21 september 2012

Ramez Naam, Arctic Sea Ice: What, Why, and What Next (Part II)

Scientific American

The Bad News

So the seas will not rise, the ocean currents won’t suddenly end, and there will be some economic benefits. What is it about the melting ice that worries us?

By now many of us have seen pictures of lonely polar bears, seemingly stranded on a patch of ice surrounded by water. There are indeed threats to polar species. But I want to focus on wider threats that extend beyond the Arctic and into the rest of the world. There are three in particular that should concern us.

1. More Extreme Weather
The most palpable impact of climate change for those of us who live far from the poles is the increase in extreme weather. Around the world, record highs are occurring at more than twice the rate of record lows. The US drought of 2012 was the worst since the Dust Bowl of the 1930s. A drought nearly as bad struck Texas and the American south in 2011, and even more destructive heat waves hit China and Russia in 2010, and Europe as a whole in 2003.

You might not think that what happens in the Artic has much bearing on what happens in Texas or Moscow or southern provinces of China, but a study published in 2012 in Geophysical Research Letters has drawn a convincing connection. Blowing around the periphery of the Arctic is the polar jet stream – a region of high speed wind that blows west to east, and helps drive wind circulation around much of the northern hemisphere. The jet stream is powered by the temperature difference in fall and winter between the Arctic and the more temperate areas just to its south. But as the Arctic ice has receded, the Arctic Ocean waters have absorbed more heat in late summer and early fall. In late fall and early winter, they’ve given that heat up, back into the atmosphere. That, in turn, has led to warmer Arctic autumns and winters, which has reduced the temperature difference that fuels the jet stream.
The result is that the jet stream is now weaker than it once was – about 14% weaker than it was in 1980.

Why does this matter? Because a slower jet stream makes it easier for ‘blocking’ weather patterns to develop. Blocking weather patterns are the ones that hover over a region rather than moving on – like the drought that basted Texas in 2011 and decimated its forests and hay and wheat crops to the tune of more than $7 billion in damage, and like the heat wave that enveloped Moscow and much of the rest of Russia for most of the summer of 2010, killing an estimated 55,000 people in July and August of that year.

Climate change is driving more extreme weather – by heating up the atmosphere, pumping more energy into storms, and heating the air to the point that it can more easily suck away moisture or concentrate it in one point. As the planet continues to warm, all of those factors will increase, leading to more heat waves, more droughts, and more floods.

And the changes to the Arctic, it seems, will exacerbate this, by slowing down the jet stream, and making it more likely that the extreme weather conditions that develop get locked in place, hammering the same regions for protracted lengths of time.

2. Accelerated Warming
The second thing to fear about loss of Arctic sea ice is the potential to accelerate climate change on a global basis.

A black object gets hotter in the sun than a white object. That much is common sense. Earlier, in describing how melting ice accelerated the melt of more ice, I talked about the fact that dark sea waters absorb up to 90% of the sun’s energy that strikes them, while snow-covered ice absorbs only 10 to 20% of that same energy. The exposure of darker waters speeds up heating of the Arctic, and thus the loss of more ice.
But the impact is larger than that. And indeed, it’s large enough to make a difference on a global scale.

In June, the Arctic ice cap covers around 2% of the Earth’s surface – around 11 million square kilometers of Arctic ice cap out of a total of 510 million square kilometers of Earth’s land and oceans. And that 2% of the Earth’s surface, for a period of roughly two months, receives more solar energy per day than even the sunniest areas on the equator.
Analyzing this, Peter Wadhams of the Global Oceans Physics Program at Cambridge calculates that the loss of the Arctic ice throughout the summer would have a warming effect roughly equivalent to all human activity to date. That is to say, with the ice gone in summer, the planet would have an additional heating effect just as large as the heating effect of all human CO2 and other greenhouse gasses to date.

In other words, the complete meltdown of the Arctic could roughly double the rate of warming of the planet as a whole.

There are important caveats and uncertainties to that analysis. First, Wadhams doesn’t take into account the effect of clouds. Darker waters absorb more energy from sunlight only if the sunlight reaches them. Cloud cover in the Arctic – something which may increase as rising temperatures enable Arctic air to carry more moisture – may reflect sunlight back into space before it ever touches the water, thus reducing the warming effect of ice melt.

Conversely, Wadhams’ calculations only take into account the loss of sea ice in the Arctic Ocean itself. If the accelerating warming of the region melts permafrost in Siberia, Canada, and Alaska – something it seems to be doing – it could well change the reflectivity of the land in those areas, and thus how much sunlight they absorb. How large a change could this be? And in what direction? Climate scientist Judith Curry points out that warming in the Arctic is also associated with more winter snowfall in Canada and Europe, which could make the land more reflective and exert a cooling influence. That change, however, would have an impact mostly in winter (when sunlight is most scarce) and in early spring. In late spring and summer the effect of a warmer Arctic on tundra is likely to be towards less light-colored permafrost and more dark-colored thaw lakes (the bodies of water that form as permafrost melts), dirt, and growing plants.

