Carbon Mitigation Initiative
CMI

CMI Science

CMI Science

CMI Science focuses on how terrestrial vegetation and the oceans soak up carbon and thereby determine the fraction of the carbon dioxide emitted into the atmosphere that actually stays there (the fraction is about one-half). CMI science increasingly features close collaboration with Princeton’s neighbor, the Geophysical Fluid Dynamics Laboratory of the U.S. Department of Commerce. A recent and growing component of CMI addresses climate variability and departures from the historical frequency of extreme events, such as heat waves, droughts and hurricanes.

Research Highlights – At a Glance

Stephen Pacala: A newly completed model for the terrestrial biosphere more accurately simulates the role of forests as a carbon sink and the accelerated growth of trees, despite the limitations from below-ground resources.

David Medvigy and Lars Hedin: Simulations reveal how nitrogen fixation determines the rate of tropical forest regrowth and amount of carbon uptake in a recovering tropical forest.

Michael Bender: Recent advances in instrumentation enable independent measurement of leaf photosynthesis and respiration rates, giving insight into leaf metabolism to model carbon uptake from the biosphere.

Jorge Sarmiento: Innovative float technology and high-resolution modeling are dramatically improving our view of the harsh and remote Southern Ocean.

François Morel: Field studies in Antarctica and lab experiments with cold-adapted microalgae yield new insight into the efficient sequestration of CO2 by high latitude oceanic ecosystems and their response to a global climate.

Michael Bender: Million-year old ice has been retrieved from Antarctic glaciers. Analyses of this ice and its trapped air shows that climate was warmer, and CO2 concentrations higher, than in more recent times.


A Durable Global Land Sink Thanks to Durable CO2 Fertilization
Principal Investigator: Stephen Pacala

At a Glance

A newly completed model for the terrestrial biosphere more accurately simulates the role of forests as a carbon sink and the accelerated growth of trees, despite the limitations from below-ground resources.

Research Highlight

In recent years, a fundamental shortcoming has been recognized by theorists of biosphere climate models. Most of the carbon in the biosphere and a large fraction of the carbon in soils is living or dead wood from forest trees, and for this reason, the future of the land carbon sink depends on the long-term enhancement of forest growth by carbon dioxide (CO2) fertilization. To model this interaction more accurately, the Pacala group completed in 2014 a fundamentally new version of the Princeton-Geophysical Fluid Dynamics Laboratory (GFDL) land model for the biosphere and its interactions with the atmosphere, its greenhouse gases and climate processes. The model has been seven years in the making, supported by CMI funding. It is expected to be the default land model at GFDL for at least a decade.

The simulation work of many labs, including the Pacala group, shows how a failure of the land carbon sink would make global climate change much worse. In a 2014 publication1, the Pacala group’s finding indicates that without a historical land sink, atmospheric CO2 concentration would currently be 80 ppm higher—an 80% increase in anthropogenic carbon accumulated in the atmosphere since the industrial revolution. The new model is based on a mathematical advance that more accurately models the competition for light among individual trees, without directly simulating each tree. Previous papers show this formulation capable of predicting the observations of forest dynamics, including changes in carbon to nitrogen ratio (C/N) that occur either along natural gradients in plant productivity or from CO2 fertilization.

Many studies predict the land sink will eventually diminish and turn to a carbon source because trees will become limited by below-ground resources (i.e. soil water, nitrogen and phosphorus). For this reason, a half dozen expensive experiments were carried out over the last two decades, in which intact forests were exposed to a doubled CO2 concentration for ten years. Contrary to expectation, the trees continued to show accelerated growth despite clear evidence of limitation from belowground resources. Two mechanisms were found to be responsible:

  • trees increased their ratio of carbon to nitrogen by increasing the amount of wood relative to fine roots and leaves (C/N ratio ~300 for wood, ~30 for leaves, ~50 for fine roots) and by increasing the C/N of leaf and root tissue; and
  • plants increased the availability of nitrogen by “priming” soil microbes with easy-to-digest carbon sources in the form of root exudates.
The root exudates caused the microbes to increase in abundance, so that there were more of them to digest undecomposed organic matter and thus liberate the nitrogen in the organic matter.

