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PERMAFROST ZONATION AND CLIMATE CHANGE IN THE NORTHERN HEMISPHERE: RESULTS FROM TRANSIENT GENERAL CIRCULATION MODELS OLEG A. ANISIMOV State Hydrological Institute, 23 Second Line, St. Petersburg, 199053, Russia

FREDERICK E. NELSON Department of Geography and Planning, State University of New York at Albany, Albany, New York 12222, U.S.A.

Abstract. Numerous studies have demonstrated that both global patterns and local details of permafrost distribution are highly responsive to climatic fluctuations, at several temporal and spatial scales. Permafrost currently underlies about one fourth of the land area of the northern hemisphere, and many qualitative predictions have been made for a severe reduction of this area in response to global warming. A map of permafrost distribution compiled using the ‘frost index’, a dimensionless number that can be related to the zonal arrangement of permafrost, shows very good correspondence with a recently published empirical map. The frost index was used in conjunction with three transient general circulation models to compile maps of permafrost zonation for conditions in the mid-21st century. Although regional patterns and local details differ substantially between the three scenarios, all result in reductions in the area occupied by each permafrost zone. Localized expansions of the area underlain by permafrost are apparent from two of the scenarios. Reductions in the areal extent of equilibrium permafrost predicted from two of the three transient models are much less than those indicated by runs using 2 CO models.



1. Introduction Models of anthropogenic climate change predict substantial warming during the next century in response to increased atmospheric concentrations of greenhouse gases. Although highly divergent results have been obtained from different models (e.g., Budyko et al., 1989; Cess et al., 1990; Randall et al., 1992; Gutowski et al., 1991), there has been general agreement among the various formulations that warming will be amplified in the high-latitude regions (Budyko and Izrael, 1987; Maxwell and Barrie, 1989; Roots, 1989). Recent work (Walsh and Crane, 1992; Bromwich et al., 1994; McGinnis and Crane, 1994) has revealed substantial shortcomings in the ability of existing general circulation models to replicate several features of the Arctic climate, however, and further refinements are likely to appear through high-resolution modeling with a specifically Arctic focus (Lynch et al., 1995). Depending on regional climate and local biologic, topographic, and edaphic parameters, pronounced warming in the high latitudes of the northern hemisphere could lead to warming, thinning, or disappearance of permafrost in locations where it now exists (Nelson et al., 1993). Evidence of spatially extensive episodes of Climatic Change 35: 241–258, 1997. c 1997 Kluwer Academic Publishers. Printed in the Netherlands.

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permafrost thawing is abundant in the geological record (e.g., P´ew´e, 1983; Velichko and Nechaev, 1992; Ballantyne and Harris, 1994), and evidence that poleward contraction is presently occurring at regional scales has been documented (Thie, 1974; Kwong and Gan, 1994; Halsey et al., 1995). Predictions about the future distribution of permafrost over areas of continental dimensions employ schematic scenarios of global warming based on (a) paleoclimatic reconstructions of particular warm episodes in the Earth’s history, used as analogs to future climate (Anisimov, 1989; Nelson and Anisimov, 1993); or (b) the results of general circulation models for conditions of atmospheric carbon dioxide doubling (Stuart, 1986; Anisimov and Nelson, 1996). Owing to limitations of the climatic scenarios used in these studies, none of them is bound to a particular time interval in the future, making estimation of permafrost degradation in real time difficult. A new class of transient general circulation models has recently come into use that appears to give more realistic results than the earlier versions (Gates et al., 1992; Greco et al., 1994). In this paper we compare contemporary permafrost distribution in the northern hemisphere, calculated using the ‘frost-index’ approach, with a recent map based on empirical data (Heginbottom, 1994). Selected transient GCMs are used, in conjunction with this climate-based permafrost model, to examine changes in permafrost distribution in the northern hemisphere that may be underway by the middle of the next century. The model’s projections are used to delineate areas within which future permafrost degradation may occur in the northern hemisphere.

