Reconstructing a lost Eocene paradise: Part I. Simulating the change in global floral distribution at the initial Eocene thermal maximum

This study utilizes the NCAR Land Surface Model (LSM1.2) integrated with dynamic global vegetation to recreate the early Paleogene global distribution of vegetation and to examine the response of the vegetation distribution to changes in climate at the Paleocene–Eocene boundary (∼ 55 Ma). We run two simulations with Eocene geography driven by climatologies generated in two atmosphere global modeling experiments: one with atmospheric pCO₂ at 560 ppm, and another at 1120 ppm. In both scenarios, the model produces the best match with fossil flora in the low latitudes. A comparison of model output from the two scenarios suggests that the greatest impact of climate on vegetation will occur in the high latitudes, in the Arctic Circle and in Antarctica. In these regions, greater accumulated summertime warmth in the 1120 ppm simulation allows temperate plant functional types to expand further poleward. Additionally, the high pCO₂ scenario produces a greater abundance of trees over grass at these high latitudes. In the middle and low latitudes, the general distribution of plant functional types is similar in both pCO₂ scenarios. Likely, a greater increment of greenhouse gases is necessary to produce the type of change evident in the mid-latitude paleobotanical record. Overall, differences between model output and fossil flora are greatest at high latitudes.     

Pre-industrial-potential and Last Glacial Maximum global vegetation simulated with a coupled climate-biosphere model: diagnosis of bioclimatic relationships

The global climate–vegetation model HadSM3_TRIFFID has been used to estimate the equilibrium states of climate and vegetation with pre-industrial and last glacial boundary conditions. The present study focuses on the evaluation of the terrestrial biosphere component (TRIFFID) and its response to changes in climate and CO₂ concentration. We also show how, by means of a diagnosis of the distribution of plant functional types according to climate parameters (soil temperature, winter temperature, growing-degree days, precipitation), it is possible to get better insights into the strengths and weaknesses of the biosphere model by reference to field knowledge of ecosystems.The model exhibits profound changes between the vegetation distribution at the Last Glacial Maximum and today that are generally consistent with palaeoclimate data, such as the disappearance of the Siberian boreal forest (taiga), an increase in shrub cover in Europe and an increase of the subtropical desert area. The effective equatorial and sub-tropical tree area is reduced by 18%. There is also an increase in cover of wooded species in North-Western Africa and in Mexico. The analysis of bioclimatic relationships turns out to be an efficient method to infer the contributions of different climatic factors to vegetation changes, both at high latitudes, where the position of the boreal treeline appears in this model to be more directly constrained by the water stress than by summer temperature, and in semi-humid areas where the contributions of temperature and precipitation changes may partly compensate each other. Our study also confirms the major contribution of the decrease in CO₂ to environmental changes and carbon storage through its selective impact on gross primary productivity of C3 and C4 plants and a reduction by 25% of water-use efficiency. Specifically, the reduction in CO₂ concentration increases the amount of precipitation necessary to sustain at least 20% of grass fraction by 50 mm/year; the corresponding threshold for trees is increased by about 150 mm/year. As a consequence, a reduction in CO₂ concentration considerably widens the climatic range where grasses and shrubs dominate.     

