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1.
CO2 climate sensitivity and snow-sea-ice albedo parameterization in an atmospheric GCM coupled to a mixed-layer ocean model 总被引:1,自引:0,他引:1
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. 相似文献
2.
Abstract Present‐day results and CO2 sensitivity are described for two versions of a global climate model (genesis) with and without sea‐ice dynamics. Sea‐ice dynamics is modelled using the cavitating‐fluid method of Flato and Hibler (1990, 1992). The atmospheric general circulation model originated from the NCAR Community Climate Model version 1, but is heavily modified to include new treatments of clouds, penetrative convection, planetary boundary‐layer mixing, solar radiation, the diurnal cycle and the semi‐Lagrangian transport of water vapour. The surface models include an explicit model of vegetation (similar to BATS and SiB), multilayer models of soil, snow and sea ice, and a slab ocean mixed layer. When sea‐ice dynamics is turned off, the CO2‐induced warming increases drastically around ~60–80°S in winter and spring. This is due to the much greater (and unrealistic) compactness of the Antarctic ice cover without dynamics, which is reduced considerably when CO2 is doubled and exposes more open ocean to the atmosphere. With dynamics, the winter ice is already quite dispersed for 1 × CO2 so that its compactness does not decrease as much when CO2 is doubled. 相似文献
3.
The impact of atmospheric and oceanic heat transports on the sea-ice-albedo instability during the Neoproterozoic 总被引:1,自引:0,他引:1
In order to simulate the climatic conditions of the Neoproterozoic, we have conducted a series of simulations with a coupled ocean–atmosphere model of intermediate complexity, CLIMBER-2, using a reduced solar constant of 6% and varied CO2 concentrations. We have also tested the impact of the breakup of the supercontinent Rodinia that has been hypothesized to play an important role in the initiation of an ice-covered Earth. Our results show that for the critical values of 89 and 149 ppm of atmospheric CO2, a snowball Earth occurs in the supercontinent case and in the dislocated configuration, respectively. The study of the sensitivity of the meridional oceanic energy transport to reductions in CO2 concentration and to the dislocation of the supercontinent demonstrates that dynamics ocean processes can modulate the CO2 threshold value, below which a snowball solution is found, but cannot prevent it. The collapse of the overturning cells and of the oceanic heat transport is mainly due to the reduced zonal temperature gradient once the sea-ice line reaches the 30° latitudinal band but also to the freshening of the tropical ocean by sea-ice melt. In term of feedbacks, the meridional atmospheric heat transport via the Hadley circulation plays the major role, all along the CO2 decrease, by increasing the energy brought in the front of the sea-ice margin but does not appear enough efficient to prevent the onset of the sea-ice-albedo instability in the case of the continental configurations tested in this contribution. 相似文献
4.
Stephen G. Warren 《Climatic change》1982,4(4):329-340
The growth and decay of ice sheets are driven by forces affecting the seasonal cycles of snowfall and snowmelt. The external forces are likely to be variations in the earth's orbit which cause differences in the solar radiation received. Radiational control of snowmelt is modulated by the seasonal cycles of snow albedo and cloud cover. The effects of orbital changes can be magnified by feedbacks involving atmospheric CO2 content, ocean temperatures and desert areas. Climate modeling of the causes of the Pleistocene ice ages involves modeling the interactions of all components of the climate system; snow, sea ice, glacier ice, the ocean, the atmosphere, and the solid earth. Such modeling is also necessary for interpreting oxygen isotope records from ice and ocean as paleoclimatic evidence. 相似文献
5.
S. J. Kim 《Climate Dynamics》2004,22(6-7):639-651
The role of reduced atmospheric CO2 concentration and ice sheet topography plus its associated land albedo on the LGM climate is investigated using a coupled atmosphere-ocean-sea ice climate system model. The surface cooling induced by the reduced CO2 concentration is larger than that by the ice sheet topography plus other factors by about 30% for the surface air temperature and by about 100% for the sea surface temperature. A large inter-hemispheric asymmetry in surface cooling with a larger cooling in the Northern Hemisphere is found for both cases. This asymmetric inter-hemispheric temperature response is consistent in the ice sheet topography case with earlier studies using an atmospheric model coupled with a mixed-layer ocean representation, but contrasts with these results in the reduced CO2 case. The incorporation of ocean dynamics presumably leads to a larger snow and sea ice feedback as a result of the reduction in northward ocean heat transport, mainly as a consequence of the decrease in the North Atlantic overturning circulation by the substantial freshening of the North Atlantic convection regions. A reversed case is found in the Southern Ocean. Overall, the reduction in atmospheric CO2 concentration accounts for about 60% of the total LGM climate change. 相似文献
6.