2. Permafrost and Carbon Storage Much of the permafrost in existence today is known to be in thermal imbalance with the present climate (e.g., Lachenbruch and Marshall, 1986). Because large quantities of carbon are sequestered in the permafrost of boreal peatlands and tundra regions (Billings, 1987; Gorham, 1991; Kolchugina and Vinson, 1993, Vompersky, 1994; Botch et al., 1995), changes in the distribution of frozen ground are likely to be an important control over feedback effects, and ultimately on the magnitude and rate of further warming. A substantial proportion of the carbon pool in tundra regions is contained in the near-surface layer of permafrost (Ping, 1996), and a systematic increase in the thickness of the seasonally thawed layer could release large amounts of CO2 to the atmosphere. Large areas of the subarctic are underlain by relict permafrost, formed during cold intervals of the Pleistocene and no longer in thermal equilibrium with the surface climate. Gradual disappearance of such permafrost will also lead to major changes in carbon fluxes. Estimates of the amount of CH4 sequestered in permafrost are highly variable (Kvenvolden and Lorenson, 1993; Moraes and Khalil, 1993; Rasmussen et al., 1993) and observations are few in number, although recently reported work in Siberia (Fukuda, 1994) suggests that the flux to the atmosphere may be substantial.

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Crude estimates of the amount of CH4 sequestered in permafrost have been made by extrapolating observed amounts via maps of the areal extent of permafrost. Besides the extreme limitations imposed by data availability, this approach suffers from reliance on maps that are in some cases constructed using arbitrary criteria, which are in turn inadequately documented. Comparisons of groups of such maps covering the eastern third of Canada (Nelson, 1989) and the territory of the Former Soviet Union (Nelson and Anisimov, 1993) showed that, even when pairings are made between maps with similar publication dates, discrepancies in the position of the ‘boundaries’ of permafrost zones involve distances of hundreds of kilometers. The implications of errors of such magnitude for estimating the contribution of highlatitude regions to the world carbon pool are serious. Maps showing the distribution of permafrost under scenarios of climate change have also been published recently (e.g., Woo et al., 1992; Chernyad’yev and Chekhovskiy, 1993), but details about criteria used in their construction were not provided.

3. Permafrost Mapping 3.1. THE FROST INDEX The problems outlined above illustrate the need for an objective, replicable method of mapping permafrost that can be linked unambiguously to climatic parameters. The concept used in this study to predict permafrost distribution is based on a ‘frost index’, described by Nelson and Outcalt (1987). The index provides a replicable vehicle for regionalization of permafrost that is in equilibrium with the widely available climate data used to drive it. Although the frost index implicitly treats climate change as a step function, it can be used in conjunction with climate-change scenarios to investigate the relatively fast reaction of the uppermost permafrost layer to global warming (Stuart, 1986; Anisimov and Nelson, 1993). The latitudinal zonation of permafrost is usually defined on the basis of its areal continuity or discontinuity. Within such classifications the ‘boundaries’ between different permafrost types are fuzzy; cartographic lines separating areas of continuous and discontinuous permafrost are actually transitional in nature, a fact particularly important to recognize when mapping has been performed on the basis of empirical data. Most classifications specify that permafrost is ‘continuous’ when more than 80–90% of an area of interest is underlain by permafrost; permafrost is said to be ‘extensive’ or ‘sporadic’ when percentages are lower (e.g., Harris, 1986; Kudryavtsev et al., 1980; Heginbottom et al., 1993). At the equatorward margins of the permafrost regions, climatic warming could induce thickening of the active layer in areas that continue to be underlain by permafrost. In seasonally frozen soils the depth of winter freezing relative to that of summer thaw may decrease. An important metric in polar and subpolar regions is, therefore, the ratio of the potential depths of seasonal freezing and thawing;