Albedo control of seasonal South Polar cap recession on Mars

Over the last few decades, General Circulation Models (GCM) have been used to simulate the current martian climate. The calibration of these GCMs with the current seasonal cycle is a crucial step in understanding the climate history of Mars. One of the main climatic signals currently used to validate GCMs is the annual atmospheric pressure cycle. It is difficult to use changes in seasonal deposits on the surface of Mars to calibrate the GCMs given the spectral ambiguities between CO₂ and H₂O ice in the visible range. With the OMEGA imaging spectrometer covering the near infra-red range, it is now possible to monitor both types of ice at a spatial resolution of about 1 km. At global scale, we determine the change with time of the Seasonal South Polar Cap (SSPC) crocus line, defining the edge of CO₂ deposits. This crocus line is not symmetric around the geographic South Pole. At local scale, we introduce the snowdrop distance, describing the local structure of the SSPC edge. Crocus line and snowdrop distance changes can now be used to calibrate GCMs. The albedo of the seasonal deposits is usually assumed to be a uniform and constant parameter of the GCMs. In this study, albedo is found to be the main parameter controlling the SSPC recession at both global and local scale. Using a defrost mass balance model (referred to as D-frost) that incorporates the effect of shadowing induced by topography, we show that the global SSPC asymmetry in the crocus line is controlled by albedo variations. At local scale, we show that the snowdrop distance is correlated with the albedo variability. Further GCM improvements should take into account these two results. We propose several possibilities for the origin of the asymmetric albedo control. The next step will be to identify and model the physical processes that create the albedo differences.     

Climate model sensitivity to atmospheric CO2 concentrations for the middle Miocene

We present results of the first middle Miocene climate modelling study using the latest NCAR Community Atmosphere Model (CAM v.3.1) and Community Land Model (CLM v.3.0) coupled to a slab ocean. We examine the sensitivity of the middle Miocene climate to varying concentrations of atmospheric carbon dioxide (180, 355 and 700 ppm). Model simulations are forced with realistic Miocene boundary conditions for continental geometry, topography and vegetation. Global annual mean surface temperature increases by 2.2 °C with each successive doubling of CO₂ which is consistent with climate sensitivity of previous paleoclimate studies and estimates for future climate. In addition to growing evidence that tropical sea surface temperatures were higher than suggested by proxy-data, our understanding of middle to high latitude warming mechanisms is still incomplete. We compare our results to the late Miocene study of Steppuhn et al. [Steppuhn, A., Micheels, A., Bruch, A., Uhl, D., Utescher, T., Mosbrugger, V., 2007. The sensitivity of ECHAM4/ML to a double CO₂ scenario for the Late Miocene and the comparison to terrestrial proxy data. Global and Planetary Change, 57, 189–212] to explore the dependence of paleoclimate model sensitivities on different software systems and boundary conditions. Our comparison shows climate sensitivity to be overall quite robust — this is as significant, as it is often unclear to what extent simulation behaviour and outputs are dependent on a particular model implementation and initial/boundary conditions. Some distinct differences in model outputs, such as our reduced latitudinal surface temperature gradient and stronger Asian monsoon system, compared to the late Miocene study of Steppuhn et al. [Steppuhn, A., Micheels, A., Bruch, A., Uhl, D., Utescher, T., Mosbrugger, V., 2007. The sensitivity of ECHAM4/ML to a double CO₂ scenario for the Late Miocene and the comparison to terrestrial proxy data. Global and Planetary Change, 57, 189–212] are shown to be closely linked to the choice of topography, vegetation and ocean heat flux.     

The sensitivity of ECHAM4/ML to a double CO2 scenario for the Late Miocene and the comparison to terrestrial proxy data

