Simulation of long-term future climate changes with the green McGill paleoclimate model: the next glacial inception

The multi-component “green” McGill Paleoclimate Model (MPM), which includes interactive vegetation, is used to simulate the next glacial inception under orbital and prescribed atmospheric CO₂ forcing. This intermediate complexity model is first run for short-term periods with an increasing atmospheric CO₂ concentration; the model's response is in general agreement with the results of GCMs for CO₂ doubling. The green MPM is then used to derive projections of the climate for the next 100 kyr. Under a constant CO₂ level, the model produces three types of evolution for the ice volume: an imminent glacial inception (low CO₂ levels), a glacial inception in 50 kyr (CO₂ levels of 280 or 290 ppm), or no glacial inception during the next 100 kyr (CO₂ levels of 300 ppm and higher). This high sensitivity to the CO₂ level is due to the exceptionally weak future variations of the summer insolation at high northern latitudes. The changes in vegetation re-inforce the buildup of ice sheets after glacial inception. Finally, if an initial global warming episode of finite duration is included, after which the atmospheric CO₂ level is assumed to stabilize at 280, 290 or 300 ppm, the impact of this warming is seen only in the first 5 kyr of the run; after this time the response is insensitive to the early warming perturbation.     

Glacial termination: sensitivity to orbital and CO2 forcing in a coupled climate system model

 To study glacial termination and related feedback mechanisms, a continental ice dynamics model is globally and asynchronously coupled to a physical climate (atmosphere-ocean-sea ice) model. The model performs well under present-day, 11 kaBP (thousand years before present) and 21 kaBP perpetual forcing. To address the ice-sheet response under the effects of both perpetual orbital and CO₂ forcing, sensitivity experiments are conducted with two different orbital configurations (11 kaBP and 21 kaBP) and two different atmospheric CO₂ concentrations (200 ppmv and 280 ppmv). This study reveals that, although both orbital and CO₂ forcing have an impact on ice-sheet maintenance and deglacial processes, and although neither acting alone is sufficient to lead to complete deglaciation, orbital forcing seems to be more important. The CO₂ forcing has a large impact on climate, not uniformly or zonally over the globe, but concentrated over the continents adjacent to the North Atlantic. The effect of increased CO₂ (from 200 ppmv to 280 ppmv) on surface air temperature has its peak there in winter associated with a reduction in sea-ice extent in the northern North Atlantic. These changes are accompanied by an enhancement in the intensity of the meridional overturning and poleward ocean heat transport in the North Atlantic. On the other hand, the effect of orbital forcing (from 21 kaBP to 11 kaBP) has its peak in summer. Since the summer temperature, rather than winter temperature, is found to be dominant for the ice-sheet mass balance, orbital forcing has a larger effect than CO₂ forcing in deglaciation. Warm winter sea surface temperature arising from increased CO₂ during the deglaciation contributes to ice-sheet nourishment (negative feedback for ice-sheet retreat) through slightly enhanced precipitation. However, the precipitation effect is totally overwhelmed by the temperature effect. Our results suggest that the last deglaciation was initiated through increasing summer insolation with CO₂ providing a powerful feedback. Received: 22 February 2000 / Accepted: 17 September 2000     

Radiative forcing and response of a GCM to ice age boundary conditions: cloud feedback and climate sensitivity

 A general circulation model is used to examine the effects of reduced atmospheric CO₂, insolation changes and an updated reconstruction of the continental ice sheets at the Last Glacial Maximum (LGM). A set of experiments is performed to estimate the radiative forcing from each of the boundary conditions. These calculations are used to estimate a total radiative forcing for the climate of the LGM. The response of the general circulation model to the forcing from each of the changed boundary conditions is then investigated. About two-thirds of the simulated glacial cooling is due to the presence of the continental ice sheets. The effect of the cloud feedback is substantially modified where there are large changes to surface albedo. Finally, the climate sensitivity is estimated based on the global mean LGM radiative forcing and temperature response, and is compared to the climate sensitivity calculated from equilibrium experiments with atmospheric CO₂ doubled from present day concentration. The calculations here using the model and palaeodata support a climate sensitivity of about 1 Wm^-2 K^-1 which is within the conventional range. Received: 8 February 1997 / Accepted: 4 June 1997     