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wherever this ratio exceeds unity for two or more consecutive years permafrost is present, by definition. Conversely, where the ratio is less than 1.0, only seasonal frost is expectable unless relict permafrost is present. The frost/thaw ratio forms the basis for the frost index, which, when normalized to the interval [0,1], can be used to approximate the boundaries of different permafrost types. The normalized ‘surface frost index’ is defined as

F+

=

1=2

1=2

DDF+ =(DDT1=2 + DDF+

);

(1)

where DDF+ represents the annual degree days of freezing, adjusted for the effects of snow cover, and DDT is the annual thawing index (degree days of thawing). The degree-day sums are integrated numerically from monthly air temperature series. The thermal adjustment for snow-cover effects is made with a negative exponential expression using computed values of winter snow depth, density, and thermal diffusivity. An average winter snow cover Z s is computed from monthly precipitation statistics as

Zs

=

sin  2

(X k i=1

)

[(Pi =r )(k

i + 1)]=k

;

(2)

where Pi is water-equivalent precipitation for the months i (i = 1, 2, : : : , k ) in which the mean temperature is 0  C or less, r is relative snow density (assumed constant in this analysis), and  is the latitude of the location under consideration. Snow falling early in winter is assigned a greater effect by Equation (2) than is snow arriving in midwinter or spring. The trigonometric term specifies that thaws are shorter and less pronounced with increasing latitude. Complete computational details of the frost index are given in Nelson and Outcalt (1987). Values of the surface frost index used to represent the threshold boundaries of continuous, extensive discontinuous, and sporadic discontinuous permafrost zones are, respectively, 0.67, 0.60, and 0.50. These values were derived by Nelson and Outcalt (1987), on the basis of extensive calculations involving a wide variety of soils by Brown (1964). Sensitivity analyses of the model were performed by Anisimov and Nelson (1996) in the context of climatic change. In much of the subarctic, where permafrost is thin and concentrated in peatlands, the frost index is adequate to assess generalized changes in the zonal arrangement of permafrost at the spatial and temporal scales considered in this paper. Simulation work by Riseborough and Smith (1993) indicated that 5 m of marginal permafrost at a location in subarctic Canada would thaw in less than 70 years under the warming scenario predicted by the Canadian Climate Centre GCM. In areas with thick, ice-rich permafrost such as West Siberia, however, the effects of latent heat (Riseborough, 1990) would cause retreat of geocryological ‘boundaries’ to be very gradual. The applicability of the frost index to problems involving short-term climate change is limited in the latter kind of area, owing to its implicit assumption of climatic stationarity. Mapping the short-term migration of permafrost at regional

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or local scales in such situations will necessarily involve explicit treatment of the permafrost thermal regime. Maps produced using the frost index are, therefore, most effective at depicting large (continental or hemispheric) areas, and are best viewed as representing potential permafrost distribution. A complementary viewpoint is that they facilitate estimation of permafrost distribution involving only the upper layer of the substrate, which is of major importance for most practical applications in engineering (Andersland and Ladanyi, 1994; Williams, 1995) and ecology (Chapin et al., 1992; Reynolds and Tenhunen, 1996). By delimiting the contemporary zonal boundaries on maps of permafrost compiled using climatechange scenarios, a ‘zone of relict permafrost’ is created implicitly; this concept is consistent with the scale and level-of-measurement conventions employed on the majority of small-scale (e.g., hemispheric) permafrost maps. Anisimov and Nelson (1996) used several models of climatic change, scaled to 2  C warming, to drive the frost-index model. These runs included the GFDL, GISS, and UKMO GCMs and the Holocene paleoreconstruction of M. Budyko (Budyko and Izrael, 1987). Results from these analyses indicated a severe impact on the zone of continuous permafrost, with reductions of its areal extent ranging between 29% and 67%. The total areal reduction of the permafrost region in these experiments was from 25% to 44%. Each run produced a north-northeastward displacement of all permafrost zones; no equatorward movement was indicated in any region. 3.2. CONTEMPORARY PERMAFROST The performance of the permafrost model and the consistency of the computed permafrost distribution with empirical data is demonstrated by the two maps shown in Figure 1. The simplest version of the permafrost model requires only two input parameters, mean monthly air temperatures and precipitation for stations located in the region of interest. Values of these parameters for the modern climate were taken from the Global Ecosystem Database (Leemans and Cramer, 1991), which was constructed using climatic data from 13,118 weather stations in both hemispheres, and provides relatively high spatial resolution (0:5  0:5 degrees of latitude and longitude). Very good agreement is apparent between the frost index (Figure 1A) and empirical (1B) maps, a result consistent with a similar application for a much smaller region (Nelson, 1986; 1990). A semi-quantitative comparison of the maps was made by reducing the classification to two categories (continuous and discontinuous) and recording the class into which each GCM node fell. Of the total of 211 points falling in the permafrost zones of the theoretical map, only 20 were in disagreement with the empirical, yielding an overall agreement between the maps of slightly more than 90%. Minor divergences can be explained by the uneven distribution of weather stations and difficulties associated with spatial interpolation in mountainous regions, as well as variations in the quality and geographic coverage

