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A decrease in the C-13/C-12 ratio of atmospheric CO2 has been documented by direct observations since 1978 and from ice core measurements since the industrial revolution. This decrease, known as the C-13-Suess effect, is driven primarily by the input of fossil fuel-derived CO2 but is also sensitive to land and ocean carbon cycling and uptake. Using updated records, we show that no plausible combination of sources and sinks of CO2 from fossil fuel, land, and oceans can explain the observed C-13-Suess effect unless an increase has occurred in the C-13/C-12 isotopic discrimination of land photosynthesis. A trend toward greater discrimination under higher CO2 levels is broadly consistent with tree ring studies over the past century, with field and chamber experiments, and with geological records of C-3 plants at times of altered atmospheric CO2, but increasing discrimination has not previously been included in studies of long-term atmospheric 13C/12C measurements. We further show that the inferred discrimination increase of 0.014 +/- 0.007% ppm(-1) is largely explained by photorespiratory and mesophyll effects. This result implies that, at the global scale, land plants have regulated their stomatal conductance so as to allow the CO2 partial pressure within stomatal cavities and their intrinsic water use efficiency to increase in nearly constant proportion to the rise in atmospheric CO2 concentration.

The stable isotope ratios of atmospheric CO(2) ((18)O/(16)O and (13)C/(12)C) have been monitored since 1977 to improve our understanding of the global carbon cycle, because biosphere-atmosphere exchange fluxes affect the different atomic masses in a measurable way. Interpreting the (18)O/(16)O variability has proved difficult, however, because oxygen isotopes in CO(2) are influenced by both the carbon cycle and the water cycle. Previous attention focused on the decreasing (18)O/(16)O ratio in the 1990s, observed by the global Cooperative Air Sampling Network of the US National Oceanic and Atmospheric Administration Earth System Research Laboratory. This decrease was attributed variously to a number of processes including an increase in Northern Hemisphere soil respiration; a global increase in C(4) crops at the expense of C(3) forests; and environmental conditions, such as atmospheric turbulence and solar radiation, that affect CO(2) exchange between leaves and the atmosphere. Here we present 30 years' worth of data on (18)O/(16)O in CO(2) from the Scripps Institution of Oceanography global flask network and show that the interannual variability is strongly related to the El Niño/Southern Oscillation. We suggest that the redistribution of moisture and rainfall in the tropics during an El Niño increases the (18)O/(16)O ratio of precipitation and plant water, and that this signal is then passed on to atmospheric CO(2) by biosphere-atmosphere gas exchange. We show how the decay time of the El Niño anomaly in this data set can be useful in constraining global gross primary production. Our analysis shows a rapid recovery from El Niño events, implying a shorter cycling time of CO(2) with respect to the terrestrial biosphere and oceans than previously estimated. Our analysis suggests that current estimates of global gross primary production, of 120 petagrams of carbon per year, may be too low, and that a best guess of 150-175 petagrams of carbon per year better reflects the observed rapid cycling of CO(2). Although still tentative, such a revision would present a new benchmark by which to evaluate global biospheric carbon cycling models.

Climate-change induced alterations in the global distribution of cool season (C3) and warm season (C4) grasses would impact the global carbon cycle and have differing, local effects on range and agricultural production. We hypothesize that a major influence on C3/C4 distribution may be the seasonal timing of water availability with respect to the different C3 and C4 growing seasons. An algorithm expressing this hypothesis (the SAW hypothesis for Seasonal Availability of Water), estimates C3 versus C4 grass biomass from climate data. Sensitivity analysis indicated that temperatures used to delineate the start and end of the C3 and C4 grass growing seasons were more important than photosynthetic responses to temperature. To evaluate the SAW hypothesis, this algorithm was applied globally on a 1°×1° latitude–longitude grid. When compared with vegetation survey data at 141 locations in North America, Argentina, Australia, and South Africa, SAW algorithm predictions yielded an R2 of 0.71. Error resulted primarily from comparing large grid cells to plot data, interannual variability of climate, and from gridding measured climate to data-sparse locations with a single lapse rate of air temperature with elevation. Application of the SAW algorithm to a climate change scenario suggested that changes in temperature and precipitation patterns could offset C3 photosynthetic advantages offered by elevated atmospheric CO2 concentrations. These results underscored the importance of accurately representing the timing and spatial distribution as well as the magnitude of temperature and precipitation in scenarios of future climate.

