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On the basis of projected losses of their essential sea-ice habitats, a United States Geological Survey research team concluded in 2007 that two-thirds of the world's polar bears (Ursus maritimus) could disappear by mid-century if business-as-usual greenhouse gas emissions continue (1-3). That projection, however, did not consider the possible benefits of greenhouse gas mitigation. A key question is whether temperature increases lead to proportional losses of sea-ice habitat, or whether sea-ice cover crosses a tipping point and irreversibly collapses when temperature reaches a critical threshold (4-6). Such a tipping point would mean future greenhouse gas mitigation would confer no conservation benefits to polar bears. Here we show, using a general circulation mode (l7), that substantially more sea-ice habitat would be retained if greenhouse gas rise is mitigated. We also show, with Bayesian network model outcomes, that increased habitat retention under greenhouse gas mitigation means that polar bears could persist throughout the century in greater numbers and more areas than in the business-as-usual case (3). Our general circulation model outcomes did not reveal thresholds leading to irreversible loss of ice (6); instead, a linear relationship between global mean surface air temperature and sea-ice habitat substantiated the hypothesis that sea-ice thermodynamics can overcome albedo feedbacks proposed to cause sea-ice tipping points (5,6,8). Our outcomes indicate that rapid summer ice losses in models (9) and observations (6,10) represent increased volatility of a thinning sea-ice cover, rather than tipping-point behaviour. Mitigation-driven Bayesian network outcomes show that previously predicted declines in polar bear distribution and numbers (3) are not unavoidable. Because polar bears are sentinels of the Arctic marine ecosystem (11) and trends in their sea-ice habitats foreshadow future global changes, mitigating greenhouse gas emissions to improve polar bear status would have conservation benefits throughout and beyond the Arctic (12)., Polar bears are dependent on the sea ice for access to their marine mammal prey (13,14), and occur only in Northern Hemisphere marine areas that are ice covered for long [...]

Seasonally averaged 200-mb circulations for recent winters (1987/88 and 1988/89) that represent opposite phases of El Nino and a zonal-mean zonal flow index cycle are diagnosed using data assimilated by the Goddard Earth Observing System (GEOS) and operational analyses of the European Centre for Medium-Range Weather Forecasts (ECMWF). The comparison is undertaken to determine whether there are significant differences in the 200-rob vorticity dynamics implied by the mean meridional circulations in the two datasets and whether these differences can be related to the Incremental Analysis Update (IAU) method used in the GEOS assimilation. The two datasets show a high degree of similarity in their depictions of the large-scale rotational flow, but there are substantial differences in the associated divergent circulations. For the zonal-mean flow, the zonal winds are substantially the same, but the meridional wind in the Tropics and subtropics is considerably weaker in the GEOS assimilation than its counterparts in both the ECMWF data and the GEOS analyses used to produce the assimilation. The authors examine the assimilation of the Hadley circulation using a zonally symmetric f-plane model. For this model, the IAU method easily assimilates the rotational flow but fails to assimilate the divergent circulation. This deficiency of the IAU method may explain the weakness of the Hadley cell in the GEOS assimilation, at least in relation to the GEOS analysis. For this simple model, an alternative assimilation method, based on constraints imposed by the analyzed potential vorticity and mean meridional circulation fields, is proposed that simultaneously assimilates both rotational and divergent flow components. Barotropic modeling suggests that an accurate representation of mean meridional flow anomalies can be important for the diagnosis of both zonal-mean and eddy rotational flow perturbations, particularly during extreme phases of the zonal-mean zonal flow fluctuation.

The contribution of the interaction between tropically forced circulation anomalies and the extratropical mountains in the generation of extratropical circulation anomalies during the 1987/88 and 1988/89 winter seasons is diagnosed using a divergent barotropic model that solves for both the zonal-mean and eddy components of the 200-mb rotational anomalies. Barotropic modeling shows that the orographic modulation of the rotational response to the 200-mb tropical divergence anomaly can be substantial over the Pacific-North American region. ---The modulation consists of a large-scale wave pattern with a ridge in the central subtropical Pacific, a trough over the Gulf of Alaska, and a weak ridge extending across North America from Baja California to Greenland. These features have an amplitude of ;40 gpm, and the orographic modulation is thus about one-third as strong as the primary wave pattern. --- The associated 200-mb zonal wind is strongest (;5 m s 21 ) in the vicinity of the eastern end of the East Asian jet, thus contributing to the southeastward jet extension during El Nino winters. ---The Himalayan-Tibetan complex is the major locus of orographic interaction in the model, for it alone accounts for all the features and over two-thirds of the amplitude modulation. --- The eddy and zonal-mean parts of the tropically forced flow anomalies make comparable contributions to orographic modulation. However, the midlatitude eddy anomalies themselves result, in part, from the interaction of the zonal-mean zonal wind anomaly and the climatological vorticity gradients, that is, from "zonal-eddy" interaction. The strength of this interaction depends on the arbitrarily specified distribution of the compensating zonal-mean subsidence in the model. These findings indicate the potential importance of secondary orographic interaction in the generation of extratropical circulation anomalies in response to tropical heating anomalies. Experiments with more compl. [ABSTRACT FROM AUTHOR]