4.1. Key IAV Results and Possible Physical Driving Mechanisms
 Our inversion attributes the main features of the global CO2 flux variability for 1988–2003, especially the release of CO2 in 1997/1998 and the relative uptake of late 1991 to early 1993, to the tropics (Figure 4) and to the tropical land regions in particular, both with high significance. The variability in the extratropical north is 2 to 3 times less, though of even higher significance given the lower uncertainty there (Figure 5). The partition of the variability into latitudinal land and ocean totals is statistically robust, although marginally so for the tropical oceans and extratropical south. When the latitude bands are partitioned into individual continents and basins, the significance of the variability becomes marginal in the north, and is lost altogether in the tropics, except for the well-observed Tropical Pacific and Australasia. On the scale of the 22 individual regions (Figures 7 and 8), Tropical Asia, Europe, and (marginally) Temperate North America are robust on land, while for the oceans, the North, East, and South Pacific, the Northern Ocean, and (marginally) the Southern Ocean and Tropical Indian are significant.
 For these regions that we believe have significant variability (i.e., reflecting a real underlying flux signal, with a signal-to-noise ratio s ≥ 2 here), we may consider what physical or biological processes may be driving it. Our results for the East Pacific have a clear physical driver: the increased uptake before and during the 1997/1998 El Niño warm phase is consistent with the capping of the tropical thermocline then, which reduces the usual outgassing of CO2-rich waters. The increased CO2 release later agrees with the opposite condition, the increased shoaling of the thermocline there during the strong La Niña of 1999–2000. The 0.4 PgC yr−1 reduction in East Pacific outflux we obtain in 1997 compared to non-El Niño conditions agrees well with a similar reduction of about 0.4 PgC yr−1 estimated from Table 2 of Feely et al.  for that year based on in situ measurements scaled up to the broader equatorial Pacific. Our strong uptake during 1997/1998 in the South Pacific suggests that this ENSO-driven effect may act south of 15°S, as well.
 The highly significant variability in Tropical Asia is also physically interpretable: the timing and magnitude of the release of CO2 centered on early 1998 agrees well with the large Indonesian fires observed at that time, driven by ENSO-induced drought conditions (Page et al.  estimate a release of 0.81–2.57 PgC from the Indonesian fires between June 1997 and March 1998). The relative uptake of carbon there in 1992–1993 could be due to effects of the June 1991 Pinatubo eruption, though the net fluxes estimated in this analysis do not by themselves allow us to distinguish between the possible causes (decreased autotrophic or heterotrophic respiration, increased photosynthesis, decreased incidence of fires).
 Patterns of variability of equally high significance are found, however, for some regions (the North and South Pacific, and the Northern Ocean) for which there are no obvious driving processes. For weakly constrained regions with high variability like the South Pacific, having all the constraining measurement sites on one corner of the region may cause a sensitivity to the data that is not fully reflected in the a posteriori uncertainties. This may be the case for the Tropical Indian Ocean, as well: It is constrained largely by a single station at Seychelles. As noted in Appendix A, we have added an extra 1.0 ppm measurement uncertainty to the Seychelles site for 1988–1996 to account for measurement problems then. This value may still be too low: If this extra uncertainty is increased to 1.5 ppm across the same span, the variability for the Tropical Indian is reduced by a factor of 2 beyond what is shown in Figure 8. Further sensitivity studies are required to investigate such regions that return significant variability with our current uncertainty estimates, but for which no clear physical drivers have been found.
 The largest IAV estimated in our inversions outside of Tropical Asia, that for the four regions in South America and Africa, is not significant on the scale of the original regions, or when they are grouped into two continents: the estimation uncertainty for these regions is too high, because of the sparsity of measurement sites near them. If grouped into a single Africa/South America region, however, the variability is highly significant. If we understand that the sparsity of data in the tropics primarily impacts these two regions, then there is no need to disregard our tropical results altogether: We may discuss robust variability of the well-observed Tropical Asia, Australia, and East Pacific regions separately, and discuss the rest of the tropics grouped together as necessary to obtain significant results. With this approach, South America and Africa together account for almost as much uptake as Tropical Asia post-Pinatubo, and at least as much outgassing during the 1997/1998 El Niño.
4.2. Comparison to Previous Studies
 As the uncertainties in the 13-model mean IAV estimates discussed above are assumed here to be times lower than those in the IAV obtained using any one of the models (assuming Nind = 5; see Appendix B for details), the significance of the single-model IAVs is even less than those discussed above. Using the Nind/(1 + s2) = 0.2 column of Table 4 to test these single-model significances (as described in Appendix B), we find that only 3 of the 22 emission regions have robust IAV: the North and East Pacific, and (marginally) Europe. On the coarser scales, only the northern and tropical land IAVs, the northern and tropical total (land+ocean) IAVs, and (marginally) the global land/ocean partition are significant. Since previously published CO2 flux IAV studies have generally used measurement and a priori constraints of approximately equal or looser magnitude than those used here, one might expect our low single-model significances to apply to their results, too. To a large extent, this may explain why there has been so little agreement between the results obtained from the different studies.