During 1984–2000, canopy tree growth in old-growth tropical rain forest at La Selva, Costa Rica, varied >2-fold among years. The trees' annual diameter increments in this 16-yr period were negatively correlated with annual means of daily minimum temperatures. The tree growth variations also negatively covaried with the net carbon exchange of the terrestrial tropics as a whole, as inferred from nearly pole-to-pole measurements of atmospheric carbon dioxide (CO 2 ) interpreted by an inverse tracer–transport model. Strong reductions in tree growth and large inferred tropical releases of CO 2 to the atmosphere occurred during the record-hot 1997–1998 El Niño. These and other recent findings are consistent with decreased net primary production in tropical forests in the warmer years of the last two decades. As has been projected by recent process model studies, such a sensitivity of tropical forest productivity to on-going climate change would accelerate the rate of atmospheric CO 2 accumulation.

Although not measured at many ground stations, the total daily solar irradiance (Rs) received at the earth’s surface is a critical component of ecosystem carbon, water and energy processes. Methods of estimating Rs from other meteorological data, particularly daily temperatures, have not worked as well in tropical and maritime areas. At Luquillo, Puerto Rico, the daily atmospheric transmittance for solar radiation was approximately equal to one minus the daily-average relative humidity (1 −rhave). From these observations, we developed a model (VP-RAD) for estimation of Rs with inputs of daily maximum and minimum air temperature, daily total precipitation, mean annual temperature, mean annual temperature range, site latitude, and site elevation. VP-RAD performed well over large areas; it showed a good agreement with the site data used for model development and for seven other warm, humid locations in the southeastern United States. Comparisons with a similar model revealed that predictions using VP-RAD had lower average errors and improved day-to-day correlation to measured solar irradiance. In a global comparison for the year 1987, VP-RAD-estimated and satellite-derived photosynthetically active radiation converged to within 1.0 MJ m −2 day − 1 at 72% of the 13072 1° latitude by 1° longitude vegetated grid cells. Although these comparisons revealed areas where VP-RAD may need improvement, VP-RAD should be a useful tool for applications globally. In addition, VP-RAD’s similarity in form to the Bristow–Campbell equation provides a convenient method to calculate the site-specific coefficients for this model that is widely used when solar irradiance data are not available. © 2001 Elsevier Science B.V. All rights reserved.

A generalized terrestrial ecosystem process model, BIOME-BGC (for BIOME BioGeoChemical Cycles), was used to simulate the global fluxes of CO 2 resulting from photosynthesis, autotrophic respiration, and heterotrophic respiration. Daily meteorological data for the year 1987, gridded to 1° by 1°, were used to drive the model simulations. From the maximum value of the normalized difference vegetation index (NDVI) for 1987, the leaf area index for each grid cell was computed. Global NPP was estimated to be 52 Pg C, and global R h was estimated to be 66 Pg C. Model predictions of the stable carbon isotopic ratio 13 C/ 12 C for C3 and C 4 vegetation were in accord with values published in the literature, suggesting that our computations of total net photosynthesis, and thus NPP, are more reliable than R h . For each grid cell, daily R h was adjusted so that the annual total was equal to annual NPP, and the resulting net carbon fluxes were used as inputs to a three-dimensional atmospheric transport model (TM2) using wind data from 1987. We compared the spatial and seasonal patterns of NPP with a diagnostic NDVI model, where NPP was derived from biweekly NDVI data and Rh was tuned to fit atmospheric CO 2 observations from three northern stations. To an encouraging degree, predictions from the BIOME-BGC model agreed in phase and amplitude with observed atmospheric CO 2 concentrations for 20° to 55°N, the zone in which the most complete data on ecosystem processes and meteorological input data are available. However, in the tropics and high northern latitudes, disagreements between simulated and measured CO 2 concentrations indicated areas where the model could be improved. We present here a methodology by which terrestrial ecosystem models can be tested globally, not by comparisons to homogeneous-plot data, but by seasonal and spatial consistency with a diagnostic NDVI model and atmospheric CO 2 observations.

