Atmospheric CO2 Variations Research Articles

Inter-annual variability of the atmospheric carbon dioxide concentrations as simulated with global terrestrial biosphere models and an atmospheric transport model

Seasonal and inter-annual variations of atmospheric CO2 for the period from 1961 to 1997 have been simulated using a global tracer transport model driven by a new version of the Biome BioGeochemical Cycle model (Biome-BGC). Biome-BGC was forced by daily temperature and precipitation from the NCEP reanalysis dataset, and the calculated monthly-averaged CO2 fluxes were used as input to the global transport model. Results from an inter-comparison with the Carnegie—Ames—Stanford Approach model (CASA) and the Simulation model of Carbon cYCle in Land Ecosystems (Sim-CYCLE) model are also reported. The phase of the seasonal cycle in the Northern Hemisphere was reproduced generally well by Biome-BGC, although the amplitude was smaller compared to the observations and to the other biosphere models. The CO2 time series simulated by Biome-BGC were compared to the global CO2 concentration anomalies from the observations at Mauna Loa and the South Pole. The modeled concentration anomalies matched the phase of the inter-annual variations in the atmospheric CO2 observations; however, the modeled amplitude was lower than the observed value in several cases. The result suggests that a significant part of the inter-annual variability in the global carbon cycle can be accounted for by the terrestrial biosphere models. Simulations performed with another climate-based model, Sim-CYCLE, produced a larger amplitude of inter-annual variability in atmospheric CO2, making the amplitude closer to the observed range, but with a more visible phase mismatch in a number of time periods. This may indicate the need to increase the Biome-BGC model sensitivity to seasonal and inter-annual changes in temperature and precipitation.

Atmospheric CO2 variations over the last three glacial-interglacial climatic cycles deduced from the Dome Fuji deep ice core, Antarctica using a wet extraction technique

A deep ice core drilled at Dome Fuji, East Antarctica was analyzed for the CO 2 concentration using a wet extraction method in order to reconstruct its atmospheric variations over the past 320 kyr, which includes three full glacial–interglacial climatic cycles, with a mean time resolution of about 1.1 kyr. The CO 2 concentration values derived for the past 65 kyr are very close to those obtained from other Antarctic ice cores using dry extraction methods, although the wet extraction method is generally thought to be inappropriate for the determination of the CO 2 concentration. The comparison between the CO 2 and Ca 2+ concentrations deduced from the Dome Fuji core suggests that calcium carbonate emitted from lands was mostly neutralized in the atmosphere before reaching the central part of Antarctica, or that only a small part of calcium carbonate was involved in CO 2 production during the wet extraction process. The CO 2 concentration for the past 320 kyr deduced from the Dome Fuji core varies between 190 and 300 ppmv, showing clear glacial–interglacial variations similar to the result of the Vostok ice core. However, for some periods, the concentration values of the Dome Fuji core are higher by up to 20 ppmv than those of the Vostok core. There is no clear indication that such differences are related to variations of chemical components of Ca 2+ , microparticle and acidity of the Dome Fuji core. DOI: 10.1034/j.1600-0889.2003.00050.x

Concentration variations of atmospheric CO2 observed at Syowa Station, Antarctica from 1984 to 2000

Concentration variations of atmospheric CO2 observed at Syowa Station, Antarctica from 1984 to 2000

Interannual variability of atmospheric CO2 in the Mediterranean: measurements at the island of Lampedusa

Interannual variability of atmospheric CO2 in the Mediterranean: measurements at the island of Lampedusa

Planktonic foraminifera, sea surface temperatures, and mechanisms of oceanic change in the Peru and south equatorial currents, 0–150 ka BP

