Modeling the interannual variability and trends in gross and net primary productivity of tropical forests from 1982 to 1999

The role of tropical ecosystems in global carbon cycling is uncertain, at least partially due to an incomplete understanding of climatic forcings of carbon fluxes. To reduce this uncertainty, we simulated and analyzed 1982–1999 Amazonian, African, and Asian carbon fluxes using the Biome-BGC prognostic carbon cycle model driven by National Centers for Environmental Prediction reanalysis daily climate data. We first characterized the individual contribution of temperature, precipitation, radiation, and vapor pressure deficit to interannual variations in carbon fluxes and then calculated trends in gross primary productivity (GPP) and net primary productivity (NPP). In tropical ecosystems, variations in solar radiation and, to a lesser extent, temperature and precipitation, explained most interannual variation in GPP. On the other hand, temperature followed by solar radiation primarily determined variation in NPP. Tropical GPP gradually increased in response to increasing atmospheric CO₂. Confirming earlier studies, changes in solar radiation played a dominant role in CO₂ uptake over the Amazon relative to other tropical regions. Model results showed negligible impacts from variations and trends in precipitation or vapor pressure deficits on CO₂ uptake.     

Simulated responses of Pinus halepensis forest productivity to climatic change and CO2 increase using a statistical model

Tree ring chronologies provide long-term records of growth in natural environmental conditions and may be used to evaluate impacts of climatic change and CO₂ increase on forest productivity. This study focuses on 21 Pinus halepensis forest stands in calcareous Provence (in the south-east of France). A chronology of net primary productivity (NPP) both for the 20th century and for each stand was estimated using tree ring data (width and density). The response of each stand to climate in terms of NPP was statistically modelled using response functions. Anomalies between estimated NPP and NPP reconstructed by response functions were calculated to evaluate the fertilising effect of CO₂ increase on tree growth. The changes in anomalies during the 20th century were attributed to the effect of CO₂ increase. A multiplying factor (β) linking CO₂ concentration and stand productivity was then calculated, on the basis of the trend observed during the 20th century. In this study, the value of the β factor obtained under natural conditions (β=0.50) is consistent with those from controlled CO₂ enrichment experiments. Both response functions and the β factor were used to predict NPP changes for a 2×CO₂ scenario. The 2×CO₂ climate was obtained using predictions from Météo France's ARPEGE atmospheric general circulation model (AGCM) downscaled to Marseilles meteorological station. NPP increased significantly for nine stands solely when the climatic effect was taken into account. The main factors responsible for this enhancement were increased winter and early spring temperatures. When the fertilising effect of the CO₂ increase was added, NPP was significantly enhanced for 14 stands (i.e. NPP enhancement ranged from 8% to 55%). Although the effects of global change were slightly detectable during the 20th century, their acceleration is likely to lead to great changes in the future productivity of P. halepensis forests.     

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.     

Spatiotemporal dynamics of forest net primary production in China over the past two decades

Forest ecosystems play an important role in global carbon cycle regulation. Clarifying the dynamics and mechanism of carbon sink is of both scientific and political importance. In this paper, we have investigated the spatiotemporal change of forest net primary production (NPP) in China for recent two decades based on the geographically weighted regression (GWR) with a cumulative remote sensing index, the maximum normalized difference vegetation index (NDVI_max). GWR is a recently developed regression method with special emphasis on spatial non-stationarity. Outputs of forest NPP at three different stages was generated by the GWR model with NDVI_max for the 1980s, early and late 1990s which were consequently analyzed. Our results indicated a wave-like pattern of change in forest NPP in the three stages with a trough-like depression for the early 1990s. The average forest NPP increased by about 0.72% from the 1980s to the late 1990s. A continuously increasing trend at a pace of 0.07% and 0.22% yr^− 1 was observed in the tropical and subtropical zones from the 1980s to late 1990s respectively, while a continuously decreasing trend (− 0.05% yr^− 1) was noted for the temperate zone. From forest type perspective, only the deciduous broadleaf forests exhibited a continuously decreasing trend of 0.18% yr^− 1. The complex spatiotemporal patterns revealed by this study suggest the need for further research in this direction in order to build in-depth insights into the revealed complexities.     