《大气与海洋》2013,51(3):224-237
Abstract The University of Victoria's (UVic) Earth System Climate model is used to conduct equilibrium atmospheric CO2 sensitivity experiments over the range 200–1600 ppm in order to explore changes in northern hemisphere snow cover and feedbacks on terrestrial surface air temperature (SAT). Simulations of warmer climates predict a retreat of snow cover over northern continents, in a northeasterly direction. The decline in northern hemisphere global snow mass is estimated to reach 33% at 600 ppm and 54% at 1200 ppm. In the most northerly regions, annual mean snow depth increases for simulations with CO2 levels higher than present day. The shift in the latitude of maximum snowfall is estimated to be inversely proportional to the CO2 concentration. The northern hemisphere net shortwave radiation changes are found to be greater over land than over the ocean, suggesting a stronger albedo feedback from changes in terrestrial snow cover than from changes in sea ice. Results also reveal high sensitivity of the snow mass balance under low CO2 conditions. The amplification feedback (defined as the zonal SAT anomaly caused by doubling CO2 divided by the equatorial anomaly) is greatest for scenarios with less than 300 ppm, reaching 1.9 at the pole for 250 ppm. The stronger feedback is attributed to the significant albedo changes over land areas. The simulation with 200 ppm triggers continuous accumulation of snow ('glaciation') in regions which, according to paleo‐reconstructions, were covered by ice during the last glacial cycle (the Canadian Arctic, Scandinavia, and the Taymir Peninsula). 相似文献
7.
Andrew J. Weaver Michael Eby Edward C. Wiebe Cecilia M. Bitz Phil B. Duffy Tracy L. Ewen 《大气与海洋》2013,51(4):361-428
Abstract A new earth system climate model of intermediate complexity has been developed and its climatology compared to observations. The UVic Earth System Climate Model consists of a three‐dimensional ocean general circulation model coupled to a thermodynamic/dynamic sea‐ice model, an energy‐moisture balance atmospheric model with dynamical feedbacks, and a thermomechanical land‐ice model. In order to keep the model computationally efficient a reduced complexity atmosphere model is used. Atmospheric heat and freshwater transports are parametrized through Fickian diffusion, and precipitation is assumed to occur when the relative humidity is greater than 85%. Moisture transport can also be accomplished through advection if desired. Precipitation over land is assumed to return instantaneously to the ocean via one of 33 observed river drainage basins. Ice and snow albedo feedbacks are included in the coupled model by locally increasing the prescribed latitudinal profile of the planetary albedo. The atmospheric model includes a parametrization of water vapour/planetary longwave feedbacks, although the radiative forcing associated with changes in atmospheric CO2 is prescribed as a modification of the planetary longwave radiative flux. A specified lapse rate is used to reduce the surface temperature over land where there is topography. The model uses prescribed present‐day winds in its climatology, although a dynamical wind feedback is included which exploits a latitudinally‐varying empirical relationship between atmospheric surface temperature and density. The ocean component of the coupled model is based on the Geophysical Fluid Dynamics Laboratory (GFDL) Modular Ocean Model 2.2, with a global resolution of 3.6° (zonal) by 1.8° (meridional) and 19 vertical levels, and includes an option for brine‐rejection parametrization. The sea‐ice component incorporates an elastic‐viscous‐plastic rheology to represent sea‐ice dynamics and various options for the representation of sea‐ice thermodynamics and thickness distribution. The systematic comparison of the coupled model with observations reveals good agreement, especially when moisture transport is accomplished through advection. Global warming simulations conducted using the model to explore the role of moisture advection reveal a climate sensitivity of 3.0°C for a doubling of CO2, in line with other more comprehensive coupled models. Moisture advection, together with the wind feedback, leads to a transient simulation in which the meridional overturning in the North Atlantic initially weakens, but is eventually re‐established to its initial strength once the radiative forcing is held fixed, as found in many coupled atmosphere General Circulation Models (GCMs). This is in contrast to experiments in which moisture transport is accomplished through diffusion whereby the overturning is reestablished