What’s the right target for CO2 in the atmosphere?

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?