The new model’s description of decomposition can also simulate the priming of microbes by plant root exudates. The new model’s predictions were tested against a wide range of data, and results agree for both diurnal and annual carbon fluxes, growth rates of individual trees in the canopy and understory, tree size distributions, and species-level population dynamics during succession. The model was also used to show how the optimal strategy for rate of wood production—namely, a strategy that can outcompete a species with any other strategy, all else being equal—shifts as a function of the atmospheric CO2 concentration. The simulations predict that carbon sinks caused by CO2 fertilization will continue despite water limitation. Figure 1.1 shows wood production (carbon for left panels, tree diameter growth for right panels) in the Northern Midwest of the US at preindustrial (top) and double preindustrial (bottom) CO2, and for several kinds of trees (with different root-leaf ratios on the horizontal axis and growing either alone (dark symbols) or in a multispecies stand (open symbols))1. The results show wood production is predicted to remain 2-3 times higher under doubled CO2.


Figure 1.1
Figure 1.1. Net Primary Productivity (NPP, left axis) and Diameter at Breast Height (DBH, right axis) versus Root/Leaf area ratio1.

Reference

  1. Weng, E., S. Malyshev, J. W. Lichstein, C.E. Farrior, R. Dybzinski, T. Zhang, E. Shevliakova, and S.W. Pacala, 2014. Scaling from individuals to ecosystems in an Earth System Model using a mathematically tractable model of height-structured competition for light. Biogeosci. Discuss., 11: 17757-17860, in review. doi:10.5194/bgd-11-17757-2014.

Individual-Based Dinitrogen Fixation and Biodiversity Interact to Determine Tropical Forest Carbon Uptake
Principal Investigators: David Medvigy and Lars Hedin

At a Glance

Simulations reveal how nitrogen fixation determines the rate of tropical forest regrowth and amount of carbon uptake in a recovering tropical forest.

Research Highlight

The premise of this research project is that nutrient dynamics strongly regulate the ability of tropical forests to sequester atmospheric carbon dioxide1-3. Recent findings imply the existence of an ecosystem-level carbon-nitrogen feedback mechanism (symbiotic with certain types of bacteria) in which dinitrogen fixing trees can provide the nitrogen needed to maintain high forest growth rates, following any type of forest disturbance that reduces tree population (see Figure 1.2.1)4. However, field-based evaluation of this feedback has been difficult because

  • the carbon pools of a forest equilibrate slowly, over decades to centuries of ecological succession;
  • experimental inhibition of nitrogen fixation is not possible in real-world forests; and
  • nitrogen fixers may influence forest succession, but this interaction is difficult to isolate.

To address these problems, the Medvigy and Hedin groups examined carbon-nitrogen feedbacks by applying a simulation model to 64 large-scale plots distributed across 300 years of forest succession at the Agua Salud Project, Panama (see Figure 1.2.2). Field observations of plant traits were used to develop a representation of plant diversity in the model. This model representation consisted of different plant functional types (PFTs), where each PFT was endowed with a characteristic combination of traits seen in the observations. The model represented the large-scale plots by assigning the observed trees to PFTs according to their traits. This trait-based approach contrasts with the conventional approach of treating all tropical forest trees identically. The model can thus resolve PFT-nutrient feedbacks by evaluating nutrient dynamics within ensembles of spatiallylinked individual trees of differing PFTs. The results showed that nitrogen fixation accelerates forest carbon accumulation, doubling the accumulation rate in early succession (0-30 years following disturbance) and increasing carbon storage in old-growth forests by 10%. An indirect effect on carbon accumulation was also found, showing how fixation interacted with the abundance of different PFTs. These results helped the Medvigy and Hedin groups to infer that nitrogen fixation can support the sequestration of a substantial quantity of carbon in the land biosphere (~24 petagrams of carbon) if extended to tropical forests worldwide.

Regrowing tropical forests currently contribute to over 40% of terrestrial carbon uptake5. These forests will remain a critically important element of the terrestrial carbon cycle as tropical deforestation continues in the coming decades6. This initiative has thus identified nitrogen fixation as essential for rapid tropical forest regrowth. This result runs counter to the conventional interpretation that tropical forests are nitrogen-rich7.

