|Working Group I: The Scientific Basis|
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4.4.4 Model Simulations of Perturbed and Y2100 Atmospheres
The OxComp workshop also defined a series of perturbations to the Y2000 atmosphere for which the models reported the monthly averaged 3-D distribution of O3 abundances and the budget for CH4, specifically the loss due to reaction with tropo-spheric OH. From these diagnostics, the research group at Oslo calculated the change in global mean tropospheric O3 (DU) and in OH (%) relative to Y2000, as shown in Table 4.11. For each model at every month, the “troposphere” was defined as where O3 abundances were less than 150 ppb in the Y2000 simulation, a reasonably conservative diagnostic of the tropopause (see Logan, 1999). Because O3 is more effective as a greenhouse gas when it lies above the surface boundary layer (SAR; Hansen et al., 1997a; Prather and Sausen, 1999; Chapter 6 of this report), the model study diagnosed the O3 change occurring in the 0 to 2 km layers of the model. This amount is typically 20 to 25% of the total change and is consistent across models and types of perturbations here.
Case A, a +10% increase in CH4 abundance for Y2000, had consistent
results across reporting models that differed little from the SAR’s Delta-CH4
model study. The adopted values for this report are -3% change in OH and +0.64
DU increase in O3, as listed under the “TAR” row in Table
Cases B-C-D are a sequence of three Y2100 atmospheres based on A2x: Case B is the full Y2100-A2x scenario; Case C is the same Y2100-A2x scenario but with unchanged (Y2000) NOx emissions; and Case D is the same but with NOx, VOC and CH4 unchanged since Y2000 (i.e., only CO emissions change). Case B (Y2100-A2x) results are available from most OxComp participants. All models predict a decrease in OH, but with a wide range from -6 to -25%, and here we adopt a decrease of -16%. Given the different distributions of the O3 increase from the OxComp models (Figures 4.12-13), the increases in globally integrated O3 were remarkably consistent, ranging from +16.6 to +26.5 DU, and we adopt +22 DU. Without the increase in NOx emissions (Case C) the O3 increase drops substantially, ranging from +4.6 to +14.5 DU; and the OH decrease is large, -37 to -43%. With only CO emissions (Case D) the O3 increase is smallest in all models, +0.4 to +5.9 DU.
This report adopts a weighted, rounded average of the changes in OH and O3 for cases A-D as shown in the bold rows in Table 4.11. The weighting includes factors about model formulation and comparison with observations. This sequence of calculations (Y2000 plus Cases A-B-C-D) allows us to define a simple linear relationship for the absolute change in tropospheric O3 and the relative change in OH as a function of the CH4 abundance and the emission rates for NOx, for CO, and for VOC. These two relationships are given in Table 4.11. Since the change in CH4 abundance and other pollutant emissions for Y2100-A2x are among the largest in the SRES scenarios, we believe that interpolation of the O3 and OH changes for different emission scenarios and years introduces little additional uncertainty.
The possibility that future emissions of CH4 and CO overwhelm the
oxidative capacity of the troposphere is tested (Case E, see Table
4.3 footnote c) with a +10% increase in CH4 on top of Y2100-A2x
(Case B). Even at 4,300 ppb CH4, the decrease in OH calculated by
two CTMs is only slightly larger than in Case A, and thus, at least for SRES
A2p, the CH4-feedback factor does not become as large as in the runaway
case (Prather, 1996). This report assumes that the CH4 feedback remains
constant over the next century; however, equivalent studies for the low-NOx
future scenarios are not assessed.
The apparent agreement on predicting the single global, annual mean tropospheric O3 increase, e.g., Case B in Table 4.11, belies the large differences as to where this increase occurs and what is its peak magnitude. The spatial distributions of the tropospheric O3 increases in July for Case B are shown in Figure 4.12 (latitude by altitude zonal average abundance, ppb) and Figure 4.13 (latitude by longitude column density, DU) for nine CTMs. The largest increase in abundance occurs near the tropopause at 40°N latitude; yet some models concentrate this increase in the tropics and others push it to high latitudes. In terms of column density, models generally predict large increases along the southern edge of Asia from Arabia to eastern China; although the increases in tropical, biomass-burning regions varies widely from model to model.
This similarity in the total, but difference in the location, of the predicted O3 increases is noted in Isaksen and Jackman (1999) and is probably due to the different transport formulations of the models as documented in previous CTM intercomparisons (Jacob et al., 1997). Possibly, the agreement on the average O3 increase may reflect a more uniform production of O3 molecules as a function of NOx emissions and CH4 abundance across all models. Nevertheless, the large model range in the predicted patterns of O3 perturbations leads to a larger uncertainty in climate impact than is indicated by Table 4.11.
The projected increases in tropospheric O3 under SRES A2 and A1FI will have serious consequences on the air quality of most of the Northern Hemisphere by year 2100. Taking only the global numbers from Figure 4.14, the mean abundance of tropospheric O3 will increase from about 52 ppb (typical mid-tropospheric abundances) to about 84 ppb in year 2100. Similar increases of about +30 ppb are seen near the surface at 40°N on a zonal average in Figure 4.12. Such increases will raise the “background” levels of O3 in the northern mid-latitudes to close to the current clean-air standard.
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