For the Tortonian, Steppuhn et al. [Steppuhn, A., Micheels, A., Geiger, G., Mosbrugger, V., 2006. Reconstructing the Late Miocene climate and oceanic heat flux using the AGCM ECHAM4 coupled to a mixed-layer ocean model with adjusted flux correction. Palaeogeography, Palaeoclimatology, Palaeoecology, 238, 399–423] perform a model simulation which considers a generally lower palaeorography, a weaker ocean heat transport and an atmospheric CO₂ concentration of 353 ppm. The Tortonian simulation of Steppuhn et al. [Steppuhn, A., Micheels, A., Geiger, G., Mosbrugger, V., 2006. Reconstructing the Late Miocene climate and oceanic heat flux using the AGCM ECHAM4 coupled to a mixed-layer ocean model with adjusted flux correction. Palaeogeography, Palaeoclimatology, Palaeoecology, 238, 399–423] demonstrates some realistic trends: the high latitudes are warmer than today and the meridional temperature gradient is reduced. However, the Tortonian run also indicates some insufficiencies such as too cool mid-latitudes which can be due to an underestimated pCO₂ in the atmosphere. As a sensitivity study, we perform a further model experiment for which we additionally increase the atmospheric carbon dioxide (700 ppm). According to this CO₂ sensitivity experiment, we find a global warming and a globally more intense water cycle as compared to the previous Tortonian run. Particularly the high latitudes are warmer in the Tortonian CO₂ sensitivity run which leads to a lower amount of Arctic sea ice and a reduced equator-to-pole temperature difference. Our Tortonian CO₂ sensitivity study basically agrees with results from recent climate model experiments which consider an increase of CO₂ during the next century (e.g. [Cubasch, U., Meehl, G.A., Boer, G.J., Stouffer, R.J., Dix, M., Noda, A., Senior, C.A., Raper, S., Yap, K.S., 2001. Projections of Future Climate Change. In: Houghton, J.T., Y. Ding, D.J. Griggs, M. Noguer, P.J. van der Linden, X. Dai, K. Maskell, C.A. Johnson (eds.), Climate Change 2001: The Scientific Basis. Contribution of Working Group I to the Third Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge University Press, Cambridge, 525–582]) suggesting that the climatic response on a higher atmospheric CO₂ concentration is almost independent from the different settings of boundary conditions (Tortonian versus today). To validate the Tortonian model simulations, we perform a quantitative comparison with terrestrial proxy data. This comparison demonstrates that the Tortonian CO₂ sensitivity experiment tends to be more realistic than the previous Tortonian simulation by Steppuhn et al. [Steppuhn, A., Micheels, A., Geiger, G., Mosbrugger, V., 2006. Reconstructing the Late Miocene climate and oceanic heat flux using the AGCM ECHAM4 coupled to a mixed-layer ocean model with adjusted flux correction. Palaeogeography, Palaeoclimatology, Palaeoecology, 238, 399–423]. However, a high carbon dioxide concentration of 700 ppm is questionable for the Late Miocene, and it cannot explain shortcomings of our Tortonian run with ‘normal’ CO₂. In order to fully understand the Late Miocene climate, further model experiments should also consider the palaeovegetation.     

2×CO2 Eastern Asia regional responses in the RSM/CCM3 modeling system

A global to regional modeling system has been developed to evaluate precipitation under doubled CO₂. The National Centers for Environmental Prediction (NCEP) regional spectral model (RSM) is initialized and forced by current and doubled CO₂ simulations from the NCAR community climate model (CCM3). Three RSM simulations, RSM0, RSM1, and RSM2, with resolution of 280, 50 and 15 km, are examined. The RSM0 setup resolution matches the T42 CCM3 simulations. The RSM2 simulation is centered over Taiwan. Due to incompatibility of the model physics, noticeable differences between RSM0 and CCM3 are found, especially in wintertime, which suggests that simulation from RSM0, rather than CCM3, should be used to contrast high-resolution regional variations produced by RSM1 or RSM2 simulations.While the spatial distributions of RSM1 and RSM2 simulations over Taiwan are greatly improved over the CCM3 simulation, the intensity of the unique wintertime drizzle is overestimated, especially in RSM2. There is also a spurious northward extension of the precipitation pattern from the subtropical warm-pool region. Thus the regional response to doubled CO₂, which consists of more summerlike wintertime precipitation characteristics over the northeastern and eastern sides of Taiwan, with increased intensity mostly in the extreme events, is still in doubt and must be examined with improved global and regional models.     

