Radiative Effects Of CO2 Research Articles

Ignoring Plant Physiological Responses to Elevated CO2 Will Overestimate Terrestrial Vertebrate Biodiversity Loss Under Global Climate Change

AbstractRising CO2 is accelerating Earth warming and drying by altering the radiation budget (CO2 radiative effect), and it is commonly thought that future biodiversity will be greatly threatened. However, elevated CO2 also modifies plant physiology including photosynthesis rate and water use efficiency (CO2 physiological effect), the impacts of which have been usually ignored. By coupling machine learning techniques and Earth system models, this study considered both of the two effects and simulated terrestrial vertebrate biodiversity changes under moderate CO2 emission pathway. Our results show that the CO2 radiative effect positively impacts terrestrial vertebrate biodiversity at high latitudes but has a negative impact at low latitudes, whereas the physiological effect generally has a positive impact that peaks near the equator. Between 45°N and 45°S, 15% of areas with diminishing terrestrial vertebrate biodiversity due to radiative effect experience a reversal in trend when physiological effect is also considered; species migration pressure is also mitigated. Additionally, the protection priority rankings of 36 biodiversity hotspots need to be readjusted with the consideration of CO2 physiological effect, for which the impacts on terrestrial vertebrate biodiversity are important and varied greatly over space. Overall, our findings highlight the critical impacts of vegetation physiology on terrestrial vertebrate biodiversity in response to elevated atmospheric CO2, which can provide a reference for resource allocation and biodiversity conservation policymaking.

Strong Nonlinearity of Land Climate‐Carbon Cycle Feedback Under a High CO2 Growth Scenario

AbstractProjections of future climate change for given CO2 and other greenhouse gas emission scenarios depend on the response of global climate‐carbon cycle feedback, which consists of carbon‐concentration feedback (e.g., CO2 physiology effect on land carbon sink) and carbon‐climate feedback (e.g., CO2 radiative effect on land carbon sink). Previous studies have assumed no significant interaction between these two feedbacks within the Earth system. This study quantifies the interaction of these two feedbacks, or the nonlinear feedback on land using the fully, biogeochemically, and radiatively coupled simulations under a 1% yr−1 CO2 increase path from nine Earth system models of the Coupled Model Intercomparison Project Phase 6 (CMIP6). The results show that the nonlinear feedback is 1.64 ± 2.92 × 10−2 GtC ppm−1 K−1 at the end of 140‐year simulation with a quadrupling CO2 (4 × CO2), where its strength is 11% ± 18% of the carbon‐concentration feedback or −27% ± 49% of the carbon‐climate feedback on land. Compared to previous assumptions that did not consider this interaction, the nonlinear feedback contributes about 8% ± 12% of the land carbon increase accumulated at the 4 × CO2. The nonlinear feedback largely results from the combined effect of increased CO2‐induced additional fertilization effect on warming‐induced additional leaf area index and vegetation productivity over the Northern Hemisphere. The magnitude of the nonlinear feedback on land decreases with an increase in atmospheric CO2 or warming under the high emission scenario. This study highlights the significance of land nonlinear climate‐carbon cycle feedback in increasing land carbon sink and slowing down future climate change.

Downwind control of oceanic air by land: the land wake and its sensitivity to CO2

Oceans are well-known to be directly altered by global climate forcings such as greenhouse gas changes, but how oceans are indirectly influenced by land and its response to such forcings remains less explored. Here, we assess the present-day and projected future state of a little-explored feature of the climate system—a ‘land wake’ in relative humidity downwind of the east coast of North America, consisting of low-humidity continental air extending roughly 1000 km over the Atlantic ocean. The wake exists throughout the year, but is supported by high continental temperatures in summer and low continental moisture in winter. The wake is well represented in an ensemble of global climate models (GCMs), qualitatively matching reanalysis data. Under increasing atmospheric CO2, the land wake intensifies in GCM simulations through two pathways: the radiative effects of CO2 on surface temperatures, and the biogeochemical effect of CO2 on terrestrial vegetation. Vegetation responses to increased CO2 alter the summer wake from Florida to Newfoundland, and both the radiative and biogeochemical effects of CO2 drive reductions in coastal cloud cover. These changes illustrate the potential of rapidly changing terrestrial climate to influence coastal regions and the ocean environment downwind of continents through both light conditions and the energy balance of the surface ocean.

Peatland Heterogeneity Impacts on Regional Carbon Flux and Its Radiative Effect Within a Boreal Landscape

AbstractPeatlands, with high spatial variability in ecotypes and microforms, constitute a significant part of the boreal landscape and play an important role in the global carbon (C) cycle. However, the effects of this peatland heterogeneity within the boreal landscape are rarely quantified. Here, we use field‐based measurements, high‐resolution land cover classification, and biogeochemical and atmospheric models to estimate the atmosphere‐ecosystem C fluxes and the corresponding radiative effect (RE) for a boreal landscape (Kaamanen) in northern Finland. Our result shows that the Kaamanen catchment currently functioned as a sink of carbon dioxide (CO2) and a source of methane (CH4). Peatlands (26% of the area) contributed 22% of the total CO2 uptake and 89% of CH4 emissions; forests (61%) accounted for 78% of CO2 uptake and offset 6% of CH4 emissions; water bodies (13%) offset 7% of CO2 uptake and contributed 11% of CH4 emissions. The heterogeneity of peatlands accounted for 11%, 88%, and 75% of the area‐weighted variability (deviation from the area‐weighted mean among different land cover types (LCTs) within the catchment) in CO2 flux, CH4 flux, and the combined RE of CO2 and CH4 exchanges over the 25‐year time horizon, respectively. Aggregating peatland LCTs or misclassifying them as nonpeatland LCTs can significantly (p < 0.05) bias the regional CH4 exchange and RE estimates, while differentiating between drier noninundated and wetter inundated peatlands can effectively reduce the bias. Current land cover products lack such details in peatland heterogeneity, which would be needed to better constrain boreal C budgets and global C‐climate feedbacks.

Winter Precipitation Changes in California Under Global Warming: Contributions of CO2, Uniform SST Warming, and SST Change Patterns

AbstractClimate models project a wetting winter over California, distinct from the drying in all other Mediterranean climate regions under warming. Dynamic effect via an eastward extension of the westerly jet stream over the North Pacific is the root cause for the wetting California. Utilizing four sets of prescribed‐SST (sea surface temperature) simulations, the relative effects of direct CO2 radiative forcing, uniform SST warming, and SST change patterns are investigated. The wetting winter is dominated by uniform SST warming mainly through its effect on the jet extension. As a fundamental response to the enhanced upper‐tropospheric warming in the tropics, the jet extension in coupled simulations is well reproduced under uniform SST warming and robust among models. An El Niño‐like SST warming pattern has a secondary contribution to the jet extension via atmospheric teleconnection, but with large inter‐model spread. Relative to the SST forcing, the direct CO2 radiative effect is negligible.

Effects of carbon dioxide on the upper flammability limits of methane in O2/CO2 atmosphere

In this paper, the effects of carbon dioxide on the upper flammability limits of oxy-methane were investigated. Firstly, the upper flammability limits of oxy-methane were measured under normal pressure and temperature. Then, the chemical effect of CO2 on the upper flammability was studied using the kinetic simulation. Finally, the thermal and radiative effects of CO2 on the upper flammability limits were discussed using energy balance analysis. The experimental upper flammability limits of CH4/O2/CO2 are from 9.5% to 33.25% in the condition of increasing oxygen concentrations from 15% to 50%. The analysis shows that the upper flammability limits of methane are mainly determined by the thermal property of CO2 and not significantly changed by the chemical and radiative effects.