This work is expected to be of broad interest to climate change scientists, ecologists, earth-system modelers, policy-makers, and practitioners conserving and restoring tropical forests. Modelers may improve the representation of biological nitrogen fixation in the next generation of earth-system models. Practitioners may ensure biodiversity and functional diversity are present in reforested landscapes. Policy-makers may use this information to make decisions about how to use tropical lands (i.e., agriculture versus reforestation).

This project is now expanding to include investigation of nitrogen-phosphorus feedbacks on ecosystem carbon accumulation. Phosphorus (P) availability is particularly low in lowland tropical forests because there is little parent material P available to provide fresh input of P through weathering. This lack of P results in P-limitation in tropical forests. Despite the importance of P, none of the climatecarbon cycle models participating in the 5th Assessment Report of the Intergovernmental Panel on Climate Change included P dynamics. This project will address this deficiency with the following objectives: (1) develop a model for P cycling in terrestrial ecosystems, including the interactions of P with nitrogen (N); (2) constrain critical model uncertainties using new and existing field and LIDAR remote-sensing measurements; and (3) incorporating P and P-N dynamics into terrestrial biosphere models and carrying out simulations to assess the impacts of coupled P-N dynamics on the simulated terrestrial carbon sink.

Figure 1.2.1
Figure 1.2.1. N2-fixing nodules on the roots of the common neotropical tree Inga help to provide more than 50% of the nitrogen needed to support 50 tons per hectare of carbon recovery in forests by 12 years following disturbance. (Photo courtesy of Sarah Batterman.)
Figure 1.2.2
Figure 1.2.2. Landscape and forests surrounding the Agua Salud Project research site in Panama. (Photo courtesy of Sarah Batterman.)

References

  1. Davidson, E.A., C.J.R. de Carvalho, A.M. Figueira, F.Y. Ishida, J.P.H.B. Ometto, G.B. Nardoto, R.T. Saba, S.N. Hayashi, E.C. Leal, I.C.G. Vieira, and L.A. Martinelli, 2007. Recuperation of nitrogen cycling in Amazonian forests following agricultural abandonment. Nature, 447: 995-998. doi:10.1038/ nature05900.
  2. Davidson, E.A., C.J.R. de Carvalho, I.C.G. Vieira, R.O. Figueiredo, P. Moutinho, F.Y. Ishida, M.T.P. dos Santos, J.B. Guerrero, K. Kalif, and R.T. Saba, 2004. Nitrogen and phosphorus limitation of biomass growth in a tropical secondary forest. Ecological Applications, 14: S150-S163. doi:10.1890/01- 6006.
  3. Wright, S.J., J.B. Yavitt, N. Wurzburger, B.L. Turner, E.V.J. Tanner, E.J. Sayer, L.S. Santiago, M. Kaspari, L.O. Hedin, K.E. Harms, M.N. Garcia, and M.D. Corre, 2011. Potassium, phosphorus, or nitrogen limit root allocation, tree growth, or litter production in a lowland tropical forest. Ecology, 92(8): 1616-1625. doi:10.1890/10-1558.1.
  4. Batterman, S.A., L.O. Hedin, M. van Breugel, J. Ransijn, D.J. Craven, and J.S. Hall, 2013. Key role of symbiotic dinitrogen fixation in tropical forest secondary succession. Nature, 502: 224-227. doi:10.1038/nature12525.
  5. Pan, Y., R.A. Birdsey, J. Fang, et al., 2011. A large and persistent sink in the world’s forests. Science, 333: 988-993. doi:10.1126/science.1201609.
  6. Soares-Filho, B.S., D.C. Nepstad, L.M. Curran, G.C. Cerqueira, R.A. Garcia, C.A. Ramos, E.Voll, A. McDonald, P. Lefebvre, and P. Schlesinger, 2006. Modeling conservation in the Amazon basin. Nature, 440: 520-523. doi:10.1038/nature04389.
  7. Hedin, L.O., E.N.J. Brookshire, D.N.L. Menge, and A.R. Barron, 2009. The Nitrogen Paradox in Tropical Forest Ecosystems. Annual Review of Ecology, Evolution, and Systematics, 40: 613-635. doi:10.1146/annurev.ecolsys.37.091305.110246.


