Mahowald Natalie M.

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Natalie M.

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  • Article
    Preindustrial-control and twentieth-century carbon cycle experiments with the Earth System Model CESM1(BGC)
    (American Meteorological Society, 2014-12-15) Lindsay, Keith ; Bonan, Gordon B. ; Doney, Scott C. ; Hoffman, Forrest M. ; Lawrence, David M. ; Long, Matthew C. ; Mahowald, Natalie M. ; Moore, J. Keith ; Randerson, James T. ; Thornton, Peter E.
    Version 1 of the Community Earth System Model, in the configuration where its full carbon cycle is enabled, is introduced and documented. In this configuration, the terrestrial biogeochemical model, which includes carbon–nitrogen dynamics and is present in earlier model versions, is coupled to an ocean biogeochemical model and atmospheric CO2 tracers. The authors provide a description of the model, detail how preindustrial-control and twentieth-century experiments were initialized and forced, and examine the behavior of the carbon cycle in those experiments. They examine how sea- and land-to-air CO2 fluxes contribute to the increase of atmospheric CO2 in the twentieth century, analyze how atmospheric CO2 and its surface fluxes vary on interannual time scales, including how they respond to ENSO, and describe the seasonal cycle of atmospheric CO2 and its surface fluxes. While the model broadly reproduces observed aspects of the carbon cycle, there are several notable biases, including having too large of an increase in atmospheric CO2 over the twentieth century and too small of a seasonal cycle of atmospheric CO2 in the Northern Hemisphere. The biases are related to a weak response of the carbon cycle to climatic variations on interannual and seasonal time scales and to twentieth-century anthropogenic forcings, including rising CO2, land-use change, and atmospheric deposition of nitrogen.
  • Article
    Carbon-nitrogen interactions regulate climate-carbon cycle feedbacks : results from an atmosphere-ocean general circulation model
    (Copernicus Publications on behalf of the European Geosciences Union, 2009-10-08) Thornton, Peter E. ; Doney, Scott C. ; Lindsay, Keith ; Moore, J. Keith ; Mahowald, Natalie M. ; Randerson, James T. ; Fung, Inez Y. ; Lamarque, J.-F. ; Feddema, J. J. ; Lee, Y.-H.
    Inclusion of fundamental ecological interactions between carbon and nitrogen cycles in the land component of an atmosphere-ocean general circulation model (AOGCM) leads to decreased carbon uptake associated with CO2 fertilization, and increased carbon uptake associated with warming of the climate system. The balance of these two opposing effects is to reduce the fraction of anthropogenic CO2 predicted to be sequestered in land ecosystems. The primary mechanism responsible for increased land carbon storage under radiatively forced climate change is shown to be fertilization of plant growth by increased mineralization of nitrogen directly associated with increased decomposition of soil organic matter under a warming climate, which in this particular model results in a negative gain for the climate-carbon feedback. Estimates for the land and ocean sink fractions of recent anthropogenic emissions are individually within the range of observational estimates, but the combined land plus ocean sink fractions produce an airborne fraction which is too high compared to observations. This bias is likely due in part to an underestimation of the ocean sink fraction. Our results show a significant growth in the airborne fraction of anthropogenic CO2 emissions over the coming century, attributable in part to a steady decline in the ocean sink fraction. Comparison to experimental studies on the fate of radio-labeled nitrogen tracers in temperate forests indicates that the model representation of competition between plants and microbes for new mineral nitrogen resources is reasonable. Our results suggest a weaker dependence of net land carbon flux on soil moisture changes in tropical regions, and a stronger positive growth response to warming in those regions, than predicted by a similar AOGCM implemented without land carbon-nitrogen interactions. We expect that the between-model uncertainty in predictions of future atmospheric CO2 concentration and associated anthropogenic climate change will be reduced as additional climate models introduce carbon-nitrogen cycle interactions in their land components.