Figure 1. (A) Permafrost zonation in the northern hemisphere, derived from the ‘frost index’, using data contained in the Global Ecosystem Database (Leemans and Cramer, 1991). Threshold values for the sporadic, extensive, and continuous permafrost zones are frost-index values of 0.5, 0.6, and 0.67, respectively. (B) Permafrost zonation in the northern hemisphere, compiled through interpolation of observational data (from Heginbottom, 1994). Figure 1B is a generalized, preliminary version of the International Permafrost Association’s map of permafrost in the northern hemisphere, which portrays the relative abundance of ground ice (Brown, 1992).

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of data in the empirical database and differences in the definition of permafrost continuity (see Nelson, 1989; Nelson and Anisimov, 1993, pp. 143–145). Given the radically different modes of map compilation, this high degree of consistency suggests that the frost index is an appropriate vehicle for assessing broad changes in permafrost distribution.

4. Transient Scenarios of Climate Change The impact studies discussed earlier were based on climatic scenarios that treated climate change as a step function. Within the framework of the paleoclimatic method, the reconstructed equilibrium climate of the previous warm epoch is the only one used to derive conclusions about the possible effects of warming. General circulation models are often employed to calculate two equilibrium climates: (a) for modern conditions and (b) for conditions of atmospheric CO2 doubling. The problem of transient climate change is beyond the scope of both of these methods. The situation has changed recently with the appearance of a new generation of transient GCMs. As in earlier versions, the main driving factor is the concentration of atmospheric CO2 . In the transient models, however, the CO2 concentration changes gradually in time according to a prescribed scenario of gas emission, which is of critical importance for the performance of the model. Significant progress in understanding transient climate change has been achieved in studies carried out under the auspices of the Intergovernmental Panel on Climate Change (IPCC) program. This work involves the cooperative efforts of scientists in the areas of climate change and its impacts on the environment (Houghton et al., 1990, 1996; Greco et al., 1994). Special attention in these studies was given to specific problems associated with climate change predictions derived from transient GCMs. Following Greco et al. (1994), these problems include: (a) CO2 emission scenarios. The transient GCM experiments used different CO2 concentration scenarios to drive the respective models. Any given CO2 concentration is, therefore, reached in different years in the integrations of the various models. (b) Climate sensitivity. The models give different estimates of the globally averaged annual air temperature sensitivity to doubling of the atmospheric CO2 concentration. These estimates are quite divergent, ranging from 1.3 to 2.3 degrees. (c) The ‘climate drift’ problem. The control integrations of the models produce values of globally averaged air temperature noticeably different from the empirical record for the modern climate. (d) The ‘cold start’ problem. Model integrations begin at a quasi-equilibrium state of modern climate, whereas the real atmosphere has been experiencing radiative forcing from anthropogenic CO2 emissions since the end of the previous century, making it inappropriate to assign a given year in the simulation to an actual calendar year.