Investigating the mechanisms leading to the deglaciation of past continental northern hemisphere ice sheets with the CLIMBER–GREMLINS coupled model

A coupling procedure between a climate model of intermediate complexity (CLIMBER-2.3) and a 3-dimensional thermo-mechanical ice-sheet model (GREMLINS) has been elaborated. The resulting coupled model describes the evolution of atmosphere, ocean, biosphere, cryosphere and their mutual interactions. It is used to perform several simulations of the Last Deglaciation period to identify the physical mechanisms at the origin of the deglaciation process. Our baseline experiment, forced by insolation and atmospheric CO₂, produces almost complete deglaciation of past northern hemisphere continental ice sheets, although ice remains over the Cordilleran region at the end of the simulation and also in Alaska and Eastern Siberia. Results clearly demonstrate that, in this study, the melting of the North American ice sheet is critically dependent on the deglaciation of Fennoscandia through processes involving switches of the thermohaline circulation from a glacial mode to a modern one and associated warming of the northern hemisphere. A set of sensitivity experiments has been carried out to test the relative importance of both forcing factors and internal processes in the deglaciation mechanism. It appears that the deglaciation is primarily driven by insolation. However, the atmospheric CO₂ modulates the timing of the melting of the Fennoscandian ice sheet, and results relative to Laurentide illustrate the existence of threshold CO₂ values, that can be translated in terms of critical temperature, below which the deglaciation is impeded. Finally, we show that the beginning of the deglaciation process of the Laurentide ice sheet may be influenced by the time at which the shift of the thermohaline circulation from one mode to the other occurs.     

Cosmic conclusions from climatic models: Can they be justified?

Climatic models are increasingly being used to answer “cosmic questions” such as the possibility of an ice-covered Earth or a runaway greenhouse effect, or to examine the coevolution of climate and life. Conclusions from these models on such issues, of course, rest on the physical parameterizations of the models. Some of the basic parameterizations are reexamined quantitatively, and it is concluded that presently believed uncertainties in these parameterizations lead to an order-of-magnitude uncertainty in estimates of the sensitivity of the present Earth's climate to external forcings (like a change in solar constant). However, seasonal simulations with present Earth models suggest that estimates of the overall sensitivity of the climate to external forcing may be narrowed (over decadal time scales) to, perhaps, a factor of 2. But the effects of glaciers, continental locations, and atmospheric composition, all of which can change on geological time scales, further enhance the uncertainties in long-term climatic sensitivity estimates from state-of-the-art models. But it is precisely these long-term estimates of climatic sensitivity which support quantitative conclusions on, for example, the possible existence of continuously habitable zones around main-sequence stars. We believe that those who draw cosmic conclusions from climatic models should at least attempt to bracket the final results by repeating their calculations over a plausible range of uncertainty in basic model parameterizations.     

Climate and carbon cycle changes under the overshoot scenario

The “overshoot scenario” is an emissions scenario in which CO₂ concentration in the atmosphere temporarily exceeds some pre-defined, “dangerous” threshold (before being reduced to non-dangerous levels). Support for this idea comes from its potential to achieve a balance between the burdens of current and future generations in dealing with global warming. Before it can be considered a viable policy, the overshoot scenario needs to be examined in terms of its impacts on the global climate and the environment. In, particular, it must be determined if climate change cause by the overshoot scenario is reversible or not, since crossing that “dangerous” CO₂ threshold could result in climate change from which we might not be able to recover. In this study, we quantify the change in several climatic and environmental variables under the overshoot scenario using a global climate model of intermediate complexity. Compared to earlier studies on the overshoot scenario, we have an explicit carbon cycle model that allows us to represent carbon-climate feedbacks and force the climate model more realistically with CO₂ emissions rates rather than with prescribed atmospheric pCO₂. Our standard CO₂ emissions rate is calculated on the basis of historical atmospheric pCO₂ data and the WRE S650 non-overshoot stabilization profile. It starts from the preindustrial year 1760, peaks in the year 2056, and ends in the year 2300. A variety of overshoot scenarios were constructed by increasing the amplitude of the control emissions peak but decreasing the peak duration so that the cumulative emissions remain essentially constant. Sensitivity simulations of various overshoot scenarios in our model show that many aspects of the global climate are largely reversible by year 2300. The significance of the reversibility, which takes roughly 200 years in our experiments, depends on the time horizon with which it is viewed or the number of future generations for whom equity is sought. At times when the overshoot scenario has emissions rates higher then the control scenario, the transient changes in atmospheric and oceanic temperatures and surface ocean pH can be significant, even for moderate overshoot scenarios that remain within IPCC SRES emissions scenarios. The large transient changes and the centennial timescale of climate reversibility suggest that the overshoot might not be the best mitigation approach, even if it technically follows the optimal economic path.     