Planktonic foraminiferal faunas of the southeast Pacific indicate that sea surface temperatures (SST) have varied by as much as 8–10°C in the Peru Current, and by ∼5–7°C along the equator, over the past 150,000 years. Changes in SST at times such as the Last Glacial Maximum reflect incursion of high‐latitude species Globorotalia inflata and Neogloboquadrina pachyderma into the eastern boundary current and as far north as the equator. A simple heat budget model of the equatorial Pacific shows that observed changes in Peru Current advection can account for about half of the total variability in equatorial SSTs. The remaining changes in equatorial SST, which are likely related to local changes in upwelling or pycnocline depth, precede changes in polar climates as recorded by δ18O. This partitioning of processes in eastern equatorial Pacific SST reveals that net ice‐age cooling here reflects first a rapid response of equatorial upwelling to insolation, followed by a later response to changes in the eastern boundary current associated with high‐latitude climate (which closely resembles variations in atmospheric CO2 as recorded in the Vostok ice core). Although precise mechanisms responsible for the equatorial upwelling component of climate change remain uncertain, one likely candidate that may operate independently of the ice sheets is insolation‐driven changes in El Niño/Southern Oscillation (ENSO) frequency. Early responses of equatorial SST detected both here and elsewhere highlight the sensitivity of tropical systems to small changes in seasonal insolation. The scale of tropical changes we have observed are substantially greater than model predictions, suggesting a need for further quantitative assessment of processes associated with long‐term climate change.

Environmental control of leaf area production: Implications for vegetation and land‐surface modeling

Leaf surface area per unit ground cover (leaf area index, LAI) is one of the major controls on plant productivity and biospheric feedbacks on atmospheric energy and water exchanges. Nearly all vegetation and land‐surface models include parameterizations of LAI, however not much research currently focuses on the validation of simulated responses of LAI to environmental change. The objective of our research was to quantitatively review the plant science literature to extract information on the response of LAI to variations in soil moisture, soil fertility and atmospheric CO2. Our synthesis confirms that LAI is likely co‐limited by a number of resources, including water, nitrogen and light. Atmospheric CO2 influences LAI in much the same manner as other plant resources. When CO2 supply is strongly limiting to gross primary production (i.e., at relatively low CO2 concentrations), LAI is strongly correlated with CO2, whereas when CO2 is abundant, LAI sensitivity to CO2 dramatically decreases. Such a nonlinear relationship between leaf area production and atmospheric CO2 may introduce a potential bias for global change modeling, particularly in the simulation of low‐density vegetation that has the potential to significantly increase canopy size without inducing self‐shading.

新生代におけるグローバル炭素循環と気候変動との関係―ＣＯ２は長期的気候変動の主要因か？―

Climate change and variations of atmospheric CO2 during Cenozoic have been discussed by many studies. The model results for global carbon cycle and climate change are generally consistent with those of many analytical studies concerning climate events in early Cenozoic, e.g., early Eocene warming events. However, the climate events proposed by analytical studies in late Cenozoic are not wholly inconsistent with the results of the model studies, e.g., Miocene climatic optimum (warming), cooling events in Eocene/Oligocene boundary and middle Miocene (15 Ma). Many problems remain in relation to evaluating CO2 flux by hydrothermal solutions at mid-ocean ridges, island-arc, and back-arc basins in the models. Also, the discrepancy may be derived from errors in estimating weathering flux, organic carbon burial, and change in vegetation. Moreover, another greenhouse effect gas such as methane, land-sea distribution, albedo variations due to the formation of ice-sheets, and temperature distribution attributed to changes in the ocean circulation system should be considered. Recent analytical studies reveal that the CO2 level since Miocene has remained relatively low in spite of the suggested climate events in this period. The mechanism of oceanic environmental change, as well as atmospheric CO2, is especially important to elucidate climate change during Cenozoic.

Regional source/sink impact on the diurnal, seasonal and inter-annual variations in atmospheric CO2 at a boreal forest site in Canada