  • Preprint
    Skill metrics for confronting global upper ocean ecosystem-biogeochemistry models against field and remote sensing data
    ( 2008-03-04) Doney, Scott C. ; Lima, Ivan D. ; Moore, J. Keith ; Lindsay, Keith ; Behrenfeld, Michael J. ; Westberry, Toby K. ; Mahowald, Natalie M. ; Glover, David M. ; Takahashi, Taro
    We present a generalized framework for assessing the skill of global upper ocean ecosystem-biogeochemical models against in-situ field data and satellite observations. We illustrate the approach utilizing a multi-decade (1979-2004) hindcast experiment conducted with the Community Climate System Model (CCSM-3) ocean carbon model. The CCSM-3 ocean carbon model incorporates a multi-nutrient, multi-phytoplankton functional group ecosystem module coupled with a carbon, oxygen, nitrogen, phosphorus, silicon, and iron biogeochemistry module embedded in a global, threedimensional ocean general circulation model. The model is forced with physical climate forcing from atmospheric reanalysis and satellite data products and time-varying atmospheric dust deposition. Data-based skill metrics are used to evaluate the simulated time-mean spatial patterns, seasonal cycle amplitude and phase, and subannual to interannual variability. Evaluation data include: sea surface temperature and mixed layer depth; satellite derived surface ocean chlorophyll, primary productivity, phytoplankton growth rate and carbon biomass; large-scale climatologies of surface nutrients, pCO2, and air-sea CO2 and O2 flux; and time-series data from the Joint Global Ocean Flux Study (JGOFS). Where the data is sufficient, we construct quantitative skill metrics using: model-data residuals, time-space correlation, root mean square error, and Taylor diagrams.
  • Article
    Satellite-detected fluorescence reveals global physiology of ocean phytoplankton
    (Copernicus Publications on behalf of the European Geosciences Union, 2009-05-08) Behrenfeld, Michael J. ; Westberry, Toby K. ; Boss, Emmanuel S. ; O'Malley, Robert T. ; Siegel, David A. ; Wiggert, Jerry D. ; Franz, Bryan A. ; McClain, Charles R. ; Feldman, G. C. ; Doney, Scott C. ; Moore, J. Keith ; Dall'Olmo, Giorgio ; Milligan, A. J. ; Lima, Ivan D. ; Mahowald, Natalie M.
    Phytoplankton photosynthesis links global ocean biology and climate-driven fluctuations in the physical environment. These interactions are largely expressed through changes in phytoplankton physiology, but physiological status has proven extremely challenging to characterize globally. Phytoplankton fluorescence does provide a rich source of physiological information long exploited in laboratory and field studies, and is now observed from space. Here we evaluate the physiological underpinnings of global variations in satellite-based phytoplankton chlorophyll fluorescence. The three dominant factors influencing fluorescence distributions are chlorophyll concentration, pigment packaging effects on light absorption, and light-dependent energy-quenching processes. After accounting for these three factors, resultant global distributions of quenching-corrected fluorescence quantum yields reveal a striking consistency with anticipated patterns of iron availability. High fluorescence quantum yields are typically found in low iron waters, while low quantum yields dominate regions where other environmental factors are most limiting to phytoplankton growth. Specific properties of photosynthetic membranes are discussed that provide a mechanistic view linking iron stress to satellite-detected fluorescence. Our results present satellite-based fluorescence as a valuable tool for evaluating nutrient stress predictions in ocean ecosystem models and give the first synoptic observational evidence that iron plays an important role in seasonal phytoplankton dynamics of the Indian Ocean. Satellite fluorescence may also provide a path for monitoring climate-phytoplankton physiology interactions and improving descriptions of phytoplankton light use efficiencies in ocean productivity models.
  • Article
    Impact of variable air-sea O2 and CO2 fluxes on atmospheric potential oxygen (APO) and land-ocean carbon sink partitioning
    (Copernicus Publications on behalf of the European Geosciences Union, 2008-06-02) Nevison, Cynthia D. ; Mahowald, Natalie M. ; Doney, Scott C. ; Lima, Ivan D. ; Cassar, Nicolas
    A three dimensional, time-evolving field of atmospheric potential oxygen (APO ~O2/N2+CO2) was estimated using surface O2, N2 and CO2 fluxes from the WHOI ocean ecosystem model to force the MATCH atmospheric transport model. Land and fossil carbon fluxes were also run in MATCH and translated into O2 tracers using assumed O2:CO2 stoichiometries. The modeled seasonal cycles in APO agree well with the observed cycles at 13 global monitoring stations, with agreement helped by including oceanic CO2 in the APO calculation. The modeled latitudinal gradient in APO is strongly influenced by seasonal rectifier effects in atmospheric transport. An analysis of the APO-vs.-CO2 mass-balance method for partitioning land and ocean carbon sinks was performed in the controlled context of the MATCH simulation, in which the true surface carbon and oxygen fluxes were known exactly. This analysis suggests uncertainty of up to ±0.2 PgC in the inferred sinks due to variability associated with sparse atmospheric sampling. It also shows that interannual variability in oceanic O2 fluxes can cause large errors in the sink partitioning when the method is applied over short timescales. However, when decadal or longer averages are used, the variability in the oceanic O2 flux is relatively small, allowing carbon sinks to be partitioned to within a standard deviation of 0.1 Pg C/yr of the true values, provided one has an accurate estimate of long-term mean O2 outgassing.
  • Article
    Exploring the sensitivity of interannual basin-scale air-sea CO2 fluxes to variability in atmospheric dust deposition using ocean carbon cycle models and atmospheric CO2 inversions
    (American Geophysical Union, 2007-05-04) Patra, Prabir K. ; Moore, J. Keith ; Mahowald, Natalie M. ; Uematsu, Mitsuo ; Doney, Scott C. ; Nakazawa, Takakiyo
    Estimates of sources/sinks of carbon dioxide (CO2) at the Earth's surface are commonly made using atmospheric CO2 inverse modeling, terrestrial and oceanic biogeochemical modeling, and inventory-based studies. In this study, we compare sea-air CO2 fluxes from the Time-Dependent Inverse (TDI) atmosphere model and the marine Biogeochemical Elemental Cycling (BEC) model to study the processes involved in ocean carbon cycling at subbasin scales. A dust generation and transport model, based on analyzed meteorology and terrestrial vegetation cover, is also used to estimate the interannual variability in dust and iron deposition to different ocean basins. Overall, a fairly good agreement is established between the TDI and BEC model results for the net annual patterns and seasonal cycle of sea-air CO2 exchange. Sensitivity studies with the ocean biogeochemical model using increased or reduced atmospheric iron inputs indicate the relative sensitivity of air-sea CO2 exchange. The simulated responses to changes in iron inputs are not instantaneous (peak response after ∼2−3 years). The TDI model derived seasonal cycles for the Southern Ocean (South Atlantic) are better matched by the BEC model by increasing (decreasing) iron inputs through atmospheric aerosols. Our results suggest that some of the interannual variability in TDI model air-sea CO2 fluxes during the past decade may be explainable by dust variability that relaxes/increases iron limitation in high-nitrate, low-chlorophyll (HNLC) ocean regions.
  • Preprint
    Dissolved iron in the vicinity of the Crozet Islands, Southern Ocean
    ( 2007-04-26) Planquette, Helene ; Statham, Peter J. ; Fones, Gary R. ; Charette, Matthew A. ; Moore, C. Mark ; Salter, Ian ; Nedelec, Florence H. ; Taylor, Sarah L. ; French, M. ; Baker, Alexander R. ; Mahowald, Natalie M. ; Jickells, T. D.
    The annual phytoplankton bloom occurring north of the Crozet Plateau provides a rare opportunity to examine the hypothesis that natural iron fertilisation can alleviate HNLC conditions normally associated with the Southern Ocean. Therefore, during CROZEX, a large multidisciplinary study performed between November 2004 and January 2005, measurements of total dissolved iron (DFe, ≤ 0.2 μm) were made on seawater from around the islands and atmospheric iron deposition estimated from rain and aerosol samples. DFe concentrations were determined by flow injection analysis with N,N-dimethyl- pphenylenediamine dihydrochloride (DPD) catalytic spectrophotometric detection. DFe concentrations varied between 0.086 nM and 2.48 nM, with low values in surface waters. Enrichment of dissolved iron (>1 nM) at close proximity to the islands suggests that the plateau and the associated sediments are a source of iron. Waters further north also appear to be affected by this input of coastal and shelf origin, although dissolved iron concentrations decrease as a function of distance to the north of the plateau with a gradient of ~0.07 at the time of sampling. Using lateral and vertical diffusion coefficients derived from Ra isotope profiles and also estimates of atmospheric inputs, it was then possible to estimate a DFe concentration of ~0.55 nM to the north of the islands prior to the bloom event, which is sufficient to initiate the bloom, the lateral island source being the largest component. A similar situation is observed for other Sub-Antarctic Islands such as Kerguelen, South Georgia, that supply dissolved iron to their surrounding waters, thus, enhancing chlorophyll concentrations.
