Bibliography - S. E. Mikaloff-Fletcher

1. Beaulieu, C., Jorge Sarmiento, S. E. Mikaloff-Fletcher, Jie Chen, and David Medvigy, 2012: Identification and characterization of abrupt changes in the land uptake of carbon. Global Biogeochemical Cycles, American Geophysical Union, 26(GB1007), doi:10.1029/2010GB004024
   [ Abstract ]

   A recent study of the net land carbon sink estimated using the Mauna Loa, Hawaii atmospheric CO₂ record, fossil fuel estimates, and a suite of ocean models suggests that the mean of the net land carbon uptake remained approximately constant for three decades and increased after 1988/1989. Due to the large variability in the net land uptake, it is not possible to determine the exact timing and nature of the increase robustly by visual inspection. Here, we develop a general methodology to objectively determine the nature and timing of the shift in the net land uptake based on the Schwarz Information Criterion. We confirm that it is likely that an abrupt shift in the mean net land carbon uptake occurred in 1988. After taking into account the variability in the net land uptake due to the influence of volcanic aerosols and the El Niño Southern Oscillation, we find that it is most likely that there is a remaining step increase at the same time (p-values of 0.01 and 0.04 for Mauna Loa and South Pole, respectively) of about 1 Pg C/yr. Thus, we conclude that neither the effect of volcanic eruptions nor the El Niño Southern Oscillation are the causes of the sudden increase of the land carbon sink. By also applying our methodology to the atmospheric growth rate of CO₂, we demonstrate that it is likely that the atmospheric growth rate of CO₂ exhibits a step decrease between two fitted lines in 1988—1989, which is most likely due to the shift in the net land uptake of carbon.

2. Rodgers, K. B., S. E. Mikaloff-Fletcher, D. Bianchi, C. Beaulieu, E. D. Galbraith, A. Gnanadesikan, A. G. Hogg, D. Iudicone, B. R. Lintner, T. Naegler, P. J. Reimer, Jorge Sarmiento, and R. D. Slater, 2011: Interhemispheric gradient of atmospheric radiocarbon reveals natural variability of Southern Ocean winds. Climate of the Past, European Geosciences Union, 7, doi:10.5194/cp-7-1123-2011 1123-1138
   [ Abstract ]

   Tree ring Δ¹⁴C data (Reimer et al., 2004; McCormac et al., 2004) indicate that atmospheric Δ¹⁴C varied on multi-decadal to centennial timescales, in both hemispheres, over the period between AD950 and 1830. The Northern and Southern Hemispheric Δ¹⁴C records display similar variability, but from the data alone is it not clear whether these variations are driven by the production of ¹⁴C in the stratosphere (Stuiver and Quay, 1980) or by perturbations to exchanges between carbon reservoirs (Siegenthaler et al., 1980). As the sea-air flux of ¹⁴CO₂ has a clear maximum in the open ocean regions of the Southern Ocean, relatively modest perturbations to the winds over this region drive significant perturbations to the interhemispheric gradient. In this study, model simulations are used to show that Southern Ocean winds are likely a main driver of the observed variability in the interhemispheric gradient over AD950-1830, and further, that this variability may be larger than the Southern Ocean wind trends that have been reported for recent decades (notably 1980-2004). This interpretation also implies that there may have been a significant weakening of the winds over the Southern Ocean within a few decades of AD1375, associated with the transition between the Medieval Climate Anomaly and the Little Ice Age. The driving forces that could have produced such a shift in the winds at the Medieval Climate Anomaly to Little Ice Age transition remain unknown. Our process-focused suite of perturbation experiments with models raises the possibility that the current generation of coupled climate and earth system models may underestimate the natural background multi-decadal- to centennial-timescale variations in the winds over the Southern Ocean.

3. Sarmiento, Jorge, M. N. Gloor, N. Gruber, C. Beaulieu, A. R. Jacobson, S. E. Mikaloff-Fletcher, Stephen W. Pacala, and K. B. Rodgers, 2010: Trends and regional distributions of land and ocean carbon sinks. Biogeosciences, www.biogeosciences.net/7/2351/2010/, 7, doi:10.5194/bg-7-2351-2010 2351-2367
   [ Abstract ]