Simulated changes in vegetation distribution, land carbon storage, and atmospheric CO2 in response to a collapse of the North Atlantic thermohaline circulation

It is investigated how abrupt changes in the North Atlantic (NA) thermohaline circulation (THC) affect the terrestrial carbon cycle. The Lund–Potsdam–Jena Dynamic Global Vegetation Model is forced with climate perturbations from glacial freshwater experiments with the ECBILT-CLIO ocean–atmosphere–sea ice model. A reorganisation of the marine carbon cycle is not addressed. Modelled NA THC collapses and recovers after about a millennium in response to prescribed freshwater forcing. The initial cooling of several Kelvin over Eurasia causes a reduction of extant boreal and temperate forests and a decrease in carbon storage in high northern latitudes, whereas improved growing conditions and slower soil decomposition rates lead to enhanced storage in mid-latitudes. The magnitude and evolution of global terrestrial carbon storage in response to abrupt THC changes depends sensitively on the initial climate conditions. These were varied using results from time slice simulations with the Hadley Centre model HadSM3 for different periods over the past 21 kyr. Changes in terrestrial storage vary between −67 and +50 PgC for the range of experiments with different initial conditions. Simulated peak-to-peak differences in atmospheric CO₂ are 6 and 13 ppmv for glacial and late Holocene conditions. Simulated changes in δ¹³C are between 0.15 and 0.25‰. These simulated carbon storage anomalies during a NA THC collapse depend on their magnitude on the CO₂ fertilisation feedback mechanism. The CO₂ changes simulated for glacial conditions are compatible with available evidence from marine studies and the ice core CO₂ record. The latter shows multi-millennial CO₂ variations of up to 20 ppmv broadly in parallel with the Antarctic warm events A1 to A4 in the South and cooling in the North.     

Sea-ice and its response to CO2 forcing as simulated by global climate models

The simulation of sea-ice in global climate models participating in the Coupled Model Intercomparison Project (CMIP1 and CMIP2) is analyzed. CMIP1 simulations are of the unpertubed control climate whereas in CMIP2, all models have been forced with the same 1% yr^–1 increase in CO₂ concentration, starting from a near equilibrium initial condition. These simulations are not intended as forecasts of climate change, but rather provide a means of evaluating the response of current climate models to the same forcing. The difference in modeled response therefore indicates the range (or uncertainty) in model sensitivity to greenhouse gas and other climatic perturbations. The results illustrate a wide range in the ability of climate models to reproduce contemporary sea-ice extent and thickness; however, the errors are not obviously related to the manner in which sea-ice processes are represented in the models (e.g. the inclusion or neglect of sea-ice motion). The implication is that errors in the ocean and atmosphere components of the climate model are at least as important. There is also a large range in the simulated sea-ice response to CO₂ change, again with no obvious stratification in terms of model attributes. In contrast to results obtained earlier with a particular model, the CMIP ensemble yields rather mixed results in terms of the dependence of high-latitude warming on sea-ice initial conditions. There is an indication that, in the Arctic, models that produce thick ice in their control integration exhibit less warming than those with thin ice. The opposite tendency appears in the Antarctic (albeit with low statistical significance). There is a tendency for models with more extensive ice coverage in the Southern Hemisphere to exhibit greater Antarctic warming. Results for the Arctic indicate the opposite tendency (though with low statistical significance).A list of the CMIP modeling groups is included in the Acknowledgements section.     

Climate Change Scenarios for the Hudson Bay Region: An Intermodel Comparison

General circulation models (GCMs) are unanimous in projecting warmer temperatures in an enhanced CO₂ atmosphere, with amplification of this warming in higher latitudes. The Hudson Bay region, which is located in the Arctic and subarctic regions of Canada, should therefore be strongly influenced by global warming. In this study, we compare the response of Hudson Bay to a transient warming scenario provided by six-coupled atmosphere-ocean models. Our analysis focuses on surface temperature, precipitation, sea-ice coverage, and permafrost distribution. The results show that warming is expected to peak in winter over the ocean, because of a northward retreat of the sea-ice cover. Also, a secondary warming peak is observed in summer over land in the Canadian and Australian-coupled GCMs, which is associated with both a reduction in soil moisture conditions and changes in permafrost distribution. In addition, a relationship is identified between the retreat of the sea-ice cover and an enhancement of precipitation over both land and oceanic surfaces. The response of the sea-ice cover and permafrost layer to global warming varies considerably among models and thus large differences are observed in the projected regional increase in temperature and precipitation. In view of the important feedbacks that a retreat of the sea-ice cover and the distribution of permafrost are likely to play in the doubled and tripled CO₂ climates of Hudson Bay, a good representation of these two parameters is necessary to provide realistic climate change scenarios. The use of higher resolution regional climate model is recommended to develop scenarios of climate change for the Hudson Bay region.     