The time-scale for core collapse in spherical star clusters

The collapse time for a cluster of equal-mass stars is usually stated to be either 330 central relaxation times (t_rc) or 12-19 half-mass relaxation times (t_rh). But the first of these times applies only to the late stage of core collapse, and the second only to low-concentration clusters. To clarify how the time depends on the density profile, the Fokker-Planck equation is solved for the evolution of a variety of isotropic cluster models, including King models, models with power-law density cusps of ρ ∼ r^−γ, and models with nuclei. The collapse times for King models vary considerably with the cluster concentration when expressed in units of t_rc or t_rh, but vary much less when expressed in units of t_rc divided by a dimensionless measure of the temperature gradient in the core. Models with cusps have larger temperature gradients and evolve faster than King models, but not all of them collapse: those with 0 < γ < 2 expand because they start with a temperature inversion. Models with nuclei collapse or expand as the nuclei would in isolation if their central relaxation times are short; otherwise their evolution is more complicated. Suggestions are made for how the results can be applied to globular clusters, galaxies, and clusters of dark objects in the centers of galaxies.Scott D. Tremaine     

The natural latitudinal distribution of atmospheric CO2

Although poorly understood, the north–south distribution of the natural component of atmospheric CO₂ offers information essential to improving our understanding of the exchange of CO₂ between the atmosphere, oceans, and biosphere. The natural or unperturbed component is equivalent to that part of the atmospheric CO₂ distribution which is controlled by non-anthropogenic CO₂ fluxes from the ocean and terrestrial biosphere. Models should be able to reproduce the true north–south gradient in CO₂ due to the natural component before they can reliably estimate present-day CO₂ sources and sinks and predict future atmospheric CO₂. We have estimated the natural latitudinal distribution of atmospheric CO₂, relative to the South Pole, using measurements of atmospheric CO₂ during 1959–1991 and corresponding estimates of anthropogenic CO₂ emissions to the atmosphere. Key features of the natural latitudinal distribution include: (1) CO₂ concentrations in the northern hemisphere that are lower than those in the southern hemisphere; (2) CO₂ concentration differences that are higher in the tropics (associated with outgassing of the oceans) than those currently measured; and (3) CO₂ concentrations over the southern ocean that are relatively uniform. This natural latitudinal distribution and its sensitivity to increasing fossil fuel emissions both indicate that near-surface concentrations of atmospheric CO₂ in the northern hemisphere are naturally lower than those in the southern hemisphere. Models that find the contrary will also mismatch present-day CO₂ in the northern hemisphere and incorrectly ascribe that region as a large sink of anthropogenic CO₂.     

The effect of climate change on carbon in Canadian peatlands

Peatlands, which are dominant features of the Canadian landscape, cover approximately 1.136 million km², or 12% of the land area. Most of the peatlands (97%) occur in the Boreal Wetland Region (64%) and Subarctic Wetland Region (33%). Because of the large area they cover and their high organic carbon content, these peatlands contain approximately 147 Gt soil carbon, which is about 56% of the organic carbon stored in all Canadian soils.A model for estimating peatland sensitivity to climate warming was used to determine both the sensitivity ratings of various peatland areas and the associated organic carbon masses. Calculations show that approximately 60% of the total area of Canadian peatlands and 51% of the organic carbon mass in all Canadian peatlands is expected to be severely to extremely severely affected by climate change.The increase in average annual air temperature of 3–5 °C over land and 5–7 °C over the oceans predicted for northern Canada by the end of this century would result in the degradation of frozen peatlands in the Subarctic and northern Boreal wetland regions and severe drying in the southern Boreal Wetland Region. In addition, flooding of coastal peatlands is expected because of the predicted rise in sea levels. As a result of these changes, a large part of the carbon in the peatlands expected to be severely and extremely severely affected by climate change could be released into the atmosphere as carbon dioxide (CO₂) and methane (CH₄), which will further increase climate warming.     