Time series of in-situ CO2 data from Fraserdale (50°N, 81°W) in the northern Ontario boreal forest is described, together with an analysis of observed variations occurring on daily to interannual timescales. Atmospheric CO2 measurements at Fraserdale reflect a complex interaction between the daily cycle of the vegetative carbon flux and the daily evolution of the boundary layer mixing dynamics. This is particularly evident during the growing season, when CO2 concentrations are influenced strongly by local and regional biospheric activities. In addition, the atmospheric CO2 measurements at Fraserdale are greatly influenced by the direction of atmospheric transport, and reflect the complex heterogeneous distribution of ecosystem types around the site. Averaged over the 7-yr period from 1990 to 1996, the seasonal cycle associated with air from west and northwest of the site shows an amplitude of ~19 ppm, while that associated with air from the south and southwest shows an amplitude of ~23 ppm; the seasonal minimum, on average, occurs about a week earlier in the latter case than in the former. This is reflective of the fact that many of the deciduous trees are located to the south and southwest of Fraserdale. Furthermore, its location in the boreal forest causes the seasonal minimum to occur on average in early August at Fraserdale, compared to late August observed at Alert and at many other background stations in the high-latitude Northern Hemisphere. At the Fraserdale site there is no statistically significant indication, during the 1990–1996 study period, of changes in the length of the growing season (as measured by zero crossing points in the seasonal cycle).

Inter-annual variability of the atmospheric carbon dioxide concentrations as simulated with global terrestrial biosphere models and an atmospheric transport model

Seasonal and inter-annual variations of atmospheric CO2 for the period from 1961 to 1997 have been simulated using a global tracer transport model driven by a new version of the Biome BioGeochemical Cycle model (Biome-BGC). Biome-BGC was forced by daily temperature and precipitation from the NCEP reanalysis dataset, and the calculated monthly-averaged CO2 fluxes were used as input to the global transport model. Results from an inter-comparison with the Carnegie—Ames—Stanford Approach model (CASA) and the Simulation model of Carbon cYCle in Land Ecosystems (Sim-CYCLE) model are also reported. The phase of the seasonal cycle in the Northern Hemisphere was reproduced generally well by Biome-BGC, although the amplitude was smaller compared to the observations and to the other biosphere models. The CO2 time series simulated by Biome-BGC were compared to the global CO2concentration anomalies from the observations at Mauna Loa and the South Pole. The modeled concentration anomalies matched the phase of the inter-annual variations in the atmospheric CO2 observations; however, the modeled amplitude was lower than the observed value in several cases. The result suggests that a significant part of the inter-annual variability in the global carbon cycle can be accounted for by the terrestrial biosphere models. Simulations performed with another climate-based model, Sim-CYCLE, produced a larger amplitude of inter-annual variability in atmospheric CO2, making the amplitude closer to the observed range, but with a more visible phase mismatch in a number of time periods. This may indicate the need to increase the Biome-BGC model sensitivity to seasonal and inter-annual changes in temperature and precipitation.

Adaptive model of DOM dynamics in the surface ocean

The bulk of marine dissolved organic matter (DOM) forms a large, old, refractory carbon pool in the deep ocean, yet a small fraction in the surface ocean is actively involved in the global carbon cycle and may contribute significantly to the biological pump. We argue that present models of plankton and DOM in the surface ocean are incompatible with current knowledge of marine DOM dynamics. We present a plankton model with an adaptive formulation of bacteria-DOM interactions which is more consistent with observations. Our model reproduces net accumulation of DOM and is the first to reconcile the prevailing reports of net consumption of inorganic nitrogen by bacteria with commonly found DOC:DON ratios in the surface ocean. Our model predicts that factors governing DOM production by phytoplankton and zooplankton have little influence on DOM accumulation in the surface ocean. Long-term accumulation, eventual export of DOM, and hence its contribution to the biological pump appear to be primarily controlled by characteristics of bacterial DOM utilization. The model implies a negative relation between temperature and DOM accumulation, which can be obscured, however, by the impact of temperature on water-column stability. On longer time scales, this negative relation could indicate a positive feedback between temperature and CO2. DOM can accumulate independently of inorganic nutrient content of the surface ocean. Therefore, the predicted positive feedback is potentially very strong and could help explain the large variations in atmospheric CO between glacial and interglacial periods.

