Peter Lehner's Blog
What’s the right target for CO2 in the atmosphere?
July 17, 2008
Posted by Peter Lehner
At a recent climate conference, scientists John Holden and Thomas Lovejoy demonstrated the current horrors of climate disruption. They also asked a simple question:
What’s the right target for CO2 in the atmosphere?
Pre-industrial levels were around 280 parts per million. We’re now at 385 ppm. Supposedly, we’re now talking about trying to stabilize the atmosphere at 450 ppm. But many view that is unrealistic and some say it is unnecessary. Instead they suggest a target of 550 ppm.
James Hansen, the foremost NASA climate scientist, however, says that we can’t tolerate even 450 ppm; we should aim to reduce from where we are now to 350 ppm. After all, he points out, even now we’re seeing rapid reduction of arctic ice, sea level rise, significant storms and droughts, floods, changed species patterns – all which come with suffering and cost. Is he right? What does 450 and 550 ppm mean?
The economist scientist, Dimitri Zenghelis who worked on one Stern report, had a slide showing the probability of temperature increase associated with each target CO2 concentration.
- At 450 ppm, there is a 50 percent chance of at least a 2° C increase, but also a 25 percent chance of 3° C and a 5 percent chance of a 5° C increase.
- At 550 ppm, there is an 80 percent chance of at least a 2° C increase, but also a 45 percent chance of 3° C and a 20 percent chance of a 5° C increase.
What do these temperature changes mean?
Again, it’s largely a question of probabilities, but the solid science (from studies released by the likes of IPCC, Stern, and others) suggest that at a 2° C change, we would see a sharp decrease in the availability of fresh water and in crop yields, increased exposure to flooding and malaria, and a high risk of extinction of Artic species like caribou and polar bear.
At 3° C change, we’d see an increase in serious droughts across Southern Europe, billions of more people suffering from water shortages, and millions at increased risk of hunger, malnutrition and coastal flooding. At this temperature increase, between 20-50 percent of species would face extinction, and the Amazon would teeter on the brink of collapse. We would lose half of the Articic tundra.
At 5° C change, we would see the disappearance of large glaciers in the Himalayas, effecting millions in India and China, a serious increase in ocean acidity, and a sea level rise that would threaten major cities like New York, London and Tokyo. Add to this a Greenland ice sheet that has begun melting irreversibly, and the risk of a collapsing West Arctic Ice Sheet and Atlantic Thermohaline Circulation – collapses that would result in dramatic sea level rises and temperature changes.
Putting this together, if we aim to limit atmospheric concentrations to 550 ppm – a level that our President and our Congress are so far unwilling to target – we face a 25 percent chance of having what could be devastating chances and a 5 percent chance – 1 in 20 – of catastrophic impacts.
In our usual lives, we do a lot to avoid catastrophic events. Think of what we do to avoid our house burning down (statistically, a 0.3 percent chance in 2006) and even then we can move to another house and rebuild; we can’t move to another planet while we rebuild the earth.
Can we really look at our kids and say we’re ok with a 25 percent chance of catastrophe during their – and probably our – lives?
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Comments
Dan Troutman — Jul 17 2008 04:46 PM
Any of you speak Chinese? Better check with them since they're now the world's largest CO2 emitting nation and probably have the most to lose. I doubt that they'll subsribe to any CO2 limit in the near future. Gotta admire those "developing" nations puffin' away in the east Pacific rim. :)
Dan Staley — Jul 17 2008 07:52 PM
Any of you speak Chinese? Better check with them since
Per capita, the US is the largest. Oh, darn, I guess that means we should give up and do nothing.
It doesn't matter who is biggest this or that if ecosystems flip and we can no longer predict crop yields. Work must be done regardless of biggest.
Best,
D
Dan Troutman — Jul 18 2008 12:23 AM
I didn't say give and do nothing. But since we're talking about taking unilateral drastic measures now for something that won't have any benefit for nearly 100 years, it's a hard sell to the average American family. I wonder how the "you should cut back now for your grandchildrens' sake" will play out during tough economic times?
We can only hope that we live long enough to find out what really happens and see if the statistics were correct. :)
Peter Lehner — Jul 18 2008 12:24 PM
That's the tough problem, Dan: How do we get people to care? Any ideas?
Many immigrants and Americans worked hard during tough times to provide a better life for their kids. Why were they willing to do so then but do we assume people are not willing to do so now?
Sure, we're out of the habit. But I know that I, and many of my colleagues here, are constantly trying to help people understand that this is a crisis that is effecting their lives now.
We're open to any suggestions you, or anyone else, may have.
Thanks,
Peter
Earl Killian — Jul 18 2008 05:39 PM
Thank you for the thoughtful post. One thing you might want to do next time is summarize Hansen's argument in a little more detail, since that detail is important. Hansen believes we need to stay under 450ppm in the short-term, and return to 350ppm over the longer term. He basically identifies from the paleo record Earth's short-term response to CO2 increases, and its long-term response. The long-term response is about twice that of the short-term. Thus Hansen is not disagreeing with the IPCC on the short-term effect (indeed his estimate form the paleo record matches the IPCC's estimate). Rather he is adding an effect that is essentially not considered by the IPCC yet.
Earl Killian — Jul 18 2008 05:45 PM
Dan Troutman, what matters to the atmosphere in cumulative emissions, not this year's emissions. By that measure the US dwarfs China, and will for some time. So you might want to ask, do any of us speak Bushlish? Or whatever the language is of the US President (someone once told me he spoke English, which really made me laugh).