Climate sensitivity due to increased CO2: experiments with a coupled atmosphere and ocean general circulation model

A version of the National Center for Atmospheric Research community climate model — a global, spectral (R15) general circulation model — is coupled to a coarse-grid (5° latitude-] longitude, four-layer) ocean general circulation model to study the response of the climate system to increases of atmospheric carbon dioxide (CO₂). Three simulations are run: one with an instantaneous doubling of atmospheric CO₂ (from 330 to 660 ppm), another with the CO₂ concentration starting at 330 ppm and increasing linearly at a rate of 1% per year, and a third with CO₂ held constant at 330 pm. Results at the end of 30 years of simulation indicate a globally averaged surface air temperature increase of 1.6° C for the instantaneous doubling case and 0.7°C for the transient forcing case. Inherent characteristics of the coarse-grid ocean model flow sea-surface temperatures (SSTs) in the tropics and higher-than-observed SSTs and reduced sea-ice extent at higher latitudes] produce lower sensitivity in this model after 30 years than in earlier simulations with the same atmosphere coupled to a 50-m, slab-ocean mixed layer. Within the limitations of the simulated meridional overturning, the thermohaline circulation weakens in the coupled model with doubled CO₂ as the high-latitude ocean-surface layer warms and freshens and westerly wind stress is decreased. In the transient forcing case with slowly increasing CO₂ (30% increase after 30 years), the zonal mean warming of the ocean is most evident in the surface layer near 30°–50° S. Geographical plots of surface air temperature change in the transient case show patterns of regional climate anomalies that differ from those in the instantaneous CO₂ doubling case, particularly in the North Atlantic and northern European regions. This suggests that differences in CO₂ forcing in the climate system are important in CO₂ response in regard to time-dependent climate anomaly regimes. This confirms earlier studies with simple climate models that instantaneous CO₂ doubling simulations may not be analogous in all respects to simulations with slowly increasing CO₂.A portion of this study is supported by the US Department of Energy as part of its Carbon Dioxide Research Program     

Transient simulation of the last glacial inception. Part II: sensitivity and feedback analysis

The sensitivity of the last glacial-inception (around 115 kyr BP, 115,000 years before present) to different feedback mechanisms has been analysed by using the Earth system model of intermediate complexity CLIMBER-2. CLIMBER-2 includes dynamic modules of the atmosphere, ocean, terrestrial biosphere and inland ice, the last of which was added recently by utilising the three-dimensonal polythermal ice-sheet model SICOPOLIS. We performed a set of transient experiments starting at the middle of the Eemiam interglacial and ran the model for 26,000 years with time-dependent orbital forcing and observed changes in atmospheric CO₂ concentration (CO₂ forcing). The role of vegetation and ocean feedback, CO₂ forcing, mineral dust, thermohaline circulation and orbital insolation were closely investigated. In our model, glacial inception, as a bifurcation in the climate system, appears in nearly all sensitivity runs including a run with constant atmospheric CO₂ concentration of 280 ppmv, a typical interglacial value, and simulations with prescribed present-day sea-surface temperatures or vegetation cover—although the rate of the growth of ice-sheets growth is smaller than in the case of the fully interactive model. Only if we run the fully interactive model with constant present-day insolation and apply present-day CO₂ forcing does no glacial inception appear at all. This implies that, within our model, the orbital forcing alone is sufficient to trigger the interglacial–glacial transition, while vegetation, ocean and atmospheric CO₂ concentration only provide additional, although important, positive feedbacks. In addition, we found that possible reorganisations of the thermohaline circulation influence the distribution of inland ice.     

