Changes In Atmospheric CO2 Concentration Research Articles

A revised evolutionary history of Poales: origins and diversification

Poales represents more than one-third of all monocotyledons (c. 20 000 species in 16 families) and constitutes a microcosm of the angiosperms. The extreme variation in species richness among the families of Poales is still not understood: Poaceae includes ∼10 000 species, whereas six families have fewer than ten species. Here, using the largest phylogenetic analysis of Poales to date, molecular dating, ancestral reconstructions and diversification analyses, we develop a macro-evolutionary and macro-ecological approach to seek correlates for changing diversification patterns. We show that the poalean families diverged in the Late Cretaceous, a time of high levels of CO2 and high rainfall. Our habitat reconstructions indicate that Poales inhabited open and dry habitats in this environment. We also demonstrate that lineages with CO2-concentrating mechanisms inhabiting dry and open environments exhibited higher diversification rates than C3, shade and wet lineages. CO2-concentrating mechanisms counteract the effects of low atmospheric CO2 and reduce phototranspiration. It is believed that the parallel evolution of C4 and CAM (Crassulacean acid metabolism) photosynthesis in Poaceae, Cyperaceae and Bromeliaceae is an adaptation to changes in atmospheric CO2 concentrations. Combinations of extrinsic and intrinsic factors might have played a role in shifts in diversification rates and may explain the variation in species richness in Poales. © 2014 The Linnean Society of London, Botanical Journal of the Linnean Society, 2014, 00, 000‐000. ADDITIONAL KEYWORDS: ancestral reconstructions ‐ Cenozoic ‐ CO2-concentrating mechanisms ‐ molecular dating ‐ open and dry habitats ‐ radiation.

Global climate change and crop resistance to aphids: contrasting responses of lucerne genotypes to elevated atmospheric carbon dioxide

AbstractPredicted increases in atmospheric carbon dioxide (CO2) concentrations could modify crop resistance to insect herbivores by altering plant quality. The short generation times of aphids may allow them to exploit such changes and colonise previously resistant plant genotypes. Lucerne (Medicago sativa) has undergone global selective breeding against aphids, including the pea aphid, Acyrthosiphon pisum. The purpose of this study was to characterise how ambient CO2 (aCO2) and elevated (eCO2) (400 and 600 µmol mol−1, respectively) affected plant physiological traits potentially linked to aphid resistance, focussing on foliar amino acid concentrations, across five M. sativa genotypes with varying resistance to A. pisum. These included susceptible (Hunter River), low (Trifecta), moderate (Aurora and Genesis) and high resistance (Sequel). Under eCO2, root nodulation doubled and essential amino acid concentrations increased by 86% in resistant Sequel, whereas essential amino acid concentrations decreased by 53% in Genesis. Moreover, concentrations of lysine, an amino acid whose deficiency has been linked previously to A. pisum resistance in M. sativa, increased by 127% in Sequel at eCO2. Compared with aCO2, aphid colonisation of Sequel plants rose from 22% to 78% and reproduction rates increased from 1.1 to 4.3 nymphs week−1 under eCO2 conditions. In contrast, Genesis became more resistant at eCO2 compared with plants at aCO2; aphid colonisation rates fell from 78% to 44% of plants and reproductive rates decreased from 4.9 to 1.7 nymphs week−1. In conclusion, predicted changes in atmospheric CO2 concentrations could either reduce (Sequel) or enhance (Genesis) resistance to aphids, which might be linked to quantitative and qualitative changes in foliar amino acids.