Decomposing East-Asian winter temperature and monsoonal circulation changes using timeslice experiments

Based on a set of pilot atmosphere-only experiments from the Cloud Feedback Model Intercomparison Project Phase 3 (CFMIP-3), in this study, the winter surface air temperature (SAT) and monsoonal circulation changes in East Asia as a response to the 4 × CO2 forcing in coupled model are decomposed into the four parts in terms of the responses to the uniform SST warming, 4 × CO2 radiative effect, SST pattern changes, and plant physiological effect. The uniform SST warming presents the most significant influence on the increase of SAT change, which strengthens the East Asian winter monsoon (EAWM) circulation. The CO2 radiative effect can also induce the SAT increase over East Asia but with a magnitude smaller compared to the uniform SST warming, in which more warming is in land than ocean and the EAWM circulation could be weakened consequently due to the decreased land–sea thermal contrast in response to the CO2 radiative effect. The SAT changes in response to the SST pattern change show inconsistencies over the eastern and southern parts of East Asia between the two models, associated with the large difference for EAWM circulation changes, indicating that the SST pattern change could be the primary source of inter-model uncertainties in the East-Asian SAT change. As for the influence of plant physiological effect, it could generate a SAT rise in many highly vegetated regions. Further analyses for different areas show that both the uniform SST warming and CO2 radiative effects could induce more intense SAT increase in northern East Asia, while the plant physiological effect has a more significant influence on that in southern/eastern part of East Asia.

Diagnosing Changes of Winter NAO in Response to Different Climate Forcings in a Set of Atmosphere-Only Timeslice Experiments

Changes of the winter North Atlantic Oscillation (NAO) variability in response to different climate forcings, and their possible causes, are decomposed and investigated using a set of atmosphere-only timeslice experiments forced by sea surface temperature (SST) from coupled runs. The results indicate that the effects of uniform SST warming and direct CO2 radiative forcing could enhance NAO variability, while SST pattern change could lead to large inter-model difference for model simulations. For the influences of uniform SST warming and the direct CO2 radiative effect, the most significant air temperature increases occur at mid-low latitudes instead of northern polar regions, which produces a greater meridional temperature gradient at mid-high latitudes, thus leading to enhanced westerly winds according to the thermal wind theory. The effects of uniform SST warming and CO2 direct radiative forcing could lead to intensification of winter NAO variability, although this result does not consider ocean-atmosphere coupling. The meridional temperature gradient decreases in most areas of the northern Atlantic under the forcing of SST pattern change, but with a larger inter-model uncertainty, which makes the change of winter NAO variability in response to SST pattern change an open issue.

Understanding the Asian summer monsoon response to greenhouse warming: the relative roles of direct radiative forcing and sea surface temperature change

Future hydroclimate projections from state-of-the-art climate models show large uncertainty and model spread, particularly in the tropics and over the monsoon regions. The precipitation and circulation responses to rising greenhouse gases involve a fast component associated with direct radiative forcing and a slow component associated with sea surface temperature (SST) warming; the relative importance of the two may contribute to model discrepancies. In this study, regional hydroclimate responses to greenhouse warming are assessed using output from coupled general circulation models in the Coupled Model Intercomparison Project-Phase 5 (CMIP5) and idealized atmospheric general circulation model experiments from the Atmosphere Model Intercomparison Project. The thermodynamic and dynamic mechanisms causing the rainfall changes are examined using moisture budget analysis. Results show that direct radiative forcing and SST change exert significantly different responses both over land and ocean. For most part of the Asian monsoon region, the summertime rainfall changes are dominated by the direct CO2 radiative effect through enhanced monsoon circulation. The response to SST warming shows a larger model spread compared to direct radiative forcing, possibly due to the cancellation between the thermodynamical and dynamical components. While the thermodynamical response of the Asian monsoon is robust across the models, there is a lack of consensus for the dynamical response among the models and weak multi-model mean responses in the CMIP5 ensemble, which may be related to the multiple physical processes evolving on different time scales.