Ice Age mystery: a proposed theory for the cause of long term climate change

Summary ?Analysis of available data shows that the duration of the glacial/interglacial cycle is determined by the time for the ocean to go through one major temperature cycle. At the start of an interglacial period, clear skies with consequent release of CO₂ from the ocean, warms the atmosphere, which in turn eventually warms the ocean to its maximum. Cloudy skies then cause the climate (land and air temperature) to cool and the CO₂ to be reabsorbed to start glaciation preliminaries. The albedo feedback effect of the glacial ice, a relatively warm ocean, which produces enhanced cloud cover, and the increased solubility of CO₂ in cold seawater ensure a long period of glaciation. Glacial periods end when pack ice spreads out on the ocean cooling it until reduced cloud cover once again allows the Sun’s heat, unreflected by cloud cover, to melt the ice and release CO₂ back into the atmosphere. Received May 22, 2002; accepted June 20, 2002     

Relative roles of climate sensitivity and forcing in defining the ocean circulation response to climate change

The response of the ocean’s meridional overturning circulation (MOC) to increased greenhouse gas forcing is examined using a coupled model of intermediate complexity, including a dynamic 3-D ocean subcomponent. Parameters are the increase in CO₂ forcing (with stabilization after a specified time interval) and the model’s climate sensitivity. In this model, the cessation of deep sinking in the north “Atlantic” (hereinafter, a “collapse”), as indicated by changes in the MOC, behaves like a simple bifurcation. The final surface air temperature (SAT) change, which is closely predicted by the product of the radiative forcing and the climate sensitivity, determines whether a collapse occurs. The initial transient response in SAT is largely a function of the forcing increase, with higher sensitivity runs exhibiting delayed behavior; accordingly, high CO₂-low sensitivity scenarios can be assessed as a recovering or collapsing circulation shortly after stabilization, whereas low CO₂-high sensitivity scenarios require several hundred additional years to make such a determination. We also systemically examine how the rate of forcing, for a given CO₂ stabilization, affects the ocean response. In contrast with previous studies based on results using simpler ocean models, we find that except for a narrow range of marginally stable to marginally unstable scenarios, the forcing rate has little impact on whether the run collapses or recovers. In this narrow range, however, forcing increases on a time scale of slow ocean advective processes results in weaker declines in overturning strength and can permit a run to recover that would otherwise collapse.     

Climate Change to the End of the Millennium

Anthropogenic climate change will continue long after anthropogenic CO₂ emissions cease. Atmospheric CO₂, global warming and ocean circulation will approach equilibrium on the millennial timescale, whereas thermal expansion of the ocean, ice sheet melt and their contributions to sea level rise are unlikely to be complete. Atmospheric CO₂ in year 3000 depends non-linearly on the total amount of CO₂ emitted and is very likely to exceed the present level of ∼380 ppmv. CO₂ is doubled for ∼2500 GtC emitted, quadrupled if all ∼5000 GtC of conventional fossil fuel resources are emitted, and increases by a factor of ∼32 if a further 20,000 GtC of exotic fossil fuel resources are emitted. Global warming in year 3000 will also depend on climate sensitivity to doubling CO₂, which is most probably ∼3 ^∘C but highly uncertain. Thermal expansion will contribute 0.5–2 m to millennial sea level rise for each doubling of CO₂. The Greenland ice sheet could melt completely within the millennium under > 8×CO₂, adding a further ∼7 m to sea level. The rate of melt depends on the magnitude of forcing above a regional warming threshold of 1–3 ^∘C. The West Antarctic ice sheet could be threatened by 4–10 ^∘C local warming, and its potential contribution to millennial sea level rise exceeds current maximum estimates of ∼1 m. The fate of the ocean thermohaline circulation may depend on the rate as well as the magnitude of forcing.     

Heat budget of the upper Arctic Ocean under a warming climate

The heat budget of the upper Arctic Ocean is examined in an ensemble of coupled climate models under idealised increasing CO₂ scenarios. All of the experiments show a strong amplification of surface air temperatures but a smaller increase in sea surface temperature than the rest of the world as heat is lost to the atmosphere as the sea-ice cover is reduced. We carry out a heat budget analysis of the Arctic Ocean in an ensemble of model runs to understand the changes that occur as the Arctic becomes ice free in summer. We find that as sea-ice retreats heat is lost from the ocean surface to the atmosphere contributing to the amplification of Arctic surface temperatures. Furthermore, heat is mixed upwards into the mixed layer as a result of increased upper ocean mixing and there is increased advection of heat into the Arctic as the ice edge retreats. Heat lost from the upper Arctic Ocean to the atmosphere is therefore replenished by mixing of warmer water from below and by increased advection of warm water from lower latitudes. The ocean is therefore able to contribute more to Arctic amplification.     

