PUAF 741

Global Environmental Problems

Spring 2006

Problem Set #8

Due: 5 April
 

Don't forget to read the technical summaries of the IPCC reports posted on the climate change page. If you want more detail on a particular topic, see the corresponding section of the full report. If you are having a hard time understanding something in the technical summary, see the summary for policymakers (SPM). The major points in the SPMs are fair game for the final exam.

  

1.

As mentioned in class (we will discuss this in more detail in coming weeks), the ultimate objective of the UN Framework Convention on Climate Change is to stabilize concentrations of greenhouse gases in the atmosphere at a "safe" level. Unfortunately, there is no consensus on what is a "safe" level, and no agreement on how stablization would be achieved.

A more modest step is to stablize emissions of greenhouse gases at current rates of emission. Below we investigate the impact of this alternative.

A.

The preindustrial concentration of methane was about 700 ppbv. The residence time of methane in the preindustrial (i.e., clean) atmosphere is estimated at about 12 years. What was the preindustrial (i.e., natural) emission rate of methane, in Tg/y?

B.

Global anthropogenic emissions of methane from all sources (energy, landfills, agriculture, ruminants, etc.) are estimated at 350 Tg/y. If anthropogenic emissions remain constant at this rate, and if natural emissions remain unchanged at the preindustrial rate, what will be the equilibrium concentration of methane?

  C. Compare your answer from part B to the preindustrial concentration, and to the 1998 concentration of 1745 ppbv. Roughly how long will it take to achieve stabilization of methane concentrations?  

D.

The radiative forcing due to the increase in methane from the preindustrial to the 1998 concentration was estimated by the IPCC at 0.48 W/m2. Additional increases add 3.7 x 10-4 W/m2 per ppbv. Estimate the radiative forcing from methane if emissions are stablized.

 
E.
 Redo A-D for nitrous oxide. The preindustrial concentration and residence time were 270 ppbv and 114 years; current estimated anthropogenic emissions are 11 Tg/y; the 1998 concentration and radiative forcing were 314 ppbv and 0.15 W/m2; additional increases add 3.1 x 10-3 W/m2 per ppbv.  
 
F.
 Carbon dioxide is more complicated, because it cannot be characterized by a single residence time. Very roughly 10% of emitted anthropogenic carbon dioxide is retained in the atmosphere with a residence time of 2 y, 25% with a residence time of 17 years, 30% with a residence time of 75 years, and the remainder with a residence time of 400 years. If anthropogenic emissions were held constant at the current level of 7 Pg/y of carbon, what would be the equilibrium concentration in the atmosphere? What would be the coresponding radiation forcing? Compare the concentration and radiative forcing to the 1998 values of 365 ppmv and 1.46 W/m2.  
 
G.
 Total up the radiative forcings from the stabilized concentrations of methane, nitrous oxide, and carbon dioxide estimated in parts D, E, and F. Estimate the equivalent carbon dioxide concentration for the total.  
 
H.
 A discussed in class, the IPCC estimates that the global-average equilibrium temperature change due to a doubling of carbon dioxide (a radiative forcing of 3.7 W/m2) would be 1.5 to 4.5 C, with a best estimate of 2.5 C. What would be the corresponding temperature increase for the stabilized greenhouse gas concentrations computed above?