IN NOVEMBER 2011 an American icebreaker, USCGC Healy, set off from Seward, Alaska, to sail north through the Arctic Circle into the Chukchi Sea. It was the beginning of the winter-long polar night. Sea ice was forming. The sun did not appear in the northern Chukchi for weeks. Those on board expected creatures to be sparse in number and entering hibernation. Instead, they found a ferment of activity.

Robert Campbell of the University of Rhode Island, one of Healey’s supercargo of scientists, outlined the details at Arctic Frontiers, a scientific conference held in Tromso, Norway, last month. His research, and that of his colleagues, showed that planktonic animals such as copepods (pictured above) and krill were abundant, active and grazing on the still smaller algae of the phytoplankton, themselves adapted to manage with the tiniest sliver of winter light. Instead of hibernating, they were developing. Larvae were turning into adults and a few species were even reproducing. This revelation of life in the middle of the polar night is one of many surprises of recent Arctic science. And that knowledge is changing people’s understanding of the world’s northernmost habitat.

The Arctic, sparsely populated and rarely visited by outsiders, is easily forgotten by those at lower latitudes. But it is where the impact of a warming climate is most vividly seen. A 1°C rise in temperatures at the equator—from, say, 25°C to 26°C—will have effects, but does not change much. A rise from 0°C to 1°C melts the ice. That changes everything, from local biology to global meteorology.

And the Arctic is melting fast. Julienne Stroeve, of the National Snow and Ice Data Centre in Boulder, Colorado, calculates that between 1953 and 2014 the average area of the Arctic sea ice shrank by 48,000km2 a year (a three-hundredth of its winter extent). Between 1979 and 2014 it shrank by 87,000km2 a year. Between 1996 and 2014, the rate rose to 148,000km2, even though the past two seasons have seen an increase in summer ice compared with the low point in 2012.

This accelerating rate involves two feedback loops—one well-known, the other less so. The well-known one is that ice reflects sunlight but water, which is darker, absorbs it. So as water replaces ice, more heat is retained. In 1980 the Arctic Ocean absorbed 38% of incoming solar energy. By 2010 it absorbed 63%. This warms the water, melting more ice from beneath.

That phenomenon triggers the second feedback loop. In the 1980s Arctic sea ice was about 3 metres thick. Some would melt in summer and freeze again in winter, but most would remain throughout the year. This is multi-year ice. Now, millions of square kilometres melt away in summer and refreeze in winter. This is first-year ice. The two have different properties. Multi-year ice is eroded by the wind and pounded from below by ocean currents, so its surface is pitted and ridged. First-year ice, in contrast, is unscarred by time.

When multi-year ice thaws, the meltwater gathers on its surface, forming ponds between the ridges. That means a fair amount of ice remains uncovered, and continues to reflect sunlight. Meltwater from first-year ice, though, spreads right across the smooth surface, reducing reflectivity far more. In 1985, 70% of Arctic ice was multi-year and 30% first-year. By 2012, those proportions were reversed. And the biggest loss is of ice five years old or more. This sort is the crinkliest, and reflects most light of all. It has shrunk from 30% of the total in 1980 to 5% now.

Let the sun shine in

For living things, this should be excellent news. More open water means more light, which means more photosynthesis. Primary production, as carbon compounds produced by this photosynthesis are known, is rising. Antje Boetius of Bremen University says the seabed at the North Pole is now green—or would be if you could see it—because so much photosynthesis has taken place at the surface and the algae have died and sunk to the bottom. According to Mar Fernández-Méndez of the Alfred Wegener Institute in Bremerhaven, in the summer of 1982 a column of Arctic seawater with a cross-sectional area of a square metre fixed 26 milligrams of carbon a day. By 2012 that figure had risen to 34-53mg.

With rising primary production, and thus more food available, you might expect the rest of the ecosystem to be flourishing, too. And some of it is. There has, for example, been a substantial increase in the number of cod—especially of sexually active cod, more than seven years old. These fish are also now found much closer to the pole than they used to be, at 79° or 80° north. Harbour seals are doing well, too. A colony on Svalbard is the species’s northernmost outpost. Unlike some other seals, harbour seals do not depend on ice to give birth or to moult, so the ice’s retreat is less bothersome to them. And as a predominantly temperate species, they benefit when waters warm up.