Studying the influence of turbulent mixing on thermonuclear burning regimes during X-ray bursts of neutron stars using the <Emphasis Type="Italic">K</Emphasis><Subscript><Emphasis Type="Italic">ε</Emphasis></Subscript> model

We study the influence of turbulent mixing on the development of thermonuclear flashes in the surface layers of neutron stars. A simple K _ε model that includes various physical processes is used to describe the turbulent processes. In contrast to the widespread mixing-length theory, the K _ε model does not require using additional dimensional parameters, traces the development of turbulence in dynamics, describes the various turbulence development scenarios (gravitational and shear instabilities, convection, semiconvection, etc.) in a unified way, and can be used in multidimensional numerical simulations. Empirical constants of the model are chosen on the basis of experimental data and direct numerical simulations of typical processes. We have used the Era and Tigr-3T software packages to numerically simulate thermonuclear flashes in the accretion-renewable atmospheres of neutron stars. Turbulence is shown to accelerate significantly the transport of released energy to the stellar surface. Mixing equalizes the concentrations of matter components throughout the burning layer and increases the amount of matter involved in the thermonuclear burning during a flash.     

The Spectrum of Torsional Oscillations of the Moon

The diagnostic potentialities of the torsional oscillations for probing the structure of the interiors of the Moon are investigated. Models with no core, a liquid core, and a solid core are considered. The profiles of compressional and shear wave velocities V _P and V _S for the lunar interior estimated by Bills and Ferrari (1977), Goins et al. (1981), and Nakamura (1983) from the Apollo lunar seismic network are used. For all these models, the periods of torsional oscillations for n = 2–100 and four overtones have been calculated. The derivatives of the dimensionless eigenfrequency with respect to the dimensionless shear modulus and density are calculated and tabulated for use. These data can be used to determine corrections to the model density and shear modulus distributions due to their small change. The damping of torsional oscillations is studied. Several trial radial distributions of the dissipative function Q are considered.     

Use of a land-surface-transfer scheme (LSX) in a global climate model: the response to doubling stomatal resistance

One response of vegetation to future increases in atmospheric CO₂ may be a widespread increase in stomatal resistance. Such a response would increase plant water usage efficiency while still allowing CO₂ assimilation at current rates. The associated reduction in transpiration rates has the potential of causing significant modifications in climate on regional and global scales.This paper describes the effects of a uniform doubling of the stomatal resistance parameterization in a global climate model (GENESIS). The model includes a land-surface transfer scheme (LSX) that accounts for the physical effects of vegetation, including stomatal resistance and transpiration, which is described in detail in an appendix. The atmospheric general circulation model is a heavily modified version of the NCAR Community Climate Model version 1 with new treatments of clouds, penetrative convection, planetary boundary layer mixing, solar radiation, the diurnal cycle, and semi-Lagrangian transport of water vapor. The other surface models include multi-layer models of soil, snow and sea ice, and a 50-m slab ocean mixed layer.The effects of doubling the stomatal resistance parameterization are largest in heavily forested regions: tropical South America, and parts of the Northern Hemispheric boreal forests in Canada, Russia and Siberia in summer. The primary surface changes are a decrease in evapotranspiration, an increase in upward sensible heat flux, and a surface-air warming. Secondary effects include shifts in the ITCZ which cause large increases in precipitation, soil moisture and runoff in western tropical South America, and decreases in these quantities in northern subtropical Africa. Noticeable changes in relative humidity, cloudiness and meridional circulation occur throughout the troposphere. The global effects on atmospheric temperature and specific humidity are small fractions of those found in other doubled CO₂ experiments. However, unlike doubled CO₂ the signs of those changes combine to give relatively large reductions in relative humidity and cloudiness. It is suggested that the stomatal-resistance effect and other plant responses to large-scale environmental perturbations should be included in models of future climate.     

