice is likely to shrink even faster than it currently is. An ice free June could be quite a bit closer than we imagine.
Just a few years ago it would have seemed incredibly alarmist to predict even a single ice free Arctic day this decade. Increasingly, it seems foolish not to accept that ice free Arctic summer days, weeks, or months this decade are, if not a certainty, then at least a very real possibility.

The Good News

So is this the end of the world? Or is it just an interesting but ultimately unimportant phenomenon we get to observe? Could it even be an opportunity?

First, some good news. The melting Arctic ice will not cause sea levels to rise to any noticeable degree. The Statue of Liberty isn’t about to be reduced to a head and single upraised arm, forlornly holding her torch just above water. The Arctic ice cap is sea ice. It floats already. And just as a melting ice cube in your drink doesn’t raise the overall level of fluid in the glass, the melting of floating ice in the Arctic won’t directly raise sea levels. (This is quite different from the effect of land-based ice, such as that on Greenland or Antarctica. The melting of ice that is currently sitting on land does raise sea levels. But such ice is also far harder to melt.)

More good news, or at least the absence of terrible news: The melting Arctic ice is unlikely to suddenly stop the “deep ocean conveyor”, the current that brings warm water to Europe and keeps the continent – much of which is at the same latitude as Canada – fairly warm and temperate. While the deep ocean conveyor belt, also known as the thermohaline circulation, does appear to be slowing a bit, calculations show that the amount of fresh water needed to stop it is far greater than the amount of water currently trapped in Arctic ice. (A breakdown of the thermohaline circulation, by the way, is the vague explanation given for the rather jumbled science of the movie The Day After Tomorrow. So, among other good news, take note that you won’t need Dennis Quaid to snowshoe across a frozen landscape to come to your rescue.)

Finally, good economic and natural resource news: The receding ice will open up new trade routes, making it easier, cheaper, faster, and more efficient to ship goods between northern Europe, Canada, Russia, and the United States. Cargo that once had to be placed on ships that passed through either the Panama Canal or the Suez Canal will now, in many cases, have a shorter route – one that saves on both time and fuel.

The opening up of the Arctic will also open up exploration for minerals and for fossil fuels. The US Geological Survey estimates that the region has the world’s largest remaining untapped reservoirs of oil and natural gas – as much as 90 billion barrels of oil and almost 1.7 trillion cubic feet of natural gas. Those numbers would make the Arctic home to 13% of the world’s remaining oil and 30% of the world’s remaining natural gas. And that is a very rich prize, spurring investment in exploration, and increasing jockeying between the world’s powers – including some countries, like China, that don’t even have a physical presence in the Arctic – to gain access to those resources.

Tapping those minerals and oil and natural gas would be an economic boon to communities in the Arctic. It would also be a win for the global economy as a whole, helping to keep energy prices lower by bringing new supply to market. But of course, burning that oil and natural gas would also accelerate climate change – the very process that has set the Arctic on the path to melting.

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