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fell because of hardship, not from any shift in our fundamental habits. In
the midst of the downturn, it was easy to predict that once the recovery
began, people would rush back to the behavior they know best. In fact,
that is exactly what happened. Researchers found that global emissions
of carbon dioxide increased by 5.9 percent in 2010, more than making up
for a slight decrease in 2009, the year in which the recession had the most
impact. 72 Clearly, the recession did not slow down the steady increase in
greenhouse gas emissions.
Fourth, even this discussion does not face the full measure of the chal-
lenge, for the simple reason that carbon dioxide is not the only green-
house gas. George Monbiot points out that according to the Potsdam
Institute for Climate Impact Research, we should aim to stabilize “green-
house gases in the atmosphere at or below the equivalent of 440 parts of
carbon dioxide per million.” When the carbon dioxide concentration was
around 380 ppm, “the other greenhouse gases raise[d] this to an equiva-
lent of 440 or 450.” 73 But in late 2013, the global monthly mean of car-
bon dioxide approached 400 ppm, so that the overall concentration of
greenhouse gases is now well above the equivalent of 450 ppm. 74 We have
already exceeded the upper limit for our contribution to the greenhouse
effect we set some years ago.
Fifth, we must take another factor into account as well. As time goes
by and we emit more carbon dioxide, the less the biosphere can absorb;
by one estimate, it will absorb fully one-third less as much by 2030. 75 As
a result, emiting a certain quantity of carbon dioxide a decade from now
will impose a greater burden on the biosphere than emiting it today—
and what is more, reducing our output will only keep up with the Earth's
capacity to absorb less. In effect, we will have to cut back our footprint an
extra portion just to take that fact into account.
Sixth, much as these estimates for how deep we must cut have to
be revised, the guess as to how soon we should achieve our target must
change as well. Monbiot's already severe estimate—that the United
Kingdom would have to cut its emissions 80 percent by 2030—was based
on a guess as to when our emissions would be so great that they would
trigger positive feedback loops and thus irreversible climate change. But
Monbiot relies on an estimate in a paper published back in 2003. 76 It's
already clear that in the intervening years we've emited far more than
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