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.     

Phytochemical changes in leaves of subtropical grasses and fynbos shrubs at elevated atmospheric CO2 concentrations

The effects of elevated atmospheric CO₂ concentrations on plant polyphenolic, tannin, nitrogen, phosphorus and total nonstructural carbohydrate concentrations were investigated in leaves of subtropical grass and fynbos shrub species. The hypothesis tested was that carbon-based secondary compounds would increase when carbon gain is in excess of growth requirements. This premise was tested in two ecosystems involving plants with different photosynthetic mechanisms and growth strategies. The first ecosystem comprised grasses from a C₄-dominated, subtropical grassland, where three plots were subjected to three different free air CO₂ enrichment treatments, i.e., elevated (600 to 800 μmol mol⁻¹), intermediate (400 μmol mol⁻¹) and ambient atmospheric CO₂. One of the seven grass species, Alloteropsis semialata, had a C₃ photosynthetic pathway while the other grasses were all C₄. The second ecosystem was simulated in a microcosm experiment where three fynbos species were grown in open-top chambers at ambient and 700 μmol mol⁻¹ atmospheric CO₂ in low nutrient acid sands typical of south western coastal and mountain fynbos ecosystems. Results showed that polyphenolics and tannins did not increase in the grass species under elevated CO₂ and only in Leucadendron laureolum among the fynbos species. Similarly, foliar nitrogen content of grasses was largely unaffected by elevated CO₂, and among the fynbos species, only L. laureolum and Leucadendron xanthoconus showed changes in foliar nitrogen content under elevated CO₂, but these were of different magnitude. The overall decrease in nitrogen and phosphorus and consequent increase in C:N and C:P ratio in both ecosystems, along with the increase in polyphenolics and tannins in L. laureolum in the fynbos ecosystem, may negatively affect forage quality and decomposition rates. It is concluded that fast growing grasses do not experience sink limitation and invest extra carbon into growth rather than polyphenolics and tannins and show small species-specific chemical changes at elevated atmospheric CO₂ concentrations. Responses of fynbos species are varied and were species-specific.     

A preliminary estimate of changing calcrete carbon storage on land since the Last Glacial Maximum

The glacial-to-interglacial shift in land carbon storage is important in understanding the global carbon cycle and history of the climate system. While organic carbon storage on land appears to have been much less than present during the cold, dry glacial maximum, calcrete (soil carbonate) carbon storage would have been greater. Here we attempt a global estimation of this change; we use published figures for present soil carbonate by biome to estimate changing global soil carbonate storage, on the basis of reconstruction of vegetation areas for four timeslices since the Last Glacial Maximum. It appears that there would most likely have been around a 30–45% decrease in calcrete carbon on land accompanying the transition between glacial and interglacial conditions. This represents a change of about 500–400 GtC (outer error limits are estimated at 750–200 GtC) . In order to be weathered into dissolved bicarbonate, this would take up an additional 500–400 GtC (750–200 GtC) in CO₂ from ocean/atmosphere sources. An equivalent amount to the carbonate leaving the caliche reservoir on land may have accumulated in coral reefs and other calcareous marine sediments during the Holocene, liberating an equimolar quantity of CO₂ back into the ocean-atmosphere system as the bicarbonate ion breaks up.     

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.     