YIELDSTAT – A spatial yield model for agricultural crops

The YIELDSTAT model for crop yields, an advanced hybrid of traditional non-linear regression approaches and expert knowledge databases, was developed to predict the spatial distribution of yields for a range of arable crops (winter wheat, winter barley, winter rye, winter triticale, spring barley, oats, potato, sugar beet, winter oil-seed rape, silage maize, clover, clover/grass mix, lucerne, lucerne/grass mix, fodder grass) and two grassland types (intensive, extensive) in eastern Germany across different scales up to the regional scale. YIELDSTAT accounts for a wide range of yield-influencing factors derived from weather, soil, relief and management data, as well as for the long-term changing atmospheric CO2 concentration and for the trend owing to progress in breeding and agro-technology. YIELDSTAT regression modules were derived from several hundred farm data sets from 1975 to 1990 and tested against recent yield observations from the Federal State of Thuringia, Germany. The model test was performed at three different spatial scales. YIELDSTAT successfully reproduced the observed data at all three scales, with a normalised mean bias error of 3.02% across all crops and scales. Model testing also revealed a number of weaknesses in the model, identifying yield-reducing factors that had not been considered previously. All in all, the model proved fitness-for-purpose for simulating spatial yields, also under assumed future climate conditions.

Scaling climate change experiments across space and time

Scaling climate change experiments across space and time

Hot climates, high sensitivity

Climate sensitivity is the Holy Grail of climate science; because CO2 is one of the principal control knobs for climate, sensitivity to changes in atmospheric CO2 concentration is of particular interest. This sensitivity is typically characterized by the change in global mean temperature per doubling of concentration. Because the determination of climate sensitivity is plagued by uncertainties about the operation of various feedbacks in the climate system—notably cloud feedback—it is natural to look to the past for clues about how well we can model those feedbacks. The chilly climate of the Last Glacial Maximum has been extensively exploited for this purpose (1), but the high CO2 hothouse climates such as the Pliocene (∼5 Mya) or Eocene (∼55 Mya) may be better analogs for where we are headed with our present adventure in turning fossil fuels into atmospheric CO2. In this issue, Caballero and Huber (2) throw some cold water on this hot topic by providing evidence that hothouse states may have different climate sensitivity per doubling of CO2 than the present state. None of this means that study of past hot climates is worthless in the Grail quest; it means only that the Parsifals of climate will have to work harder to extract the treasure. Better information about the past wouldn’t hurt either. As Caballero and Huber note, there have already been great strides in understanding the magnitude and pattern of warmth in hothouse climates, which have helped resolve some earlier modeling paradoxes, but much remains to be done. In particular, narrowing the broad error bars on past atmospheric CO2 is crucial to relating these climates to what is going on at present.

Nonlinear precipitation response to El Niño and global warming in the Indo-Pacific

Precipitation changes over the Indo-Pacific during El Nino events are studied using an Atmospheric General Circulation Model forced with sea-surface temperature (SST) anomalies and changes in atmospheric CO2 concentrations. Linear increases in the amplitude of the El Nino SST anomaly pattern trigger nonlinear changes in precipitation amounts, resulting in shifts in the location and orientation of the Intertropical Convergence Zone (ITCZ) and the South Pacific Convergence Zone (SPCZ). In particular, the maximum precipitation anomaly along the ITCZ and SPCZ shifts eastwards, the ITCZ shifts south towards the equator, and the SPCZ becomes more zonal. Precipitation in the equatorial Pacific also increases nonlinearly. The effect of increasing CO2 levels and warming SSTs is also investigated. Global warming generally enhances the tropical Pacific precipitation response to El Nino. The precipitation response to El Nino is found to be dominated by changes in the atmospheric mean circulation dynamics, whereas the response to global warming is a balance between dynamic and thermodynamic changes. While the dependence of projected climate change impacts on seasonal variability is well-established, this study reveals that the impact of global warming on Pacific precipitation also depends strongly on the magnitude of the El Nino event. The magnitude and structure of the precipitation changes are also sensitive to the spatial structure of the global warming SST pattern.

