Climate modelling is difficult and is dogged by uncertainties. But un-
certainty about exactly how the climate will respond to extra greenhouse
gases is no justification for inaction. If you were riding a fast-moving mo-
torcycle in fog near a cliff-edge, and you didn’t have a good map of the
cliff, would the lack of a map justify not slowing the bike down?

So, who should slow the bike down? Who should clean up carbon
emissions? Who is responsible for climate change? This is an ethical ques-
tion, of course, not a scientific one, but ethical discussions must be founded
on facts. Let’s now explore the facts about greenhouse gas emissions. First,
a word about the units in which they are measured. Greenhouse gases
include carbon dioxide, methane, and nitrous oxide; each gas has dif-
ferent physical properties; it’s conventional to express all gas emissions
in “equivalent amounts of carbon dioxide,” where “equivalent” means
“having the same warming effect over a period of 100 years.” One ton
of carbon-dioxide-equivalent may be abbreviated as “1 t CO2e,” and one
billion tons (one thousand million tons) as “1 Gt CO2e” (one gigaton). In
this book 1 t means one metric ton (1000 kg). I’m not going to distinguish
imperial tons, because they differ by less than 10% from the metric ton or

In the year 2000, the world’s greenhouse gas emissions were about 34
billion tons of CO2-equivalent per year. An incomprehensible number.
But we can render it more comprehensible and more personal by divid-
ing by the number of people on the planet, 6 billion, so as to obtain the
greenhouse-gas pollution per person, which is about 5½ tons CO2e per year
per person. We can thus represent the world emissions by a rectangle
whose width is the population (6 billion) and whose height is the per-
capita emissions.

Greenhouse gas pollution