Nature 448:136, 2007
July 12, 2007

http://www.nature.com/nature/journal/v448/n7150/full/448136a.html

BOOK REVIEW – Six Degrees: Our Future on a Hotter Planet
by Mark Lynas
Fourth Estate: 2007. 384 pp. £12.99

Review by Stefan Rahmstorf

This book is not for the faint-hearted. British
writer Mark Lynas ventures where few scientists
would dare to tread. He sets out to answer the
question that many of us climatologists ponder in
private and often get asked by journalists, but
usually shy away from answering: what will it
really be like to live on a warmer planet? In
Lynas’s words: “Will we all, as some
environmentalists suggest, be reduced to eking
out a living from shattered remains of
civilization in Arctic refuges, or will life go
on much as before – if only a little warmer?”

Degrees of change

Lynas sets out to answer this systematically on
the basis of his extensive reading of the
scientific literature. He has spent many months
in Oxford University’s Radcliffe Science Library
trawling through thousands of papers. The result
is arranged in six chapters, one for every degree
Celsius of potential global warming. His
statements are referenced throughout, and, as a
palaeoclimatologist, I was familiar with fewer
than half of the 500 or so papers he cites. That
is the nature of scientific specialization: few
researchers could afford the time for such a
wide-ranging literature review.

One of the best aspects of Six Degrees is that it
pulls together data from past climate changes in
Earth’s history to get an idea of what a warmer
climate might look like. The often dramatic
natural climate changes of the past are sometimes
cited by those opposed to reducing greenhouse-gas
emissions as evidence of why we need not worry
about the ongoing warming. But the latest
Intergovernmental Panel on Climate Change (IPCC)
report (http://www.ipcc.ch), which contains a
65-page chapter reviewing the main palaeoclimatic
findings, shows that the first lesson from the
past is that Earth’s climate system is very
sensitive. It has always responded strongly to
natural changes in Earth’s energy budget – the
balance between the absorbed incoming energy from
the Sun and the outgoing long-wave (thermal)
radiation from Earth. So we have every reason to
believe that it will respond strongly again to
the growing perturbation to the radiation balance
that humans are now causing. Six Degrees hammers
home the second lesson to be drawn from the past:
mega-droughts or sea-level changes of tens of
metres accompanied past cooling and warming
episodes of similar magnitude (albeit probably
not as rapid) as that expected this century.

I have my quibbles with some of Lynas’s
interpretations and there is the odd error, but
such complaints seem petty in view of the overall
achievement and importance of this book. Lynas
avoids the obvious pitfall of cherry-picking the
most dramatic possibilities; Six Degrees is
alarming but not alarmist. He tries to give a
level-headed account of what we may expect,
mentioning scientific controversies where they
exist. For example, he gives balanced discussions
about the future of the Sahel region of northern
Africa, citing studies that conclude that global
warming may end the ongoing drought there, and
about the risk of a shut-down of the North
Atlantic Current, to which the new IPCC report
assigns a probability of up to 10% this century.

Six Degrees is essentially about risks, because
much remains uncertain about the future. My major
beef is that he often makes risks sound like
truths. After a sensible discussion of amplifying
feedbacks from the carbon cycle and methane
release, in which he states how uncertain and
hard to quantify these still are, he then claims
that three degrees Celsius of warming “inexorably
leads to four degrees, which leads inexorably to
five”. Even a small risk of this happening is bad
enough, without making it sound so definite.

Lynas, who is not a natural scientist, must be
highly commended for basing his book thoroughly
on science – more so, in fact, than some popular
books on climate change written by scientists.
Gloomy as his story sounds, in some cases he may
even be too optimistic. The possibility of
violent conflict in regions struck by drought and
food shortages is only mentioned in his
three-degree-warming chapter. A recent report by
the German Advisory Council on Global Change
(http://www.wbgu.de/wbgu_jg2007_engl.html) sees
this risk arising much sooner.

The book ends with a lucid description of the
state of denial about climate change that
humanity is still in (but hopefully now emerging
from), and a good account of the policy options
available to us to stop global warming. Lynas is
interested in leaving the reader ready for
action, rather than depressed.

Lynas is a gripping story-teller, making the book
infinitely less tedious than the papers it is
based on. A must-read for those who can stomach
it.

– Stefan Rahmstorf is a climatologist at the
Potsdam Institute for Climate Impact Research,
Telegrafenberg, 14412 Potsdam, Germa

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