The modification of greenhouse gas warming by the direct effect of sulphate aerosols

 The Canadian Centre for Climate Modelling and Analysis (CCCma) second generation climate model (GCMII) consists of an atmospheric GCM coupled to mixed layer ocean. It is used to investigate the climate response to a doubling of the CO₂ concentration together with the direct effect of scattering by sulphate aerosols. As expected, the aerosols offset some of the greenhouse gas (GHG) warming; the global annual mean screen temperature change due to doubled CO₂ is 3.4 °C in this model and this is reduced to 2.7 °C when an estimate of the direct effect of anthropogenic sulphate aerosols is included. The pattern of climate response to the comparatively localized aerosol forcing is not itself localized, and it bears a striking resemblance to the response pattern that arises from the globally distributed change in GHG forcing. This “non-local” response to “localized” forcing indicates that the pattern of climate response is determined, to first order, by the overall magnitude of the change in forcing rather than its detailed nature or structure. Feedback processes operating in the system apparently determine this pattern by locally amplifying and suppressing the response to the magnitude of the change in forcing. The influence of the location of the change in forcing is relatively small. These “non-local” and “local” effects of aerosol forcing are characterized and displayed and some of their consequences discussed. Effects on the moisture budget and on the energetics of the global climate are also examined. Received: 10 June 1997 / Accepted: 8 January 1998     

CO<Subscript>2</Subscript> and non-CO<Subscript>2</Subscript> radiative forcings in climate projections for twenty-first century mitigation scenarios

Climate is simulated for reference and mitigation emissions scenarios from Integrated Assessment Models using the Bern2.5CC carbon cycle–climate model. Mitigation options encompass all major radiative forcing agents. Temperature change is attributed to forcings using an impulse–response substitute of Bern2.5CC. The contribution of CO₂ to global warming increases over the century in all scenarios. Non-CO₂ mitigation measures add to the abatement of global warming. The share of mitigation carried by CO₂, however, increases when radiative forcing targets are lowered, and increases after 2000 in all mitigation scenarios. Thus, non-CO₂ mitigation is limited and net CO₂ emissions must eventually subside. Mitigation rapidly reduces the sulfate aerosol loading and associated cooling, partly masking Greenhouse Gas mitigation over the coming decades. A profound effect of mitigation on CO₂ concentration, radiative forcing, temperatures and the rate of climate change emerges in the second half of the century.     

On the sensitivity of global warming experiments to the parametrisation of sub-grid scale ocean mixing

An ocean general circulation model coupled to an energy-moisture balance atmosphere model is used to investigate the sensitivity of global warming experiments to the parametrisation of sub-grid scale ocean mixing. The climate sensitivity of the coupled model using three different parametrisations of sub-grid scale mixing is 3°C for a doubling of CO₂ (6°C for a quadrupling of CO₂). This suggests that the ocean has only a weak feedback on global mean surface air temperature although significant regional differences, notably at high latitudes, exist with different sub-grid scale parametrisations. In the experiment using the Gent and McWilliams parametrisation for mixing associated with mesoscale eddies, an enhancement of the surface response in the Southern Ocean is found. This enhancement is largely due to the existence of more realistic sea-ice in the climatological control integration and the subsequent enhanced ice-albedo feedback upon warming. In accordance with earlier analyses, the Gent and McWilliams scheme decreases the global efficiency of ocean heat uptake. During the transient phase of all experiments, the North Atlantic overturning initially weakened but ultimately recovered, surpassing its former strength. This suggests that in the region around the North Atlantic the ocean acts as a negative feedback on local warming during the transient phase but a positive feedback at equilibrium. During the transient phase of the experiments with a more sophisticated and realistic parametrisation of sub-grid scale mixing, warmed Atlantic water was found to penetrate at depth into the Arctic, consistent with recent observations in the region. Received: 14 October 1998 / Accepted: 27 April 1999     

A nonlinear impulse response model of the coupled carbon cycle-climate system (NICCS)

 Impulse-response-function (IRF) models are designed for applications requiring a large number of climate change simulations, such as multi-scenario climate impact studies or cost-benefit integrated-assessment studies. The models apply linear response theory to reproduce the characteristics of the climate response to external forcing computed with sophisticated state-of-the-art climate models like general circulation models of the physical ocean-atmosphere system and three-dimensional oceanic-plus-terrestrial carbon cycle models. Although highly computer efficient, IRF models are nonetheless capable of reproducing the full set of climate-change information generated by the complex models against which they are calibrated. While limited in principle to the linear response regime (less than about 3 ^∘C global-mean temperature change), the applicability of the IRF model presented has been extended into the nonlinear domain through explicit treatment of the climate system's dominant nonlinearities: CO₂ chemistry in ocean water, CO₂ fertilization of land biota, and sublinear radiative forcing. The resultant nonlinear impulse-response model of the coupled carbon cycle-climate system (NICCS) computes the temporal evolution of spatial patterns of climate change for four climate variables of particular relevance for climate impact studies: near-surface temperature, cloud cover, precipitation, and sea level. The space-time response characteristics of the model are derived from an EOF analysis of a transient 850-year greenhouse warming simulation with the Hamburg atmosphere-ocean general circulation model ECHAM3-LSG and a similar response experiment with the Hamburg carbon cycle model HAMOCC. The model is applied to two long-term CO₂ emission scenarios, demonstrating that the use of all currently estimated fossil fuel resources would carry the Earth's climate far beyond the range of climate change for which reliable quantitative predictions are possible today, and that even a freezing of emissions to present-day levels would cause a major global warming in the long term. Received: 28 January 2000 / Accepted: 9 March 2001     

Sensitivity of Hudson Bay Sea ice and ocean climate to atmospheric temperature forcing

A regional sea-ice?Cocean model was used to investigate the response of sea ice and oceanic heat storage in the Hudson Bay system to a climate-warming scenario. Projections of air temperature (for the years 2041?C2070; effective CO₂ concentration of 707?C950?ppmv) obtained from the Canadian Regional Climate Model (CRCM 4.2.3), driven by the third-generation coupled global climate model (CGCM 3) for lateral atmospheric and land and ocean surface boundaries, were used to drive a single sensitivity experiment with the delta-change approach. The projected change in air temperature varies from 0.8°C (summer) to 10°C (winter), with a mean warming of 3.9°C. The hydrologic forcing in the warmer climate scenario was identical to the one used for the present climate simulation. Under this warmer climate scenario, the sea-ice season is reduced by 7?C9?weeks. The highest change in summer sea-surface temperature, up to 5°C, is found in southeastern Hudson Bay, along the Nunavik coast and in James Bay. In central Hudson Bay, sea-surface temperature increases by over 3°C. Analysis of the heat content stored in the water column revealed an accumulation of additional heat, exceeding 3?MJ?m^?3, trapped along the eastern shore of James and Hudson bays during winter. Despite the stratification due to meltwater and river runoff during summer, the shallow coastal regions demonstrate a higher capacity of heat storage. The maximum volume of dense water produced at the end of winter was halved under the climate-warming perturbation. The maximum volume of sea ice is reduced by 31% (592?km3) while the difference in the maximum cover is only 2.6% (32,350?km²). Overall, the depletion of sea-ice thickness in Hudson Bay follows a southeast?Cnorthwest gradient. Sea-ice thickness in Hudson Strait and Ungava Bay is 50% thinner than in present climate conditions during wintertime. The model indicates that the greatest changes in both sea-ice climate and heat content would occur in southeastern Hudson Bay, James Bay, and Hudson Strait.     

