Tuesday, April 19, 2011

James Hansen: A $100 m Measurement Needed

From Mr. Lalloobhoy Battliwala

A nice draft from Hansen (also available on his website). I understand in small parts, which is dangerous. Some excerpt and highlight below.
"It is remarkable and untenable that the second largest forcing that drives global climate change remains unmeasured."
One thing I don't understand is the role of ozone - and thereby methane. My understanding is that methane's short-term impact is via ozone. I wonder if the near-term warming of ozone and BC is roughly matched by the cooling of the (cooling) aerosols.
I also don't understand why he mentions "without the requirement for undue government review panels".
The GHG forcing is about +3 W/m2. 2. But the large uncertainty in the aerosol forcing implies that the net forcing is very uncertain, so uncertain that either value suggested by Hansen's grandchildren for the net forcing (Sophie's +2 W/m2 or Connor's +1 W/m2) could be correct.
The correct answer defines the terms of humanity's 'Faustian aerosol bargain' (Hansen and Lacis, 1990). Global warming has been limited, as aerosol cooling partially offsets GHG warming. But aerosols remain airborne only several days, so they must be pumped into the air faster and faster to keep pace with increasing long-lived GHGs.
Unfortunately, Hansen et al. (1984) chose to estimate climate sensitivity from paleoclimate data by treating the aerosol change between glacial and interglacial conditions as a forcing. There is nothing inherently wrong with asking the question: what is the sensitivity of the remaining processes in the system if we consider ice sheets, GHGs, and aerosols to be specified forcings, even though the ice sheets and GHGs are slow feedbacks and aerosol changes are a fast feedback. The problem is that it is impossible to get an accurate answer to that question. The aerosol forcing depends sensitively on how much the aerosols absorb (the aerosol single scatter albedo) and on the altitude distribution of the aerosols, but, worse, it depends on how the aerosols modify cloud properties. The large uncertainty in the value of the aerosol forcing causes the resulting empirical climate sensitivity to have a large error bar.
Earth's energy imbalance and its changes will determine the future of Earth's climate. It is thus imperative to measure Earth's energy imbalance and the factors that are changing it.
We also must quantify the causes of changes of Earth's energy imbalance. The two dominant causes are changes of greenhouse gases, which are measured very precisely, and changes of atmospheric aerosols. It is remarkable and untenable that the second largest forcing that drives global climate change remains unmeasured. We refer to the direct and indirect effects of human-made aerosols.
We have inferred indirectly, from the planet's energy imbalance and global temperature change, that aerosols are probably causing a forcing of about ‒1.6 W/m2 in 2010. Our estimated uncertainty, necessarily partly subjective, is ± 0.3 W/m2, thus a range of aerosol forcing from ‒1.3 to ‒1.9 W/m2.
These analyses tend to confirm that aerosol forcing is large and negative, but cannot tell us what aerosols are causing the forcing, how much of the forcing is due to indirect effects on clouds, and how the aerosol forcing is changing. Aerosol climate forcing is complex (Ramanathan et al., 2001; Ramaswamy et al., 2001), in part because there are many different aerosol compositions distributed inhomogeneously around the planet. Different compositions have different effects on solar radiation, and, via their effects on clouds, they have different effects on terrestrial thermal radiation.
The total aerosol climate forcing that we have inferred implies that the aerosol indirect climate forcing exceeds the direct aerosol forcing, possibly by a large amount. There is no simple relationship between the direct and indirect forcings, which each are likely to be strongly dependent on aerosol composition.
No practical way to determine the aerosol direct and indirect climate forcings has been proposed other than simultaneous measurement of the reflected solar and emitted thermal radiation fields as described above. The two instruments must be looking at the same area at essentially the same time. Such a mission concept has been well-defined (Hansen et al., 1992) and if carried out by the private sector without a requirement for undue government review panels it could be achieved within a cost of about $100M.

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