  • Article
    Impacts of increasing anthropogenic soluble iron and nitrogen deposition on ocean biogeochemistry
    (American Geophysical Union, 2009-08-28) Krishnamurthy, Aparna ; Moore, J. Keith ; Mahowald, Natalie M. ; Luo, Chao ; Doney, Scott C. ; Lindsay, Keith ; Zender, Charles S.
    We present results from transient sensitivity studies with the Biogeochemical Elemental Cycling (BEC) ocean model to increasing anthropogenic atmospheric inorganic nitrogen (N) and soluble iron (Fe) deposition over the industrial era. Elevated N deposition results from fossil fuel combustion and agriculture, and elevated soluble Fe deposition results from increased atmospheric processing in the presence of anthropogenic pollutants and soluble Fe from combustion sources. Simulations with increasing Fe and increasing Fe and N inputs raised simulated marine nitrogen fixation, with the majority of the increase in the subtropical North and South Pacific, and raised primary production and export in the high-nutrient low-chlorophyll (HNLC) regions. Increasing N inputs alone elevated small phytoplankton and diatom production, resulting in increased phosphorus (P) and Fe limitation for diazotrophs, hence reducing nitrogen fixation (∼6%). Globally, the simulated primary production, sinking particulate organic carbon (POC) export. and atmospheric CO2 uptake were highest under combined increase in Fe and N inputs compared to preindustrial control. Our results suggest that increasing combustion iron sources and aerosol Fe solubility along with atmospheric anthropogenic nitrogen deposition are perturbing marine biogeochemical cycling and could partially explain the observed trend toward increased P limitation at station ALOHA in the subtropical North Pacific. Excess inorganic nitrogen ([NO3 −] + [NH4 +] − 16[PO4 3−]) distributions may offer useful insights for understanding changing ocean circulation and biogeochemistry.
  • Article
    Are the impacts of land use on warming underestimated in climate policy?
    (IOP Publishing, 2017-09-18) Mahowald, Natalie M. ; Ward, Daniel S. ; Doney, Scott C. ; Hess, Peter G. ; Randerson, James T.
    While carbon dioxide emissions from energy use must be the primary target of climate change mitigation efforts, land use and land cover change (LULCC) also represent an important source of climate forcing. In this study we compute time series of global surface temperature change separately for LULCC and non-LULCC sources (primarily fossil fuel burning), and show that because of the extra warming associated with the co-emission of methane and nitrous oxide with LULCC carbon dioxide emissions, and a co-emission of cooling aerosols with non-LULCC emissions of carbon dioxide, the linear relationship between cumulative carbon dioxide emissions and temperature has a two-fold higher slope for LULCC than for non-LULCC activities. Moreover, projections used in the Intergovernmental Panel on Climate Change (IPCC) for the rate of tropical land conversion in the future are relatively low compared to contemporary observations, suggesting that the future projections of land conversion used in the IPCC may underestimate potential impacts of LULCC. By including a ‘business as usual’ future LULCC scenario for tropical deforestation, we find that even if all non-LULCC emissions are switched off in 2015, it is likely that 1.5 ◦C of warming relative to the preindustrial era will occur by 2100. Thus, policies to reduce LULCC emissions must remain a high priority if we are to achieve the low to medium temperature change targets proposed as a part of the Paris Agreement. Future studies using integrated assessment models and other climate simulations should include more realistic deforestation rates and the integration of policy that would reduce LULCC emissions.
  • Article
    Multicentury changes in ocean and land contributions to the climate-carbon feedback
    (John Wiley & Sons, 2015-06-02) Randerson, James T. ; Lindsay, Keith ; Munoz, E. ; Fu, W. ; Moore, J. Keith ; Hoffman, Forrest M. ; Mahowald, Natalie M. ; Doney, Scott C.
