Monday, May 20, 2013

So How Much More Time Does Otto Give Us?

Given the lower estimates of climate sensitivity in the Otto et al paper in Nature Geosciences, how much more time does the world have to dilly-dally about cutting emissions?

Well, none really. The 2°C limit which has become the fallback position isn't something to take lightly -- after all it's about 1/4th-1/3th of an inverse ice age, which is hardly something to scoff at, and with the warming and melting it will ultimately raise sea level by 25-50 feet meters (over hundreds of years).

But let's play along. Let's assume atmospheric carbon levels are increasing exponentially (which they are, once you look beyond a 10-15 year window), with a doubling time of D.

And let's assume the radiative forcing (and hence temperature change) is proportional to the logarithm of the atmosphere's carbon-equivalent level.

Then just a little algebra shows that the time t to reach a certain temperature T is

t = DT/S

where S is climate sensitivity.

Using greenhouse gas forcings since 1979 from NOAA, I find that CO2 is increasing at an annual rate of 0.46%, but CO2-equivalent is increasing at 0.66%, which gives a doubling time of D = 105 years.

The Otto paper reduces transient climate sensitivity (TCR) from a value of 1.6°C (based on data through the 1990s) to 1.3°C (based on data up through 2009).

So if we want to limit T to 2°C, the difference between these two TCRs gives us an additional 30 years.

3 decades. That's a fair bit of time.

Will we use those 30 years wisely and take climate change seriously, to finally do something about carbon dioxide, to build a clean, sustainable, noncarbon world for the many future generations who will live after us?


N.b. a lower climate sensitivity doesn't change the ocean acidification problem.


A.Grinsted said...

What is the relevance of the sea level figure to the blog post and otto et als paper?

Anyway I have already commented on that graph pointing out some issues

David Appell said...

The relevance is explained in the second paragraph.