Climate, as always, is incredibly difficult to model. But the main takeaway here is that the replacement of white snow and ice in the Arctic with dark colored water is in the ballpark where it could rival – and add to – all human greenhouse gas emissions to date.

Global climate models, it should be said, don’t anticipate the ice being gone until well after 2100, and so they don’t include this added heating effect when they calculate future temperatures. Thus, they may underestimate the temperature changes to come.

That is also troubling because current proposals for tackling climate change involve dramatically ratcheting down human greenhouse gas emissions, and allowing greenhouse gas concentrations in the atmosphere to gradually drop. That plan assumes that those greenhouse gas emissions are the major source of human caused warming. The addition of a new source of warming may dash any hopes of arresting climate change by reducing greenhouse gas emissions. If we want to keep global temperature increase below two degrees Celsius we may need to either stop the melting of the Arctic – something which may no longer be possible – or find ways to combat climate change that go beyond simply reducing our greenhouse gas emissions.

3. The Arctic Methane Bomb
The final risk is the largest in the very long term, though the extent to which it will affect us in the coming years and decades is still a matter of great uncertainty.

The Arctic and the region immediately surrounding it are home to immense amounts of buried carbon. The permafrost of Siberia, Canada, and Alaska is estimated to hold around 1.7 trillion tons of carbon – mostly in the form of dead plant matter. The sea bed beneath the shallower parts of the Arctic Ocean holds anywhere up to another 10 trillion tons of carbon trapped in a semi-frozen state called methane hydrates.

By contrast, all human CO2 emissions over the last century amount to only 1.1 trillion tons of carbon. The permafrost carbon, alone, could exceed the effect of all human burning of fossil fuels. The Arctic Sea bed deposits are close to ten times all carbon humans have released. What’s worse is that much of that carbon will end up released as methane (CH4) instead of carbon dioxide (CO2).

A molecule of methane absorbs and traps roughly a hundred times as much heat as a molecule of carbon dioxide. Fortunately, methane degrades quickly in the atmosphere, lasting on average for around 10 years before being converted into CO2, which can last for a hundred years or more. Even so, over the course of a century, a molecule of methane released today will have 25 to 30 times the heating impact of a molecule of CO2 released today.

If even 10% of the northern permafrost’s buried carbon were released as methane, it would have a heating effect over the next decade equivalent to ten times all human greenhouse emissions to date, and over the next century equivalent to roughly four times all human greenhouse emissions to date.

And the permafrost is melting. In Fairbanks, Alaska, ground that’s been frozen solid for 10,000 years is melting, opening up sink holes. In the town of Newtok, Alaska, the permafrost melt has been so bad that the residents recently voted to move the entire town rather than stay and watch it sink into the once frozen land.

Historically, climate modelers haven’t expected the bulk of the carbon in northern permafrost to be released any time soon. Instead, as recently as 2006, climate scientists expected only around 100 billion tons of permafrost carbon to make its way into the atmosphere this century. How much warming effect that carbon release will have will depend on how much of it emerges as CO2 and how much emerges as methane. When plant matter decays in the presence of oxygen, the carbon will be produced as CO2. When plant matter decays anaerobically, without oxygen (for example, in a pool of slushy soil that was once permafrost) then much of the carbon will emerge as methane. If one third of that 100 billion tons expected this century were released as methane, then the heating effect each year would be roughly equivalent to that of the amount of CO2 and other greenhouse gases human civilization releases each year.

That is to say, we could end all our burning of fossil fuels – take them all the way to zero – and still see greenhouse gas levels rising just as rapidly as they are today.

Unfortunately, climate models today don’t take into account this release of carbon. As a paper in 2011 on permafrost melt noted, “none of the climate projections in the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report, none of the recent permafrost projections, and none of the projections of the terrestrial carbon cycle account for” the release of carbon from melting permafrost.
Worse, even the predictions of 100 billion tons of carbon entering the atmosphere from melting permafrost may be out of date and too conservative. In 2008, a study from the National Center for Atmospheric Research (NCAR) and the National Snow and Ice Data Center (NSIDC), published in Geophysical Research Letters, found that melting sea ice threatened permafrost as much as 900 miles inland, and that in past periods of rapid sea ice loss, Arctic land warmed three and a half times as fast as the warming that models predict for the 21st century.

Permafrost melt models haven’t taken into account the rapid feedback cycles driving the warming of the Arctic. They haven’t even taken into account the effect that greenhouse gases released from the permafrost itself will have.

The projections of the last few paragraphs deal with the on-land deposits of carbon in the permafrost alone. I’ve ignored the even vaster methane deposits in methane hydrates frozen below shallow waters of the Arctic Ocean’s continental shelves. That store of carbon is enough that, if all of it were to go, it would have a warming effect equivalent to hundreds of times the total human carbon emissions to date.

The Arctic sea shelf is definitely giving off methane. In 2011, a Russian expedition found kilometer-wide plumes of methane bubbles rising up from the sea floor. Yet the total quantity given off remains quite low today – at most a tiny fraction of overall greenhouse gas emissions. Indeed, it’s possible that those methane plumes have been there for decades or centuries, and are only newly discovered rather than new. Climate realists point out other reasons to remain calm in the face of the methane at the bottom of the Arctic Ocean.

First, while most of the methane is believed to be buried roughly 200 meters below the sea bed, only the top 25 meters or so of sea-bed are currently thawed, and thawing seems to have only progressed by about one meter in the last 25 years – a pace that suggests that the large bulk of the buried methane will stay in place for centuries to come.
Second, several thousand years ago, when orbital mechanics maximized Arctic warmth, the area around the North Pole is believed to have been roughly 4 degrees Celsius warmer than it is today and covered in less sea ice than today. Yet there’s no evidence of a massive amount of methane release in this time. (Though it must be said that the Arctic is set to pass those temperatures sometime in the next few decades, and keep soaring beyond them, at which point we’ll be in territory uncharted over the past few hundred thousand years.)

Third, the last time methane was released in vast quantities into the atmosphere – during the Paleocene-Eocene Thermal Maximum 56 million years ago – the process didn’t happen overnight. It took thousands of years.

Put those facts together, and we are probably not in danger of a methane time bomb going off any time soon.

However, even a slow, gradual release of just a tiny fraction of the methane buried beneath the Arctic Ocean could significantly add to the pace of climate change. If the Arctic sea floor methane deposits started to outgas at a rate that would empty them into the atmosphere in 10,000 years, that would still be an added annual warming effect roughly on par with the amount of carbon humans emit into the atmosphere each year. If the rate of Arctic sea floor methane release were faster – more like a 1,000 year pace to empty those deposits – then we’d be looking at a warming effect each year from that methane outgassing that would be many times greater than the warming from the fossil fuels we burn.

The Triple Whammy, and the Perils of Prediction

So, in addition to the increased persistence of severe weather from a slowing jet stream, we face a triple whammy of raw warming effect.
1. Warming from the greenhouse gases we emit already.
2. Warming from the loss of ice and permafrost in the Arctic, and the exposure of dark water and dark land below.
3. Warming from the release of more carbon into the atmosphere as the permafrost and the Arctic sea floor methane begin to go.

The first of these is certain.

The second, the darkening of the Arctic and the warming that will come with it, is high confidence, though we still have questions about the exact magnitude. That factor alone means our current climate change projections for the coming century may be out of date and overly conservative. It means that we may, in the next 10 to 20 years, reach a point where no matter what reduction we make in greenhouse gases, the planet will keep warming.

The third whammy, the risk of more carbon entering the atmosphere, is the most speculative. There’s a range of possibilities. At the low end, published work suggests that, at the very minimum, CO2 release from melting permafrost will add 10-15% to human emissions. In the middle of the range is the possibility that permafrost will melt more rapidly than expected, or that at least a few percent of the carbon it gives off will be in the form of methane. In that range, permafrost melt alone could add a greenhouse effect equivalent to all human greenhouse gas emissions (in addition, of course, to the heating effect from a darker Arctic and the heating effect from human-released greenhouse gases). And at the high end there’s the small but non-zero chance of much more rapid methane release from the permafrost or from the oceans, with the release even a small fraction of the methane trapped there leading to a warming effect that exceeds human contributions significantly.

It seems pretty likely then, that if the ice cap continues on its way towards rapid disappearance, we’re on path to a rate of warming faster than current climate models. And at worst, far beyond that.
“It’s tough to make predictions,” Yogi Berra once said, “especially about the future.” Climate is an incredibly complex system, with feedback loops in every direction, with variables that are tough to model, and with random noise in the year-to-year data that can make trends look slower or faster than they really are. It’s quite possible that random fluctuations are making the trends in the ice look worse than they truly are. It’s possible that Arctic ice will settle into a new and stable state at this reduced level. It’s possible that the permafrost melt will be on the slow end of projections, or even well below any projections we have today. Perhaps increased snowfall from higher humidity in the Arctic will more than offset the higher temperatures, or perhaps cloud cover increase will more than cancel out the darkening of the Arctic.

But our recent track record in predicting what happens to the ice has not been good. The reality of changes to the Arctic has far outstripped most predictions. Only a few years ago, in the 2007 Intergovernmental Panel on Climate Change report, the bulk of models showed the Arctic ice cap surviving in summer until well past 2100. Now it’s not clear that the ice will survive in summer past 2020. The level of sea ice we saw this September, in 2012, wasn’t expected by the mean of IPCC models until 2065. The melting Arctic has outpaced the predictions of almost everyone – everyone except the few who were called alarmists.

