ANTARCTICA is the coldest, windiest place on Earth. Every year, 20 million square kilometres of sea â an area twice the size of the US â freezes during the winter. But come October, the few scientists who have spent the winter months isolated on this frigid land are joined by an influx of summer visitors making their way towards the beaches.
The incomers waddle over the ice in short lines, or toboggan on their stomachs. Leaders stop to look around, and then head off in seemingly random directions. But these AdĂ©lie penguins know exactly where they are going â they come back to the same place every year. Soon cities of penguins will form: pockets of noise, chaos and smell in this empty continent.
With their white fronts and black heads and backs, the penguins may look like they are wearing tuxedos, but this is no party. Life in the Antarctic is harsh, and the penguins are experts in survival, moving seasonally with the ebb and flow of their icy environment. Having spent the winter on the margins between ice and sea, they need ice-free ground for their nests, and ready access to open waters to feed their chicks.
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The AdĂ©lie penguinâs survival is intimately linked with ice, but it is a love-hate relationship. During the winter, ice is a platform to live on and the best place to find food but it can also act as a barrier, blocking access to traditional nesting grounds and feeding areas. Now, parts of the Antarctic are experiencing their own version of a heatwave, and rogue icebergs are creating havoc. Can the AdĂ©lie penguin weather the changes?
At the height of the most recent ice age, around 19,000 years ago, the ice had a stronger grip on Antarctica than it does today. A thick sheet covered the continent, extending far beyond the land and dropping off into the sea in high cliffs. Most of the Antarctic mainland was blocked off to AdĂ©lie penguins, forcing them to retreat to offshore islands. But slowly the world entered the warm interglacial phase that weâre now in, and huge icebergs calved off the ice shelves, exposing rocky beaches. AdĂ©lie penguins swept southwards from their refuges to recolonise the continent. Today there are almost 5 million of them in over 150 colonies.
After the thawing ice shelves shrank back to reveal suitable beaches for AdĂ©lie penguins to nest on, their fate was controlled by another form of ice â sea ice. Outside the nesting season the birds make their home among the broken chunks of pack ice at the edge of the frozen sea. Although it appears barren, the sea ice is a vital food source for penguins. The underside is covered in a lawn of algae and the upside-down larder is full of krill â shrimp-like crustaceans that are a key food for penguins.
Antarctic sea ice always behaves erratically, and it is impossible to predict how much will form each year. But one region has experienced drastic changes that seem to be part of a longer-term trend. The Antarctic Peninsula â the horn of land extending northwards from the continent towards South America â has warmed rapidly in the past 50 years. âAir temperatures in the west peninsula have increased dramatically, about 2.5 to 3 °C since the 1950s,â says John King, from the British Antarctic Survey based in Cambridge. âOver this timescale, it is probably the most rapid warming seen anywhere in the world.â
An increase in temperature of just a few degrees can have a huge effect on icy regions. âThe entire food chain has been affected,â says Wayne Trivelpiece, an ecologist with the US Antarctic Marine Living Resources Program in La Jolla, California. The peninsula itself has become visibly greener. Grasses and mosses are growing in places they have never been seen before. And as the air warms up the sea-ice cover is diminishing. âThe sea used to be heavily covered in ice most winters, but now the sea ice is following a cyclical pattern, with strong ice winters for two years in a row, then three to five years of warm winters with little ice,â says Trivelpiece.
Itâs bad news for the AdĂ©lie. For them, breeding involves long fasts, so they must build up their fat reserves during the winter if they are to successfully raise their chicks. And the sea ice is their main source of food. âNo ice means a major loss of penguins,â says Trivelpiece. Take the colonies on King George Island off the tip of the peninsula, which Trivelpiece has visited for the past 25 years. Less than half of the young penguins survive and return to breed at the colonies compared with 10 years ago. âSomething major has happened. The krill have crashed, and thatâs tied into the ice story,â he says. Itâs a similar picture at many colonies along the peninsula.
The signs are ominous. Biologists regard AdĂ©lie penguins as a vital indicator of the health of this ecosystem, a âcanary in the coal mineâ. âAdĂ©lie penguins show us what will happen to other krill-dependent species like whales and seals, which are not so easy to monitor,â says Trivelpiece.
Adélie penguins only arrived at the northern part of the Antarctic Peninsula 600 years ago. Their days there may be numbered if the decline in sea ice continues. Over on the other side of the continent, however, it has been a totally different story, with changes in sea ice leading to a huge rise in penguin numbers.
Ross Island was used by Scott and Shackleton as a base for their expeditions, and the huts they built are still standing today. It is also the site of Antarcticaâs largest and most active volcano, Mount Erebus, and its biggest âtownâ, the American research station McMurdo. At the height of summer, over 1000 personnel live here. But thatâs nothing compared with the numbers of penguins. A soaring population means that Ross Island is now a nesting site for around 300,000 AdĂ©lie.
This boom may seem contradictory, but it illustrates how finely balanced is the relationship between penguins and their icy environment. Ross Island is far colder than the Antarctic Peninsula, with average yearly temperatures hovering around â20 °C. During the 1960s, the sea ice was extensive and persistent, and the AdĂ©lie population on Ross Island was low but stable. When temperatures rose by a few degrees in the 1970s and the sea ice retreated, penguin numbers rocketed, to perhaps three times those in the 1960s.