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These factors complicate interpretation of transient GCM results. Applications of transient scenarios to environmental impact studies were very limited until recently, when an original approach for minimizing these difficulties was suggested by the IPCC Working Group II on climate scenarios. This ‘simple linked method’ (Greco et al., 1994) ties the full three-dimensional GCMs to a fast one-dimensional climate model in such a way that this simpler model, with given sensitivity of mean global temperature to atmospheric CO2 concentration, starts with a pre-industrial climate and produces a realistic rate of global climate change. The rate of climate change is used further to adjust the GCMs, thus harmonizing the model’s results for different temporal intervals. The ‘linked’ method was used in conjunction with three transient GCMs to simulate the climate for a decade around 2050. The models used are (1) the GFDL89 model of the Geophysical Fluid Dynamic Laboratory (Manabe et al., 1991); (2) the ECHAM1-A model of the Max Planck Institute in Germany (Cubasch et al., 1992), and the UKTR model of the United Kingdom Meteorological Office (Murphy, 1994; Murphy and Mitchell, 1994). More computational details can be found in the original publications, as well as the compendium from which the model results were obtained for this study (Greco et al., 1994). These models and associated climate-change experiments are particularly well documented, and were selected to provide a standard context for comparative impact studies (Greco et al., 1994, p. 2). Transient models continue to evolve rapidly, however, and impact studies based on these scenarios are likely to be modified substantially when repeated with updated versions of the GCMs. A recent publication by Mitchell et al. (1995), for example, reported that inclusion of anthropogenic sulphate aerosols in a coupled ocean-atmosphere model substantially reduced the magnitude of the simulated warming in northern midlatitude continental locations. Forcing by anthropogenic aerosols is regionally heterogeneous, and may have important effects on temperatures in the Arctic (Blanchet, 1995), although the uncertainties surrounding this topic are large (Schneider, 1994; Santer et al., 1996; Schwartz and Andreae, 1996). A comprehensive review of climate-change modeling, including the effects of radiative forcing by atmospheric aerosols, is contained in the volume edited by Houghton et al. (1996). The frost-index permafrost model was used, in conjunction with modern climatic data and the three transient scenarios of climate change, to investigate potential changes in the distribution of permafrost caused by global warming by the middle of the next century. The coarse resolution of the GCMs was improved through use of high-resolution supplementary data for modern climate (Leemans and Cramer, 1991). Values of future temperature and precipitation were calculated at each node of a 0:5  0:5 grid as sums of their modern values and projected increments derived from the closest node of the GCM.

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Table I Area (km2 106 ) of terrestrial northern hemisphere occupied by each permafrost zone. Values in parenthesis indicate percentage of contemporary values



Model

All zones

Continuous

Extensive

Sporadic

Contemporary GFDL89 ECHAM1-A UKTR

25.5 (100) 22.4 (88) 22.4 (88) 19.8 (78)

11.7 (100) 10.3 (88) 10.3 (88) 7.8 (66)

5.6 (100) 4.9 (87) 4.8 (86) 4.7 (85)

8.1 (100) 7.2 (88) 7.3 (90) 7.3 (90)