Effects of High Concentrations of CO2 on the Lower Flammability Limits of Oxy-methane Mixtures

In this paper, the effects of high concentrations of CO2 on the lower flammability limits (LFL) of the CH4/O2/CO2 mixture were studied. For comparison, the LFL of the CH4/O2/N2 mixture were studied in the same way. First, the LFL of gas mixtures were measured using a cylindrical quartz glass tube in the condition of various oxygen concentrations. The experimental values of LFL of CH4/O2/CO2 decreased with the increase of oxygen concentrations, but the decreasing rate was small. Then, the chemical, thermal, and radiative effects of high concentrations of CO2 on the LFL were analyzed with the energy balance analysis. The thermal property of the gas mixture played the major role in the determination of the LFL. The radiative effect of CO2 on the LFL was much smaller than the thermal effect. The chemical effect of CO2 has little impact on the LFL of CH4/O2/CO2. Finally, the LFL of the CH4/O2/CO2 mixture were well-predicted using the calculated flame temperature method with a fixed critical temperature.

Global modelling of the early martian climate under a denser CO2 atmosphere: Water cycle and ice evolution

We discuss 3D global simulations of the early martian climate that we have performed assuming a faint young Sun and denser CO2 atmosphere. We include a self-consistent representation of the water cycle, with atmosphere–surface interactions, atmospheric transport, and the radiative effects of CO2 and H2O gas and clouds taken into account. We find that for atmospheric pressures greater than a fraction of a bar, the adiabatic cooling effect causes temperatures in the southern highland valley network regions to fall significantly below the global average. Long-term climate evolution simulations indicate that in these circumstances, water ice is transported to the highlands from low-lying regions for a wide range of orbital obliquities, regardless of the extent of the Tharsis bulge. In addition, an extended water ice cap forms on the southern pole, approximately corresponding to the location of the Noachian/Hesperian era Dorsa Argentea Formation. Even for a multiple-bar CO2 atmosphere, conditions are too cold to allow long-term surface liquid water. Limited melting occurs on warm summer days in some locations, but only for surface albedo and thermal inertia conditions that may be unrealistic for water ice. Nonetheless, meteorite impacts and volcanism could potentially cause intense episodic melting under such conditions. Because ice migration to higher altitudes is a robust mechanism for recharging highland water sources after such events, we suggest that this globally sub-zero, ‘icy highlands’ scenario for the late Noachian climate may be sufficient to explain most of the fluvial geology without the need to invoke additional long-term warming mechanisms or an early warm, wet Mars.

Effects of increased CO2 on land water balance from 1850 to 1989

Numerous studies have shown that increased atmospheric CO2 concentration is one of the most important factors altering land water balance. In this study, we investigated the effects of increased CO2 on global land water balance using the dataset released by the Coupled Model Intercomparison Project Phase 5 derived from the Canadian Centre for Climate Modelling and Analysis second-generation Earth System Model. The results suggested that the radiative effect of CO2 was much greater than the physiological effect on the water balance. At the model experiment only integrating CO2 radiative effect, the precipitation, evapotranspiration (ET) and runoff had significantly increased by 0.37, 0.12 and 0.31 mm year−2, respectively. Increases of ET and runoff caused a significant decrease of soil water storage by 0.05 mm year−2. However, the results showed increases of runoff and decreases of precipitation and ET in response to the CO2 fertilisation effect, which resulted into a small, non-significant decrease in the land water budget. In the Northern Hemisphere, especially on the coasts of Greenland, Northern Asia and Alaska, there were obvious decreases of soil water responding to the CO2 radiative effect. This trend could result from increased ice–snow melting as a consequence of warmer surface temperature. Although the evidence suggested that variations in soil moisture and snow cover and vegetation feedback made an important contribution to the variations in the land water budget, the effect of other factors, such as aerosols, should not be ignored, implying that more efforts are needed to investigate the effects of these factors on the hydrological cycle and land water balance.