Studies of Photosynthesis and Respiration in Leaves
Principal Investigator: Michael Bender

At a Glance

Recent advances in instrumentation enable independent measurement of leaf photosynthesis and respiration rates, giving insight into leaf metabolism to model carbon uptake from the biosphere.

Research Highlight

Until recently, the gross rates of photosynthesis and respiration by leaves (how much carbon is adsorbed versus how much released) was immeasurable in sunlight; instead, only the net rate of carbon assimilation was measurable. Using an instrument and methods developed over several years (see Figure 1.3), it is now possible to study differences of leaf respiration rates in the light and in the dark, and infer the causes for differences in these rates. The instrument is centered around a mass spectrometer that makes ultra-high precision measurements of the oxygen O2 concentration of air (rises due to photosynthesis, falls due to respiration). It also measures the abundance of an isotope tracer of O2 that is unaffected by respiration and reflects the gross rate of photosynthesis. The measured rates of photosynthesis and respiration record basic plant processes, allowing for the calculation of other properties that describe the function of leaves. David Medvigy, a plant modeler working with Michael Bender and postdoctoral associate Paul Gauthier, has begun installing new processes in his models with guidance from these experiments. This work to date confirms the basic paradigm of photosynthesis and respiration in leaves that is used in all models of the land biosphere. It also suggests an important modification associated with the fact that the process of respiration slows down in the light, and reveals the mechanistic basis for this slowdown.

The ultimate goal of this work is to measure rates of photosynthesis and respiration in leaves, interpret the biochemical controls on these processes, use the data to characterize net carbon dioxide (CO2) uptake by leaves, and implement the new insights into models of the land biosphere. These models are basic tools that are used to estimate how the land biosphere will change with time in global warming scenarios. The results of these studies are being implemented into models used to formulate policies for the management of forests and grasslands. They may have implications for agricultural yields as well.

Experiments performed with these innovative methods will help advance understanding of a range of basic processes, including the movement of CO2 molecules in leaves, activities of photosynthetic enzymes, and the efficiency of different modes of carbon fixation, in addition to photosynthesis and respiration.

Figure 1.3
Figure 1.3. A bean leaf being illuminated in a cuvette. Photosynthesis and respiration rates are measured from the change in the concentration of O2 and isotope tracers as air flows through the cuvette. (Photo courtesy of the Bender group.)

The Southern Ocean Carbon and Climate Observations and Modeling (SOCCOM) Project
Principal Investigator: Jorge Sarmiento

At a Glance

Innovative float technology and high-resolution modeling are dramatically improving our view of the harsh and remote Southern Ocean.

Research Highlight

The new Southern Ocean Carbon and Climate Observations and Modeling (SOCCOM) project combines cutting edge robotic float technology with high-resolution earth system modeling to expand our understanding of the Southern Ocean. Headed by Jorge Sarmiento of CMI’s Carbon Science Group and funded by the National Science Foundation, SOCCOM involves 25 researchers at 13 institutions around the U.S. and has an operating budget of $21 million over six years. The impetus for the project comes primarily from modeling research, including key studies carried out by the Sarmiento Group with CMI support, that suggests the Southern Ocean surrounding Antarctica plays a very important role in the planet’s carbon and climate cycles. Such theoretical studies indicate that