to a strength that is greater than its initial condition. When applied to the climate of the Last Glacial Maximum (LGM), the model obtains tropical cooling (30°N‐30°S), relative to the present, of about 2.1°C over the ocean and 3.6°C over the land. These are generally cooler than CLIMAP estimates, but not as cool as some other reconstructions. This moderate cooling is consistent with alkenone reconstructions and a low to medium climate sensitivity to perturbations in radiative forcing. An amplification of the cooling occurs in the North Atlantic due to the weakening of North Atlantic Deep Water formation. Concurrent with this weakening is a shallowing of, and a more northward penetration of, Antarctic Bottom Water. Climate models are usually evaluated by spinning them up under perpetual present‐day forcing and comparing the model results with present‐day observations. Implicit in this approach is the assumption that the present‐day observations are in equilibrium with the present‐day radiative forcing. The comparison of a long transient integration (starting at 6 KBP), forced by changing radiative forcing (solar, CO2, orbital), with an equilibrium integration reveals substantial differences. Relative to the climatology from the present‐day equilibrium integration, the global mean surface air and sea surface temperatures (SSTs) are 0.74°C and 0.55°C colder, respectively. Deep ocean temperatures are substantially cooler and southern hemisphere sea‐ice cover is 22% greater, although the North Atlantic conveyor remains remarkably stable in all cases. The differences are due to the long timescale memory of the deep ocean to climatic conditions which prevailed throughout the late Holocene. It is also demonstrated that a global warming simulation that starts from an equilibrium present‐day climate (cold start) underestimates the global temperature increase at 2100 by 13% when compared to a transient simulation, under historical solar, CO2 and orbital forcing, that is also extended out to 2100. This is larger (13% compared to 9.8%) than the difference from an analogous transient experiment which does not include historical changes in solar forcing. These results suggest that those groups that do not account for solar forcing changes over the twentieth century may slightly underestimate (~3% in our model) the projected warming by the year 2100. 相似文献
8.
Responses of ocean circulation and ocean carbon cycle in the course of a global glaciation from the present Earth conditions are investigated by using a coupled climate-biogeochemical model. We investigate steady states of the climate system under colder conditions induced by a reduction of solar constant from the present condition. A globally ice-covered solution is obtained under the solar constant of 92.2% of the present value. We found that because almost all of sea water reaches the frozen point, the ocean stratification is maintained not by temperature but by salinity just before the global glaciation (at the solar constant of 92.3%). It is demonstrated that the ocean circulation is driven not by the surface cooling but by the surface freshwater forcing associated with formation and melting of sea ice. As a result, the deep ocean is ventilated exclusively by deep water formation in southern high latitudes where sea ice production takes place much more massively than northern high latitudes. We also found that atmospheric CO2 concentration decreases through the ocean carbon cycle. This reduction is explained primarily by an increase of solubility of CO2 due to a decrease of sea surface temperature, whereas the export production weakens by 30% just before the global glaciation. In order to investigate the conditions for the atmospheric CO2 reduction to cause global glaciations, we also conduct a series of simulations in which the total amount of carbon in the atmosphere?Cocean system is reduced from the present condition. Under the present solar constant, the results show that the global glaciation takes place when the total carbon decreases to be 70% of the present-day value. Just before the glaciation, weathering rate becomes very small (almost 10% of the present value) and the organic carbon burial declines due to weakened biological productivity. Therefore, outgoing carbon flux from the atmosphere?Cocean system significantly decreases. This suggests the atmosphere?Cocean system has strong negative feedback loops against decline of the total carbon content. The results obtained here imply that some processes outside the atmosphere?Cocean feedback loops may be required to cause global glaciations. 相似文献
9.