The possible response of life zones in China under global climate change

The response of natural vegetation to climate change is of global concern. In this research, an aggregated Holdridge Life Zone System was used to study the possible response of life zones in China under doubled atmospheric CO₂ concentration with the input climatic parameters at 0.5×0.5° resolution of longitude and latitude from NCAR regional climate model 2 (RegCM2) coupled with the CSIRO global climate model. The results indicate that the latitudinal distribution of life zones would become irregular because of the complicated climate change. In particular, new life zones, such as subtropical desert (SD), tropical desert (TDE) and tropical thorn woodland (TTW), would appear. Subtropical evergreen broadleaved forest (SEBF), tropical rainforest and monsoon forest (TRF), SD, TDE and TTW zones would appear in the northeastern China. Cool-temperate mixed coniferous and broadleaved forest (CMC) and warm-temperate deciduous broadleaved forest (WDBF) zones would appear at latitudes 25–35°N. The temperate desert (TD) in the western China would become Tibetan high-cold plateau (THP), SEBF, WDBF and temperate steppe (TS), and a large part of THP would be replaced by TRF, TDE, SEBF, TS and TTW. The relative area (distribution area/total terrestrial area) of CMC, TRF, TDE and TTW zone would increase about 3%, 21%, 3% and 6%, respectively. However, the relative area of SEBF, TS, TD and THP would decrease about 5%, 3%, 19% and 4%, respectively. In all, the relative area of forests (CCF, CMC, WDBF, SEBF, TRF) would increase about 15%, but the relative area of desert (TD, SD, TDE, and TTW) and THP would decrease about 9% and 4%, respectively. Therefore, responses of different life zones in China to climate change would be dramatic, and nationwide corridors should be considered for the conservation of migrating species under climate change.     

Continental-scale comparisons of terrestrial carbon sinks estimated from satellite data and ecosystem modeling 1982–1998

A simulation model based on satellite observations of monthly vegetation cover was used to estimate monthly carbon fluxes in terrestrial ecosystems from 1982 to 1998. The NASA–CASA model was driven by vegetation properties derived from the Advanced Very High Resolution Radiometer (AVHRR) and radiative transfer algorithms that were developed for Moderate Resolution Imaging Spectroradiometer (MODIS). For the terrestrial biosphere, predicted net ecosystem production (NEP) flux for atmospheric CO₂ has varied widely between an annual source of −0.9 Pg C per year and a sink of +2.1 Pg C per year. The southern hemisphere tropical zones (SHT, between 0° and 30°S) have a major influence over the predicted global trends in interannual variability of NEP. In contrast, the terrestrial NEP sink for atmospheric CO₂ on the North American (NA) continent has been fairly consistent between +0.2 and +0.3 Pg C per year, except during relatively cool annual periods when continental NEP fluxes are predicted to total to nearly zero. The predicted NEP sink for atmospheric CO₂ over Eurasia (EA) increased notably in the late 1980s and has been fairly consistent between +0.3 and +0.55 Pg C per year since 1988. High correlations can be detected between the El Niño Southern Oscillation (ENSO) and predicted NEP fluxes on the EA continent and for the SHT latitude zones, whereas NEP fluxes for the North American continent as a whole do not correlate strongly with ENSO events over the same time series since 1982. These observations support the hypothesis that regional climate warming has had notable but relatively small-scale impacts on high latitude ecosystem (tundra and boreal) sinks for atmospheric CO₂.     

Variability in terrestrial carbon sinks over two decades: Part 2 — Eurasia

We have analyzed 17 yr (1982–1998) of net carbon flux predictions from a simulation model based on satellite observations of monthly vegetation cover. The NASA-CASA model was driven by vegetation cover properties derived from the Advanced Very High Resolution Radiometer and radiative transfer algorithms that were developed for the Moderate Resolution Imaging Spectroradiometer (MODIS). We report that although the terrestrial ecosystem sink for atmospheric CO₂ for the Eurasian region has been fairly consistent at between 0.3 and 0.6 Pg C per year since 1988, high interannual variability in net ecosystem production (NEP) fluxes can be readily identified at locations across the continent. Ten major areas of highest variability in NEP were detected: eastern Europe, the Iberian Peninsula, the Balkan states, Scandinavia, northern and western Russia, eastern Siberia, Mongolia and western China, and central India. Analysis of climate anomalies over this 17-yr time period suggests that variability in precipitation and surface solar irradiance could be associated with trends in carbon sink fluxes within such regions of high NEP variability.     

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.     