   We show here an updated estimate of the net land carbon sink (NLS) as a function of time from 1960 to 2007 calculated from the difference between fossil fuel emissions, the observed atmospheric growth rate, and the ocean uptake obtained by recent ocean model simulations forced with reanalysis wind stress and heat and water fluxes. Except for interannual variability, the net land carbon sink appears to have been relatively constant at a mean value of −0.27 PgC yr⁻¹ between 1960 and 1988, at which time it increased abruptly by −0.88 (−0.77 to −1.04) PgC yr⁻¹ to a new relatively constant mean of −1.15 PgC yr⁻¹ between 1989 and 2003/7 (the sign convention is negative out of the atmosphere). This result is detectable at the 99% level using a t-test. The land use source (LU) is relatively constant over this entire time interval. While the LU estimate is highly uncertain, this does imply that most of the change in the net land carbon sink must be due to an abrupt increase in the land sink, LS = NLS – LU, in response to some as yet unknown combination of biogeochemical and climate forcing. A regional synthesis and assessment of the land carbon sources and sinks over the post 1988/1989 period reveals broad agreement that the Northern Hemisphere land is a major sink of atmospheric CO₂, but there remain major discrepancies with regard to the sign and magnitude of the net flux to and from tropical land.

4. Gruber, N., M. N. Gloor, S. E. Mikaloff-Fletcher, S. C. Doney, S. Dutkiewicz, M. Follows, M. Gerber, A. R. Jacobson, F. Joos, K. Lindsay, , and , et al., 2009: Oceanic Sources, Sinks, and Transport of Atmospheric CO₂. Global Biogeochemical Cycles, 23(GB 1005), doi:10.1029/2008GB003349
   [ Abstract ]

   We synthesize estimates of the contemporary net air-sea CO₂ flux on the basis of an inversion of interior ocean carbon observations using a suite of 10 ocean general circulation models (Mikaloff Fletcher et al., 2006, 2007) and compare them to estimates based on a new climatology of the air-sea difference of the partial pressure of CO₂ (pCO₂) (Takahashi et al., 2008). These two independent flux estimates reveal a consistent description of the regional distribution of annual mean sources and sinks of atmospheric CO₂ for the decade of the 1990s and the early 2000s with differences at the regional level of generally less than 0.1 Pg C a^-1. This distribution is characterized by outgassing in the tropics, uptake in midlatitudes, and comparatively small fluxes in the high latitudes. Both estimates point toward a small (˜ -0.3 Pg C a^-1) contemporary CO₂ sink in the Southern Ocean (south of 44°S), a result of the near cancellation between a substantial outgassing of natural CO₂ and a strong uptake of anthropogenic CO₂. A notable exception in the generally good agreement between the two estimates exists within the Southern Ocean: the ocean inversion suggests a relatively uniform uptake, while the pCO₂-based estimate suggests strong uptake in the region between 58°S and 44°S, and a source in the region south of 58°S. Globally and for a nominal period between 1995 and 2000, the contemporary net air-sea flux of CO₂ is estimated to be -1.7 ± 0.4 Pg C a^-1 (inversion) and -1.4 ± 0.7 Pg C a^-1 (pCO₂-climatology), respectively, consisting of an outgassing flux of river-derived carbon of ˜+0.5 Pg C a^-1, and an uptake flux of anthropogenic carbon of -2.2 ± 0.3 Pg C a^-1 (inversion) and -1.9 ± 0.7 Pg C a^-1 ((pCO₂-climatology). The two flux estimates also imply a consistent description of the contemporary meridional transport of carbon with southward ocean transport throughout most of the Atlantic basin, and strong equatorward convergence in the Indo-Pacific basins. Both transport estimates suggest a small hemispheric asymmetry with a southward transport of between -0.2 and -0.3 Pg C a^-1 across the equator. While the convergence of these two independent estimates is encouraging and suggests that it is now possible to provide relatively tight constraints for the net air-sea CO₂ fluxes at the regional basis, both studies are limited by their lack of consideration of long-term changes in the ocean carbon cycle, such as the recent possible stalling in the expected growth of the Southern Ocean carbon sink.

5. Jacobson, A. R., S. E. Mikaloff-Fletcher, N. Gruber, Jorge Sarmiento, and M. N. Gloor, 2007: A joint atmosphere-ocean inversion for surface fluxes of carbon dioxide: I. Methods and global-scale fluxes. Global Biogeochemical Cycles, 21(GB1019), doi:10.1029/2005GB002556
   [ Abstract ]

