OK. Fossil fuel burning increases CO₂ concentrations significantly. But does it matter? "Carbon is nature!", the oilspinners remind us, "Carbon is life!" If CO₂ had no harmful effects, then indeed carbon emissions would not matter. However, carbon dioxide is a greenhouse gas. Not the strongest greenhouse gas, but a significant one nonetheless. Put more of it in the atmosphere, and it does what greenhouse gases do: it absorbs infrared radiation (heat) heading out from the earth and reemits it in a random direction; the effect of this random redirection of the atmospheric heat traffic is to impede the flow of heat from the planet, just like a quilt. So carbon dioxide has a warming effect. This fact is based not on complex historical records of global temperatures but on the simple physical properties of CO₂ molecules. Greenhouse gases are a quilt, and CO₂ is one layer of the quilt.

So, if humanity succeeds in doubling or tripling CO₂ concentrations (which is where we are certainly heading, under business as usual), what happens? Here, there is a lot of uncertainty. Climate science is difficult. The climate is a complex, twitchy beast, and exactly how much warming CO₂-doubling would produce is uncertain. The consensus of the best climate models seems to be that doubling the CO₂ concentration would have roughly the same effect as increasing the intensity of the sun by 2%, and would bump up the global mean temperature by something like 3 °C. This would be what historians call a Bad Thing. I won't recite the whole litany of probable drastic effects, as I am sure you've heard it before. The litany begins "the Greenland icecap would gradually melt, and, over a period of a few 100 years, sea-level would rise by about 7 metres." The brunt of the litany falls on future generations. Such temperatures have not been seen on earth for at least 100 000 years, and it's conceivable that the ecosystem would be so significantly altered that the earth would stop supplying some of the goods and services that we currently take for granted.

Climate modelling is difficult and is dogged by uncertainties. But uncertainty about exactly how the climate will respond to extra greenhouse gases is no justification for inaction. If you were riding a fast-moving motorcycle 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 question, 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 different 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 tonne.

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 dividing by the number of people on the planet, 6 billion, so as to obtain the greenhouse-gas pollution per person, which is about 5.5 tons CO₂ e 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.

Now, all people are created equal, but we don't all emit 5.5 tons of CO₂ per year. We can break down the emissions of the year 2000, showing how the 34-billion-ton rectangle is shared between the regions of the world:

This picture, which is on the same scale as the previous one, divides the world into eight regions. Each rectangle's area represents the greenhouse gas emissions of one region. The width of the rectangle is the population of the region, and the height is the average per-capita emissions in that region. In the year 2000, Europe's per-capita greenhouse gas emissions were twice the world average; and North America's were four times the world average. We can continue subdividing, splitting each of the regions into countries. This is where it gets really interesting:

The major countries with the biggest per-capita emissions are Australia, the USA, and Canada. European countries, Japan, and South Africa are notable runners up. Among European countries, the United Kingdom is resolutely average. What about China, that naughty "out of control" country? Yes, the area of China's rectangle is about the same as the USA's, but the fact is that their per-capita emissions are below the world average. India's per-capita emissions are less than half the world average. Moreover, it's worth bearing in mind that much of the industrial emissions of China and India are associated with the manufacture of stuff for rich countries. So, assuming that "something needs to be done" to reduce greenhouse gas emissions, who has a special responsibility to do something? As I said, that's an ethical question. But I find it hard to imagine any system of ethics that denies that the responsibility falls especially on the countries to the left hand side of this diagram - the countries whose emissions are two, three, or four times the world average. Countries that are most able to pay. Countries like Britain and the USA, for example.

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