Monday, August 24, 2009

Smoke gets in Eli's ears

There have been interesting developments in the big smoke. To start with something old, Gunnar Myhre has narrowed the uncertainty for directs aerosol forcing from -0.9 to -0.1 W/m2 to -0.3 + 0.2 W/m2. This means that not only is the negative direct aerosol forcing less than previously thought but that the range is much smaller.

In the Intergovernmental Panel on Climate Change Fourth AssessmentReport, the direct aerosol effect is reported to have a radiative forcing estimate of –0.5 Watt per square meter (W m–2), offsetting the warming from CO2 by almost one-third. The uncertainty,however, ranges from –0.9 to –0.1 W m–2, which is largely due to differences between estimates from global aerosol models and observation-based estimates, with the latter tending to have stronger (more negative) radiative forcing. This study demonstrates consistency between a global aerosol model and adjustment to an observation-based method, producing a global and annual mean radiative forcing that is weaker than –0.5 W m–2, with a best estimate of –0.3 W m–2. The physical explanation for the earlier discrepancy is that the relative increase in anthropogenic black carbon(absorbing aerosols) is much larger than the overall increase in the anthropogenic abundance of aerosols.
Myhre compares results from observation based estimates of aerosol optical depth and those derived from modeling. The observation based estimates start with measured aerosol optical depth (AOD) or radiative emission. Estimates are made of the human contribution to the scattering. In the second approach aerosol models are used to estimate how humans have changed the amount and type of aerosol with radiative transfer models that calculate the forcing. Previously there was a significant difference between the two. A key assumption for the observational models has been that the nature of pre-industrial aerosols was the same as today but the amount has simply increased. Myhre observes that the black carbon component has grown roughly twice as fast as the other anthropogenic aerosols. When the observational models are corrected for this, they come into agreement with the model based estimates.
The much stronger increase in the anthropogenic fraction of absorption AOD than total AOD will to some extent be model-dependent but will be robust because of the much stronger growth in the BC emissions than the growth in the rest of the aerosols and their precursors. This suggests that the global annual average single scattering albedo of the aerosols has been reduced because of human activity. The global mean annual average single scattering albedo computed in the model for all aerosols at 5 µm is 0.986 at preindustrial conditions and 0.970 at present-day conditions. Thus, aerosol in present times is approximately twice as absorbing as that in preindustrial conditions.
The net effect would be to slightly lower the estimated greenhouse gas forcing since a higher aerosol forcing would have to be counteracted by a larger CO2 forcing. Watch the bounding ball.


Rattus Norvegicus said...

Since I am unable to read the full text, does he account for the positive feedback effect of atmospheric black carbon on ice-albedo, as Hansen has demonstrated?

Duae Quartunciae said...

Eli, you conclude: "The net effect would be to slightly lower the estimated greenhouse gas forcing since a higher aerosol forcing would have to be counteracted by a larger CO2 forcing"

I do not believe that is true. I don't see it in the papers, and the premise for your inference is, in my opinion, incorrect.

We don't know the total forcing at all well.

In the IPCC 4AR (page 204, table 2.12), the total forcing is given as from 0.6 to 2.4 W/m^2. That's a very large range of possible totals... and given that the total forcing is so uncertain there seems to be no basis at all for thinking that you must "counteract" a smaller estimate of the aerosol forcing. All that happens is that the net forcing estimate becomes a better constrained and a bit reduced.

The greenhouse forcing is much better known than the aerosol forcing! I don't think the greenhouse forcing estimates will change in the slightest.

Other variables that have a lot of uncertainty include the climate sensitivity to forcings, and the excess imbalance being absorbed into the ocean. It is because of THESE uncertainties that we cannot easily constrain the total forcing.

Hence if the net forcing is a bit reduced, this might be taken as an indication that perhaps sensitivity is high, or the ocean imbalance is high. These seem far more likely that a revision to the relativity straightforward matter of greenhouse forcing. Frankly, I think everything still remains within the already stated uncertainty bounds, as given in the 4AR.

James Annan said...

I believe DQ is right, the updated estimate might (if it becomes generally accepted) help to rein in the wilder reaches of some estimates (eg climate sensitivity) but won't have a big impact on the mean. Basically it implies we are more confident that the net forcing has been fairly large, therefore sensitivity is not quite as big as it might have been if the aerosols had cancelled out more of the GHGs.

EliRabett said...

Eli thought that is what he was saying, that this places, if not an upper bound on climate sensitivity, then a restraint.

Oh well.

Duae Quartunciae said...

Sorry to beat a dead horse, Eli... but a restraint on climate sensitivity is not at all the same thing as a change in the greenhouse forcing, which is what you mentioned in your post.

Anonymous said...

What would be the impact of this potential revision on the already-made projections and the tuning that precedes projection?


Hank Roberts said...

> slightly lower

The upper tail end of the probability distribution, without making much difference about the middle

> the wilder reaches

(has anyone noticed that probability distribution functions have two tails, a large middle, and no head? It almost sounds like a legislature. But I digress.)

Anonymous said...

Note that climate sensitivity estimations are based on much more than just model fitting past-century climate change, and indeed, a number of people note that past-century climate does not bound the upper end of climate sensitivity well at all (eg, see Forest et al. papers, and look at the CS pdfs that do not include expert priors). Therefore, reducing the magnitude of estimtaed aerosol forcing might not have much effect on CS estimation as one would expect.

The impact on projections is another question: there, it depends on how aerosol emissions are projected to change over the century. If I read this right, it means that reducing both sulfates and BC together for air pollution purposes will not result in much "unmasking" of warming, which would be somewhat reassuring.

Also note that this is _direct_ aerosol forcing: indirect aerosol effects are still the elephant in the china shop.