sponds to over a third of the average person’s carbon emissions.

I like to base my calculations on everyday knowledge rather than on
trawling through impersonal national statistics. For example, if I want
to estimate the typical wind speeds in Cambridge, I ask “is my cycling
speed usually faster than the wind?” The answer is yes. So I can deduce
that the wind speed in Cambridge is only rarely faster than my typical
cycling speed of 20km/h. I back up these everyday estimates with other
peoples’ calculations and with ofﬁcial statistics. (Please look for these in
each chapter’s end-notes.) This book isn’t intended to be a deﬁnitive store
of super-accurate numbers. Rather, it’s intended to illustrate how to use
approximate numbers as a part of constructive consensual conversations.

In the calculations, I’ll mainly use the United Kingdom and occasion-
ally Europe, America, or the whole world, but you should ﬁnd it easy to
redo the calculations for whatever country or region you are interested in.

Let me close this chapter with a few more warnings to the reader.
Not only will we make a habit of approximating the numbers we cal-
culate; we’ll also neglect all sorts of details that investors, managers, and
economists have to attend to, poor folks. If you’re trying to launch a re-
newable technology, just a 5% increase in costs may make all the difference
between success and failure, so in business every detail must be tracked.
But 5% is too small for this book’s radar. This is a book about factors of
2 and factors of 10. It’s about physical limits to sustainable energy, not
current economic feasibility. While economics is always changing, the fun-
damental limits won’t ever go away. We need to understand these limits.

Debates about energy policy are often confusing and emotional because
people mix together factual assertions and ethical assertions.

Examples of factual assertions are “global fossil-fuel burning emits 34
billion tons of carbon dioxide equivalent per year;” and “if CO2 concen-
trations are doubled then average temperatures will increase by 1.5–5.8°C
in the next 100 years;” and “a temperature rise of 2°C would cause the
Greenland ice cap to melt within 500 years;” and “the complete melting of
the Greenland ice cap would cause a 7-metre sea-level rise.”

A factual assertion is either true or false; ﬁguring out which may be dif-
ﬁcult; it is a scientiﬁc question. For example, the assertions I just gave are
either true or false. But we don’t know whether they are all true. Some of
them are currently judged “very likely.” The difﬁculty of deciding which
factual assertions are true leads to debates in the scientiﬁc community. But
given sufﬁcient scientiﬁc experiment and discussion, the truth or falsity of
most factual assertions can eventually be resolved, at least “beyond rea-
sonable doubt.”

Examples of ethical assertions are “it’s wrong to exploit global re-
sources in a way that imposes signiﬁcant costs on future generations;” and
“polluting should not be free;” and “we should take steps to ensure that
it’s unlikely that CO2 concentrations will double;” and “politicians should
agree a cap on CO2 emissions;” and “countries with the biggest CO2 emis-

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