  • the Southern Ocean accounts for half of the planetary ocean uptake of anthropogenic carbon from the atmosphere1 and the majority of its uptake of heat (Sarmiento group analysis of the Fifth Coupled Model Intercomparison Project, CMIP 5; Refs. 2,3);
  • Southern Ocean upwelling delivers nutrients to lower latitude surface waters that are critical for ocean ecosystems around the world4,5; and
  • the impacts of ocean acidification from rising carbon dioxide (CO2) are projected to be most severe in the Southern Ocean, approaching ecosystem tipping points within a few decades6,7.
Until now, the biogeochemical (BGC) observations needed to test these model-based hypotheses have been sparse due to the harsh environment limiting access to the region by research vessels, particularly during the Southern hemisphere winter. To escape the limitations of ship-based measurements, the SOCCOM project is taking advantage of Argo autonomous float technology, which has already been widely deployed throughout the world’s oceans. SOCCOM scientists have augmented conventional robotic Argo floats (which measure ocean temperature and salinity) with newly developed biogeochemical sensors to measure carbon (indirectly determined by measuring pH), nitrate nutrients, and oxygen (see Figure 1.4.1). SOCCOM is the world’s first large-scale BGC Argo deployment and will increase the number of biogeochemical measurements made monthly in the Southern Ocean by a factor of 10-30 (with the higher increase in the Southern Hemisphere winter, when observations are scarcest).

A total of 200 floats are planned for deployment over the six-year term of the project. Since the spring of 2014, the SOCCOM team has deployed 24 BGC floats via two Southern Ocean cruises that sailed from Hobart, Tasmania and Cape Town, South Africa. Figure 1.4.2 shows a snapshot of pH provided by measurements taken along the trajectory of the Hobart-based GO-SHIP repeat hydrography cruise P16S (angled axis). The blue low pH water in the lower waters of the southernmost float is due to upwelling of deep water rich in dissolved inorganic carbon from decomposition of organic matter. The red high pH water in the surface ocean is due to low dissolved inorganic carbon from biological uptake. The deepening of this high pH water in Float 9095 is due to a combination of seasonal biological uptake, and the horizontal movement of this float across the so-called Polar Front which marks a boundary between waters with different properties. These floats are now complementing cruise data with unprecedented continuous monitoring of pH over time; the floats are providing the first annual record of the combined chemical and biological changes over broad regions of the Southern Ocean, and analysis of the newly collected data is underway.

Data from the floats are being made available to the public in real time at the SOCCOM website (http://soccom.princeton.edu) and will also be incorporated into the global Argo data system to provide easy access to researchers around the world. The project will also transfer sensor technology to commercial float developers and will work to ensure that the findings of the SOCCOM project reach the widest possible audience, including policy makers and the general public.

Several more BGC floats will be deployed this spring on cruises from Argentina and Tasmania, and the SOCCOM team is currently working with collaborators to organize the deployment of 37 floats to be built in 2015. The remainder of the floats will be launched between now and 2020. Combined with high-resolution modeling carried out under the project, SOCCOM’s leading-edge observations will help researchers better understand the inner workings of the Southern Ocean and its current impacts on Earth’s climate and biosphere. Predictive model simulations carried out under SOCCOM will also help researchers anticipate how changes in the Southern Ocean will impact global climate in the future.

Figure 1.4.1
Figure 1.4.1. Loading (inset) and deployment of a BGC Argo robotic float. (Inset photo courtesy of Hannah Zanowski, Princeton University. Outset photo courtesy of Annie Wong, University of Washington.)
Figure 1.4.2
Figure 1.4.2. pH measurements collected by BGC floats at three southern latitudes, launched from a GO-SHIP cruise along the south to north P16S cruise track. The map shows the south to north P16S cruise track as well as the locations of measurements made by BGC floats 9092, 9095, 9254, which are denoted by the black, green, and yellow dots, respectively. The vertical section cutting diagonally across the figure is pH measured at depths ranging from 0-1000 m, collected along the ship track at the time the floats were deployed. The vertical sections projecting out to the right show the time history of pH from BGC floats 9092, 9095, 9254 between April 2014 and January 2015. Figure courtesy of Ken Johnson, Monterey Bay Aquarium Research Institute (MBARI).

References

  1. Gruber, N., M. Gloor, S. E. Mikaloff Fletcher, S. Dutkiewicz, M. Follows, S. C. Doney, M. Gerber, A. R. Jacobson, K. Lindsay, D. Menemenlis, A. Mouchet, S. A. Mueller, J. L. Sarmiento, and T. Takahashi, 2009. Oceanic sources and sinks for atmospheric CO2. Glob. Biogeochem. Cycles, 23: GB1005. doi:10.1029/2008GB003349.
  2. Frölicher, T. L., M. