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The Two-Mile Time Machine by Richard Alley, Princeton, £15.95, ISBN
0691004935

TEN thousand years ago, the great thaw that ended the last ice age suddenly
flooded the world. It took just a few years for mountains to become islands and
plains to turn to seas. Since then, luckily for us, the global climate has
altered only sluggishly. Any changes have occurred over decades if not
centuries, giving us time to adapt or migrate.

This state of affairs is exceptional, it turns out. Cores drilled through the
ice sheet shrouding Greenland revealed that in the more distant past our
planet’s climate could change dramatically within a year or two—and these
climate flips occurred again and again. Go back 100,000 years and you see a
world in which the climate was inherently unstable, particularly round the North
Atlantic.

This unexpected finding galvanised researchers. In search of evidence to back
up the results from Greenland, they have scrutinised oceans, coral reefs, peat
bogs and lake sediments. Combining global data inspired a new generation of
models to show how the changes in ocean temperature and atmospheric gases worked
together to alter the world’s climate. Just in time, it seems. We need models
like these to get a grip on what’s likely to happen to climate as a result of
the greenhouse gases we are now pouring into the atmosphere.

In The Two-Mile Time Machine Richard Alley, professor of geosciences
at Pennsylvania State University, shakes any lingering complacency we might feel
as we move into an uncharted world. Carbon dioxide levels have now reached their
highest levels for more than 400,000 years. Ice cores tell us that during most
of that time temperatures have tracked carbon dioxide and methane levels
remarkably closely.

Alley speeds us through the principles of ice-core research and the early
developments that led to the headline discoveries in central Greenland. To
illustrate the story, he uses the work of the American scientists who analysed
one of two cores drilled to bedrock in central Greenland between 1992 and 1993.
With the work of the European team that analysed the other core, this provides
the most detailed and accurately dated record of climate through the past
100,000 years ever achieved.

Analyses of more than 30 constituents in the ice, including atmospheric
dusts, dissolved acids and salts, natural isotopes and trapped bubbles of air,
have given us a new picture of the larger-scale changes in the Earth’s
environment. They make it possible for the first time to examine the nature of
the rapid climate jumps that pervaded much of the last glacial period.

Alley devotes the central part of his book to discussing the possible
mechanisms driving global climate change on both short and very long timescales.
What emerges is a vivid picture of the complex dynamics linking ocean,
atmospheric circulation and the ice sheets. This, it is now believed, involves a
sensitive switching mechanism centred on ocean currents in the North Atlantic.
Whether this switch is vulnerable in our present climate is still an open
question, but at least we are now aware that it exists.

With a highly readable style designed to capture and stimulate the
imagination of his students, Alley explains some of the complexities of Earth
system science with a minimum of jargon. This book is not just for students: it
will be readily accessible to a wide audience that should be aware of its
contents. He has taken pains to keep an open mind when debating the key issues,
and to focus on the leading ideas and questions rather than the loudest
voices.

As we settle down to our seasonal festivities, perhaps we should reflect with
Alley that we are privileged to benefit from a few centuries of easy living
fuelled by a few hundred million years’ worth of stored solar energy.