 Bousquet et al.  (B00) have attributed the anomalous post-Pinatubo global uptake to the northern extratropical land regions, especially North America. We do find a sharp uptake spike in the combined North America region (boreal+temperate) of 0.5 PgC yr−1 here, but it is in 1994, too long after the June 1991 eruption to be directly related to it. Our uptake for Temperate North America is roughly 0.6 PgC yr−1 greater before 1995 than after, but is roughly constant across 1989–1995; no clear post-Pinatubo signal is seen. In our results, the feature in the north most likely to be related to the Pinatubo eruption is the 0.5 PgC yr−1 uptake event in Europe in 1992. Part of the explanation for the strong post-Pinatubo North American CO2 sink found in B00 may be due to the TM2 model they used. In our study here, the North American xIAV for the TM2 model was offset −0.4 PgC yr−1 across 1992–1994 compared to the 13-model mean. Our global land flux total looks broadly similar to theirs, but our global ocean total is quite different: B00 have a negative ocean excursion in 1995–1996 and a positive excursion in 1997/1998, while we have a large negative excursion in 1997 during the early edge of the El Niño. Our Tropical Pacific results also look broadly similar, with outgassing in 1989 trending toward uptake by 1997, but our tropical land totals show little resemblance to theirs: differences in station selection (we use the Tokyo-Sydney flight data, they use the South China Sea ship tracks) and the temporal coverage of the measurement time series may be especially important for this underconstrained area.
 Additional details on the B00 results have been given by Peylin et al. , including land and ocean totals for three broad latitude bands. In general, there is little agreement between our latitudinal land/ocean IAV results and theirs; this may be partially due to the somewhat tighter a priori uncertainties they apply to South America and Africa, which may drive some of the IAV out of the tropics into the north. Their results for the North Atlantic agree broadly with ours, however, including uptake there in 1995.
 The most thorough study of regional CO2 flux estimates for the full globe published to date is that of Rödenbeck et al. . They used fewer measurement times series than we do (from 16 to 35 CMDL sites in overlapping fixed networks across 1986–2000) and solved for more regions (∼800, at the ∼8° × 10° resolution of the model); thus more of an explicit a priori constraint was required (i.e., in terms of explicit correlations between neighboring regions in the a priori covariance matrix, rather than implicitly in the shape of the prespecified fluxes inside each region, as is done here). They used a priori uncertainties proportional to net primary production (NPP) over land, and flat uncertainty fields over the ocean, with interregion correlations given by exponential decay with e-folding lengths of 1275 km over land and 1912 km over the ocean. This choice of a priori constraint allowed the largest deviations from the prior where the local NPP was largest: over the tropical land regions, and over Amazonia in particular. Their strongest flux IAV was in fact obtained in the tropical land regions, especially South America, with less in the northern land and relatively little in the oceans (especially after 1996 when they used the most sites). Our global land IAV agrees well with theirs, though our ocean IAVs differ more. We obtain ocean uptake in 1997, for example, that they do not get. Interestingly, we both obtain a 1 PgC yr−1 ocean outgassing event in 1993–1994, though ours occurs a few months earlier.
 At the continent/basin scale, the only areas for which we clearly agree with Rödenbeck et al.  are in North America and Tropical Asia/Australia, both from the mid-1990s on. We find the same timing of outgassing around the 1997/1998 El Niño from Tropical Asia, but have a somewhat larger response; they place more of the tropical response at that time in South America than we do, perhaps due to the different constraint approach and the fewer number of tropical measurements used. On the scale of our original 22 regions, we show fair agreement for the IAV of Temperate North America going all the way back to 1991, and good agreement in Tropical Asia and Europe back to 1996. Interestingly, these are also the only land regions for which we feel our IAVs are significant. It is perhaps also significant that the agreement in Tropical Asia and Europe drops off before 1996, when Rödenbeck et al. go from using 35 to 26 sites. For the ocean regions, we have very good agreement for the Tropical Indian ocean across the full span, but little similarity elsewhere, because of the very low IAV they obtain. In particular, there is almost no similarity in the East Pacific, a region where we feel we are obtaining physically meaningful results. Given that the a posteriori uncertainty estimates presented by Rödenbeck et al. are generally higher than ours for similarly sized regions, we suspect that the IAVs for at most only a few of their regions would be considered significant according to the χ2 test performed here, even if lower transport uncertainties more appropriate for a model driven by more accurate analyzed winds were considered. Thus the disagreement between most of our estimates is actually in agreement with the low measures of significance given by our χ2 test. The agreement that we do have occurs only for those regions with significant IAV according to the χ2 test.
 Our finding that the tropical/southern land biosphere is driving the largest features in the global flux IAV is supported by the latitudinal CO2-13C inversions of Piper et al. [2001a, 2001b] and John Miller (personal communication, 2004), which attribute the global CO2 release of 1997/1998 to the tropical land biosphere, as well as obtaining a secondary release peak in 1995 from the same region. These inversions do not seek to separate the land and ocean areas by geographical region, but rather in terms of land and ocean flux totals inside each latitude band using 13C. Despite this agreement, our inversion emphasizes the difficulty of separating the land and ocean by region inside a given latitude band using the CO2 data alone.