Publications

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2018
Rodenbeck, C, Zaehle S, Keeling R, Heimann M.  2018.  How does the terrestrial carbon exchange respond to inter-annual climatic variations? A quantification based on atmospheric CO2 data Biogeosciences. 15:2481-2498.   10.5194/bg-15-2481-2018   AbstractWebsite

The response of the terrestrial net ecosystem exchange (NEE) of CO2 to climate variations and trends may crucially determine the future climate trajectory. Here we directly quantify this response on inter-annual timescales by building a linear regression of inter-annual NEE anomalies against observed air temperature anomalies into an atmospheric inverse calculation based on long-term atmospheric CO2 observations. This allows us to estimate the sensitivity of NEE to inter-annual variations in temperature (seen as a climate proxy) resolved in space and with season. As this sensitivity comprises both direct temperature effects and the effects of other climate variables co-varying with temperature, we interpret it as "inter-annual climate sensitivity". We find distinct seasonal patterns of this sensitivity in the northern extratropics that are consistent with the expected seasonal responses of photosynthesis, respiration, and fire. Within uncertainties, these sensitivity patterns are consistent with independent inferences from eddy covariance data. On large spatial scales, northern extratropical and tropical interannual NEE variations inferred from the NEE-T regression are very similar to the estimates of an atmospheric inversion with explicit inter-annual degrees of freedom. The results of this study offer a way to benchmark ecosystem process models in more detail than existing effective global climate sensitivities. The results can also be used to gap-fill or extrapolate observational records or to separate inter-annual variations from longer-term trends.

2016
Welp, LR, Patra PK, Rodenbeck C, Nemani R, Bi J, Piper SC, Keeling RF.  2016.  Increasing summer net CO2 uptake in high northern ecosystems inferred from atmospheric inversions and comparisons to remote-sensing NDVI. Atmospheric Chemistry and Physics. 16:9047-9066.   10.5194/acp-16-9047-2016   AbstractWebsite

Warmer temperatures and elevated atmospheric CO2 concentrations over the last several decades have been credited with increasing vegetation activity and photosynthetic uptake of CO2 from the atmosphere in the high northern latitude ecosystems: the boreal forest and arctic tundra. At the same time, soils in the region have been warming, permafrost is melting, fire frequency and severity are increasing, and some regions of the boreal forest are showing signs of stress due to drought or insect disturbance. The recent trends in net carbon balance of these ecosystems, across heterogeneous disturbance patterns, and the future implications of these changes are unclear. Here, we examine CO2 fluxes from northern boreal and tundra regions from 1985 to 2012, estimated from two atmospheric inversions (RIGC and Jena). Both used measured atmospheric CO2 concentrations and wind fields from interannually variable climate reanalysis. In the arctic zone, the latitude region above 60 degrees N excluding Europe (10 degrees W-63 degrees E), neither inversion finds a significant long-term trend in annual CO2 balance. The boreal zone, the latitude region from approximately 50-60 degrees N, again excluding Europe, showed a trend of 8-11 Tg C yr(-2) over the common period of validity from 1986 to 2006, resulting in an annual CO2 sink in 2006 that was 170-230 Tg C yr(-1) larger than in 1986. This trend appears to continue through 2012 in the Jena inversion as well. In both latitudinal zones, the seasonal amplitude of monthly CO2 fluxes increased due to increased uptake in summer, and in the arctic zone also due to increased fall CO2 release. These findings suggest that the boreal zone has been maintaining and likely increasing CO2 sink strength over this period, despite browning trends in some regions and changes in fire frequency and land use. Meanwhile, the arctic zone shows that increased summer CO2 uptake, consistent with strong greening trends, is offset by increased fall CO2 release, resulting in a net neutral trend in annual fluxes. The inversion fluxes from the arctic and boreal zones covering the permafrost regions showed no indication of a large-scale positive climate-carbon feedback caused by warming temperatures on high northern latitude terrestrial CO2 fluxes from 1985 to 2012.

Nevison, CD, Manizza M, Keeling RF, Stephens BB, Bent JD, Dunne J, Ilyina T, Long M, Resplandy L, Tjiputra J, Yukimoto S.  2016.  Evaluating CMIP5 ocean biogeochemistry and Southern Ocean carbon uptake using atmospheric potential oxygen: Present-day performance and future projection. Geophysical Research Letters. 43:2077-2085.   10.1002/2015gl067584   AbstractWebsite

Observed seasonal cycles in atmospheric potential oxygen (APO similar to O-2+1.1 CO2) were used to evaluate eight ocean biogeochemistry models from the Coupled Model Intercomparison Project (CMIP5). Model APO seasonal cycles were computed from the CMIP5 air-sea O-2 and CO2 fluxes and compared to observations at three Southern Hemisphere monitoring sites. Four of the models captured either the observed APO seasonal amplitude or phasing relatively well, while the other four did not. Many models had an unrealistic seasonal phasing or amplitude of the CO2 flux, which in turn influenced APO. By 2100 under RCP8.5, the models projected little change in the O-2 component of APO but large changes in the seasonality of the CO2 component associated with ocean acidification. The models with poorer performance on present-day APO tended to project larger net carbon uptake in the Southern Ocean, both today and in 2100.

