Into the freezer that sustains us and other giant sponges

In this beautifully illustrated book, David Thomas introduces us to the mysterious and inaccessible world of the pack ice. This floating ecosystem at its greatest extent covers nearly 11 million square miles of the Earth's surface. The Antarctic winter sea ice alone holds the equivalent of 12,300 chest freezers full of ice for each human on the planet.

The book provides a pleasant romp through the physics, chemistry and biology of pack ice without an equation or quantitative table in sight.

Though largely qualitative, the book is scientific in its approach as it illuminates the central role that the pack ice plays in the ecology of the Arctic and the Antarctic and explores the similarities and differences that characterise these two extreme environments.

An eloquent chapter describes the freezing of sea water through frazil, grease, nilas, finger and pancake ice as the winter sets in. The resulting pack ice is riddled with microscopic channels filled with the brine expelled. In the Arctic, this simple process repeated over thousands of square miles produces dense, cold saline water that sinks rapidly to the sea bed and moves south, pulling the Gulf Stream northwards. This not only ameliorates our own climate but also provides the driving force behind the deep circulation of the world's oceans.

The heart of the book is an exploration of the life dependent on the restless, unpredictable environment of the pack ice. The microbial life within the porous structure of the ice, which provides the basis of the food web, is briefly but informatively described. This rich world contains large populations of viruses, bacteria (some active down to - 20C), algae and microscopic grazers that can turn the ice brown. Larger animals graze the base of the ice and feed on the fallout from this food web. Krill, crustaceans just a few centimetres long, over-winter under the ice in vast numbers in the Southern Ocean and form the major feedstock for many mammals and birds. The maximum biomass of krill is three times the mass of all humans on the planet. When the ice melts in spring, it releases a rich cargo of organic matter that gives a kick-start to production in the open water and, through falling particles, provides a feeding bonanza for the animals that live on the seabed. The effects on organisms from seaweeds and ancient giant sponges to penguins and whales are clearly described. The impact of human activity on these high latitudes is briefly addressed, although global warming is not really given the consideration it deserves.

The book could have been significantly enriched by annotations within the text to the sources of the many intriguing facts introduced. Nonetheless, Thomas has made the intricate and remote world of the pack ice accessible to us through this enjoyable book.

It seems ironic that, just as we are beginning to appreciate the importance of ice ecosystems, we seem hell-bent on removing them from the face of the earth through accelerated global warming.

Michael Whitfield is honorary visiting professor in marine science, Plymouth University.