Importance of carbon dioxide physiological forcing to future climate change

An increase in atmospheric carbon dioxide (CO(2)) concentration influences climate both directly through its radiative effect (i.e., trapping longwave radiation) and indirectly through its physiological effect (i.e., reducing transpiration of land plants). Here we compare the climate response to radiative and physiological effects of increased CO(2) using the National Center for Atmospheric Research (NCAR) coupled Community Land and Community Atmosphere Model. In response to a doubling of CO(2), the radiative effect of CO(2) causes mean surface air temperature over land to increase by 2.86 +/- 0.02 K (+/- 1 standard error), whereas the physiological effects of CO(2) on land plants alone causes air temperature over land to increase by 0.42 +/- 0.02 K. Combined, these two effects cause a land surface warming of 3.33 +/- 0.03 K. The radiative effect of doubling CO(2) increases global runoff by 5.2 +/- 0.6%, primarily by increasing precipitation over the continents. The physiological effect increases runoff by 8.4 +/- 0.6%, primarily by diminishing evapotranspiration from the continents. Combined, these two effects cause a 14.9 +/- 0.7% increase in runoff. Relative humidity remains roughly constant in response to CO(2)-radiative forcing, whereas relative humidity over land decreases in response to CO(2)-physiological forcing as a result of reduced plant transpiration. Our study points to an emerging consensus that the physiological effects of increasing atmospheric CO(2) on land plants will increase global warming beyond that caused by the radiative effects of CO(2).

A broader view of the role of humans in the climate system

The 2007 report from the Intergovernmental Panel on Climate Change Working Group I presents a narrow view of the state of climate science.1 Attempts to significantly influence regional and local-scale climate based on controlling carbon dioxide emissions alone cannot succeed since humans are significantly altering the global climate in a variety of diverse ways beyond the radiative effect of CO2. The IPCC assessments have been too conservative in recognizing the importance of these human climate forcings as they alter regional and global climate. When the IPCC focuses its policy attention on CO2, it neglects other important aspects of the impact of human activities on climate.

Impact of CO2 concentration changes on the biosphere‐atmosphere system of West Africa

AbstractVegetation dynamics plays a critical role in causing the decadal variability of precipitation over the Sahel region of West Africa. However, the potential impact of changes in CO2 concentration on vegetation dynamics and precipitation variability of this region has not been addressed by previous studies. In this paper, we explore the role of CO2 concentration in the regional climate system of West Africa using a zonally symmetric, synchronously coupled biosphere‐atmosphere model. We first document the response of precipitation and vegetation to incremental changes of CO2 concentration; the impact of CO2 concentration on the variability of the regional biosphere‐atmosphere system is then addressed using the second half of the twentieth century as an example. An increase of CO2 concentration causes the regional biosphere‐atmosphere system to become wetter and greener, with the radiative effect of CO2 and improved plant‐water relation dominant in the Sahelian grassland region and the direct enhancement of leaf carbon assimilation dominant in the tree‐covered region to the south. Driven by the observed sea surface temperature (SST) of the tropical Atlantic Ocean during the period 1950–97 and with CO2 concentration prescribed at a pre‐industrial level 300ppmv, the model simulates a persistent Sahel drought during the period of 1960s−1990s. The simulated drought takes place in the form of a transition of the coupled biosphere‐atmosphere system from a wet/green regime in the 1950s to a dry/barren regime after the 1960s. This climate transition is triggered by SST forcing and materialized through vegetation‐climate interactions. The same SST forcing does not produce such a persistent drought when a constant modern CO2 concentration of 350ppmv is specified, indicating that the biosphere‐atmosphere system at higher CO2 level is more resilient to drought‐inducing external forcings. This finding suggests that the regional climate in Sahel, which tends to alternate between dry and wet spells, may experience longer (or more frequent) wet episodes and shorter (or less frequent) dry episodes in the future than in the past. Our study has significant implications regarding the impact of climate change on regional socio‐economic development.