    Improved constraints on carbon cycle responses to climate change are needed to inform mitigation policy, yet our understanding of how these responses may evolve after 2100 remains highly uncertain. Using the Community Earth System Model (v1.0), we quantified climate-carbon feedbacks from 1850 to 2300 for the Representative Concentration Pathway 8.5 and its extension. In three simulations, land and ocean biogeochemical processes experienced the same trajectory of increasing atmospheric CO2. Each simulation had a different degree of radiative coupling for CO2 and other greenhouse gases and aerosols, enabling diagnosis of feedbacks. In a fully coupled simulation, global mean surface air temperature increased by 9.3 K from 1850 to 2300, with 4.4 K of this warming occurring after 2100. Excluding CO2, warming from other greenhouse gases and aerosols was 1.6 K by 2300, near a 2 K target needed to avoid dangerous anthropogenic interference with the climate system. Ocean contributions to the climate-carbon feedback increased considerably over time and exceeded contributions from land after 2100. The sensitivity of ocean carbon to climate change was found to be proportional to changes in ocean heat content, as a consequence of this heat modifying transport pathways for anthropogenic CO2 inflow and solubility of dissolved inorganic carbon. By 2300, climate change reduced cumulative ocean uptake by 330 Pg C, from 1410 Pg C to 1080 Pg C. Land fluxes similarly diverged over time, with climate change reducing stocks by 232 Pg C. Regional influence of climate change on carbon stocks was largest in the North Atlantic Ocean and tropical forests of South America. Our analysis suggests that after 2100, oceans may become as important as terrestrial ecosystems in regulating the magnitude of the climate-carbon feedback.
  • Article
    North-South asymmetry in the modeled phytoplankton community response to climate change over the 21st century
    (John Wiley & Sons, 2013-12-20) Marinov, Irina ; Doney, Scott C. ; Lima, Ivan D. ; Lindsay, Keith ; Moore, J. Keith ; Mahowald, Natalie M.
    Here we analyze the impact of projected climate change on plankton ecology in all major ocean biomes over the 21st century, using a multidecade (1880–2090) experiment conducted with the Community Climate System Model (CCSM-3.1) coupled ocean-atmosphere-land-sea ice model. The climate response differs fundamentally in the Northern and Southern Hemispheres for diatom and small phytoplankton biomass and consequently for total biomass, primary, and export production. Increasing vertical stratification in the Northern Hemisphere oceans decreases the nutrient supply to the ocean surface. Resulting decreases in diatom and small phytoplankton biomass together with a relative shift from diatoms to small phytoplankton in the Northern Hemisphere result in decreases in the total primary and export production and export ratio, and a shift to a more oligotrophic, more efficiently recycled, lower biomass euphotic layer. By contrast, temperature and stratification increases are smaller in the Southern compared to the Northern Hemisphere. Additionally, a southward shift and increase in strength of the Southern Ocean westerlies act against increasing temperature and freshwater fluxes to destratify the water-column. The wind-driven, poleward shift in the Southern Ocean subpolar-subtropical boundary results in a poleward shift and increase in the frontal diatom bloom. This boundary shift, localized increases in iron supply, and the direct impact of warming temperatures on phytoplankton growth result in diatom increases in the Southern Hemisphere. An increase in diatoms and decrease in small phytoplankton partly compensate such that while total production and the efficiency of organic matter export to the deep ocean increase, total Southern Hemisphere biomass does not change substantially. The impact of ecological shifts on the global carbon cycle is complex and varies across ecological biomes, with Northern and Southern Hemisphere effects on the biological production and export partially compensating. The net result of climate change is a small Northern Hemisphere-driven decrease in total primary production and efficiency of organic matter export to the deep ocean.
  • Article
    Desert dust and anthropogenic aerosol interactions in the Community Climate System Model coupled-carbon-climate model
    (Copernicus Publications on behalf of the European Geosciences Union, 2011-02-15) Mahowald, Natalie M. ; Lindsay, Keith ; Rothenberg, D. ; Doney, Scott C. ; Moore, J. Keith ; Thornton, Peter E. ; Randerson, James T. ; Jones, C. D.
    Coupled-carbon-climate simulations are an essential tool for predicting the impact of human activity onto the climate and biogeochemistry. Here we incorporate prognostic desert dust and anthropogenic aerosols into the CCSM3.1 coupled carbon-climate model and explore the resulting interactions with climate and biogeochemical dynamics through a series of transient anthropogenic simulations (20th and 21st centuries) and sensitivity studies. The inclusion of prognostic aerosols into this model has a small net global cooling effect on climate but does not significantly impact the globally averaged carbon cycle; we argue that this is likely to be because the CCSM3.1 model has a small climate feedback onto the carbon cycle. We propose a mechanism for including desert dust and anthropogenic aerosols into a simple carbon-climate feedback analysis to explain the results of our and previous studies. Inclusion of aerosols has statistically significant impacts on regional climate and biogeochemistry, in particular through the effects on the ocean nitrogen cycle and primary productivity of altered iron inputs from desert dust deposition.
