person by 2050. This is such a deep cut, I suggest the best way to think

about it is *no more fossil fuels.*

One last thing about the climate-change motivation: while a range of

human activities cause greenhouse-gas emissions, the biggest cause by far

is **energy use**. Some people justify not doing anything about their energy

use by excuses such as “methane from burping cows causes more warming

than jet travel.” Yes, agricultural by-products contributed one eighth of

greenhouse-gas emissions in the year 2000. But energy-use contributed

three quarters (ﬁgure 1.9). The climate change problem is principally an

energy problem.

OK, enough about climate change. I’m going to assume we are motivated

to get off fossil fuels. Whatever your motivation, the aim of this book

is to help you ﬁgure out the numbers and do the arithmetic so that you

can evaluate policies; and to lay a factual foundation so that you can see

*which proposals add up*. I’m not claiming that the arithmetic and numbers

in this book are new; the books I’ve mentioned by Goodstein, Lomborg,

and Lovelock, for example, are full of interesting numbers and back-of-

envelope calculations, and there are many other helpful sources on the

internet too (see the notes at the end of each chapter).

What I’m aiming to do in this book is to make these numbers simple

and memorable; to show you how you can ﬁgure out the numbers for

yourself; and to make the situation so clear that any thinking reader will

be able to draw striking conclusions. I don’t want to feed you my own con-

clusions. Convictions are stronger if they are self-generated, rather than

taught. Understanding is a creative process. When you’ve read this book

I hope you’ll have reinforced the conﬁdence that you can ﬁgure anything

out.

I’d like to emphasize that the calculations we will do are deliberately

imprecise. Simpliﬁcation is a key to understanding. First, by rounding the

numbers, we can make them easier to remember. Second, rounded num-

bers allow quick calculations. For example, in this book, the population

of the United Kingdom is 60 million, and the population of the world is

6 billion. I’m perfectly capable of looking up more accurate ﬁgures, but

accuracy would get in the way of ﬂuent thought. For example, if we learn

that the world’s greenhouse gas emissions in 2000 were 34 billion tons of

CO_{2}-equivalent per year, then we can instantly note, without a calculator,

that the average emissions per person are 5 or 6 tons of CO_{2}-equivalent per

person per year. This rough answer is not exact, but it’s accurate enough to

inform interesting conversations. For instance, if you learn that a round-

trip intercontinental ﬂight emits nearly two tons of CO_{2} per passenger,

then knowing the average emissions yardstick (5-and-a-bit tons per year

per person) helps you realize that just one such plane-trip per year corre-

Man”