Carbon–Concentration and Carbon–Climate Feedbacks in CMIP5 Earth System Models

AbstractThe magnitude and evolution of parameters that characterize feedbacks in the coupled carbon–climate system are compared across nine Earth system models (ESMs). The analysis is based on results from biogeochemically, radiatively, and fully coupled simulations in which CO2 increases at a rate of 1% yr−1. These simulations are part of phase 5 of the Coupled Model Intercomparison Project (CMIP5). The CO2 fluxes between the atmosphere and underlying land and ocean respond to changes in atmospheric CO2 concentration and to changes in temperature and other climate variables. The carbon–concentration and carbon–climate feedback parameters characterize the response of the CO2 flux between the atmosphere and the underlying surface to these changes. Feedback parameters are calculated using two different approaches. The two approaches are equivalent and either may be used to calculate the contribution of the feedback terms to diagnosed cumulative emissions. The contribution of carbon–concentration feedback to diagnosed cumulative emissions that are consistent with the 1% increasing CO2 concentration scenario is about 4.5 times larger than the carbon–climate feedback. Differences in the modeled responses of the carbon budget to changes in CO2 and temperature are seen to be 3–4 times larger for the land components compared to the ocean components of participating models. The feedback parameters depend on the state of the system as well the forcing scenario but nevertheless provide insight into the behavior of the coupled carbon–climate system and a useful common framework for comparing models.

Climate Change Would Increase the Water Intensity of Irrigated Corn Ethanol

Changes in atmospheric CO2 concentrations, temperature, and precipitation affect plant growth and evapotranspiration. However, the interactive effects of these factors are relatively unexplored, and it is important to consider their combined effects at geographic and temporal scales that are relevant to policymaking. Accordingly, we estimate how climate change would affect water requirements for irrigated corn ethanol production in key regions of the U.S. over a 40 year horizon. We used the geographic-information-system-based environmental policy integrated climate (GEPIC) model, coupled with temperature and precipitation predictions from five different general circulation models and atmospheric CO2 concentrations from the Special Report on Emissions Scenarios A2 emission scenario of the Intergovernmental Panel on Climate Change, to estimate changes in water requirements and yields for corn ethanol. Simulations infer that climate change would increase the evaporative water consumption of the 15 billion gallons per year of corn ethanol needed to comply with the Energy Independency and Security Act by 10%, from 94 to 102 trillion liters/year (tly), and the irrigation water consumption by 19%, from 10.22 to 12.18 tly. Furthermore, on average, irrigation rates would increase by 9%, while corn yields would decrease by 7%, even when the projected increased irrigation requirements were met. In the irrigation-intensive High Plains, this implies increased pressure for the stressed Ogallala Aquifer, which provides water to seven states and irrigates one-fourth of the grain produced in the U.S. In the Corn Belt and Great Lakes region, where more rainfall is projected, higher water requirements could be related to less frequent rainfall, suggesting a need for additional water catchment capacity. The projected increases in water intensity (i.e., the liters of water required during feedstock cultivation to produce 1 L of corn ethanol) because of climate change highlight the need to re-evaluate the corn ethanol elements of the Renewable Fuel Standard.

Isotopic evidence for long term warmth in the Mesozoic

Atmospheric CO2 concentrations appear to have been considerably higher than modern levels during much of the Phanerozoic and it has hence been proposed that surface temperatures were also higher. Some studies have, however, suggested that Earth's temperature (estimated from the isotopic composition of fossil shells) may have been independent of variations in atmospheric CO2 (e.g. in the Jurassic and Cretaceous). If large changes in atmospheric CO2 did not produce the expected climate responses in the past, predictions of future climate and the case for reducing current fossil-fuel emissions are potentially undermined. Here we evaluate the dataset upon which the Jurassic and Cretaceous assertions are based and present new temperature data, derived from the isotopic composition of fossil brachiopods. Our results are consistent with a warm climate mode for the Jurassic and Cretaceous and hence support the view that changes in atmospheric CO2 concentrations are linked with changes in global temperatures.