Other species, though, are being squeezed by competition from these newcomers and by changing conditions in general. In the waters where cod thrive, minke whales and harp seals are struggling, at least to judge by the condition of their blubber. A study by Bjarte Bogstad of Norway’s Institute of Marine Research found that the blubber of both species had declined from being more than 40mm thick in the early 1990s to less than 35mm now.

The whales may be suffering because the Arctic zooplankton on which they feed are being replaced by more temperate species that are harder to catch or less nutritious. Harp seals suffer directly because they depend on ice (they give birth on it, moult on it and use it for rest while hunting). If the ice is too thin, their pups fall into the water and drown. They also suffer indirectly, through the loss of their main prey, a fish called the capelin, which has begun to swim northward in search of cooler waters. Garry Stenson of Fisheries and Oceans Canada, a government agency, reckons that the number of harp-seal miscarriages has risen greatly in response.

Harp seals exemplify the difficulty of adapting to changing conditions. The obvious response for an ice-dependent animal such as a harp seal would be to follow the retreating ice—in its case, northward from its breeding grounds near Newfoundland. But that would bring the seals closer to their main predator, the polar bear (which eats seal pups). Dr Stenson says harp seals will swim north, but only if there is no ice in March, as happened in 2010. In 2011 there was a little March ice and they stayed put—only to be almost wiped out when the ice melted later in the summer.

This sort of thing, of course, is all part of the cut and thrust of ecology. More worrying to those who study the Arctic’s biological awakening is that the rise in primary production which sustains it might suddenly be capped.

Light is not the only requirement for life. The phytoplankton that sit at the bottom of the food chain need nutrients such as nitrates (to make proteins) and silicates (to make protective shells). Dr Fernández-Méndez thinks a shortage of these may limit the Arctic Ocean’s productivity even if most of the ice melts and exposes the water beneath to light.

Jon Lawrence of Britain’s National Oceanography Centre has looked at what might happen to nutrient availability and productivity in an ice-free Arctic. He concluded that, as the ice melted, primary production would rise by only 11%, not 300%, as a linear extrapolation would suggest.

The reason is the green seabed. The dead algae have taken the nitrate with them. Something similar happens in other oceans, of course, but in the Arctic the lost surface nitrates do not seem to be being replenished—perhaps because of a lack of the upwelling currents found elsewhere. Mr Lawrence calculates that almost all the extra productivity the Arctic is likely to see will thus take place at least 20 metres below the surface, where it is pretty dark regardless of ice. Productivity at the surface itself, by contrast, is flat.

Another reason the Arctic’s creatures may not flourish in the extra sunlight is that they have evolved to take advantage of a short summer. This season is so compressed that the timing of plant growth, animal births and feeding cycles has to be closely synchronised. A few days makes a difference. Copepods, for example, emerge just before the peak bloom of algae. If the ice melts early, the algae will bloom early too and then die. The newly emerging copepods will thus not have enough to eat. And since copepods are food for predators, the ecosystem as a whole will suffer.

To capture the impact of changing environmental conditions on food webs as a whole (rather than on individual species), two recent studies built computer models to describe them. These looked at how many species are in a web, how many links there are between these species (ie, who eats whom), where the species live, and whether that is changing. Both models suggested that the condition of some Arctic food webs is coming under strain.

Building ecological models is, however, a tricky business. Even in well-studied habitats all the variables are rarely understood. And evolution can happen quickly if the pressure is great enough. For example, copepods whose internal clocks better matched changed conditions would spread rapidly.

Winds of change

All of which is fascinating. From the human, rather than the copepod point of view, though, the biggest question about the Arctic is this: is it affecting other parts of the Earth’s climate—those regions where people live? Jennifer Francis of Rutgers University argues that it is.

Because the poles are cold, warm air flows from the equator north and south in order to equalise temperatures. The Earth’s rotation disturbs these air currents, creating (in the northern hemisphere) the jet stream, which flows around the planet in a wavy pattern, bringing up warm air and making north-west Europe hotter than other places at the same latitude.