Forkel, M, Carvalhais N, Rodenbeck C, Keeling R, Heimann M, Thonicke K, Zaehle S, Reichstein M.  2016.  Enhanced seasonal CO2 exchange caused by amplified plant productivity in northern ecosystems. Science. 351:696-699.   10.1126/science.aac4971   AbstractWebsite

Atmospheric monitoring of high northern latitudes (above 40 degrees N) has shown an enhanced seasonal cycle of carbon dioxide (CO2) since the 1960s, but the underlying mechanisms are not yet fully understood. The much stronger increase in high latitudes relative to low ones suggests that northern ecosystems are experiencing large changes in vegetation and carbon cycle dynamics. We found that the latitudinal gradient of the increasing CO2 amplitude is mainly driven by positive trends in photosynthetic carbon uptake caused by recent climate change and mediated by changing vegetation cover in northern ecosystems. Our results underscore the importance of climate-vegetation-carbon cycle feedbacks at high latitudes; moreover, they indicate that in recent decades, photosynthetic carbon uptake has reacted much more strongly to warming than have carbon release processes.

2009
Rafelski, LE, Piper SC, Keeling RF.  2009.  Climate effects on atmospheric carbon dioxide over the last century. Tellus Series B-Chemical and Physical Meteorology. 61:718-731.   10.1111/j.1600-0889.2009.00439.x   AbstractWebsite

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.

2008
Hamme, RC, Keeling RF.  2008.  Ocean ventilation as a driver of interannual variability in atmospheric potential oxygen. Tellus Series B-Chemical and Physical Meteorology. 60:706-717.   10.1111/j.1600-0889.2008.00376.x   AbstractWebsite

We present observations of interannual variability on 2-5 yr timescales in atmospheric potential oxygen (APO approximate to O(2) + CO(2)) from the Scripps Institution of Oceanography global flask sampling network. Interannual variations in the tracer APO are expected to arise from air-sea fluxes alone, because APO is insensitive to exchanges with the terrestrial biosphere. These interannual variations are shown to be regionally coherent and robust to analytical artefacts. We focus on explaining a feature dominant in records from the Northern Hemisphere stations, marked by increasing APO in the late 1990s, followed by an abrupt drawdown in 2000-2001. The timing of the drawdown matches a renewal of deep convection in the North Atlantic, followed the next year by a severe winter in the western North Pacific that may have allowed ventilation of denser isopycnals than usual. We find a weak correlation between changes in the interhemispheric APO difference and El Nino indices, and the observations show no strong features of the 1997-98 El Nino. Comparisons with estimates of variations in ocean productivity and ocean heat content demonstrate that these processes are secondary influences at these timescales. We conclude that the evidence points to variability in ocean ventilation as the main driver of interannual variability in APO.

2006
Manning, AC, Keeling RF.  2006.  Global oceanic and land biotic carbon sinks from the Scripps atmospheric oxygen flask sampling network. Tellus Series B-Chemical and Physical Meteorology. 58:95-116.   10.1111/j.1600-0889.2006.00175.x   AbstractWebsite

Measurements of atmospheric O-2/N-2 ratio and CO2 concentration are presented over the period 1989-2003 from the Scripps Institution of Oceanography global flask sampling network. A formal framework is described for making optimal use of these data to estimate global oceanic and land biotic carbon sinks. For the 10-yr period from 1990 to 2000, the oceanic and land biotic sinks are estimated to be 1.9 +/- 0.6 and 1.2 +/- 0.8 Pg C yr(-1), respectively, while for the 10-yr period from 1993 to 2003, the sinks are estimated to be 2.2 +/- 0.6 and 0.5 +/- 0.7 Pg C yr(-1), respectively. These estimates, which are also compared with earlier results, make allowance for oceanic O-2 and N-2 outgassing based on observed changes in ocean heat content and estimates of the relative outgassing per unit warming. For example, for the 1993-2003 period we estimate outgassing of 0.45 x 10(14) mol O-2 yr(-1) and 0.20 x 10(14) mol N-2 yr(-1), which results in a correction of 0.5 Pg C yr(-1) on the oceanic and land biotic carbon sinks. The basis for this oceanic outgassing correction is reviewed in the context of recent model estimates. The main contributions to the uncertainty in the global sinks averages are from the estimates for oceanic outgassing and the estimates for fossil fuel combustion. The oceanic sink of 2.2 Pg C yr(-1) for 1993-2003 is consistent, within the uncertainties, with the integrated accumulation of anthropogenic CO2 in the ocean since 1800 as recently estimated from oceanic observations, assuming the oceanic sink varied over time as predicted by a box-diffusion model.

