Philippe Bousquet

Multiple constraints on regional CO2 flux variations over land and oceans

[1]xa0To increase our understanding of the carbon cycle, we compare regional estimates of CO2 flux variability for 1980–1998 from atmospheric CO2 inversions and from process-based models of the land (SLAVE and LPJ) and ocean (OPA and MIT). Over the land, the phase and amplitude of the different estimates agree well, especially at continental scale. Flux variations are predominantly controlled by El Nino events, with the exception of the post-Pinatubo period of the early 1990s. Differences between the two land models result mainly from the response of heterotrophic respiration to precipitation and temperature. The “Lloyd and Taylor” formulation of LPJ [Lloyd and Taylor, 1994] agrees better with the inverse estimates. Over the ocean, inversion and model results agree only in the equatorial Pacific and partly in the austral ocean. In the austral ocean, an increased CO2 sink is present in the inversion and OPA model, and results from increased stratification of the ocean. In the northern oceans, the inversions estimate large flux variations in line with time-series observations of the subtropical Atlantic, but not supported by the two model estimates, thus suggesting that the CO2 variability from high-latitude oceans needs further investigation.

Influence of transport uncertainty on annual mean and seasonal inversions of atmospheric CO2 data

[1]xa0Inversion methods are often used to estimate surface CO2 fluxes from atmospheric CO2 concentration measurements, given an atmospheric transport model to relate the two. The published estimates disagree strongly on the location of the main sources and sinks, however. Are these differences due to the different time spans considered, or are they artifacts of the method and data used? Here we assess the uncertainty in such estimates due to the choice of time discretization of the measurements and fluxes, the spatial resolution of the fluxes, and the transport model. A suite of 27 Bayesian least squares inversions has been run, given by varying the number of flux regions solved for (7, 12, and 17), the time discretization (annual/annual, annual/monthly, and monthly/monthly for the fluxes/data), and the transport model (TM2, TM3, and GCTM), while holding all other inversion details constant. The estimated fluxes from this ensemble of inversions for the land + ocean sum are stable over large zonal bands, but the spread in the results increases when considering the longitudinal flux distribution inside these bands. On average for 1990–1994 the inversions place a large CO2 uptake north of 30°N (3.2 ± 0.3 GtC yr−1), mostly over the land regions, with more in Eurasia than North America. The ocean fluxes are generally smaller than given by Takahashi et al. [1999], especially south of 15°S and in the global total, where they are less than half as large. A small uptake is found for the tropical land regions, suggesting that growth more than compensates for deforestation there. The results for the different transport models are consistent with their known mixing properties; the longitudinal pattern of their land biosphere rectifier, in particular, strongly influences the regional partitioning of the flux in the north. While differences between the transport models contribute significantly to the spread of the results, an equivalent or even larger spread is due to the time discretization method used: Solving for annual mean fluxes with monthly mean measurements tended to give spurious land/ocean flux partition in the north. We suggest then that this time discretization method be avoided. Overall, the uncertainty quoted for the estimated fluxes should include not only the random error calculated by the inversion equations but also all the systematic errors in the problem, such as those addressed in this study.

Horizontal displacement of carbon associated with agriculture and its impacts on atmospheric CO2

[1]xa0The growth of crops represents a sink of atmospheric CO2, whereas biomass is consumed by humans and housed animals, yielding respiratory sources of CO2. This process induces a lateral displacement of carbon and creates geographic patterns of CO2 sources and sinks at the surface of the globe. We estimated the global carbon flux harvested in croplands to be 1290 TgC/yr. Most of this carbon is transported into domestic trade, whereas a small fraction (13%) enters into international trade circuits. We then calculated the global patterns of CO2 fluxes associated with food and feedstuff trade, using country-based agricultural statistics and activity maps of human and housed animal population densities. The CO2 flux maps show regional dipoles of sources and sinks in Asia and North America. The effect of these fluxes on atmospheric CO2 was simulated using a global atmospheric transport model. The mean latitudinal CO2 gradients induced by the displacement of crop products are fairly small (≈0.2 ppm) compared with observations (4–5 ppm), indicating that this process has a only a small influence in explaining the latitudinal distribution of CO2 fluxes. On the other hand, the simulated longitudinal mean atmospheric CO2 gradients at northern midlatitudes (≈ up to 0.5 ppm) are comparable to the ones measured between atmospheric stations, suggesting that CO2 fluxes from crop products trade are an important component of continental- and regional-scale CO2 budgets. Thus they should be accounted for as prior information in regional inversions.

Evaluation of SF6, C2Cl4, and CO to approximate fossil fuel CO2 in the Northern Hemisphere using a chemistry transport model

The distribution of the fossil fuel component in atmospheric CO2 cannot be measured directly at a cheap cost. Could anthropogenic tracers with source patterns similar to fossil fuel CO2 then be used for that purpose? Here we present and evaluate a methodology using surrogate tracers, CO, SF6, and C2Cl4, to deduce fossil fuel CO2. A three-dimensional atmospheric chemistry transport model is used to simulate the relationship between each tracer and fossil fuel CO2. In summertime the regression slopes between fossil fuel CO2 and surrogate tracers show large spatial variations for chemically active tracers ( CO and C2Cl4), although C2Cl4 presents less scatter than CO. At two tall tower sites in the United States ( WLEF, Wisconsin, and WITN, North Carolina), we found that in summertime the C2Cl4 ( CO) versus fossil CO2 slope is on average up to 15% ( 25%) higher than in winter. We show that for C2Cl4 this seasonal variation is due to OH oxidation. For CO the seasonal variation is due to both chemistry and mixing with nonanthropogenic CO sources. In wintertime the three surrogate tracers SF6, C2Cl4, and CO are about equally as good indicators of the presence of fossil CO2. However, our model strongly underestimates the variability of SF6 at both towers, probably because of unaccounted for emissions. Hence poor knowledge of emission distribution hampers the use of SF6 as a surrogate tracer. From a practical point of view we recommend the use of C2Cl4 as a proxy of fossil CO2. We also recommend the use of tracers to separate fossil CO2. Despite the fact that the uncertainty on the regression slope is on the order of 30%, the tracer approach is likely to have less bias than when letting one model with one inventory emission map calculate the fossil CO2 distribution.

Quantifier les puits et sources des gaz à effet de serre: une nouvelle ambition pour la télédétection spatiale

EnglishThe long lifetime of some greenhouse gases in the atmosphere, such as carbon dioxide, methane and nitrous oxide, gives them a critical role in climate change. At the same time, the accumulation of these gases increases their background level and therefore reduces their space-time variability relative to this level, which leads to particularly challenging specifications on the expected accuracy of such measurements to bring new information. Their remote sensing from space has therefore started late, in the 2000s. The measurement of their atmospheric concentration is essentially motivated by the quantification of the amount of gas exchanged between the atmosphere and the Earths surface. Such an objective requires the development of sophisticated processing chains, including atmospheric chemistrytransport models and other complementary sources of information on the fluxes of these gases. francaisLa longue duree de vie de certains gaz a effet de serre dans latmosphere, comme le dioxyde de carbone, le methane et le protoxyde dazote, donne a ceux-ci un role critique dans levolution du climat. Par ailleurs, laccumulation de ces gaz augmente le niveau de fond et reduit ainsi leur variabilite spatio-temporelle relative a celui-ci, ce qui conduit a des specifications particulierement ambitieuses sur la precision attendue des mesures, pour quelles apportent une information nouvelle. Leur teledetection spatiale a donc debute tardivement, au cours de la decennie 2000. La mesure de leurs concentrations atmospheriques est essentiellement motivee par lestimation de la quantite nette de gaz echangee entre latmosphere et la surface du globe. Un tel objectif impose la conception de chaines de traitement sophistiquees, faisant appel a des modeles de chimie-transport atmospherique et dautres sources dinformation complementaires sur les flux de ces gaz.