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Five reasons why the speed of Arctic sea ice loss matters

  • 22 Mar 2013, 15:45
  • Roz Pidcock and Freya Roberts

In the not too distant future, scientists predict the Arctic Ocean will be largely sea ice free in summer - but pinpointing when is more difficult. But does it matter if it happens sooner rather than later? Here are five reasons why it might. 

Arctic sea ice is declining rapidly, with the seasonal low in summer shrinking particularly quickly. Scientists have different ways to predict Arctic sea ice decline. If melting continues as it has in recent years, it could be as soon as 2020, but climate models suggest it won't happen until 2040 or later. So far, it's not clear which is more likely - see here for more.

But why does it matter how quickly it happens?

1. Loss of ice means more heat is absorbed

Albedo is a measure of how well the earth's surface reflects sunlight. Snow-covered sea ice has a high albedo and reflects 85 per cent of sunlight. But the open water revealed as ice melts is darker and absorbs more - reflecting just seven per cent. The less sunlight is reflected, the more heat the planet absorbs.

 NASA

The albedo effect in the Arctic. Source: NASA 

Professor Jennifer Francis from Rutgers University explained recently that losing reflective sea ice can, in turn, speed up surface warming - what's known as a positive feedback. She says:

"As sea ice retreats, sunshine that would have been reflected back to space by the bright ice is instead absorbed by the ocean, which heats up, melting even more ice."

The heat doesn't only warm the oceans. Francis explains excess heat entering the oceans during summer is later released back into the atmosphere - raising atmospheric temperatures too. It's one important reason why the Arctic is warming faster than the rest of the planet.

2. Melting Greenland ice sheet raises sea levels

In fact, the loss of reflective sea ice is part of the reason Arctic temperature has risen three times faster than the global average in recent decades. This effect, known as Arctic amplification, has consequences for nearby land ice, too.

As land ice melts, it adds freshwater to the oceans causing sea levels to rise, and surface melt from Greenland is increasing, as the image below shows. Satellite data suggest over the last 20 years, the Greenland ice sheet has lost 140 billion tonnes of ice per year.

This has raised sea levels by about 7.8 millimetres - 12 per cent of total sea level rise in that time. The newest climate models project that by 2100, Greenland's ice sheet could contribute four to nine centimetres to sea level. Earlier estimates had put that figure anywhere between one and 12 centimetres.

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Number of melt days on the Greenland ice sheet on average (1979-2007) and last year (2012). Source: National Snow and Ice Data Centre

Warmer, sea ice free oceans could also increase melting from Greenland's glaciers. Professor Andrew Shepherd from Leeds University tells us:

"[S]mall increases in ocean temperature can quickly destabilise glaciers that flow into the oceans, and so we could expect further ice losses due to that effect too."

The quicker sea ice loss causes Arctic temperatures to rise, the faster the Greenland ice sheet is likely to melt - and while it isn't going to disappear any time soon, this contributes more to sea level rise.

3. Thawing permafrost amplifies warming

Rising Arctic temperatures are thawing once-frozen ground in the Arctic - known as permafrost. Scientists are concerned carbon dioxide and methane released from the carbon-rich permafrost could cause additional warming by adding to greenhouse gases already in the atmosphere - another positive feedback. Frozen methane also exists in methane hydrates in the sea floor.

But there is a lot of uncertainty about how much carbon these frozen deposits hold, and how quickly they are thawing. So far, only a small amount of methane in the atmosphere can be traced to thawing permafrost. Most results from human activity, like agriculture and landfill, with a smaller amount released naturally from wetlands.

At the moment, warming is mainly driven by carbon dioxide emissions. But the sooner sea ice loss causes land and ocean to warm in the Arctic, the sooner methane from thawing permafrost could start contributing more to warming.

4. Ocean circulations could change

Another impact of Arctic and Greenland ice melt could be that the freshwater runoff into the ocean disrupts part of a major circulation system known as the Atlantic Meridional Overturning Circulation ( AMOC). The AMOC carries warm surface water northward, giving Europe its mild climate.

AMOC

Warm surface waters sink in the waters surrounding greenland, forming the Meridional Overturning Current. Source: Skeptical Science

Climate models predict the AMOC could slow down by an average of 25 per cent by 2100. This is mainly due to changes in temperature and precipitation patterns, but scientists say freshwater from melting ice could become increasingly important with climate change.

Models suggest a slowdown in the AMOC could cool the northern hemisphere this century, but the effect is likely to be outweighed by greenhouse gas warming. But the AMOC is just one part of a giant conveyor belt which currently redistributes warmth around the world, so losing ice could affect regional climates worldwide.

Changes in the strength of the AMOC can also affect sea ice melt by transporting more or less warm surface water into the region. So it's a complicated picture and scientists we spoke to said while the rate of Arctic melting will almost certainly affect the AMOC, more research is needed to understand the full extent of the impacts.

5. Melting sea ice can influence winter weather

As temperatures rise faster in the Arctic than at lower latitudes, this changes large-scale temperature and pressure gradients - which has consequences for northern hemisphere winter weather.

Professor Charles Greene from Cornell University tells us:

"Global warming has increased the loss of summer sea ice in the Arctic, which has altered atmospheric conditions in a manner thatstacks the deck in favor ofmore severe winter outbreaks".

The pressure change alters atmospheric circulations, including the jet stream - a stream of fast-flowing air in the atmosphere. It also makes the jet stream's path meander more, which allows cold Arctic air to reach further south, affecting the climate in the mid-latitudes.

Scientists are just beginning to understand the effect quicker Arctic sea ice melt could have on northern hemisphere climate. While it's difficult to attribute any particular event to effects of the changing Arctic, professor Francis tells us:

"If the summer ice covers disappears sooner than climate models project, I would expect to see Arctic amplification intensify sooner, and the effects on the large-scale circulation would become more conspicuous."

Global effects

Melting sea ice is likely to have global consequences by unlocking new shipping routes and exposing more fossil fuel reserves. But it's clear that although Arctic sea ice only covers a small fraction of the earth's surface, there may well be serious climate-related impacts too.

And it doesn't stop with the climate. The loss of sea ice will also have consequences for people and ecosystems in the Arctic region. As Dr Stephen Vavrus from the Centre for Climatic Research at the University of Wisconsin-Madison puts it:

"[T]he longer we can delay the loss of sea ice cover, the longer we should be able to keep these consequences at bay".

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