Atmospheric CO2 variations over the last three glacial’interglacial climatic cycles deduced from the Dome Fuji deep ice core, Antarctica using a wet extraction technique

A deep ice core drilled at Dome Fuji, East Antarctica was analyzed for the CO2 concentration using a wet extraction method in order to reconstruct its atmospheric variations over the past 320 kyr, which includes three full glacial—interglacial climatic cycles, with a mean time resolution of about 1.1 kyr. The CO2 concentration values derived for the past 65 kyr are very close to those obtained from other Antarctic ice cores using dry extraction methods, although the wet extraction method is generally thought to be inappropriate for the determination of the CO2 concentration. The comparison between the CO2 and Ca2× concentrations deduced from the Dome Fuji core suggests that calcium carbonate emitted from lands was mostly neutralized in the atmosphere before reaching the central part of Antarctica, or that only a small part of calcium carbonate was involved in CO2 production during the wet extraction process. The CO2 concentration for the past 320 kyr deduced from the Dome Fuji core varies between 190 and 300 ppmv, showing clear glacial—interglacial variations similar to the result of the Vostok ice core. However, for some periods, the concentration values of the Dome Fuji core are higher by up to 20 ppmv than those of the Vostok core. There is no clear indication that such differences are related to variations of chemical components of Ca2×, microparticle and acidity of the Dome Fuji core.

Constraining temperature variations over the last millennium by comparing simulated and observed atmospheric CO2

The response of atmospheric CO2 and climate to the reconstructed variability in solar irradiance and radiative forcing by volcanoes over the last millennium is examined by applying a coupled physical–biogeochemical climate model that includes the Lund-Potsdam-Jena dynamic global vegetation model (LPJ-DGVM) and a simplified analogue of a coupled atmosphere–ocean general circulation model. The modeled variations of atmospheric CO2 and Northern Hemisphere (NH) mean surface temperature are compatible with reconstructions from different Antarctic ice cores and temperature proxy data. Simulations where the magnitude of solar irradiance changes is increased yield a mismatch between model results and CO2 data, providing evidence for modest changes in solar irradiance and global mean temperatures over the past millennium and arguing against a significant amplification of the response of global or hemispheric annual mean temperature to solar forcing. Linear regression (r = 0.97) between modeled changes in atmospheric CO2 and NH mean surface temperature yields a CO2 increase of about 12 ppm for a temperature increase of 1 °C and associated precipitation and cloud cover changes. Then, the CO2 data range of 12 ppm implies that multi-decadal NH temperature changes between 1100 and 1700 AD had to be within 1 °C. Modeled preindustrial variations in atmospheric δ13C are small compared to the uncertainties in ice core δ13C data. Simulations with natural forcings only suggest that atmospheric CO2 would have remained around the preindustrial concentration of 280 ppm without anthropogenic emissions. Sensitivity experiments show that atmospheric CO2 closely follows decadal-mean temperature changes when changes in ocean circulation and ocean-sediment interactions are not important. The response in terrestrial carbon storage to factorial changes in temperature, the seasonality of temperature, precipitation, and atmospheric CO2 has been determined.

Sensitivity of inter-annual variation of CO2 seasonal cycle at Mauna Loa to atmospheric transport

Origins of the inter-annual variations of the Mauna Loa atmospheric CO2 seasonal cycle related to atmospheric transport were examined using a global atmospheric transport model with prescribed land biota CO2 source functions at 11 land sections. On average, the seasonal variation of atmospheric CO2 at Mauna Loa is influenced mostly by the Siberian CO2 flux, followed by temperate Asia and North America. The inter-annual variability of the seasonal cycle is caused mainly by the inter-annual variation in the transport of the Siberian signal to Mauna Loa. The characteristics of the simulated seasonal cycle and its inter-annual variability at Mauna Loa are found to be sensitive to the quality of the wind data used to drive the transport model. Implication of this result is that for studying a long-term variations of atmospheric transport a meteorological data set for driving an atmospheric transport model should be obtained from the same production procedure.

Concentration variations of atmospheric CO2 observed at Syowa Station, Antarctica from 1984 to 2000

Systematic and continuous measurements of the atmospheric CO2 concentration have been carried out at Syowa Station, Antarctica since February 1984. The measurement system was renewed in 1995, but the continuity of the data from the two systems was confirmed by operating them simultaneously. The CO2 data taken for 17 years from 1984 to 2000 showed clear evidence for a seasonal cycle, a secular trend and interannual variations. The seasonal cycle was variable from year to year, with especially larger amplitudes in 1992 and 1998 and a large phase delay in 1993. A rapid increase in the CO2 concentration was observed in 1987, 1994 and 1998 in association with ENSO events. The average rate of the secular CO2 increase for the last 17 years was calculated to be 1.49 ppmv yr−1. Short-term CO2 variations with amplitudes of around 1.0 ppmv were found in the austral summer season of several years after 1990, probably due to an intrusion of CO2-depleted air mass into the Antarctic region.