   We have constructed an inverse estimate of surface fluxes of carbon dioxide using both atmospheric and oceanic observational constraints. This global estimate is spatially resolved into 11 land regions and 11 ocean regions, and is calculated as a temporal mean for the period 1992–1996. The method interprets in situ observations of carbon dioxide concentration in the ocean and atmosphere with transport estimates from global circulation models. Uncertainty in the modeled circulation is explicitly considered in this inversion by using a suite of 16 atmospheric and 10 oceanic transport simulations. The inversion analysis, coupled with estimates of river carbon delivery, indicates that the open ocean had a net carbon uptake from the atmosphere during the period 1992–96 of 1.7 PgC yr^-1, consisting of an uptake of 2.1 PgC yr^-1 of anthropogenic carbon and a natural outgassing of about 0.5 PgC yr^-1 of carbon fixed on land and transported through rivers to the open ocean. The formal uncertainty on this oceanic uptake, despite a comprehensive effort to quantify sources of error due to modeling biases, uncertain riverine carbon load, and biogeochemical assumptions, is driven down to 0.2 PgC yr^-1 by the large number and relatively even spatial distribution of oceanic observations used. Other sources of error, for which quantifiable estimates are not currently available, such as unresolved transport and large region inversion bias, may increase this uncertainty.

6. Jacobson, A. R., S. E. Mikaloff-Fletcher, N. Gruber, Jorge Sarmiento, and M. N. Gloor, 2007: A joint atmosphere-ocean inversion for surface fluxes of carbon dioxide: II. Regional results. Global Biogeochemical Cycles, 21(GB1020), doi:10.1029/2006GB002703
   [ Abstract ]

   We report here the results from a coupled ocean-atmosphere inversion, in which atmospheric CO₂ gradients and transport simulations are combined with observations of ocean interior carbon concentrations and ocean transport simulations to provide a jointly constrained estimate of air-sea and air-land carbon fluxes. While atmospheric data have little impact on regional air-sea flux estimates, the inclusion of ocean data drives a substantial change in terrestrial flux estimates. Our results indicate that the tropical and southern land regions together are a large source of carbon, with a 77% probability that their aggregate source size exceeds 1 PgC yr^-1. This value is of similar magnitude to estimates of fluxes in the tropics due to land-use change alone, making the existence of a large tropical CO₂ fertilization sink unlikely. This terrestrial result is strongly driven by oceanic inversion results that differ from flux estimates based on ΔpCO₂ climatologies, including a relatively small Southern Ocean sink (south of 44°S) and a relatively large sink in the southern temperate latitudes (44°S–18°S). These conclusions are based on a formal error analysis of the results, which includes uncertainties due to observational error transport and other modeling errors, and biogeochemical assumptions. A suite of sensitivity tests shows that these results are generally robust, but they remain subject to potential sources of unquantified error stemming from the use of large inversion regions and transport biases common to the suite of available transport models.

7. Mikaloff-Fletcher, S. E., N. Gruber, A. R. Jacobson, M. N. Gloor, S. C. Doney, S. Dutkiewicz, M. Gerber, M. Follows, F. Joos, K. Lindsay, D. Menemenlis, and A. Mouchet, et al., 2007: Inverse estimates of the oceanic sources and sinks of natural CO₂ and the implied oceanic carbon transport. Global Biogeochemical Cycles, 21(GB1010), doi:10.1029/2006GB002751
   [ Abstract ]

   We use an inverse method to estimate the global-scale pattern of the air-sea flux of natural CO₂, i.e., the component of the CO₂ flux due to the natural carbon cycle that already existed in preindustrial times, on the basis of ocean interior observations of dissolved inorganic carbon (DIC) and other tracers, from which we estimate ΔC_gasex, i.e., the component of the observed (DIC that is due to the gas exchange of natural CO₂. We employ a suite of 10 different Ocean General Circulation Models (OGCMs) to quantify the error arising from uncertainties in the modeled transport required to link the interior ocean observations to the surface fluxes. The results from the contributing OGCMs are weighted using a model skill score based on a comparison of each model’s simulated natural radiocarbon with observations. We find a pattern of air-sea flux of natural CO₂ characterized by outgassing in the Southern Ocean between 44°S and 59°S, vigorous uptake at midlatitudes of both hemispheres, and strong outgassing in the tropics. In the Northern Hemisphere and the tropics, the inverse estimates generally agree closely with the natural CO₂ flux results from forward simulations of coupled OGCM-biogeochemistry models undertaken as part of the second phase of the Ocean Carbon Model Intercomparison Project (OCMIP-2). The OCMIP-2 simulations find far less air-sea exchange than the inversion south of 20°S, but more recent forward OGCM studies are in better agreement with the inverse estimates in the Southern Hemisphere. The strong source and sink pattern south of 20°S was not apparent in an earlier inversion study, because the choice of region boundaries led to a partial cancellation of the sources and sinks. We show that the inversely estimated flux pattern is clearly traceable to gradients in the observed ΔC_gasex, and that it is relatively insensitive to the choice of OGCM or potential biases in ΔC_gasex. Our inverse estimates imply a southward interhemispheric transport of 0.31 ± 0.02 Pg C yr^-1, most of which occurs in the Atlantic. This is considerably smaller than the 1 Pg C yr^-1 of Northern Hemisphere uptake that has been inferred from atmospheric CO₂ observations during the 1980s and 1990s, which supports the hypothesis of a Northern Hemisphere terrestrial sink.