In the past two decades, numbers have fluctuated widely, with more chicks raised after poor sea-ice years than after good ones. Thatâs because when there is extensive sea ice in the Ross Sea, it lies on top of the best feeding grounds, which are found at the edge of a current that flows right round the Antarctic mainland. âThereâs a balance of advantages and disadvantages with regard to sea ice,â says John Croxall from the British Antarctic Survey. âIn the Ross Sea, we are moving towards more optimal conditions, but in the peninsula we are moving away from them.â
But it isnât all good news for the Ross Island birds, which face a host of other challenges. Fierce storms can literally blow them off their nests in unsheltered spots. And leopard seals patrol the pack ice waiting to pick off unsuspecting penguins. These threats are tame, though, compared with the decimation that can be caused by icebergs. If these get stuck in the wrong place they pose a risk to entire penguin colonies â and that is exactly what is happening at Ross Island.
In March 2000 a section of the ice shelf the size of Jamaica broke away from the Ross Sea coast. As winds blew it up the coast, it rammed into the ice shelf further along, knocking off another huge block. Later in the year the two icebergs ran aground north of Ross Island, altering the winds and currents in the surrounding area. When spring came, the sea ice didnât blow away as it usually does, which was a disaster for the penguins.
âThe birds had to get through 800 kilometres of thick pack ice to reach their breeding sites, and this requires a lot of walking,â says David Ainley of ecological consultants HT Harvey & Associates in San Jose, California. With their short legs, they can cover less than two kilometres an hour when they waddle over the ice, compared with over seven kilometres an hour when they swim. âAll the birds arrived at their breeding sites later than usual, and some arrived too late to lay eggs,â says Ainley. Many of those that did manage to lay eggs faced a 140-kilometre round trip from the colony to the open water to get food for their chicks.
By Christmas youâd expect the Cape Crozier colony â the biggest colony on Ross Island and the fifth largest in the world â to be humming with the sounds and smells of over 100,000 chicks, but this time last year there were barely 10,000. Over at the smaller Cape Royds colony, there were just a handful of chicks when you would expect over 4000. âThings are looking bleak for the penguins this year too,â says Ainley. âSome very large icebergs are still grounded in the region. Thereâs heavy pack ice throughout the Ross Sea, and solid sea ice covering water around Ross Island.â
Should we brace ourselves for headlines such as âkiller bergs wiped out our penguinsâ? The experts say itâs too soon to write off the Ross Island AdĂ©lie just yet. âPenguins can afford a couple of years of no productivity. Itâs not the end of the world,â says Croxall. AdĂ©lie are long-lived once theyâve survived their teenage years. âThey have six to eight more chances to produce young and replace themselves,â he says. And even if the ice holds out, new generations of penguins may overcome the problem. The older ones will always try to come back to the same colony but their young will probably look for new nesting grounds elsewhere. âThey donât want to do all that walking, and will find a place thatâs clear of ice,â says Ainley. âIf the ice persists, there will be a slow decrease in the colony size as the older birds die off and are not replaced.â
But Steve Emslie of the University of North Carolina in Wilmington is less sanguine about the AdĂ©lieâs prospects at Ross Island. âIf there were a few more seasons of heavy ice, the colonies there could be wiped out,â he says. He has, quite literally, been digging into penguin history, and his findings show that the current rises and falls in AdĂ©lie populations have been repeated over thousands of years. Whatâs more, the present situation at Ross Island may be the key to a mysterious crash in numbers that Emslie has been trying to explain.
Direct observations of Antarctic penguins go back only about 50 years, but the remains of their colonies are preserved in the cold, dry environment. The layers of excrement, food scraps, feathers, eggshells and bones retain a pungent aroma even after 500 years. âSome people say they can still smell 6000-year-old soils,â says Emslie. And that isnât the only distraction in this line of work. âThe birds are very curious. They come over and watch the excavation, then jump into the pit to see whatâs going on,â he says.
Using radiocarbon dating, Emslie has found that AdĂ©lie penguins first colonised the coast of the Ross Sea about 6000 years ago, when the continental ice retreated after a cool period. A boom, known as the âpenguin optimumâ, happened between 3000 and 4000 years ago, with 10 new colonies founded in the south Ross Sea region. The good times lasted until about 2000 years ago, when the colonies were completely abandoned. âThereâs a thousand-year gap in occupation,â he says.
Penguins eventually trickled back into the region, but no one knows what caused them to disappear in the first place. Ice-core records hint that there may have been a slight cooling at this time, and Emslie suspects that semi-permanent sea ice could have prevented access to the colonies â much as the rogue glaciers are doing at the moment. âThe situation at Ross Island is a huge natural experiment,â he says. âThis could explain the disappearance of the penguins in this region 2000 years ago.â
Even if Emslie is correct, thereâs a crucial difference between what happened 2000 years ago and whatâs happening now. Then the continent was cooling, now it is expected to get warmer. Climate modellers think that global warming will begin to take effect on the Antarctic continent within the next 50 years, affecting the sea-ice zone first. Wandering icebergs aside, this could work in the AdĂ©liesâ favour. âIf sea ice continued to dissipate then we could actually witness a repeat of the penguin optimum of 3000 to 4000 years ago,â says Ainley. Croxall is also optimistic. âIn a warmer climate there is potential for new breeding habitats to open up,â he says.
Of course, this is no good if the icy feeding grounds donât survive. And as yet the situation is far too complex to call. We donât have enough information, even on a regional basis, to predict how things will pan out. So when it comes to this penguinâs survival, the jury will be out for another decade. The only thing thatâs black and white is the AdĂ©lie.
- The AdĂ©lie penguin, bellwether of climate change by David Ainley, Columbia University Press (2002) âEnvironmental change and Antarctic seabird populationsâ by John Croxall and others, Science, vol 297, p 30 (2002)