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<DIV><FONT face=Arial><STRONG><FONT size=4>Science puts a number on
survival</HEADLINE></FONT></STRONG></DIV>
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<DIV class=articleDetails><BYLINE><FONT size=2>Bill
McKibben</BYLINE><BR><DATE>January 2, 2008</DATE><BR></FONT>
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<DIV class=pageprint id=contentSwap1 style="DISPLAY: none"><FONT size=2>The past
month might have been the most important yet in the two-decade history of the
fight against global warming. Al Gore received the Nobel prize; international
negotiators made real progress on a treaty in Bali and the US worked up the
nerve to raise petrol mileage standards for cars.</FONT></DIV></P></DIV>
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<P><FONT size=2>But what may turn out to be the most crucial development went
largely unnoticed. It happened at an academic conclave in San Francisco. A NASA
scientist named James Hansen offered a simple, straightforward and mind-blowing
bottom line for the planet: 350, as in parts per million carbon dioxide in the
atmosphere. It is a number that may make what happened in Bali seem quaint and
nearly irrelevant. It is the number that may define our future.</FONT></P>
<P><FONT size=2>To understand what it means, you need a little
background.</FONT></P>
<P><FONT size=2>Twenty years ago Hansen kicked off this issue by testifying
before the US Congress that the planet was warming and that people were the
cause. At the time we could only guess how much warming it would take to put us
in real danger. Since the pre-Industrial Revolution concentration of carbon in
the atmosphere was roughly 275 parts per million, scientists and policy makers
focused on what would happen if that number doubled - 550 was a crude and
mythical red line, but politicians and economists set about trying to see if we
could stop short of that point. The answer was: not easily, but it could be
done.</FONT></P>
<P><FONT size=2>However, in the past five years scientists began to worry that
the planet was reacting more quickly than they had expected to the relatively
small temperature increases we have already seen. The rapid melt of most glacial
systems, for instance, convinced many that 450 parts per million was a more
prudent target. That is what the European Union and many big environmental
groups have been proposing in recent years, and the economic modelling makes
clear that achieving it is possible, though the chances diminish with every new
coal-fired power plant.</FONT></P>
<P><FONT size=2>But the data just keep getting worse. The news this (northern)
autumn that Arctic sea ice was melting at an off-the-charts pace, and data from
Greenland suggesting that its giant ice sheet was starting to slide into the
ocean, make even 450 look too high. Consider: we are already at 383 parts per
million, and it is knocking the planet off kilter in substantial
ways.</FONT></P>
<P><FONT size=2>So, what does that mean? Hansen says it means we have gone too
far.</FONT></P>
<P><FONT size=2>"The evidence indicates we've aimed too high - that the safe
upper limit for atmospheric CO<INF />2 is no more than 350 ppm," he said after
his presentation.</FONT></P></DIV>
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<P><FONT size=2>The last time the Earth warmed two or three degrees - which is
what 450 parts per million implies - sea levels rose by tens of metres,
something that would shake the foundations of the human enterprise should it
happen again.</FONT></P>
<P><FONT size=2>And we are already past 350. Does that mean we are doomed? Not
quite. Not any more than your doctor telling you that your cholesterol is far
too high means the game is over. Much as the way your body will thin its blood
if you give up fried chips, so the Earth naturally gets rid of some of its
carbon dioxide each year. We just need to stop putting more in and, over time,
the number will fall, perhaps fast enough to avert the worst damage.</FONT></P>
<P><FONT size=2>That "just" hides the biggest political and economic task we
have ever faced: weaning ourselves from coal, gas and oil. The difference
between 550 and 350 is that the weaning has to happen now, and everywhere. No
more passing the buck. The gentle measures bandied about at Bali do not come
close. Hansen called for an immediate ban on new coal-fired power plants that do
not capture carbon, the phasing out of old coal-fired generators, and a tax on
carbon high enough to make sure that we leave tar sands and oil shale in the
ground. To use the medical analogy: we are not talking statins to reduce your
cholesterol; we are talking huge changes in every aspect of your daily
life.</FONT></P>
<P><FONT size=2>Perhaps too huge. The problems of global equity alone may be too
much: the Chinese are not going to stop burning coal unless we give them another
way to raise people out of poverty. And we simply might have waited too
long.</FONT></P>
<P><FONT size=2>But at least we are homing in on the right number. Three hundred
and fifty is the number every person needs to know.</FONT></P>
<P><B><FONT size=2>The Washington Post</FONT></B></P></DIV><A
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