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 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.     

Climate forced atmospheric CO2 variability in the early Holocene: A stomatal frequency reconstruction

The dynamic climate in the Northern Hemisphere during the early Holocene could be expected to have impacted on the global carbon cycle. Ice core studies however, show little variability in atmospheric CO₂. Resolving any possible centennial to decadal CO₂ changes is limited by gas diffusion through the firn layer during bubble enclosure. Here we apply the inverse relationship between stomatal index (measured on sub-fossil leaves) and atmospheric CO₂ to complement ice core records between 11,230 and 10,330 cal. yr BP. High-resolution sampling and radiocarbon dating of lake sediments from the Faroe Islands reconstruct a distinct CO₂ decrease centred on ca. 11,050 cal. yr BP, a consistent and steady decline between ca. 10,900 and 10,600 cal. yr BP and an increased instability after ca. 10,550 cal. yr BP. The earliest decline lasting ca. 150 yr is probably associated with the Preboreal Oscillation, an abrupt climatic cooling affecting much of the Northern Hemisphere a few hundred years after the end of the Younger Dryas. In the absence of known global climatic instability, the decline to ca. 10,600 cal. yr BP is possibly due to expanding vegetation in the Northern Hemisphere. The increasing instability in CO₂ after 10,600 cal. yr BP occurs during a period of increasing cooling of surface waters in the North Atlantic and some increased variability in proxy climate indicators in the region.The reconstructed CO₂ changes also show a distinct similarity to indicators of changing solar activity. This may suggest that at least the Northern Hemisphere was particularly sensitive to changes in solar activity during this time and that atmospheric CO₂ concentrations fluctuated via rapid responses in climate.     

New atmospheric pCO2 estimates from palesols during the late Paleocene/early Eocene global warming interval

The late Paleocene to early Eocene was one of the warmest intervals in Earth's history. Superimposed on this long-term warming was an abrupt short-term extreme warm event at or near the Paleocene/Eocene boundary and centered in the higher latitudes. This short-term climate warming was associated with a major benthic foraminiferal extinction and a dramatic 3–4% drop in the ocean's carbon isotopic composition. It has been suggested that the late paleocene/early Eocene global warming was caused by an enhanced greenhouse effect associated with higher levels of atmospheric CO₂ relative to present levels. We present carbon isotopic data from the co-existing paleosols organic matter and carbonates from a terrestrial sequence in the Paris Basin, France that contradict the notion that an increase in atmospheric CO₂ level was the cause of extreme warming for this time interval. Atmospheric pCO₂ estimates for the Late Paleocene/early Eocene estimated from the terrestrial carbon isotopic record spanning the Paleocene/Eocene transition, are indistinguishable from each other and were generally between 300 and 700 ppm.     

Tolerance of allogromiid Foraminifera to severely elevated carbon dioxide concentrations: Implications to future ecosystem functioning and paleoceanographic interpretations