The role of land surface dynamics in glacial inception: a study with the UVic Earth System Model 总被引:5,自引:2,他引:3
The first results of the UVic Earth System Model coupled to a land surface scheme and a dynamic global vegetation model are presented in this study. In the first part the present day climate simulation is discussed and compared to observations. We then compare a simulation of an ice age inception (forced with 116 ka BP orbital parameters and an atmospheric CO2 concentration of 240 ppm) with a preindustrial run (present day orbital parameters, atmospheric [CO2] = 280 ppm). Emphasis is placed on the vegetations response to the combined changes in solar radiation and atmospheric CO2 level. A southward shift of the northern treeline as well as a global decrease in vegetation carbon is observed in the ice age inception run. In tropical regions, up to 88% of broadleaf trees are replaced by shrubs and C4 grasses. These changes in vegetation cover have a remarkable effect on the global climate: land related feedbacks double the atmospheric cooling during the ice age inception as well as the reduction of the meridional overturning in the North Atlantic. The introduction of vegetation related feedbacks also increases the surface area with perennial snow significantly. 相似文献
10.
Reinhard Calov Andrey Ganopolski Vladimir Petoukhov Martin Claussen Victor Brovkin Claudia Kubatzki 《Climate Dynamics》2005,24(6):563-576
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. 相似文献
11.
L. G. Bell 《Theoretical and Applied Climatology》2003,74(3-4):235-244
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 相似文献
12.
Timothy M. Lenton 《Climatic change》2006,76(1-2):7-29
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. 相似文献
13.
Claudia Kubatzki Martin Claussen Reinhard Calov Andrey Ganopolski 《Climate Dynamics》2006,27(4):333-344
We investigate the sensitivity of simulations of the last glacial inception (LGI) with respect to initial (size of the Greenland ice sheet) and surface (state of ocean/vegetation) conditions and two different CO2 reconstructions. Utilizing the CLIMBER-2 Earth system model, we obtain the following results: (a) ice-sheet expansion in North America at the end of the Eemian can be reduced or even completely suppressed when pre-industrial or Eemian ocean/vegetation is prescribed. (b) A warmer surrounding ocean and, in particular, a large Laurentide ice sheet reduce the size of the Greenland ice sheet before and during the LGI. (c) A changing ocean contributes much stronger to the expansion of the Laurentide ice sheet when we apply the CO2 reconstruction according to Barnola et al. (Nature 329:408–414, 1987) instead of Petit et al. (Nature 399:429–436, 1999). (d) In the fully coupled model, the CO2 reconstruction used has only a small impact on the simulated ice sheets but it does impact the course of the climatic variables. (e) For the Greenland ice sheet, two equilibrium states exist under the insolation and CO2 forcing at 128,000 years before present (128 kyear BP); the one with an ice sheet reduced by about one quarter as compared to its simulated pre-industrial size and the other with nearly no inland ice in Greenland. (f) Even the extreme assumption of no ice sheet in Greenland at the beginning of our transient simulations does not alter the simulated expansion of northern hemispheric ice sheets at the LGI. 相似文献
14.
Abstract The most common method used to evaluate climate models involves spinning them up under perpetual present‐day forcing and comparing the model results with present‐day observations. This approach clearly ignores any potential long‐term memory of the model ocean to past climatic conditions. Here we examine the validity of this approach through the 6000‐year integration of a coupled atmosphere–ocean–sea‐ice model. The coupled model is initially spun‐up with atmospheric CO2 concentrations and orbital parameters applicable for 6KBP. The model is then integrated forward in time to 2100. Results from this transient coupled model simulation are compared with the results from two additional simulations, in which the model is spun up with perpetual 1850 (preindustrial) and 1998 (present‐day) atmospheric CO2 concentrations and orbital parameters. This comparison leads to substantial differences between the equilibrium climatologies and the transient simulation, even at 1850 (in weakly ventilated regions), prior to any significant changes in atmospheric CO2. When compared to the present‐day equilibrium climatology, differences are very large: the global mean surface air and sea surface temperatures are ,0.5°C and ,0.4°C colder, respectively, deep ocean temperatures are substantially cooler, Southern Hemisphere sea‐ice cover is 38% larger, and the North Atlantic conveyor 16% weaker in the transient case. These differences are due to the long timescale memory of the deep ocean to climatic conditions which prevailed throughout the late Holocene, as well as to its large thermal inertia. It is also demonstrated that a ‘cold start’ global warming simulation (one that starts from a 1998 equilibrium climatology) underestimates the global temperature increase at 2100 by ,10%. Our results question the accuracy of current techniques for climate model evaluation and underline the importance of using paleoclimatic simulations in parallel with present‐day simulations in this evaluation process. 相似文献
15.
