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Frozen Planet: the climate science top six

  • 09 Dec 2011, 17:00
  • Ros Donald

The BBC's landmark series Frozen Planet ended yesterday with a stark exploration of the effects of climate change already apparent in the Arctic and Antarctic, where warming is occurring twice as fast as it is in the rest of the world. Here's our rundown of six scientific concepts covered in the episode.

1. First, Attenborough says sea ice thickness has halved since the 1980s and Arctic sea ice extent has reduced. We're now able to judge how far Arctic sea ice thickness has reduced, Attenborough says, because the Russian government has allowed access to data collected by submarine crews during the Cold War.

Studies show that since the 1970s summer Arctic sea ice extent has decreased by nearly a third, the average Arctic sea ice thickness at the end of the melt season has roughly halved since the 1980s, and that the Arctic Ocean could be ice-free in the summer months within 30 years.

The National Snow and Ice Data Center's arctic sea ice news and analysis page estimates that average Arctic sea ice extent for November 2011 was 502,000 square miles below the 1979-2000 average. The article recommends a paper by Cohen and Jones which attempted to produce a more accurate index for winter predictions as further reading, so we shall do the same.

2. All this has got to be good news for somebody. And it is: according to Attenborough, trans-Arctic summertime shipping lanes are likely to open up over the next few decades, cutting journey time between Europe and Asia.

Climate blogger Tamino, who uses figures on Arctic sea ice from the National Snow and Ice Data Center (NSIDC), notes that the downward trend in sea ice extent continues, and notes: "It's bad luck … that the trend continues. It's because of global warming, and that's bad. For all of us."

Another paper by Kinnard et al, which we look at in more depth here, found that the decline in Arctic sea ice over the last few decades is "unprecedented" over the last 1450 years. This kind of data provides evidence for the so-called 'Arctic amplification' effect Attenborough mentioned at the beginning, confirming climate model simulations that project temperatures will rise at double the global average in the Arctic.

3. Next, the melting of the Greenland Ice Sheet. Cue stunning shots of sapphire-blue lakes of melt water in the second-largest ice mass on the planet - six times the size of the UK. New satellite data, which we wrote about here, shows the Greenland ice sheet lost mass at the edges over the past decade.

During the episode, Attenborough met Aberystwyth University glaciologist Alun Hubbard on the ice sheet. According to the university's Greenland ice sheet website, the sheet is losing mass in three ways:

"1) through direct melt of the ice due to increased solar radiation; 2) through increased calving of icebergs into the ocean due to warm ocean currents; and 3) by dynamic thinning of the ice sheet [thinning of the ice sheet due to acceleration of the ice sheet's outlet glaciers]."

We see Hubbard observing part of the dynamic thinning phenomenon - during summer, the meltwater lakes overflow into rivers, which eventually "plunge through the ice sheet via vertical pipes calles moulins."

Glaciologists believe water lubricates the bed of the ice sheet, causing it to slide - transporting it to warmer, lower locations until it's "lost to the sea". According to the Aberystwyth University site: "As the ice sheet extends it becomes thinner, hence the term dynamic thinning."

Although the sheet has lost mass overall, it's hard to make 'big picture' assessments of the ice sheet's health, because ice loss varies from area to area, meaning that regional climate conditions strongly affect the ice sheet. A recent study of three Greenland glaciers, for example, showed vastly different rates of ice loss over the last decade.

4. The programme also documents glacier retreat in South Georgia. As Attenborough writes in the Radio Times:

"I first went to South Georgia, the mountainous, glacier-draped island just north of the Antarctic Circle, in 1988. At that time, the Cook Glacier that flows down to St Andrew's Bay reached the waterline. Last year, a Frozen Planet team discovered that the glacier had retreated by 400 metres."

The blog Glacierchange documents the retreat of two South Georgian glaciers. Interestingly, the retreat is affecting the local wildlife. This paper estimates 97 per cent of 103 South Georgian glaciers have retreated since records began, leading to the depletion in numbers of "ground nesting birds" such as penguins and the spread of rats.

5. The programme also observes thatpenguin populations are reacting to glacier retreat in different ways. Attenborough writes:

"the [St Andrew's] Bay's king penguin colony now has far more beach front and is thriving as a result."Further south, however, another species of penguin - the Adélie - is suffering. The southern part of the Antarctic Peninsula where they once flourished is now the most rapidly warming region in the whole of the southern hemisphere."

Scientists have linked trends in Adélie penguin abundance with the amount of krill available. As the sea warms around the Arctic Peninsula Adélies' habitat, krill - an ice-loving species - biomass is decreasing, having a knock-on effect on the penguins' numbers. Colonies Attenborough visited in 1992 have now disappeared altogether.

6. The loss of thousands of kilometers of the permanent ice shelf around the Antarctic Peninsula provides some amazing footage. According to a briefing by the British Antarctic Survey that looks at The Antarctic Peninsula's retreating ice shelves, since the 1950s, a total of 25,000 km2 of ice shelf has been lost from around the Antarctic Peninsula as the amount of meltwater reaching them becomes too great for them to tolerate. According to the survey:

"The breakout in March 2008 of the Wilkins Ice Shelf on the Antarctic Peninsula is the latest drama in a region that has experienced unprecedented warming over the last 50 years. In the past 30 years seven floating ice shelves retreated, with very little of their area now remaining."

Accelerated melting is likely to contribute to sea level rise. As Attenborough writes:

"This collapse is only the most recent in a wave of similar events that have travelled southwards down the peninsula. Next in line could be some of the ice shelves that act as plugs for the immense body of ice lying on the continent itself. If any of these collapse, vast quantities of land ice and melt-water will slide into the sea and cause a major rise in sea levels around the globe."

According to a 2009 paper by Vermeer and Rahmstorf, global temperature rises and global sea level rises are linked. The authors project "a sea-level rise ranging from 75 to 190 cm for the period 1990-2100", although it's worth noting there is a range of opinion amongst the scientific community on this - a recent paper suggested that both Antarctic and Greenland ice sheets continuing to melt at their current melt rate could cause the average global sea level to rise by around half a metre, while the IPCC says 18-59cm across a range of future scenarios, although they also note that this may well be an underestimate.

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