2003
Le Quere, C, Aumont O, Bopp L, Bousquet P, Ciais P, Francey R, Heimann M, Keeling CD, Keeling RF, Kheshgi H, Peylin P, Piper SC, Prentice IC, Rayner PJ.  2003.  Two decades of ocean CO2 sink and variability. Tellus Series B-Chemical and Physical Meteorology. 55:649-656.   10.1034/j.1600-0889.2003.00043.x   AbstractWebsite

Atmospheric CO2 has increased at a nearly identical average rate of 3.3 and 3.2 Pg C yr(-1) for the decades of the 1980s and the 1990s, in spite of a large increase in fossil fuel emissions from 5.4 to 6.3 Pg C yr(-1). Thus, the sum of the ocean and land CO2 sinks was 1 Pg C yr(-1) larger in the 1990s than in to the 1980s. Here we quantify the ocean and land sinks for these two decades using recent atmospheric inversions and ocean models. The ocean and land sinks are estimated to be, respectively, 0.3 (0.1 to 0.6) and 0.7 (0.4 to 0.9) Pg C yr(-1) larger in the 1990s than in the 1980s. When variability less than 5 yr is removed, all estimates show a global oceanic sink more or less steadily increasing with time, and a large anomaly in the land sink during 1990-1994. For year-to-year variability, all estimates show 1/3 to 1/2 less variability in the ocean than on land, but the amplitude and phase of the oceanic variability remain poorly determined. A mean oceanic sink of 1.9 Pg C yr(-1) for the 1990s based on O-2 observations corrected for ocean outgassing is supported by these estimates, but an uncertainty on the mean value of the order of +/-0.7 Pg C yr(-1) remains. The difference between the two decades appears to be more robust than the absolute value of either of the two decades.

1999
Langenfelds, RL, Francey RJ, Steele LP, Battle M, Keeling RF, Budd WF.  1999.  Partitioning of the global fossil CO2 sink using a 19-year trend in atmospheric O2. Geophysical Research Letters. 26:1897-1900.   10.1029/1999gl900446   AbstractWebsite

O-2/N-2 is measured in the Cape Grim Air Archive (CGAA), a suite of tanks filled with background air at Cape Grim, Tasmania (40.7 degrees S, 144.8 degrees E) between April 1978 and January 1997. Derived trends are compared with published O-2/N-2 records and assessed against limits on interannual variability of net terrestrial exchanges imposed by trends of delta(13)C in CO2. Two old samples from 1978 and 1987 and eight from 1996/97 survive critical selection criteria and give a mean 19-year trend in delta(O-2/N-2) of -16.7 +/- 0.5 per meg y(-1), implying net storage of +2.3 +/- 0.7 GtC (10(15) g carbon) yr(-1) of fossil fuel CO2 in the oceans and +0.2 +/- 0.9 GtC yr(-1) in the terrestrial biosphere. The uptake terms are consistent for both O-2/N-2 and delta(13)C tracers if the mean C-13 isotopic disequilibrium flux, combining terrestrial and oceanic contributions, is 93 +/- 15 GtC parts per thousand yr(-1).

1996
Keeling, RF, Piper SC, Heimann M.  1996.  Global and hemispheric CO2 sinks deduced from changes in atmospheric O2 concentration. Nature. 381:218-221.   10.1038/381218a0   AbstractWebsite

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 biota(1,25). A similar budgeting problem has been found for the Northern Hemisphere alone(2,3), suggesting that northern land biota may be the sought-after sink, although this interpretation is not unique(2-5); to distinguish oceanic and land carbon uptake, the budgets rely variously, and controversially, on ocean models(2,6,7), (CO2)-C-13/(CO2)-C-12 data(2,4,5), sparse oceanic observations of p(CO2) (ref. 3) or C-13/C-12 ratios of dissolved inorganic carbon, (4,5,8) or single-latitude trends in atmospheric O-2 as detected from changes in O-2/N-2 ratio.(9,10). Here we present an extensive O-2/N-2 data set which shows simultaneous trends in O-2/N-2 in both northern and southern hemispheres and allows the O-2/N-2 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.