Atmospheric CO2 Variations over the Last Climatic Cycle (160 000 Years), Deduced from the Vostok Ice Core, Antarctica (Abstract)

This is a summary of the main CO2 results obtained from the Vostok core which have been presented in two papers recently published (Barnola and others 1987; Genthon and others 1987). Previous results of ice-core analysis have already provided valuable information on atmospheric CO2 variations associated with anthropogenic activities (Neftel and others 1985, Raynaud and Barnola 1985[a], Pearman and others 1986) and with climatic variations back to about 40 ka ago (Delmas and others 1980, Neftel and others 1982, Raynaud and Barnola 1985[b]). The Antarctic Vostok ice core provides a unique opportunity for extending the ice record of atmospheric CO2 variations over the last glacial–interglacial cycle back to the end of the penultimate ice age, about 160 ka ago. CO2 measurements were made at 66 different depth levels on the air enclosed in the 2083 m long core taken at Vostok Station. The air was extracted by crushing the ice, under vacuum, in a cold-room, and analysed by gas chromatography (Barnola and others 1983). The selected sampling corresponds to a time resolution between two neighbouring levels which range approximately from 2000 to 4500 years. The ages quoted in this abstract are based on the Vostok ice chronology given by Lorius and others (1985) and take into account the fact that the air is trapped in the firn well after snow deposition (between about 2500 and 4300 years after precipitation in the case of Vostok). The CO2 variations observed are compared directly with the changes in Antarctic temperature as depicted by the stable-isotope record of the Vostok ice (Jouzel and others 1988, this volume). Furthermore, a CO2-orbital forcing-climate interaction is suggested by spectral analysis of the CO2 and temperature profiles, which both show a concentration of variance around orbital frequencies. The temperature profile is clearly dominated by a 40 ka period (which can be related to the obliquity frequency) (Jouzel and others 1988, this volume), whereas the CO2 record exhibits a well-defined 21 ka peak (which can be related to the precession frequencies) and only a weak and doubtful 40 ka peak. To check the relative influence of CO2 and orbital forcings on the temperature at Vostok, we modelled the temperature signal deduced from the stable-isotope record of the ice as a response to CO2, Northern Hemisphere ice volume and local insolation forcings. The results indicate that more than 90% of the temperature variance can be explained by these three kinds of forcing and that the contribution of the CO2 radiative effect associated with an amplification factor (which should reflect the long-term feed-back mechanisms) lies between 27 and 85% of the explained variance. This approach stresses the important role that CO2 may generally have played in determining the Earth’s climate during the late Pleistocene. The most obvious feature of the Vostok CO2 record lies in its high correlation (r2 = 0.79) with the climatic record. The results obtained show high CO2 concentrations during warm periods (mean CO2 values of 263 ppm volume for the Holocene and 272 ppm volume for the last interglacial period) and low concentrations (between about 240 and 190 ppm volume) over glacial periods. Within the last glaciation, the CO2 concentrations are higher during the first part (mean CO2 value of 230 ppm volume between about 110–65 ka B.P.) than during the second part (203 ppm volume between 65–15 ka B.P.); the second part also indicates that climatic conditions were colder. Our results point to some limitation on the possible mechanisms driving the atmospheric CO2 variations and, in particular, the influence of some oceanic areas or of changes in sea-level (see, for example, Broecker and Peng 1986). The weak 41 ka cycle (this cycle seems to be a characteristic of the spectra of the proxy data for high latitudes) in our CO2 record suggests that high latitudes may not have a major influence on CO2 variations. Furthermore, the phase relationship between CO2 and the temperature variations indicates that at the beginning of the two deglaciations around 145ka B.P. and 15ka B.P., taking into account the time resolution of our profile, the CO2 increases roughly in phase with the Vostok temperature. As surface-temperature changes around Antarctica are expected to lead to changes in sea-level (see, for instance, CLIMAP Project Members 1984), our results suggest that the CO2 increase cannot lag the increase in sea-level and thus that this parameter cannot initiate the CO2 variation recorded at the beginning of those two deglaciations. Nevertheless, this does not rule out influence of variations in sea-level on atmospheric CO2 for other periods of interest, in particular during the last interglacial–glacial transition, where the CO2 lags the Vostok temperature.

Atmospheric CO2 Variations over the Last Climatic Cycle (160 000 Years), Deduced from the Vostok Ice Core, Antarctica (Abstract)

This is a summary of the main CO2 results obtained from the Vostok core which have been presented in two papers recently published (Barnola and others 1987; Genthon and others 1987).Previous results of ice-core analysis have already provided valuable information on atmospheric CO2 variations associated with anthropogenic activities (Neftel and others 1985, Raynaud and Barnola 1985[a], Pearman and others 1986) and with climatic variations back to about 40 ka ago (Delmas and others 1980, Neftel and others 1982, Raynaud and Barnola 1985[b]). The Antarctic Vostok ice core provides a unique opportunity for extending the ice record of atmospheric CO2 variations over the last glacial–interglacial cycle back to the end of the penultimate ice age, about 160 ka ago.CO2 measurements were made at 66 different depth levels on the air enclosed in the 2083 m long core taken at Vostok Station. The air was extracted by crushing the ice, under vacuum, in a cold-room, and analysed by gas chromatography (Barnola and others 1983). The selected sampling corresponds to a time resolution between two neighbouring levels which range approximately from 2000 to 4500 years. The ages quoted in this abstract are based on the Vostok ice chronology given by Lorius and others (1985) and take into account the fact that the air is trapped in the firn well after snow deposition (between about 2500 and 4300 years after precipitation in the case of Vostok). The CO2 variations observed are compared directly with the changes in Antarctic temperature as depicted by the stable-isotope record of the Vostok ice (Jouzel and others 1988, this volume).Furthermore, a CO2-orbital forcing-climate interaction is suggested by spectral analysis of the CO2 and temperature profiles, which both show a concentration of variance around orbital frequencies. The temperature profile is clearly dominated by a 40 ka period (which can be related to the obliquity frequency) (Jouzel and others 1988, this volume), whereas the CO2 record exhibits a well-defined 21 ka peak (which can be related to the precession frequencies) and only a weak and doubtful 40 ka peak. To check the relative influence of CO2 and orbital forcings on the temperature at Vostok, we modelled the temperature signal deduced from the stable-isotope record of the ice as a response to CO2, Northern Hemisphere ice volume and local insolation forcings. The results indicate that more than 90% of the temperature variance can be explained by these three kinds of forcing and that the contribution of the CO2 radiative effect associated with an amplification factor (which should reflect the long-term feed-back mechanisms) lies between 27 and 85% of the explained variance. This approach stresses the important role that CO2 may generally have played in determining the Earth’s climate during the late Pleistocene.The most obvious feature of the Vostok CO2 record lies in its high correlation (r2 = 0.79) with the climatic record. The results obtained show high CO2 concentrations during warm periods (mean CO2 values of 263 ppm volume for the Holocene and 272 ppm volume for the last interglacial period) and low concentrations (between about 240 and 190 ppm volume) over glacial periods. Within the last glaciation, the CO2 concentrations are higher during the first part (mean CO2 value of 230 ppm volume between about 110–65 ka B.P.) than during the second part (203 ppm volume between 65–15 ka B.P.); the second part also indicates that climatic conditions were colder.Our results point to some limitation on the possible mechanisms driving the atmospheric CO2 variations and, in particular, the influence of some oceanic areas or of changes in sea-level (see, for example, Broecker and Peng 1986). The weak 41 ka cycle (this cycle seems to be a characteristic of the spectra of the proxy data for high latitudes) in our CO2 record suggests that high latitudes may not have a major influence on CO2 variations. Furthermore, the phase relationship between CO2 and the temperature variations indicates that at the beginning of the two deglaciations around 145ka B.P. and 15ka B.P., taking into account the time resolution of our profile, the CO2 increases roughly in phase with the Vostok temperature. As surface-temperature changes around Antarctica are expected to lead to changes in sea-level (see, for instance, CLIMAP Project Members 1984), our results suggest that the CO2 increase cannot lag the increase in sea-level and thus that this parameter cannot initiate the CO2 variation recorded at the beginning of those two deglaciations. Nevertheless, this does not rule out influence of variations in sea-level on atmospheric CO2 for other periods of interest, in particular during the last interglacial–glacial transition, where the CO2 lags the Vostok temperature.