CO2 climate sensitivity and snow-sea-ice albedo parameterization in an atmospheric GCM coupled to a mixed-layer ocean model

The snow-sea-ice albedo parameterization in an atmospheric general circulation model (GCM), coupled to a simple mixed-layer ocean and run with an annual cycle of solar forcing, is altered from a version of the same model described by Washington and Meehl (1984). The model with the revised formulation is run to equilibrium for 1 × CO₂ and 2 × CO₂ experiments. The 1 ×CO₂ (control) simulation produces a global mean climate about 1° warmer than the original version, and sea-ice extent is reduced. The model with the altered parameterization displays heightened sensitivity in the global means, but the geographical patterns of climate change due to increased carbon dioxide (CO₂) are qualitatively similar. The magnitude of the climate change is affected, not only in areas directly influenced by snow and ice changes but also in other regions of the globe, including the tropics where sea-surface temperature, evaporation, and precipitation over the oceans are greater. With the less-sensitive formulation, the global mean surface air temperature increase is 3.5 °C, and the increase of global mean precipitation is 7.12%. The revised formulation produces a globally averaged surface air temperature increase of 4.04 °C and a precipitation increase of 7.25%, as well as greater warming of the upper tropical troposphere. Sensitivity of surface hydrology is qualitatively similar between the two cases with the larger-magnitude changes in the revised snow and ice-albedo scheme experiment. Variability of surface air temperature in the model is comparable to observations in most areas except at high latitudes during winter. In those regions, temporal variation of the sea-ice margin and fluctuations of snow cover dependent on the snow-ice-albedo formulation contribute to larger-than-observed temperature variability. This study highlights an uncertainty associated with results from current climate GCMs that use highly parameterized snow-sea-ice albedo schemes with simple mixed-layer ocean models.The National Center for Atmospheric Research is sponsored by the National Science Foundation.     

Implications of CO2 global warming on great lakes ice cover

Statistical ice cover models were used to project daily mean basin ice cover and annual ice cover duration for Lakes Superior and Erie. Models were applied to a 1951–80 base period and to three 30-year steady double carbon dioxide (2 × CO₂) scenarios produced by the Geophysical Fluid Dynamics Laboratory (GFDL), the Goddard Institute of Space Studies (GISS), and the Oregon State University (OSU) general circulation models. Ice cover estimates were made for the West, Central, and East Basins of Lake Erie and for the West, East, and Whitefish Bay Basins of Lake Superior. Average ice cover duration for the 1951– 80 base period ranged from 13 to 16 weeks for individual lake basins. Reductions in average ice cover duration under the three 2 × CO₂ scenarios for individual lake basins ranged from 5 to 12 weeks for the OSU scenario, 8 to 13 weeks for the GISS scenario, and 11 to 13 weeks for GFDL scenario. Winters without ice formation become common for Lake Superior under the GFDL scenario and under all three 2 × CO₂ scenarios for the Central and East Basins of Lake Erie. During an average 2 × CO₂ winter, ice cover would be limited to the shallow areas of Lakes Erie and Superior. Because of uncertainties in the ice cover models, the results given here represent only a first approximation and are likely to represent an upper limit of the extent and duration of ice cover under the climate change projected by the three 2 × CO₂scenarios. Notwithstanding these limitations, ice cover projected by the 2 × CO₂ scenarios provides a preliminary assessment of the potential sensitivity of the Great Lakes ice cover to global warming. Potential environmental and socioeconomic impacts of a 2 × CO₂ warming include year-round navigation, change in abundance of some fish species in the Great Lakes, discontinuation or reduction of winter recreational activities, and an increase in winter lake evaporation.     

The role of sea ice in the temperature-precipitation feedback of glacial cycles

The response of the hydrological cycle to climate variability and change is a critical open question, where model reliability is still unsatisfactory, yet upon which past climate history can shed some light. Sea ice is a key player in the climate system and in the hydrological cycle, due to its strong albedo effect and its insulating effect on local evaporation and air-sea heat flux. Using an atmospheric general circulation model with specified sea surface temperature and sea-ice distribution, the role of sea ice in the hydrological cycle is investigated under last glacial maximum (LGM) and present day conditions, and by studying its contribution to the “temperature-precipitation feedback”. By conducting a set of sensitivity experiments in which the albedo and thickness of the sea ice are varied, the various effects of sea ice in the hydrological cycle are isolated. It is demonstrated that for a cold LGM like state, a warmer climate (as a result of reduced sea-ice cover) leads to an increase in snow precipitation over the ice sheets. The insulating effect of the sea ice on the hydrological cycle is found to be larger than the albedo effect. These two effects interact in a nonlinear way and their total effect is not equal to summing their separate contribution.