Isabella Bordi Klaus Fraedrich Alfonso Sutera Xiuhua Zhu 《Theoretical and Applied Climatology》2012,109(1-2):253-259
Although ozone appears in the Earth’s atmosphere in a small abundance, it plays a key role in the energy balance of the planet through its involvement in radiative processes. Its absorption of solar radiation leads to the temperature increase with height defining the tropopause and the stratosphere. Moreover, excluding water vapor, O3 is the third most important contributor (after CO2 and CH4) to the greenhouse radiative forcing. Thus, the total removal of O3 content in an Earth-like atmosphere may cause interesting response of the climate system that deserves further investigation. The present paper addresses this issue by means of a global climate model where the atmosphere is coupled with a passive ocean of a given depth. The model, after reaching the statistical equilibrium under present climate conditions, is perturbed by a sudden switch off of the O3 content. Results obtained for the new equilibrium suggest that the model gets in a colder state mainly because of the water vapor content decrease. Most of the cooling occurs in the Southern Hemisphere while in the Northern Hemisphere the ice cap melts quite consistently. This process appears to be governed by the northward cross-equatorial heat transports induced by changes in the general circulation. 相似文献
16.
The responses of the CSIRO coupled atmosphere-ocean-sea ice model to two greenhouse gas induced warming scenarios are described
and compared to a control run with the current CO2 level. In one scenario, denoted IS92a, the atmospheric CO2 increases such that it reaches doubling after 128 years. In the other, the CO2 increases at 1% per year compounding (doubling after 70 y). As the CO2 increases in both scenarios, the top-of-atmosphere outgoing longwave radiation increases giving enhanced cooling of the coupled
system, while the outgoing short wave radiation decreases contributing to a warming of the system. The latter overcompensates
the former leading to a global mean net radiative heat gain. The distribution of this heat gain produces the well-known interhemispheric
asymmetry in warming, despite a decrease in the sea ice around Antarctica in this model. It is found that the volume mean
temperature response over the southern ocean is greater than that over the northern hemispheric oceans, and a maximum warming
takes place at the subsurface rather at the surface of the ocean in the southern mid-to-high latitude region. The enhanced
high-latitude freshening associated with the strengthened hydrological cycle significantly affects the latitudinal distribution
of warming and other responses. It enhances the warming immediately equatorward of the deep water formation regions while
produces a reduced warming, even a cooling, in these regions. In both runs, there is a decrease in the large-scale oceanic
currents which have a significant thermohaline-driven component. The reduction in these currents reduces the poleward transport
of salt out of the tropical and subtropical regions of these oceans. This and the enhanced evaporation contribute to considerable
increases in surface salinity in the tropical and subtropical regions. In IS92a, the warming rate before doubling is smaller
than that in 1% scenario, but the cumulative effects of the two experiments at the time of doubling are similar. Nevertheless,
significant contrasts exist. For example, at the time of doubling in IS92a, the warming of the upper ocean is greater because
a more developed temperature-albedo feedback occurs. In addition, a longer time is allowed for heat anomalies to spread downward,
and so the effective heat penetration depth is greater than that in the 1% scenario. Thus the oceanic response is influenced
by the CO2 increase scenario used.
Received: 2 September 1997 / Accepted: 21 January 1998 相似文献
17.
Guillaume Le Hir Yannick Donnadieu Gerhard Krinner Gilles Ramstein 《Climate Dynamics》2010,35(2-3):285-297
The current state of knowledge suggests that the Neoproterozoic snowball Earth is far from deglaciation even at 0.2 bars of CO2. Since understanding the termination of the fully ice-covered state is essential to sustain, or not, the snowball Earth theory, we used an Atmospheric General Climate Model (AGCM) to explore some key factors which could induce deglaciation. After testing the models’ sensitivity to their parameterizations of clouds, CO2 and snow, we investigated the warming effect caused by a dusty surface, associated with ash release during a mega-volcanic eruption. We found that the snow aging process, its dirtiness and the ash deposition on the snow-free ice are key factors for deglaciation. Our modelling study suggests that, under a CO2 enriched atmosphere, a dusty snowball Earth could reach the deglaciation threshold. 相似文献
18.