5. Results 5.1. CHANGES IN PERMAFROST ZONATION The projected distribution of permafrost in the northern hemisphere produced by the three climate-change scenarios is shown in Figure 2. Different panels correspond to transient climatic scenarios for 2050 derived from the GFDL89 (Figure 2a), ECHAM1-A (Figure 2b), and UKTR (Figure 2c) models. The ‘simple linked’ method was used to harmonize the transient GCMs, and to adjust their results to a mid-range climate sensitivity of 2.5  C global temperature increase to the year 2100 under the standardized CO2 emission scenario, as suggested by Greco et al. (1994). The rate of climatic change was estimated using a one-dimensional climate model, which produced a 1.16  C mean global air temperature increase for 2050, relative to 1990 (Wigley et al., 1991; Wigley and Raper, 1992). All three maps predict a significant reduction of the area underlain by permafrost in the northern hemisphere. Table I provides estimates of the area occupied by each of the three permafrost zones within the circumarctic region, for contemporary conditions and for the three GCM scenarios. Results obtained for North America are in better agreement than those for Eurasia, where the three climate scenarios diverge substantially in depicting patterns of permafrost distribution. The UKTR scenario results in the most extreme reduction of permafrost, while both GFDL89 and ECHAM1-A produce only small changes in the permafrost distribution of Eurasia, the latter scenarios even predicting some expansion of areas underlain by sporadic permafrost in Siberia (compare permafrost areas with the southern limit of modern permafrost, depicted by a solid line in Figure 2). Except for the continuous zone under the UKTR scenario, the reduction in the dimensions of all zones is smaller than that suggested by several 2  CO2 GCMs (Anisimov and Nelson 1996). 5.2. REGIONAL DETAILS The regional detail apparent in Figure 2 is a significant departure from other depictions of the impact of climatic warming on the distribution of permafrost (e.g.,

Figure 2. Zonation of equilibrium (near surface) permafrost in the northern hemisphere by the middle of the 21st century, predicted using data from the Global Ecosystem Database (Leemans and Cramer, 1991) and transient GCM output: (A) GFDL89; (B) ECHAM1-A; (C) UKTR. Data representing GCM runs were obtained from Greco et al. (1994). Threshold values on frost index scale are identical to those of Figure 1. Solid line depicts position of contemporary southern limit of permafrost as shown in Figure 1; area between this and stippled pattern can be regarded as a ‘zone of relict permafrost’.

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Woo et al., 1992; Chernyad’yev and Chekhovskiy, 1993), which show simple and nearly uniform latitudinal offset of the zonal boundaries. Although examination of Figure 2 reveals subtle reconfigurations of the zonal boundaries, in some cases leading to local southward expansions, strict comparison of minor detail is inadvisable owing to differences in the spatial resolution of the three GCMs (Greco et al., 1994, Table I). Discussion in this section is limited, therefore, to major shifts in the positions of zonal boundaries, and to significant changes in the area underlain by permafrost within specified areas. Names for specific regions are made with reference to Heginbottom’s (1994) map Permafrost and Ground Ice Regions of the Northern Hemisphere, which appears as Figure 1B in this paper. For brevity, these regions are also referred to in places by the numbers given them in Heginbottom’s map.

5.2.1. GFDL89 North America: Results from this run show poleward retreat of the southern boundary of equilibrium permafrost throughout North America. This contraction is most pronounced east of James Bay (Region 4), where evidence of climaterelated permafrost degradation exists, although localized aggradation of permafrost has also been documented (Michaud et al., 1995). A general northward retreat of the sporadic zone is also apparent throughout central Canada, although scattered patches of several thousand km2 remain in the Interior Plains (Region 3). These changes are induced by increased winter temperature, accompanied by substantially heavier winter precipitation. Farther west, the position of the southern margin is offset more selectively in Alaska. Conversely, the extensive discontinuous permafrost zone is little affected in this scenario within central Canada (Region 4), except in the area east of Hudson Bay, where it separates into two distinct areas, centered on the Labrador and Ungava Peninsulas. This situation appears to reflect similar temperature increases in summer and winter in the western sector, and warming occurring primarily in summer to the east. A major contraction of the extensive discontinuous zone is apparent in the west-central portion of Alaska, except in the extreme northern reaches of the Seward Peninsula. An ‘island’ falling in the sporadic class develops just east of the 150th meridian. As with the extensive zone, the zone of continuous permafrost shows little net change in the central regions (3 and 4) and eastern Arctic (Region 11) of Canada. The exceptions are the Labrador and Ungava Peninsulas, where outliers of this zone shrink conspicuously. The North American Cordillera (Region 6), shown at the circumpolar scale as an unbroken, southeastward-trending extension of the continuous zone on the maps of contemporary permafrost (Figure 1), falls for the most part into the extensive discontinuous category under the GFDL89 scenario, with only the higher elevations experiencing severe enough conditions to appear as areas of ‘continuous’ permafrost. The position of the continuous zone is little