THE global budget for sources and sinks of anthropogenic CO2 has been found to be out of balance unless the oceanic sink is supplemented by an additional 'missing sink', plausibly associated with land biota1,25. A similar budgeting problem has been found for the Northern Hemisphere alone2,3, suggesting that northern land biota may be the sought-after sink, although this interpretation is not unique2–5; to distinguish oceanic and land carbon uptake, the budgets rely variously, and controversially, on ocean models2,6,7, 13CO2/12CO2 data2,4,5, sparse oceanic observations of (ref. 3) or 13C/12C ratios of dissolved inorganic carbon,4,5,8 or single-latitude trends in atmospheric O2 as detected from changes in O2/N2 ratio.9,10 Here we present an extensive O2/N2 data set which shows simultaneous trends in O2/N2 in both northern and southern hemispheres and allows the O2/N2 gradient between the two hemispheres to be quantified. The data are consistent with a budget in which, for the 1991–94 period, the global oceans and the northern land biota each removed the equivalent of approximately 30% of fossil-fuel CO2 emissions, while the tropical land biota as a whole were not a strong source or sink.

The main objective of this project was to continue research to develop carbon cycle relationships related to the land biosphere based on remote measurements of atmospheric CO2 concentration and its isotopic ratios 13C/12C, 18O/16O, and 14C/12C. The project continued time-series observations of atmospheric carbon dioxide and isotopic composition begun by Charles D. Keeling at remote sites, including Mauna Loa, the South Pole, and eight other sites. Using models of varying complexity, the concentration and isotopic measurements were used to study long-term change in the interhemispheric gradients in CO2 and 13C/12C to assess the magnitude and evolution of the northern terrestrial carbon sink, to study the increase in amplitude of the seasonal cycle of CO2, to use isotopic data to refine constraints on large scale changes in isotopic fractionation which may be related to changes in stomatal conductance, and to motivate improvements in terrestrial carbon cycle models. The original proposal called for a continuation of the new time series of 14C measurements but subsequent descoping to meet budgetary constraints required termination of measurements in 2007.

An analysis is carried out of the longest available records of atmospheric CO 2 and its 13 C/ 12 C ratio from the Scripps Institution of Oceanography network of fixed stations, augmented by data in the 1950s and 1960s from ships and ice floes. Using regression analysis, we separate the interhemispheric gradients of CO 2 and 13 C/ 12 C into: (1) a stationary (possibly natural) component that is constant with time, and (2) a time-evolving component that increases in proportion to fossil fuel emissions. Inverse calculations using an atmospheric transport model are used to interpret the components of the gradients in terms of land and ocean sinks. The stationary gradients in CO 2 and 13 C/ 12 C are both satisfactorily explained by ocean processes, including an ocean carbon loop that transports 0.5 PgC yr -1 southwards in the ocean balanced by an atmospheric return flow. A stationary northern land sink appears to be ruled out unless its effect on the gradient has been offset by a strong rectifier effect, which seems doubtful. A growing northern land sink is not ruled out, but has an uncertain magnitude (0.3–1.7 PgC yr -1 centred on year 2003) dependent on the rate at which CO 2 from fossil fuel burning is dispersed vertically and between hemispheres. DOI: 10.1111/j.1600-0889.2010.00507.x

The buildup of atmospheric CO 2 since 1958 is surprisingly well explained by the simple premise that 57% of the industrial emissions (fossil fuel burning and cement manufacture) has remained airborne. This premise accounts well for the rise both before and after 1980 despite a decrease in the growth rate of fossil fuel CO 2 emissions, which occurred at that time, and by itself should have caused the airborne fraction to decrease. In contrast, the buildup prior to 1958 was not simply proportional to cumulative fossil fuel emissions, and notably included a period during the 1940s when CO 2 growth stalled despite continued fossil fuel emissions. Here we show that the constancy of the airborne fraction since 1958 can be in part explained by decadal variations in global land air temperature, which caused a warming-induced release of CO 2 from the land biosphere to the atmosphere. We also show that the 1940s plateau may be related to these decadal temperature variations. Furthermore, we show that there is a close connection between the phenomenology producing CO 2 variability on multidecadal and El Nino timescales. DOI: 10.1111/j.1600-0889.2009.00439.x

The effect of north African savanna fires on atmospheric CO2 is investigated using a tracer transport model. The model uses winds from operational numerical weather prediction analyses and provides CO2 concentrations as a function of space and time. After a spin-up period of several years, biomass-burning sources are added, and model experiments are run for an additional year, utilizing various estimates of CO2 sources. The various model experiments show that biomass burning in the north African savannas significantly affects CO2 concentrations in South America. The effect is more pronounced during the period from January through March, when biomass burning in South America is almost nonexistent. During this period, atmospheric CO2 concentrations in parts of South America typically may increase by 0.5 to 0.75 ppm at 970 mbar, the average pressure of the lowest model layer. These figures are above the probable uncertainty level, as model runs with biomass-burning sources estimated from independent studies using distinct data sets and techniques indicate. From May through September, when severe biomass burning occurs in South America, the effect of north African savanna fires over South America has become generally small at 970 mbar, but north of the equator it may be of the same magnitude or larger than the effect of South American fires. The CO2 concentration increase in the extreme northern and southern portions of South America, however, is mostly due to southern African fires, whose effect may be 2-3 times larger than the effect of South American fires at 970 mbar. Even in the central part of the continent, where local biomass-burning emissions are maximum, southern African fires contribute to at least 15% of the CO2 concentration increase at 970 mbar. At higher levels in the atmosphere, less CO2 emitted by north African savanna fires reaches South America, and at 100 mbar no significant amount of CO2 is transported across the Atlantic Ocean. The vertical structure of the CO2 concentration increase due to biomass burning differs substantially, depending on whether sources are local or remote. A prominent maximum of CO2 concentration increase in the lower layers characterizes the effect of local sources, whereas a more homogeneous profile of CO2 concentration increase characterizes the effect of remote sources. The results demonstrate the strong remote effects of African biomoass burning which, owing to the general circulation of the atmosphere, are felt as far away as South America.

We agree with Idso that CO2 fertilization of terrestrial ecosystems is of great importance in the understanding of the behaviour of the amplitude of the seasonal cycle of atmospheric CO2 as analyzed in our paper (Kohlmaier et al., 1989, henceforth called II). DOI: 10.1034/j.1600-0889.1991.t01-1-00008.x

An observed increase of the amplitude of the seasonal cycle of atmospheric CO 2 at Mauna Loa Observatory has been examined. Previous global carbon cycle studies seemed to suggest, at least qualitatively, that the observed increase may be entirely due to a CO 2 fertilization effect on the terrestrial biota. The present detailed theoretical investigation shows that a pure CO 2 stimulation of the net primary production may indeed lead to an amplitude increase of 0.15 plus 0.17% yr -1 and minus 0.10% yr -1 as compared to the measured value of 0.67 ± 0.25% yr -1 for the period between AD 1958 and AD 1987. This result is based on the assumption of a long term fertilization factor β with a range between 0.15 and 0.60, taken to be equal to the mesaured short term fertilization factor β, obtained in CO 2 enrichment studies with exposure times generally smaller than 1 year. The experimental evaluation of the difference between the relative increase of the summer (peak to trough) and winter (trough to peak) amplitude gives information on the carbon sequestered annually by the terrestrial ecosystems. With the estimated respiration response factor, which characterizes the fraction of the additional production that is consumed by ecosystem respiration, equal to 0.75, and an assumed long-term CO 2 fertilization factor, β, equal to 0.375, acceptable agreement between prediction and measurements is obtained, amounting to a mean annual increase in living biomass of 0.7 Gt C, while a corresponding portion may be stored in soils. The fact that the fertilization alone predicts the observed difference but not the absolute value of both the summer and winter amplitude, leads to the conclusion that other external effects are operative, which are not directly related to the fertilization phenomenon of the vegetation and which influence to an equal extent the summer and winter amplitude. Two such external contributions have been identified; they are both related to the increased fossil fuel carbon input into the Northern Hemisphere: (1) the seasonality of the fossil fuel consumption in the Northern Hemisphere is approximately in phase with the relative uptake or release of CO 2 by the land vegetation and leads to a small contribution to the increase in amplitude, in the range between 0.01 and 0.08% yr -1 ; (2) the seasonally different transequatorial transport of fossil fuel carbon creates a seasonal behavior which again is approximately in phase with the biota and leads to an additional contribution in the range of 0 to 0.31% per year depending on the wind fields used. DOI: 10.1111/j.1600-0889.1989.tb00137.x