  • Article
    Contribution of ocean, fossil fuel, land biosphere, and biomass burning carbon fluxes to seasonal and interannual variability in atmospheric CO2
    (American Geophysical Union, 2008-02-12) Nevison, Cynthia D. ; Mahowald, Natalie M. ; Doney, Scott C. ; Lima, Ivan D. ; van der Werf, Guido R. ; Randerson, James T. ; Baker, David F. ; Kasibhatla, Prasad S. ; McKinley, Galen A.
    Seasonal and interannual variability in atmospheric carbon dioxide (CO2) concentrations was simulated using fluxes from fossil fuel, ocean and terrestrial biogeochemical models, and a tracer transport model with time-varying winds. The atmospheric CO2 variability resulting from these surface fluxes was compared to observations from 89 GLOBALVIEW monitoring stations. At northern hemisphere stations, the model simulations captured most of the observed seasonal cycle in atmospheric CO2, with the land tracer accounting for the majority of the signal. The ocean tracer was 3–6 months out of phase with the observed cycle at these stations and had a seasonal amplitude only ∼10% on average of observed. Model and observed interannual CO2 growth anomalies were only moderately well correlated in the northern hemisphere (R ∼ 0.4–0.8), and more poorly correlated in the southern hemisphere (R < 0.6). Land dominated the interannual variability (IAV) in the northern hemisphere, and biomass burning in particular accounted for much of the strong positive CO2 growth anomaly observed during the 1997–1998 El Niño event. The signals in atmospheric CO2 from the terrestrial biosphere extended throughout the southern hemisphere, but oceanic fluxes also exerted a strong influence there, accounting for roughly half of the IAV at many extratropical stations. However, the modeled ocean tracer was generally uncorrelated with observations in either hemisphere from 1979–2004, except during the weak El Niño/post-Pinatubo period of the early 1990s. During that time, model results suggested that the ocean may have accounted for 20–25% of the observed slowdown in the atmospheric CO2 growth rate.
  • Article
    Atmospheric carbon dioxide variability in the Community Earth System Model : evaluation and transient dynamics during the twentieth and twenty-first centuries
    (American Meteorological Society, 2013-07-01) Keppel-Aleks, Gretchen ; Randerson, James T. ; Lindsay, Keith ; Stephens, Britton B. ; Moore, J. Keith ; Doney, Scott C. ; Thornton, Peter E. ; Mahowald, Natalie M. ; Hoffman, Forrest M. ; Sweeney, Colm ; Tans, Pieter P. ; Wennberg, Paul O. ; Wofsy, Steven C.
    Changes in atmospheric CO2 variability during the twenty-first century may provide insight about ecosystem responses to climate change and have implications for the design of carbon monitoring programs. This paper describes changes in the three-dimensional structure of atmospheric CO2 for several representative concentration pathways (RCPs 4.5 and 8.5) using the Community Earth System Model–Biogeochemistry (CESM1-BGC). CO2 simulated for the historical period was first compared to surface, aircraft, and column observations. In a second step, the evolution of spatial and temporal gradients during the twenty-first century was examined. The mean annual cycle in atmospheric CO2 was underestimated for the historical period throughout the Northern Hemisphere, suggesting that the growing season net flux in the Community Land Model (the land component of CESM) was too weak. Consistent with weak summer drawdown in Northern Hemisphere high latitudes, simulated CO2 showed correspondingly weak north–south and vertical gradients during the summer. In the simulations of the twenty-first century, CESM predicted increases in the mean annual cycle of atmospheric CO2 and larger horizontal gradients. Not only did the mean north–south gradient increase due to fossil fuel emissions, but east–west contrasts in CO2 also strengthened because of changing patterns in fossil fuel emissions and terrestrial carbon exchange. In the RCP8.5 simulation, where CO2 increased to 1150 ppm by 2100, the CESM predicted increases in interannual variability in the Northern Hemisphere midlatitudes of up to 60% relative to present variability for time series filtered with a 2–10-yr bandpass. Such an increase in variability may impact detection of changing surface fluxes from atmospheric observations.
