ON SEPTEMBER 16th 2012, at the height of the summer melt, the Arctic Ocean’s ice sheet had shrunk to an area of 3.41m square kilometres (1.32m square miles), half what it was in 1979. And its volume had shrunk faster still, to a quarter of what it was in 1979, for the sheet is getting thinner as well as smaller. One culprit is global warming, which is fiercer at the poles than elsewhere. The world’s average temperature in 2012 was nearly 0.5°C above the average for 1951-80. In the Arctic, it was up almost 2°C.

This sudden warming is like the peeling back of a lid to reveal a new ocean underneath. That prospect is spreading alarm (among greens) and excitement (at the natural resources and other economic opportunities that could be unveiled). Though most of the excitement has been about oil and gas, and the opening of sea routes between the Atlantic and the Pacific, some people hope for a fishing bonanza, too, as warmth and light bring ecological renewal to what is now an icy desert. But they may be disappointed.

At the moment, the waters around the Arctic account for a fifth of the world’s catch. There are few fish, however, under the ice itself. A fishing bonanza would require big ecological change. Arctic Frontiers, a conference organised at the University of Tromso in January, looked at how warming will change the ecology, to estimate whether it will bring one about. The consensus was that it won’t—not because the Arctic will change too little, but because it will change too much.

Change and decay

At first sight, this is counterintuitive. As the ice melts, more light can reach the water, and that means more photosynthesis by marine algae. In the past, algae began to grow under the ice sheet in May and continued to do so until late September. Now, such growth starts in mid-March and continues until October. These ice algae, attached to the sheet itself, account for half the mass of living things in Arctic waters. Much of the rest is unattached algae, known as phytoplankton, and tiny animals, known as zooplankton. Both sorts of plankton support, directly or indirectly, the fish and mammals that live in the Arctic Ocean. And the plankton, too, are flourishing thanks to global warming. The Arctic phytoplankton bloom, which used to run from June to September, now runs from April to September.

The upshot is more plankton, farther north. That attracts more fish. In 2000 Atlantic cod were caught throughout the Barents Sea. By 2012 their distribution was skewed towards the northern part of that sea. Stocks of capelin (a small fish eaten by cod) used to be concentrated south of Svalbard, at latitude 75°N. In 2012 this had moved to 78°N. Some found their way as far up as 80°N.

Which all sounds most promising. But many researchers think it will not continue. First, the central Arctic is too deep for some important species, such as the polar cod (which belongs to a different genus from the Atlantic cod, and can live farther north). Young polar cod (those less than a year old) are pelagic, meaning they live at or near the surface. Those one or more years old are benthic, meaning they live near the bottom. In the Beaufort that bottom is 200 metres down. In the central Arctic it descends to about 4,000 metres, which is too deep for polar cod to survive.

A second reason why there may be no bonanza is acidification of the ocean. When water absorbs carbon dioxide, it produces carbonic acid. More CO₂ means oceans everywhere are becoming more acidic, but the phenomenon is particularly marked at high latitudes because cold water absorbs CO₂ more readily than warm water does. The retreat of the ice also exposes ever more sea to do the absorbing. Cruises by the United States Geological Survey and the University of South Florida over the past three years have found rising carbonic-acid levels north of Alaska. They have also discovered that the shells of many organisms in the area are short of aragonite, a form of calcium carbonate that gives them strength, but whose formation acid discourages. Weaker shells means fewer shelled organisms and less food for fish.

The most important reason, though, for thinking that global warming will not produce an Arctic feeding frenzy is that it may increase ocean stratification. This is the tendency of seawater to separate into layers, because fresh water is lighter than salt and cold water heavier than warm. The more stratified water is, the less nutrients in it move around.

Most free-swimming sea creatures are pelagic. Algae need light, so must live near the surface—as must the zooplankton and other animals that need the phytoplankton. When they die, all these organisms sink to the bottom, where they become food for benthic creatures. Once they have been consumed their component molecules, including nutrients such as nitrates, phosphates and iron, are stuck in Davy Jones’s locker. For the surface to be productive, the locker must be opened and the nutrients lifted back up, so that they can feed the growth of phytoplankton.

Walking the plankton

One of the most important ways this happens is by upwellings of water from the bottom—great churning columns caused by the collision of cold and temperate waters. Two of the most important are in the Arctic: south of Greenland on the Atlantic side and south of the Bering Strait on the Pacific side. Nitrates are abundant at the surface in both places, which is why they are among the world’s richest fishing grounds. There are few upwellings in the tropics, which are thus nutrient-poor.

Stratification threatens this recycling system by suppressing the vertical movement of water. And global warming encourages stratification because it turns the ice into a layer of fresh water that sits on the surface. Imagine the ocean as a Tequila sunrise sitting on a warm bar. The ice cubes at the top are melting away and the orange juice is sinking to the bottom.

At the conference, a paper by Jean-Éric Tremblay and Marcel Babin of Laval University, in Quebec, described the effect by reporting the density difference of water at the surface and at a depth of 100 metres in different oceans. This density difference is an index of ocean stratification.

Parts of the Arctic seem to be getting badly stratified (see chart). In winter, there is almost no density difference in the North Atlantic and the Barents Sea—as you would expect given the upwelling there. But in summer, the northern part of the Barents Sea is even more stratified than the tropical Atlantic and Pacific. And the Beaufort Sea’s stratification is high in both summer and winter. Dr Tremblay concludes that the replenishment of nutrients is already limited by stratification, especially at high latitudes, and that global warming will make things worse.

For Arctic productivity, the consequences are likely to be dire. Paul Wassmann of the University of Tromso looked at the production of organic matter by algae (“primary production”) in different parts of the European Arctic, and used a climate model to predict the future. The area is divided into five economic zones. By 2050, according to the model, primary production is likely to have fallen in three of them, to be flat in one and to rise only in the Russian zone (the Kara Sea and part of the Barents Sea). Primary production is measured as the weight of carbon fixed by photosynthesis per square metre of the Earth’s surface. At the moment, in the most productive area of the Arctic, the Norwegian Sea, that figure is 142 grams a square metre a year. The model predicts this will fall to 128 grams. And by 2100, according to the model, things will be worse. By then, four of the five zones will have experienced a loss in primary production. Only Russia will benefit.

A warming Arctic will not, in other words, be full of fish. It will simply be an ice-free version of the desert it already is.