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changed in northern Alaska, a realistic prediction in light of the climatic barrier formed by the Brooks Range (Stretin, 1973). Eurasia: Of the three GCMs, permafrost distribution in Eurasia is closest to that shown for contemporary conditions in Figure 1. Permafrost is absent in the eastern reaches of the European North province (Region 17), in an area where it is shown as sporadic on both contemporary maps. Poleward movement of the southern boundary of the sporadic zone is also evident in Mongolia (Region 24) and Northeast China (Region 25), as well as in Kamchatka. As in North America, the extensive discontinuous zone in Eurasia undergoes considerable contraction in its western portion but, except for local detail, is little changed in the continental interior of Asia. The alignment of the southwesterntrending extension of this zone in the Southern Siberia/Trans-Baikal region (23) is unchanged, although its area is diminished somewhat. Outliers of this category, prominent on the contemporary map (Figure 1A), have disappeared entirely from Kamchatka. Far more striking than local changes in the position of the continuous zone is the apparent stability of its geographical position throughout nearly its entire extent in Eurasia, a result that agrees with the conclusions of Vyalov et al. (1993). The GFDL89 scenario is also consistent in this respect on a circumarctic basis, by showing only minor changes in the overall spatial pattern of the continuous permafrost zone. This is a major departure from results obtained with scenarios produced by the climatic paleoreconstruction and 2  CO2 GCMs (Nelson and Anisimov, 1993; Anisimov and Nelson, 1996). 5.2.2. ECHAM1-A North America: The ECHAM1-A scenario occupies an intermediate position with respect to the contraction of permafrost regions produced by the three climate models. There is substantial diminution of the area occupied by the sporadic discontinuous zone in North America; the contrast between results obtained under this scenario and those from the GFDL89 model are particularly pronounced in Alaska, where the southern limit of equilibrium permafrost retreats hundreds of kilometers northward from the position shown in Figure 1A. The poleward displacement of the southern boundary is also substantially greater in the Interior Plains (Region 3) and Cordilleran (Region 6) areas of central Canada than is the case for GFDL89. The patterns formed by the extensive and continuous zones west of Hudson Bay are similar to those from GFDL89, except that the Cordilleran region north of 60 N falls primarily in the extensive category. East of Hudson Bay, zonal displacements are smaller in the ECHAM1-A scenario than in the other two, although the area occupied by the extensive discontinuous zone is again bifurcated into separate areas in the Labrador and Ungava peninsulas. Eurasia: Changes of the zonal patterns in Eurasia are more pronounced under the ECHAM1-A scenario than are those from GFDL89. Permafrost has disappeared from the western part of the European North sector (Region 17) and Kamchatka,