  • Article
    Observed 20th century desert dust variability : impact on climate and biogeochemistry
    (Copernicus Publications on behalf of the European Geosciences Union, 2010-11-19) Mahowald, Natalie M. ; Kloster, S. ; Engelstaedter, S. ; Moore, J. Keith ; Mukhopadhyay, S. ; McConnell, Joseph R. ; Albani, S. ; Doney, Scott C. ; Bhattacharya, A. ; Curran, M. A. J. ; Flanner, M. G. ; Hoffman, Forrest M. ; Lawrence, David M. ; Lindsay, Keith ; Mayewski, P. A. ; Neff, Jason C. ; Rothenberg, D. ; Thomas, E. ; Thornton, Peter E. ; Zender, Charles S.
    Desert dust perturbs climate by directly and indirectly interacting with incoming solar and outgoing long wave radiation, thereby changing precipitation and temperature, in addition to modifying ocean and land biogeochemistry. While we know that desert dust is sensitive to perturbations in climate and human land use, previous studies have been unable to determine whether humans were increasing or decreasing desert dust in the global average. Here we present observational estimates of desert dust based on paleodata proxies showing a doubling of desert dust during the 20th century over much, but not all the globe. Large uncertainties remain in estimates of desert dust variability over 20th century due to limited data. Using these observational estimates of desert dust change in combination with ocean, atmosphere and land models, we calculate the net radiative effect of these observed changes (top of atmosphere) over the 20th century to be −0.14 ± 0.11 W/m2 (1990–1999 vs. 1905–1914). The estimated radiative change due to dust is especially strong between the heavily loaded 1980–1989 and the less heavily loaded 1955–1964 time periods (−0.57 ± 0.46 W/m2), which model simulations suggest may have reduced the rate of temperature increase between these time periods by 0.11 °C. Model simulations also indicate strong regional shifts in precipitation and temperature from desert dust changes, causing 6 ppm (12 PgC) reduction in model carbon uptake by the terrestrial biosphere over the 20th century. Desert dust carries iron, an important micronutrient for ocean biogeochemistry that can modulate ocean carbon storage; here we show that dust deposition trends increase ocean productivity by an estimated 6% over the 20th century, drawing down an additional 4 ppm (8 PgC) of carbon dioxide into the oceans. Thus, perturbations to desert dust over the 20th century inferred from observations are potentially important for climate and biogeochemistry, and our understanding of these changes and their impacts should continue to be refined.
  • Article
    Interactions between land use change and carbon cycle feedbacks
    (John Wiley & Sons, 2017-01-23) Mahowald, Natalie M. ; Randerson, James T. ; Lindsay, Keith ; Munoz, Ernesto ; Doney, Scott C. ; Lawrence, Peter ; Schlunegger, Sarah ; Ward, Daniel S. ; Lawrence, David ; Hoffman, Forrest M.
    Using the Community Earth System Model, we explore the role of human land use and land cover change (LULCC) in modifying the terrestrial carbon budget in simulations forced by Representative Concentration Pathway 8.5, extended to year 2300. Overall, conversion of land (e.g., from forest to croplands via deforestation) results in a model-estimated, cumulative carbon loss of 490 Pg C between 1850 and 2300, larger than the 230 Pg C loss of carbon caused by climate change over this same interval. The LULCC carbon loss is a combination of a direct loss at the time of conversion and an indirect loss from the reduction of potential terrestrial carbon sinks. Approximately 40% of the carbon loss associated with LULCC in the simulations arises from direct human modification of the land surface; the remaining 60% is an indirect consequence of the loss of potential natural carbon sinks. Because of the multicentury carbon cycle legacy of current land use decisions, a globally averaged amplification factor of 2.6 must be applied to 2015 land use carbon losses to adjust for indirect effects. This estimate is 30% higher when considering the carbon cycle evolution after 2100. Most of the terrestrial uptake of anthropogenic carbon in the model occurs from the influence of rising atmospheric CO2 on photosynthesis in trees, and thus, model-projected carbon feedbacks are especially sensitive to deforestation.