I bring up this case of climate change happening far faster than we expected here, at the end of this article, to convey a key point. The future is uncertain. Changes in climate can at times move far more slowly than we expect. They can also move far more rapidly. The most important thing for us to understand is that we don’t know, for certain, what changes will come. We only know the range of possibilities. And at one end of that range, things may not be so terrible. At the other end of the range we have deep reason for concern.

“Hope for the best,” goes the English proverb, “but prepare for the worst.”

Let’s hope the Arctic sea ice stabilizes, or reverses course. But let’s not count on it. An ounce of prevention is worth a pound of cure. Every step we take to cut greenhouse gas emissions today is far easier than fighting the triple whammy we could be facing just a few years in the future.

Ramez Naam is a computer scientist and award-winning author. He believes innovation can save the planet and lift billions into prosperity, but only if we make the right choices to embrace it. His next non-fiction book, The Infinite Resource: The Power of Ideas on a Finite Planet, lays out the path to harnessing innovation to maximize our odds of overcoming climate change, finite fossil fuels, and the host of other environmental and natural resource challenges that face us. He blogs at rameznaam.com. This piece was originally published at the Scientific American blog and was reprinted with permission from the author.

References and Further Reading

Information about the state of the Arctic ice can be retrieved from the US National Snow and Ice Data Center (NSIDC) at http://nsidc.org/. In particular, for data on the coverage of Arctic ice, I’ve relied on NSIDC’s Sea Ice Index (http://nsidc.org/data/seaice_index/archives/index.html).

Sea ice volume data comes from the Pan-Arctic Ice Ocean Modeling and Assimilation System (PIOMAS) operated by the Polar Science Center at the University of Washington. You can access that data here: http://psc.apl.washington.edu/wordpress/research/projects/projections-of-an-ice-diminished-arctic-ocean/data-piomas/

The finding that human activity is responsible for roughly 60% of the Arctic ice melt from 1979 – 2011 is from: Stroeve, J. C., V. Kattsov, A. Barrett, M. Serreze, T. Pavlova, M. Holland, and W. N. Meier (2012), Trends in Arctic sea ice extent from CMIP5, CMIP3 and observations, Geophys. Res. Lett., 39, L16502, doi:10.1029/2012GL052676. You can find that paper here.

Long term reconstructions of Arctic ice coverage for the 1,450 years are from: Christophe Kinnard, Christian M. Zdanowicz, David A. Fisher, Elisabeth Isaksson, Anne de Vernal & Lonnie G. Thompson, Reconstructed changes in Arctic sea ice over the past 1,450 years, Nature 479, 509–512 (24 November 2011) doi:10.1038/nature10581. You can find that paper here.

Note that the graphic showing that trend is mine, and combines data from Kinnard’s study with contemporary ice coverage data from NSIDC. Any error there is mine.

Peter Wadhams’ observations about the heating effect of a darkening Arctic have been repeated widely in the press. I know of no primary source in the literature, but one version of the math can be found in a blog post here. I find the math in the blog post generally right but wrong in specifics. An easier approach is to take the fraction of the Earth’s surface covered by the Arctic ice cap in the height of summer (about 2% of the planet) and multiply that by the average insolation the region receives (170 – 180 watts / m^2) and then by the plausible change in albedo (perhaps 0.5). That gives a change in the amount of energy captured by the earth – before taking into account clouds and such – of about 1.75 watts / m^2 (averaged across the whole planet). That compares to 1.6 watts / m^2 of heating effect caused by humans via other changes to the earth system. The math is slightly different than Wadhams’, but the answer is roughly the same – a warming effect (a ‘climate forcing’ in the parlance of the field) roughly as large as all current human-caused warming.

On the topic of permafrost, three important papers on the rate of permafrost thaw and the amount of carbon which could be released are Vulnerability of Permafrost Carbon to Climate Change: Implications for the Global Carbon Cycle by Edward Schuur and colleagues, Amount and timing of permafrost carbon release in response to climate warming by Kevin Schaefer and colleagues, and Accelerated Arctic land warming and permafrost degradation during rapid sea ice loss by David Lawrence and colleagues.

On the topic of methane hydrate deposits on the sea floor, a relevant “don’t panic yet” paper is Siberian shelf methane emissions not tied to modern warming by Colin Schultz. A reminder that some periods of the last 10,000 years have been a bit warmer than our present, seemingly without triggering runaway explosive release of Arctic methane, can be found in Ice free Arctic Ocean, an Early Holocene analogue by Svend Hunder. (With the caveat that the planet seems to be well on its way p

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