A simulation of the possible consequences of a doubling of the CO2 content of the atmosphere has been performed with a low resolution global climatic model. The model included the diurnal and seasonal cycles, computed sea ice amount and cloud cover, and used implied oceanic heat fluxes to represent transport processes in the oceans. A highly responsive 2-layer soil moisture formulation was also incorporated. Twenty year equilibrated simulations for control (1 × CO2) and greenhouse (2 × CO2) conditions were generated. The major emphasis of the analysis presented here is on the intra-annual and interannual variability of the greenhouse run with respect to the control run. This revealed considerable differences from the time-averaged results with occasions of marked positive and negative temperature deviations. Of particular interest were the periods of negative temperature departures compared to the control run which were identified, especially over the Northern Hemisphere continents. Temporal and spatial precipitation and soil moisture anomalies also occurred, some of which were related to the surface temperature changes. Substantial sea surface temperature anomalies were apparent in the greenhouse run, indicating that a source of climatic forcing existed in addition to that due to doubling of the CO2. Comparison of the intra-annual and interannual variability of the control run with that of the greenhouse run suggests that, in many situations, it will be difficult to identify a greenhouse signal against the intrinsic natural variability of the climatic system. 相似文献
19.
A frequently made assumption in simple models of long-term climatic behavior (e.g. ‘energy balance’ models) is that, owing to instability engendered by the ice-albedo feedback, the climatic system (surface temperature, in particular) can exhibit multiple steady states within the paleoclimatologically observed range of temperature. Here we show that, for a more comprehensive model than an energy balance model, the CCM1 general circulation model, such a bimodality does not exist for present values of the solar constant and atmospheric CO2 even if one excludes the existence of a seasonal cycle of radiative forcing. Thus, no evidence is found to support the whole class of ice-age theories tuned to present CO2 levels that depend on this bimodality. As a corollary, support is found for the idea that surficial temperature and snow-sea-ice cover are essentially ‘slaved’, fast-response, climatic variables that equilibrate uniquely with the prescribed external forcing and the slow-response climatic variables (e.g. the ice sheets and deep ocean state). It is also implied that, although care should be exercised in the choice of initial conditions to minimize computer time, the selection of initial conditions is not likely to affect the final outcome of general circulation model studies of climate for fixed (near-present) values of the solar constant and CO2 forcing. 相似文献
20.
The OSU global coupled atmosphere-ocean general circulation model has been used to investigate a 2xCO2-induced climate change. A previous analysis of the simulated 2xCO2–1xCO2 temperature differences showed that the CO2-induced warming penetrated into the ocean and thereby caused a delay in the equilibration of the climate system with an estimatede-folding time of 50–75 years. The objective of the present study is to determine by what pathways and through which physical processes the simulated ocean general circulation produces the penetration of the CO2-induced warming into the ocean.A global-mean oceanic heat budget analysis shows that the ocean gains heat at a rate of 3 W/m2 due to the CO2 doubling, and that this heat penetrates downward into the ocean predominantly through the reduction in the convective overturning. A zonal-mean oceanic heat budget analysis shows that the surface warming increases from the tropics toward the midlatitudes of both hemispheres and gradually penetrated into the deeper ocean, with a greater penetration in the subtropics and midlatitudes than in the equatorial region. The zonal-mean heat budget analysis also shows that the CO2-induced warming of the ocean occurs predominantly through the down-ward transport of heat, with the meridional heat flux being only of secondary importance. In the tropics the penetration of the CO2-induced heating is minimized by the upwelling of cold water. In the subtropics the heating is transported down-ward more readily by the downwelling existing there. In the high latitudes the suppressed convection plays the dominant role in the downward penetration of the CO2-induced heating. The latter result should be considered as tentative, however, as the ocean component of the coupled model employed a prescribed surface salinity field and did not include the mechanism of brine rejection when sea water freezes into sea ice. 相似文献