About a week ago, maybe a bit more, NASA, nay Goddard Space Flight Center released a breathless press release which spread near and far even unto our buddies on the other side of reality and some on our side.
Eli is here to tell you that just about everyone missed the real discoveries in the paper and underlying work, and the paper itself let alone the press release did not tell the whole story. According to everybunny else the take home was
However, the new research shows worldwide emissions of CCl4 average 39 kilotons per year, approximately 30 percent of peak emissions prior to the international treaty going into effect.
It has been long known that there are significant fugitive emissions of just about all CFCs and ilk and that bottom up inventories of atmospheric emissions always fall short of reality. This has consequences for our understanding. It should be no real surprise that the emissions are higher than officially reported. Of course, the issue is quantifying the amount of emissions and tracing them back to their source and here the Liang paper makes an important contribution. Yes, IF the Montreal Protocols (MP) were being rigidly enforced globally we would not be seeing such emissions, and we would be seeing a faster decline, but the best of times has not yet arrived. OTOH, it is vitally important to know if the fugitive emissions themselves are decreasing.
Buried down at the bottom of the press release is this afterthought
In addition to unexplained sources of CCl4, the model results showed the chemical stays in the atmosphere 40 percent longer than previously thought. The research was published online in the Aug. 18 issue of Geophysical Research Letters.Geez, even tho it is a press release they could have provided a link. To Eli this is the most important of the results in the paper. If one simplistically looks at the problem as a one box model, to explain the slower than expected falls in CCl4 there are two possibilities, either the destruction rate is slower than expected or the emissions rate is higher or some combination of the two. The problem with deciding which is which, is that the two interact. If your lifetime is too short it will look like the amount of emissions are high, and if the lifetime is too long the amount of emissions will look too low. Liang et al have a nice way of showing this
Figure 2. CCl4 global mean trend (ppt/yr) as a function of total lifetime and emissions from the two-box model (gray contours). Purple contours indicate the emissions and τCCl4 ranges that yield IHGs within the observed 1.1–2.0 ppt range (2-σ) between 2000 and 2012, using the current best estimate EFn of 0.94. Red (Advanced Global Atmospheric Gases Experiment (AGAGE)-based) and blue (GMD-based) numbers show emissions and lifetimes derived using the observed IHG and trend for individual years (2000–2012). The dark (light) gray shading outlines the range of emissions and τCCl4 that can be reconciled with the observations for EFn of 0.94 (0.88–1.00). The black diamond symbol shows our current best estimate for τ (thick and thin red bars indicate 1-σ and 2-σ uncertainties, respectively) and the upper limit bottom-up potential emissions for 2007–2012 (thick blue bar shows 1-σ variance) with 1-σ uncertainty shown in black-hatched shading.IHG- Inter hemispherical gradient EFn - fraction of emissions in the Northern Hemisphere.
The black diamond indicates the results one would get using a bottoms up (reported emissions) estimate of emissions and the previous best estimate of the Total removal lifetime of 25 years. Eli has added the purple dot showing the estimates of Liang, et al for 2007-2012 emissions with a 35 year CCl4 total lifetime. The green dot is the result of an earlier study of Xiao, et al on emissions between 1996 and 2004 that used a lifetime which is too short. Xiao's estimate of emission rates was 74 Gg/yr on average between 1996 and 2004 using a total lifetime of 25 + ~5 years, which is shown by the green dot far to the right. OTOH, moving up the 0.9 ppm/year contour to the new inferred lifetime of Liang et al., 35 years, brings that estimate of emissions down to ~ 40 Gg/year.
Liang et al divide the world into two boxes, the Northern and Southern hemisphere. Since most of the emission is in the Northern Hemisphere, there is an inter hemispherical gradient which can be used to calculate the total emissions and lifetime.
Prominent in the abstract and the press release is that the average emissions over the 2000-2012 time period were 393445 Gg/yr. Somewhat less, well a lot less, prominent, you have to read the paper, is that emissions have been decreasing. Between 2007 and 2012 they decreased to between 31 and 45 Gg/yr. Simply taking the average of these gives an average of 35.5 Gg/yr. Simple math tells us that the average emissions between 2000 and 2007 would then be an average of 41.5 Gg/yr which is consistent with the numbers shown in Liang's Figure 2 above, with the earlier years clustering to the right of the graph in the 40 Gg.year area and the later ones to the left. Emissions are decreasing. Not as fast as we would like, but they are decreasing.
Where is of course the question all bunnies want to know. Several jumped to the conclusion that all the fugitive emissions are from China and India.
Inverse 3D modeling is IEHO the best choice for quantifying sources total emissions, reported and unreported, and in 2010, X. Xiao and about 20 friends took a shot in Atmos. Chem. Phys. 10, 10421, for the period 1996-2004. As with all such things, that paper was not perfect, and with the passage of time, some of the problems with it have become clearer, but taken together with the new Liang paper there are a number of take homes. By looking at the time history of CCl4 at stations around the globe Xiao et al was able to infer the location and average carbon tet emissions from various locations during the study period. By comparison, if you simply want global emissions, the advantage of the Liang, et al method is that the two box model is robust at the price of resolution
For convenience, Xiao et al divided the world up into eight boxes and tried to trace emissions geographically on a finer 64 x 128 point grid
So yes, CCl4 in the atmosphere is decreasing ~1% per year, slower than bunnies expected, due to nature (a longer atmospheric lifetime) and fugitive emissions (which are also decreasing).