Increases in the partial pressure of carbon dioxide (pCO₂) in the atmosphere will significantly affect a wide variety of terrestrial fauna and flora. Because of tight atmospheric–oceanic coupling, shallow-water marine species are also expected to be affected by increases in atmospheric carbon dioxide concentrations. One proposed way to slow increases in atmospheric pCO₂ is to sequester CO₂ in the deep sea. Thus, over the next few centuries marine species will be exposed to changing seawater chemistry caused by ocean–atmospheric exchange and/or deep-ocean sequestration. This initial case study on one allogromiid foraminiferal species (Allogromia laticollaris) was conducted to begin to ascertain the effect of elevated pCO₂ on benthic Foraminifera, which are a major meiofaunal constituent of shallow- and deep-water marine communities. Cultures of this thecate foraminiferan protist were used for 10–14-day experiments. Experimental treatments were executed in an incubator that controlled CO₂ (15 000; 30 000; 60 000; 90 000; 200 000 ppm), temperature and humidity; atmospheric controls (i.e., ~ 375 ppm CO₂) were executed simultaneously. Although the experimental elevated pCO₂ values are far above foreseeable surface water pCO₂, they were selected to represent the spectrum of conditions expected for the benthos if deep-sea CO₂ sequestration becomes a reality. Survival was assessed in two independent ways: pseudopodial presence/absence and measurement of adenosine triphosphate (ATP), which is an indicator of cellular energy. Substantial proportions of A. laticollaris populations survived 200 000 ppm CO₂ although the mean of the median [ATP] of survivors was statistically lower for this treatment than for that of atmospheric control specimens. After individuals that had been incubated in 200 000 ppm CO₂ for 12 days were transferred to atmospheric conditions for ~ 24 h, the [ATP] of live specimens (survivors) approximated those of the comparable atmospheric control treatment. Incubation in 200 000 ppm CO₂ also resulted in reproduction by some individuals. Results suggest that certain Foraminifera are able to tolerate deep-sea CO₂ sequestration and perhaps thrive as a result of elevated pCO₂ that is predicted for the next few centuries, in a high-pCO₂ world. Thus, allogromiid foraminiferal “blooms” may result from climate change. Furthermore, because allogromiids consume a variety of prey, it is likely that they will be major players in ecosystem dynamics of future coastal sedimentary environments.     

Carbon dioxide-water clathrate as a reservoir of CO2 on Mars

It has been suggested that the residual polar caps of Mars contain a reservoir of permanently frozen carbon dioxide which is controlling the atmospheric pressure. However, observational data and models of the polar heat balance suggest that the temperatures of the Martian poles are too high for solid CO₂ to survive permanently. On the other hand, the icelike compound carbon dioxide-water clathrate (CO₂ · 6H₂O) could function as a CO₂ reservoir instead of solid CO₂, because it is stable at higher temperatures. This paper shows that the permanent polar caps may contain several millibars of CO₂ in the form of clathrate, and discusses the implications of this permanent clathrate reservoir for the present and past atmospheric pressure on Mars.     

Effects of a 2 × CO2 climate on two large lake systems: Pyramid Lake, Nevada, and Yellowstone Lake, Wyoming

The possible effects of trace-gas induced climatic changes on Pyramid and Yellowstone Lakes are assessed using a model of lake temperature. The model is driven by years of hourly meteorological data obtained directly from the output of double-CO₂ experiments (2 × CO₂) conducted with a regional climate model nested in a general circulation model. The regional atmospheric model is the climate version of the National Center for Atmospheric Research/Pennsylvania State University mesoscale model, MM4.Average annual surface temperature of Pyramid Lake for the 2 × CO₂ climate is 15.5 ± 5.4°C (±1 σ), 2.8°C higher than the control. Annual overturn of the lake ceases as a result of these higher temperatures for the 2 × CO₂ climate. Evaporation increases from 1400 mm yr⁻¹ in the control to 1595 mm yr⁻¹ in the 2 × CO₂ simulation, but net water supplied to the Pyramid Lake basin increases from −6 mm yr⁻¹ in the control to +27 mm yr⁻¹ in the 2 × CO₂ simulation due to increased precipitation.For the open water periods, the average annual surface temperature of Yellowstone Lake is 13.2 ± 5.1°C for the 2 × CO₂ climate, a temperature 1.6°C higher than the control. The annual duration of ice cover on the lake is 152 days in the 2 × CO₂ simulation, a reduction of 44 days relative to the control. Warming of the lake for the 2 × CO₂ climate is mostly confined to the near-surface. Simulated spring overturn for the 2 × CO₂ climate occurs earlier in the year and fall overturn later than in the control. Evaporation increases from 544 mm yr⁻¹ to 600 mm yr⁻¹ in the 2 × CO₂ simulation, but net water supplied to the Yellowstone Lake basin increases from +373 mm yr⁻¹ in the control to +619 mm yr⁻¹ due to increased precipitation. The effects of these climatic changes suggest possible deterioration of water quality and productivity in Pyramid Lake and possible enhancement of productivity in Yellowstone Lake.