Climate control on Venus: Comparison of the carbonate and pyrite models

We review two models describing the Venus climate system: the carbonate and pyrite models. It has been argued carbonate and pyrite are potentially important minerals controlling the climate of Venus, though existence of either minerals has not been confirmed. Although it used to be proposed that carbonation reaction might explain the Venus’ atmospheric CO₂ abundance, it is unlikely Venus’ surface is reactive enough to control the Venus’ massive CO₂ atmosphere. Venus’ surface carbonate is also able to affect the climate through the reaction with atmospheric SO₂ to form anhydrite. Under the carbonate model the climate state is not in equilibrium and would be unstable due to the reaction between carbonate and SO₂. On the other hand, pyrite-magnetite reaction is proposed to explain the Venus’ atmospheric SO₂ abundance. Under pyrite-magnetite reaction, however, the climate would be stabilized such that the existing climate state is maintained over a geological timescale, while some observational facts such as atmospheric abundance of SO₂ and surface temperature could also be reasonably explained.     

Diffuse Interstellar Band at 862 nm as a Reddening Tracer for GAIA

We investigate the plausibility of using diffuse interstellar band at862 nm for tracing interstellar extinction with the ESA's astrometric space mission GAIA. For this purpose we perform numerical tests to simulate the conditions of real observations, covering a wide range of stellar parameters and different amounts of interstellar extinction. Our simulations indicate that with the present Radial Velocity Spectrometer setup the uncertainty in color excess of σ_E(B-V)≤ 0.05 can be achieved only for the interstellar reddening tracers brighter than V ∼ 13. None of the plausible tracers can provide accurate color excesses (σ _E(B-V) ≤ 0.05) at the distances beyond 2 kpc. We therefore conclude that with the currently planned instrumentation onboard GAIA this method can not be used as a stand-alone approach for probing interstellar extinction on the Galactic distance scales within the framework of the GAIA mission. This revised version was published online in July 2006 with corrections to the Cover Date.     

Role of H2O and CO2 Ices in Martian Climate Changes

In order to study the stability of martian climate, we constructed a two-dimensional (horizontal-vertical) energy balance model. The long-term CO₂ mass exchange process between the atmosphere and CO₂ ice caps is investigated with particular attention to the effect of planetary ice distribution on the climate stability. Our model calculation suggests that high atmospheric pressure presumed for past Mars would be unstabilized if H₂O ice widely prevailed. As a result, a cold climate state might have been achieved by the condensation of atmospheric CO₂ onto ice caps. On the other hand, the low atmospheric pressure, which is buffered by the CO₂ ice cap and likely close to the present pressure, would be unstabilized if the CO₂ ice albedo decreased. This may have led the climate into a warm state with high atmospheric pressure owing to complete evaporation of CO₂ ice cap. Through the albedo feedback mechanisms of H₂O and CO₂ ices in the atmosphere-ice cap system, Mars may have experienced warm and cold climates episodically in its history.     

Quantifying the response of forest carbon balance to future climate change in Northeastern China: Model validation and prediction

In this study, we report on the validation of process-based forest growth and carbon and nitrogen model of TRIPLEX against observed data, and the use of the model to investigate the potential impacts and interaction of climate change and increasing atmospheric CO₂ on forest net primary productivity (NPP) and carbon budgets in northeast of China. The model validation results show that the simulated tree total volume, NPP, total biomass and soil carbon are consistent with observed data across the Northeast of China, demonstrating that the improved TRIPLEX model is able to simulate forest growth and carbon dynamics of the boreal and temperate forest ecosystems at regional scale. The climate change would increase forest NPP and biomass carbon but decrease overall soil carbon under all three climate change scenarios. The combined effects of climate change and CO₂ fertilization on the increase of NPP were estimated to be 10–12% for 2030s and 28–37% in 2090s. The simulated effects of CO₂ fertilization significantly offset the soil carbon loss due to climate change alone. Overall, future climate change and increasing atmospheric CO₂ would have a significant impact on the forest ecosystems of Northeastern China.