Interannual variability of atmospheric CO2 in the Mediterranean: measurements at the island of Lampedusa

The atmospheric concentration of carbon dioxide and other greenhouse gases has been measured weekly since 1992 at the island of Lampedusa, in the Mediterranean sea. Lampedusa is a small island located approximately 100 km east of Tunisia, and 250 km south of Sicily. The 10-yr CO2 data set has been analysed to quantify trends, and characterize semi-annual, annual and inter-annual variability. The data show an average trend of ×1.7 ppmv yr−1; the average annual cycle has an amplitude of about 9 ppmv. In the period of investigation the annual growth rate varies between 0.5 and 4.5 ppmv yr−1, and the amplitude of the annual cycle between 7 and 11 ppmv yr−1. By comparing the observed growth rate with recent estimates of carbon dioxide emissions, it is calculated that 58–61% of the emitted CO2 remains in the atmosphere. The CO2 growth rate appears to be related to large-scale dynamic phenomena, primarily El Niño/Southern Oscillation (ENSO). An evident signature of the 1997–98 El Niño is apparent in the CO2 record, and corresponds to a weakening of the exchange with the biosphere. A high correlation between the global average temperature and the 12-month average carbon dioxide growth rate is also found. Wind direction displays a significant inter-annual variability throughout the measurement period, possibly influencing the observed evolution of the CO2 concentration.

Evaluation of terrestrial carbon cycle models with atmospheric CO2 measurements: Results from transient simulations considering increasing CO2, climate, and land‐use effects

An atmospheric transport model and observations of atmospheric CO2 are used to evaluate the performance of four Terrestrial Carbon Models (TCMs) in simulating the seasonal dynamics and interannual variability of atmospheric CO2 between 1980 and 1991. The TCMs were forced with time varying atmospheric CO2 concentrations, climate, and land use to simulate the net exchange of carbon between the terrestrial biosphere and the atmosphere. The monthly surface CO2 fluxes from the TCMs were used to drive the Model of Atmospheric Transport and Chemistry and the simulated seasonal cycles and concentration anomalies are compared with observations from several stations in the CMDL network. The TCMs underestimate the amplitude of the seasonal cycle and tend to simulate too early an uptake of CO2 during the spring by approximately one to two months. The model fluxes show an increase in amplitude as a result of land‐use change, but that pattern is not so evident in the simulated atmospheric amplitudes, and the different models suggest different causes for the amplitude increase (i.e., CO2 fertilization, climate variability or land use change). The comparison of the modeled concentration anomalies with the observed anomalies indicates that either the TCMs underestimate interannual variability in the exchange of CO2 between the terrestrial biosphere and the atmosphere, or that either the variability in the ocean fluxes or the atmospheric transport may be key factors in the atmospheric interannual variability.

Simulation of atmospheric CO2 over Europe and western Siberia using the regional scale model REMO

The spatial distribution and the temporal variability of atmospheric CO2 over Europe and western Siberia are investigated using the regional atmospheric model, REMO. The model, of typical horizontal resolution 50 km, is part of a nested modelling framework that has been established as a concerted action during the EUROSIBERIAN CARBONFLUX project. In REMO, the transport of CO2 is simulated together with climate variables, which offers the possibility of calculating at each time step the land atmosphere CO2 fluxes as driven by the modelled meteorology. The uptake of CO2 by photosynthesis is calculated using a light use efficiency formulation, where the absorbed photosynthetically active solar radiation is inferred from satellite measurements. The release of CO2 from plant and soil respiration is driven by the simulated climate and assumed to be in equilibrium with photosynthesis over the course of one year. Fossil CO2 emissions and air–sea fluxes within the model domain are prescribed, whereas the influence of sources outside the model domain is computed from as a boundary condition CO2 fields determined a global transport model. The modelling results are compared against pointwise eddy covariance fluxes, and against atmospheric CO2 records. We show that a necessary condition to simulate realistically the variability of atmospheric CO2 over continental Europe is to account for the diurnal cycle of biospheric exchange. Overall, for the study period of July 1998, REMO realistically simulates the short-term variability of fluxes and of atmospheric mixing ratios. However, the mean CO2 gradients from western Europe to western Siberia are not correctly reproduced. This latter deficiency points out the key role of boundary conditions in a limited-area model, as well as the need for using more realistic geographic mean patterns of biospheric carbon fluxes.