A model-based constraint on CO<sub>2</sub> fertilisation

Abstract. We derive a constraint on the strength of CO2 fertilisation of the terrestrial biosphere through a "top-down" approach, calibrating Earth system model parameters constrained by the post-industrial increase of atmospheric CO2 concentration. We derive a probabilistic prediction for the globally averaged strength of CO2 fertilisation in nature, for the period 1850 to 2000 AD, implicitly net of other limiting factors such as nutrient availability. The approach yields an estimate that is independent of CO2 enrichment experiments. To achieve this, an essential requirement was the incorporation of a land use change (LUC) scheme into the GENIE Earth system model. Using output from a 671-member ensemble of transient GENIE simulations, we build an emulator of the change in atmospheric CO2 concentration change since the preindustrial period. We use this emulator to sample the 28-dimensional input parameter space. A Bayesian calibration of the emulator output suggests that the increase in gross primary productivity (GPP) in response to a doubling of CO2 from preindustrial values is very likely (90% confidence) to exceed 20%, with a most likely value of 40–60%. It is important to note that we do not represent all of the possible contributing mechanisms to the terrestrial sink. The missing processes are subsumed into our calibration of CO2 fertilisation, which therefore represents the combined effect of CO2 fertilisation and additional missing processes. If the missing processes are a net sink then our estimate represents an upper bound. We derive calibrated estimates of carbon fluxes that are consistent with existing estimates. The present-day land–atmosphere flux (1990–2000) is estimated at −0.7 GTC yr−1 (likely, 66% confidence, in the range 0.4 to −1.7 GTC yr−1). The present-day ocean–atmosphere flux (1990–2000) is estimated to be −2.3 GTC yr−1 (likely in the range −1.8 to −2.7 GTC yr−1). We estimate cumulative net land emissions over the post-industrial period (land use change emissions net of the CO2 fertilisation and climate sinks) to be 66 GTC, likely to lie in the range 0 to 128 GTC.

Interactions between reducing CO 2 emissions, CO 2 removal and solar radiation management

We use a simple carbon cycle-climate model to investigate the interactions between a selection of idealized scenarios of mitigated carbon dioxide emissions, carbon dioxide removal (CDR) and solar radiation management (SRM). Two CO(2) emissions trajectories differ by a 15-year delay in the start of mitigation activity. SRM is modelled as a reduction in incoming solar radiation that fully compensates the radiative forcing due to changes in atmospheric CO(2) concentration. Two CDR scenarios remove 300 PgC by afforestation (added to vegetation and soil) or 1000 PgC by bioenergy with carbon capture and storage (removed from system). Our results show that delaying the start of mitigation activity could be very costly in terms of the CDR activity needed later to limit atmospheric CO(2) concentration (and corresponding global warming) to a given level. Avoiding a 15-year delay in the start of mitigation activity is more effective at reducing atmospheric CO(2) concentrations than all but the maximum type of CDR interventions. The effects of applying SRM and CDR together are additive, and this shows most clearly for atmospheric CO(2) concentration. SRM causes a significant reduction in atmospheric CO(2) concentration due to increased carbon storage by the terrestrial biosphere, especially soils. However, SRM has to be maintained for many centuries to avoid rapid increases in temperature and corresponding increases in atmospheric CO(2) concentration due to loss of carbon from the land.

Reconstruction of summer Palmer Drought Severity Index from δ13C of larch tree rings in East Siberia

We present a dendroclimatic reconstruction of Palmer Drought Severity Index (PDSI) in Yakutsk (62° 14′N, 129° 37′E), East Siberia using a larch tree-ring (latewood) δ13C chronology (1780–2008 AD). The correlation function analysis showed significant positive correlations between earlywood, latewood and total ring width and July PDSI of the previous summer. On the other hand, tree-ring δ13C from earlywood, latewood and total ring showed significant negative correlations with July PDSI in current summer, of which latewood δ13C showed highest correlation with PDSI. A moving-window correlation analysis demonstrated that the temporal stability in correlation between tree-ring variables and PDSI was highest when July PDSI was compared with latewood δ13C. Tree-ring δ13C values were corrected by taking account of both the changes in the isotope ratio of atmospheric CO2 (δ13Ca) and the plant physiological response to changes in atmospheric CO2 concentration (Ca). This correction significantly improved the sensitivity of larch tree-ring δ13C to PDSI, demonstrating that the correction is useful in building a reliable model to estimate past changes in PDSI. Our study shows that larch tree-ring δ13C has enough potential to reconstruct July PDSI series. Using a liner regression model, July PDSI was reconstructed from latewood δ13C over the last 229 years. The reconstruction showed an interannual to decadal wet/dry fluctuation with a decreasing trend from 1780 to 1960, suggesting that aridification progressed at least after the late Little Ice Age in East Siberia to the modern period of global warming.

Environmental relationships with the interannual and seasonal variation of soil respiration in a cedar (Platycladus orientalis) plantation in northern China

The relationships between soil respiration and environmental factors determine the effect of warming soil on the carbon balance in temperate forest ecosystems and on changes in atmospheric CO2 concentrations. Here, we used 3 years of data regarding soil respiration rates (Rs), soil temperature (Ts), and soil volumetric water content (θ) from a 50-year-old mature cedar ( Platycladus orientalis L.) plantation at Jiufeng Mountain, Beijing, China, to demonstrate the seasonal and interannual variation of Rs dependence on Ts and θ throughout the period 2008–2010. We used the exponential model to calculate the temperature sensitivity indicator Q10 and we examined the annual and seasonal patterns of Rs and Q10. The Rs correlated with Ts annually (p < 0.05). The Rs–Ts exponential relationship was significant in the autumn and winter (p < 0.05), while the combined Ts and θ relationships with Rs were significant in the spring and summer (p < 0.001). The spring Rs anomalies caused by drought appeared to have carryover effects that translated to Rs anomalies in the following summer. Finally, the summer Rs, which was influenced by the coincident precipitation and θ anomalies, determined the magnitude of the annual total amount of soil respiration. This result has implications for how abiotic factors may drive shifts in seasonal patterns of soil respiration under a changing climate.

Effect of long-term free-air CO2 enrichment on the diversity and activity of soil methanogens in a periodically waterlogged grassland

As atmospheric CO2 levels continue to rise researchers now identify concomitant changes in plant biomass and diversity, which are postulated to alter the quality and quantity of the organic carbon entering the soil. In anoxic soils, CH4 is the end product of the degradation of organic carbon and the system's terminal electron sink. Some soils (such as wetlands) are usually waterlogged and therefore constitute permanent CH4 sources, while others (hydromorphic soils) are only occasionally saturated with water and alternate between acting as net CH4 sources or sinks. Since methanogenesis is ultimately dependent on soil organic carbon, we hypothesized that a long term alteration of the latter will cause changes in type and magnitude of the former. To test this, we studied the effect of 10 years of atmospheric CO2 enrichment on the methanogenic potential and community in a hydromorphic temperate grassland soil at the experimental Free Air Carbon dioxide Enrichment (FACE) site in Giessen, Germany. While all soils demonstrated methanogenic potential, we detected no significant changes in CH4 production rates, lag times, methanogenic pathways, diversity, or population sizes in soils that were exposed to either 20 or 30% elevated ambient CO2. Our findings suggest that the methanogenic potential of the soil and the methanogenic community might be insensitive to changes in atmospheric CO2 concentrations, at least not on a decadal timescale. Thus, if our results can be extrapolated to other temporarily flooded or even wetland ecosystems, the often-observed increase in CH4 emissions under elevated CO2 may simply be due to an increase in labile-C input via living root and increasing fresh litter deposition, but not due to shifts in the microbial population characteristics. This could make it easier to model and extrapolate the global effect of elevated CO2 on terrestrial CH4 emissions.

Stomatal pore length change in leaves of Eotrigonobalanus furcinervis (Fagaceae) from the Late Eocene to the Latest Oligocene and its impact on gas exchange and CO2 reconstruction

It has repeatedly been reported that stomatal pore length changes with changing atmospheric CO2 concentration. Since stomatal pore length affects stomatal conductance, the question arises of how pore length change interacts with the application of stomatal density as a CO2 proxy. In this study, stomatal density, pore length and maximum stomatal conductance are evaluated for Eotrigonobalanus furcinervis (Fagaceae) from the late Eocene, late Oligocene and latest Oligocene. Then a model that simulates plant gas exchange by treating transpiration and photosynthesis as coupled processes and allows also for CO2 reconstruction was applied to the data of E. furcinervis. The model is based on an optimization approach which calculates plant gas exchange as providing a maximum amount of photosynthesis rate under given climate conditions. Stomatal density increases from the late Eocene onwards in E. furcinervis and stomatal pore length decreases. Maximum stomatal conductance which results from both stomatal pore length and density is highest for the late Eocene and lowest for the latest Oligocene. Since both stomatal conductance and stomatal density are usually negatively correlated with CO2, the stomatal data of E. furcinervis appear to be in conflict with a CO2 decrease over that period, as inferred from other evidence. The model results indicate that the stomatal data of E. furcinervis are in fact linked to a decrease of CO2 from the late Eocene to the Oligocene, in accordance with other proxy data although the late Eocene CO2values of this study are lower than usually found for this time period (range between 259 and 640ppm for the late Eocene, 213–581 ppm for the late Oligocene and 258–611 ppm for the latest Oligocene). The seemingly inconsistency between changes of stomatal density, stomatal pore length and stomatal conductance as found in this study is due to the effect on gas exchange exerted by other climate parameters than CO2. The results of this study strongly suggest that any interpretation of stomatal parameters as indicators of CO2 should consider the influence of climate on stomatal conductance and gas exchange.

Why is there a short-term increase in global precipitation in response to diminished CO2forcing?

[1] Recently, it was found that a reduction in atmospheric CO2 concentration leads to a temporary increase in global precipitation. We use the Hadley Center coupled atmosphere-ocean model, HadCM3L, to demonstrate that this precipitation increase is a consequence of precipitation sensitivity to changes in atmospheric CO2 concentrations through fast tropospheric adjustment processes. Slow ocean cooling explains the longer-term decrease in precipitation. Increased CO2 tends to suppress evaporation/precipitation whereas increased temperatures tend to increase evaporation/precipitation. When the enhanced CO2 forcing is removed, global precipitation increases temporarily, but this increase is not observed when a similar negative radiative forcing is applied as a reduction of solar intensity. Therefore, transient precipitation increase following a reduction in CO2-radiative forcing is a consequence of the specific character of CO2 forcing and is not a general feature associated with decreases in radiative forcing.

Isoprene emissions from plants are mediated by atmospheric CO2 concentrations

The tropical African tree species Acacia nigrescens Oliv. was grown in environmentally controlled growth chambers at three CO2 concentrations representative of the Last Glacial Maximum (∼180 ppmv), the present day (∼380 ppmv), and likely mid-21st century (∼600 ppmv) CO2 concentrations. Isoprene (C5H8) emissions, per unit leaf area, were greater at lower-than-current CO2 levels and lower at higher-than-current CO2 levels relative to controls grown at 380 ppmv CO2. Changes in substrate availability and isoprene synthase (IspS) activity were identified as the mechanisms behind the observed leaf-level emission response. In contrast, canopy-scale emissions remained unaltered between the treatments as changes in leaf-level emissions were offset by changes in biomass and leaf area. Substrate concentration and IspS activity-CO2 responses were used in a biochemical model, coupled to existing isoprene emission algorithms, to model isoprene emissions from A. nigrescens grown for over 2 years at three different CO2 concentrations. The addition of the biochemical model allowed for the use of emission factors measured under present day CO2 concentrations across all three CO2 treatments. When isoprene emissions were measured from A. nigrescens in response to instantaneous changes in CO2 concentration, the biochemical model satisfactorily represented the observed response. Therefore, the effect of changes in atmospheric CO2 concentration on isoprene emission at any timescale can be modelled and predicted.

Long tree-ring chronologies reveal 20th century increases in water-use efficiency but no enhancement of tree growth at five Iberian pine forests

We investigated the tree growth and physiological response of five pine forest stands in relation to changes in atmospheric CO2 concentration (ca) and climate in the Iberian Peninsula using annually resolved width and δ13C tree-ring chronologies since ad 1600. 13C discrimination (Δ≈ci/ca), leaf intercellular CO2 concentration (ci) and intrinsic water-use efficiency (iWUE) were inferred from δ13C values. The most pronounced changes were observed during the second half of the 20th century, and differed between stands. Three sites kept a constant ci/ca ratio, leading to significant ci and iWUE increases (active response to ca); whereas a significant increase in ci/ca resulted in the lowest iWUE increase of all stands at a relict Pinus uncinata forest site (passive response to ca). A significant decrease in ci/ca led to the greatest iWUE improvement at the northwestern site. We tested the climatic signal strength registered in the δ13C series after removing the low-frequency trends due to the physiological responses to increasing ca. We found stronger correlations with temperature during the growing season, demonstrating that the physiological response to ca changes modulated δ13C and masked the climate signal. Since 1970 higher δ13C values revealed iWUE improvements at all the sites exceeding values expected by an active response to the ca increase alone. These patterns were related to upward trends in temperatures, indicating that other factors are reinforcing stomatal closure in these forests. Narrower rings during the second half of the 20th century than in previous centuries were observed at four sites and after 1970 at all sites, providing no evidence for a possible CO2‘fertilization’ effect on growth. The iWUE improvements found for all the forests, reflecting both a ca rise and warmer conditions, seem to be insufficient to compensate for the negative effects of the increasing water limitation on growth.

The Effect of Terrestrial Photosynthesis Down Regulation on the Twentieth-Century Carbon Budget Simulated with the CCCma Earth System Model

Abstract The simulation of atmospheric–land–ocean CO2 exchange for the 1850–2000 period offers the possibility of testing and calibrating the carbon budget in earth system models by comparing the simulated changes in atmospheric CO2 concentration and in land and ocean uptake with observation-based information. In particular, some of the uncertainties associated with the treatment of land use change (LUC) and the role of down regulation in affecting the strength of CO2 fertilization for terrestrial photosynthesis are assessed using the Canadian Centre for Climate Modelling and Analysis Earth System Model (CanESM1). LUC emissions may be specified as an external source of CO2 or calculated interactively based on estimated changes in crop area. The evidence for photosynthetic down regulation is reviewed and an empirically based representation is implemented and tested in the model. Four fully coupled simulations are performed: with and without terrestrial photosynthesis down regulation and with interactively determined or specified LUC emissions. Simulations without terrestrial photosynthesis down regulation yield 15–20 ppm lower atmospheric CO2 by the end of the twentieth century, compared to observations, regardless of the LUC approach used because of higher carbon uptake by land. Implementation of down regulation brings simulated values of atmospheric CO2 and land and ocean carbon uptake closer to observation-based values. The use of specified LUC emissions yields a large source in the tropics during the 1981–2000 period, which is inconsistent with studies suggesting the tropics to be near-neutral or small carbon sinks. The annual cycle of simulated global averaged CO2, dominated by the Northern Hemisphere terrestrial photosynthesis and respiration cycles, is reasonably well reproduced, as is the latitudinal distribution of CO2 and the dependence of interhemispheric CO2 gradient on fossil fuel emissions. The empirical approach used here offers a reasonable method of implementing down regulation in coupled carbon–climate models in the absence of a more explicit biogeochemical representation.

Quantifying Carbon Cycle Feedbacks

Abstract Perturbations to the carbon cycle could constitute large feedbacks on future changes in atmospheric CO2 concentration and climate. This paper demonstrates how carbon cycle feedback can be expressed in formally similar ways to climate feedback, and thus compares their magnitudes. The carbon cycle gives rise to two climate feedback terms: the concentration–carbon feedback, resulting from the uptake of carbon by land and ocean as a biogeochemical response to the atmospheric CO2 concentration, and the climate–carbon feedback, resulting from the effect of climate change on carbon fluxes. In the earth system models of the Coupled Climate–Carbon Cycle Model Intercomparison Project (C4MIP), climate–carbon feedback on warming is positive and of a similar size to the cloud feedback. The concentration–carbon feedback is negative; it has generally received less attention in the literature, but in magnitude it is 4 times larger than the climate–carbon feedback and more uncertain. The concentration–carbon feedback is the dominant uncertainty in the allowable CO2 emissions that are consistent with a given CO2 concentration scenario. In modeling the climate response to a scenario of CO2 emissions, the net carbon cycle feedback is of comparable size and uncertainty to the noncarbon–climate response. To quantify simulated carbon cycle feedbacks satisfactorily, a radiatively coupled experiment is needed, in addition to the fully coupled and biogeochemically coupled experiments, which are referred to as coupled and uncoupled in C4MIP. The concentration–carbon and climate–carbon feedbacks do not combine linearly, and the concentration–carbon feedback is dependent on scenario and time.