8. Battle, M., S. E. Mikaloff-Fletcher, Michael Bender, R. F. Keeling, A. C. Manning, N. Gruber, P. P. Tans, M. B. Hendricks, D. T. Ho, C. Simonds, R. Mika, and B. Paplawsky, 2006: Atmospheric potential oxygen: New observations and their implications for some atmospheric and oceanic models. Global Biogeochemical Cycles, 20(GB1010), doi:10.1029/2005GB002534
   [ Abstract ]

   Measurements of atmospheric O₂/N₂ ratios and CO₂ concentrations can be combined into a tracer known as atmospheric potential oxygen (APO ≈ O₂/N₂ + CO₂) that is conservative with respect to terrestrial biological activity. Consequently, APO reflects primarily ocean biogeochemistry and atmospheric circulation. Building on the work of Stephens et al. (1998), we present a set of APO observations for the years 1996–2003 with unprecedented spatial coverage. Combining data from the Princeton and Scripps air sampling programs, the data set includes new observations collected from ships in the low-latitude Pacific. The data show a smaller interhemispheric APO gradient than was observed in past studies, and different structure within the hemispheres. These differences appear to be due primarily to real changes in the APO field over time. The data also show a significant maximum in APO near the equator. Following the approach of Gruber et al. (2001), we compare these observations with predictions of APO generated from ocean O₂ and CO₂ flux fields and forward models of atmospheric transport. Our model predictions differ from those of earlier modeling studies, reflecting primarily the choice of atmospheric transport model (TM3 in this study). The model predictions show generally good agreement with the observations, matching the size of the interhemispheric gradient, the approximate amplitude and extent of the equatorial maximum, and the amplitude and phasing of the seasonal APO cycle at most stations. Room for improvement remains. The agreement in the interhemispheric gradient appears to be coincidental; over the last decade, the true APO gradient has evolved to a value that is consistent with our time-independent model. In addition, the equatorial maximum is somewhat more pronounced in the data than the model. This may be due to overly vigorous model transport, or insufficient spatial resolution in the air-sea fluxes used in our modeling effort. Finally, the seasonal cycles predicted by the model of atmospheric transport show evidence of an excessive seasonal rectifier in the Aleutian Islands and smaller problems elsewhere.

9. Mikaloff-Fletcher, S. E., N. Gruber, A. R. Jacobson, S. C. Doney, S. Dutkiewicz, M. Gerber, M. Follows, F. Joos, K. Lindsay, D. Menemenlis, A. Mouchet, S. A. Müller, and Jorge Sarmiento, 2006: Inverse estimates of anthropogenic CO₂ uptake, transport, and storage by the ocean. Global Biogeochemical Cycles, 20(GB2002), doi:10.1029/2005GB002530
   [ Abstract ]

   Regional air-sea fluxes of anthropogenic CO₂ are estimated using a Green’s function inversion method that combines data-based estimates of anthropogenic CO₂ in the ocean with information about ocean transport and mixing from a suite of Ocean General Circulation Models (OGCMs). In order to quantify the uncertainty associated with the estimated fluxes owing to modeled transport and errors in the data, we employ 10 OGCMs and three scenarios representing biases in the data-based anthropogenic CO₂ estimates. On the basis of the prescribed anthropogenic CO₂ storage, we find a global uptake of 2.2 ± 0.25 Pg C yr^-1, scaled to 1995. This error estimate represents the standard deviation of the models weighted by a CFC-based model skill score, which reduces the error range and emphasizes those models that have been shown to reproduce observed tracer concentrations most accurately. The greatest anthropogenic CO₂ uptake occurs in the Southern Ocean and in the tropics. The flux estimates imply vigorous northward transport in the Southern Hemisphere, northward cross-equatorial transport, and equatorward transport at high northern latitudes. Compared with forward simulations, we find substantially more uptake in the Southern Ocean, less uptake in the Pacific Ocean, and less global uptake. The large-scale spatial pattern of the estimated flux is generally insensitive to possible biases in the data and the models employed. However, the global uptake scales approximately linearly with changes in the global anthropogenic CO₂ inventory. Considerable uncertainties remain in some regions, particularly the Southern Ocean.

Direct link to page: http://cmi.princeton.edu/bibliography/results.php?author=3591