Interannual growth rate variations of atmospheric CO2 and its δ13C, H2, CH4, and CO between 1992 and 1999 linked to biomass burning

High‐precision, multispecies measurements of flask air samples since 1992 from CSIRO's global sampling network reveal strong correlation among interannual growth rate variations of CO2 and its δ13C, H2, CH4, and CO. We show that a major fraction of the variability is consistent with two emission pulses coinciding with large biomass burning events in 1994/1995 and 1997/1998 in tropical and boreal regions, and observations of unusually high levels of combustion products in the overlying troposphere at these times. Implied pulse strengths and multispecies emission ratios are not consistent with any other single process, but do not exclude possible contributions from covarying processes that are linked through climatic forcing. Comparison of CO2 with its δ13C indicates that most of the CO2 variation is from terrestrial exchange, but does not distinguish forcing by biomass burning from imbalance in photosynthesis/respiration of terrestrial ecosystems. Partitioning of terrestrial CO2 fluxes is constrained by H2, CH4, and CO, all of which are products of biomass burning but which have no direct link to net respiration of CO2. While CO is a strong indicator of biomass burning, its short lifetime prevents it from usefully constraining the magnitude of CO2 emissions. If the H2 and CH4 variations were dominated by biomass burning, they would imply associated carbon emissions in excess of mean annual levels of other years, of 0.6–3.5 and 0.8–3.7 Pg C for 1994/1995 and 1997/1998, respectively. The large range in emission estimates mainly reflects uncertainty in H2/CO2 and CH4/CO2 emission ratios of fires in these years.

Temporal variations in atmospheric CO 2 concentrations in Kuwait City, Kuwait with comparisons to Phoenix, Arizona, USA

Hourly atmospheric carbon dioxide (CO 2) concentration measurements are available from 1996 to present for a suburban site within the growing metropolitan area of Kuwait City. Analyses of this record reveal (a) an annual cycle with highest values in February and lowest values in September reflecting the growth and decay of vegetation in the Northern Hemisphere as well as fluctuations in motor traffic, (b) a weekly cycle with highest values during the weekdays and lowest values during weekends, and (c) a diurnal cycle with highest values after sunset when the local atmosphere becomes more stable following vehicular emission of CO 2 throughout the day and lowest values in late afternoon following several hours of relatively unstable conditions. During the daytime, CO 2 concentrations are related to wind direction, with westerly winds (coming from the desert) promoting lowest CO 2 concentrations. At night, lowest CO 2 levels are associated with higher wind speeds and winds from the north. The findings from the Kuwait City area, particularly when contrasted with the situation in Phoenix, further our understanding of the dynamics of CO 2 levels in urban environments.

Quantifying ecosystem‐atmosphere carbon exchange with a 14C label

The role of terrestrial ecosystems as sources or sinks for carbon to the atmosphere and their contribution to inter‐annual variations in atmospheric CO2 remain hotly‐debated topics. Carbon enters terrestrial ecosystems through a single process, photosynthesis, but it is returned to the atmosphere by the combined metabolic activity of plants, animals, and microbes (Figure l). The largest uncertainties in our understanding of terrestrial carbon cycling are in these return processes, especially how CO2 losses from ecosystems are divided among respiration by living plants—termed autotrophic respiration—versus microbial and faunal decomposition of plant residues—termed heterotrophic respiration (Figure 1); and how seasonal and climatic factors that change plant physiological status and soil conditions influence that partitioning.