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and retreat from the contemporary southern limit of equilibrium permafrost is much more in evidence in the Mongolian and North China regions (24 and 25). In West Siberia (Region, 19), however, the southern limit of permafrost remains largely unchanged, as is the case for the GFDL89 scenario. This result is of great interest owing to the ubiquity, thickness, and potential sensitivity of ground ice in this region (e.g., Parmuzin and Shamonova, 1985; Zemtsov and Shamakhov, 1992; Mahaney et al., 1995). Changes in the configuration and extent of the extensive and continuous zones, however, are the most striking aspect of the Eurasian portion of Figure 2B, and reflect large increases in winter temperature. The south- and westward extension of these zones into southern Yakutia (23), so characteristic a feature on all maps of contemporary permafrost (see Nelson and Anisimov, 1993), has disappeared entirely; in its place is only the sporadic discontinuous category. Further west, (around the 90 meridian) areas occupied by the extensive and continuous categories are expanded slightly, and the limit of the sporadic discontinuous zone has moved south- and westward, substantially beyond its limit on the contemporary map. Moreover, eastward to the 120th meridian, the continuous permafrost zone has become far less coherent than on the GFDL89 document, having degenerated from a nearly zonal configuration into a series of areal units connected by narrow corridors. This reduction in the extent of the continuous zone is compensated by a southward expansion to nearly 60 N just east of 90 E; the net effect is a reduction in the area of this zone by an amount virtually identical to that produced under the GFDL89 scenario (Table I). 5.2.3. UKTR North America: Of the three climate scenarios, the UKTR model (Figure 2C) yields the largest reduction in the overall extent of permafrost in North America. The poleward retreat of the southern limit of permafrost evident from the ECHAM1-A scenario is nearly matched in Alaska and central Canada, but is far more pronounced east of James Bay. The extensive discontinuous zone contracts severely in Alaska and in the Canadian Shield west of Hudson Bay. The continuous zone contracts more than for the other scenarios in the Canadian Eastern Arctic and Shield (Region 4) on both sides of Hudson Bay. A substantial increase in the area occupied by discontinuous permafrost is also evident in southwestern Greenland. Eurasia: Of the three scenarios, the UKTR model also produces the most profound shrinkage of zonal areas in Eurasia. The southern limit of equilibrium permafrost retreats several hundred kilometers throughout the region, reflecting a large increase in summer temperature. The only notable exception to this is the Southern Siberia/Trans-Baikal area, where a localized expansion occurs. As with the ECHAM1-A scenario, the southwest-trending wedge of extensive and continuous permafrost disappears in this region, to be replaced by the sporadic category. Unlike the other scenarios, UKTR predicts the onset of a climatic regime that is not conducive to the maintenance of permafrost over extensive areas of West

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Siberia (Region 19). Such a development would have very serious ramifications to engineered works in this region, owing to the extensive area underlain by massive ground ice. The continuous zone assumes an even more complex configuration than that produced under the ECHAM1-A scenario, and its area is reduced to approximately two-thirds that of its extent under contemporary conditions. The reduction of the extent of permafrost in the region of the Tibet Plateau is also greater than for the other two scenarios.

6. Conclusions Climate scenarios based on transient general circulation models suggest major changes in the distribution of permafrost in the northern hemisphere. Although a general poleward movement of permafrost zones is apparent in each of the three scenarios used, details of the configuration of these zones differ appreciably. Localized expansion of the sporadic permafrost zone is suggested for central Asia by two of the scenarios. An extensive, nearly circumpolar zone of relict permafrost develops in each scenario, but only one (UKTR) predicts that the West Siberian Plain, an environmentally sensitive area with a high incidence of massive ice formed during the Pleistocene, would begin to experience a significant impact by the middle of the 21st century. Results obtained from the transient climate models employed in this study generally suggest a more restricted poleward retreat of the permafrost zones than those derived from 2CO2 models. Unlike results obtained with the latter group of GCMs (Anisimov and Nelson, 1996), there is also considerable variation in the form of the zonal configuration between the transient scenarios. The poleward retreat of zonal boundaries obtained from the experiments with transient models is much smaller than that obtained with climate-change scenarios based on paleoreconstruction (Nelson and Anisimov, 1993). The results obtained in this experiment represent only an initial step in evaluating the effects of global warming on the circumarctic distribution of permafrost. Use of more recent general circulation models that include the effects of anthropogenic aerosols is likely to alter the arrangement of permafrost zones and will provide more regional detail. The equilibrium model of permafrost used in this study is also a limiting factor, although impact studies in all topical fields relating to global change pass through such a stage (Monserud and Leemans, 1992). However, it is important to realize that the current distribution of permafrost is not the result of a long period of climatic stability (e.g., Vitt et al., 1994); results from the model have a very high degree of correspondence with those obtained on an empirical basis. The authors are currently involved in integrating a dynamic model of subsurface thermal evolution into the framework of investigations concerned with the impact of climate change on permafrost distribution.

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