Stephen C. Piper

Global net carbon exchange and intra-annual atmospheric CO2 concentrations predicted by an ecosystem process model and three-dimensional atmospheric transport model

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.

Variations in modeled atmospheric transport of carbon dioxide and the consequences for CO2 inversions

Carbon dioxide concentrations due to fossil fuel burning and CO 2 exchange with the terrestrial biosphere have been modeled with 12 different three-dimensional atmospheric transport models. The models include both on-line and off-line types and use a variety of advection algorithms and subgrid scale parameterizations. A range of model resolutions is also represented. The modeled distributions show a large range of responses. For the experiment using the fossil fuel source, the annual mean meridional gradient at the surface vases by a factor of 2. This suggests a factor of 2 variation in the efficiency of surface interhemispheric exchange as much due to differences in model vertical transport as to horizontal differences. In the upper troposphere, zonal mean gradients within the northern hemisphere vary in sign. In the terrestrial biotic source experiment, the spatial distribution of the amplitude and the phase of the seasonal cycle of surface CO 2 concentration vary little between models. However, the magnitude of the amplitudes varies similarly to the fossil case. Differences between modeled and observed seasonal cycles in the northern extratropics suggest that the terrestrial biotic source is overestimated in late spring and underestimated in winter. The annual mean response to the seasonal source also shows large differences in magnitude. The uncertainty in hemispheric carbon budgets implied by the differences in interhemispheric exchange times is comparable to those quoted by the Intergovernmental Panel on Climate Change for fossil fuel and ocean uptake and smaller than those for terrestrial fluxes. We outline approaches which may reduce this component in CO 2 budget uncertainties.

Enhanced seasonal exchange of CO2 by northern ecosystems since 1960.

Downs and Ups Every spring, the concentration of CO2 in the atmosphere of the Northern Hemisphere decreases as terrestrial vegetation grows, and every fall, CO2 rises as vegetation dies and rots. Climate change has destabilized the seasonal cycle of atmospheric CO2 such that Graven et al. (p. 1085, published online 8 August; see the Perspective by Fung) have found that the amplitude of the seasonal cycle has exceeded 50% at some latitudes. The only way to explain this increase is if extratropical land ecosystems are growing and shrinking more than they did half a century ago, as a result of changes in the structure and composition of northern ecosystems. The amplitude of the seasonal cycle of carbon dioxide in high northern latitudes has increased by 50% since 1960. [Also see Perspective by Fung] Seasonal variations of atmospheric carbon dioxide (CO2) in the Northern Hemisphere have increased since the 1950s, but sparse observations have prevented a clear assessment of the patterns of long-term change and the underlying mechanisms. We compare recent aircraft-based observations of CO2 above the North Pacific and Arctic Oceans to earlier data from 1958 to 1961 and find that the seasonal amplitude at altitudes of 3 to 6 km increased by 50% for 45° to 90°N but by less than 25% for 10° to 45°N. An increase of 30 to 60% in the seasonal exchange of CO2 by northern extratropical land ecosystems, focused on boreal forests, is implicated, substantially more than simulated by current land ecosystem models. The observations appear to signal large ecological changes in northern forests and a major shift in the global carbon cycle.

Interannual variability in the oxygen isotopes of atmospheric CO2 driven by El Nino

The stable isotope ratios of atmospheric CO2 (18O/16O and 13C/12C) 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 18O/16O variability has proved difficult, however, because oxygen isotopes in CO2 are influenced by both the carbon cycle and the water cycle. Previous attention focused on the decreasing 18O/16O 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 C4 crops at the expense of C3 forests; and environmental conditions, such as atmospheric turbulence and solar radiation, that affect CO2 exchange between leaves and the atmosphere. Here we present 30 years’ worth of data on 18O/16O in CO2 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 18O/16O ratio of precipitation and plant water, and that this signal is then passed on to atmospheric CO2 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 CO2 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 CO2. Although still tentative, such a revision would present a new benchmark by which to evaluate global biospheric carbon cycling models.

A globally applicable model of daily solar irradiance estimated from air temperature and precipitation data

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.

The influence of seasonal water availability on global C3 versus C4 grassland biomass and its implications for climate change research

Abstract 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.

A gridded global data set of daily temperature and precipitation for terrestrial biospheric modeling

The first global terrestrial gridded data set of the daily average and range of temperature and daily precipitation has been developed, intended for use in terrestrial biospheric modeling. Data for the year 1987 are shown to illustrate our methodology. Daily station data, primarily from the World Meteorological Organization global synoptic surface network of stations, have been extensively quality checked and interpolated to a 1×1 degree grid by using a nearest neighbors interpolation scheme. Annual averages of the daily average temperatures have been compared with 1987 temperatures constructed from data supplied by P.D. Jones (personal communication, 1996). Agreement between these two data sets is good, except in some areas of the southern hemisphere where station coverage is poor. Monthly and annual totals of the daily precipitation data have been compared with the monthly 1987 data set produced by the Global Precipitation Climatology Centre. Agreement between the two data sets is good over much of the northern hemisphere and South America; however, large discrepancies are seen in east-central and south-central Africa and in most of Australia, primarily due to the poor station coverage there. Comparison of the time series from individual stations with those from the gridded data set indicate that the day-to-day variation of temperature and the fraction of wet days are preserved, except in the tropics where wet days are overestimated. Station densities have been tabulated in terms of total annual net primary productivity to identify countries where increases in station data will be most effective for terrestrial biospheric modeling.

Evolution of natural and anthropogenic fluxes of atmospheric CO2 from 1957 to 2003

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

Climate effects on atmospheric carbon dioxide over the last century

The buildup of atmospheric CO2 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 CO2 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 CO2 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 CO2 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 CO2 variability on multidecadal and El Niño timescales.

Atmospheric evidence for a global secular increase in carbon isotopic discrimination of land photosynthesis

Significance Climate change and rising CO2 are altering the behavior of land plants in ways that influence how much biomass they produce relative to how much water they need for growth. This study shows that it is possible to detect changes occurring in plants using long-term measurements of the isotopic composition of atmospheric CO2. These measurements imply that plants have globally increased their water use efficiency at the leaf level in proportion to the rise in atmospheric CO2 over the past few decades. While the full implications remain to be explored, the results help to quantify the extent to which the biosphere has become less constrained by water stress globally. A decrease in the 13C/12C 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 13C-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 13C-Suess effect unless an increase has occurred in the 13C/12C 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 C3 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.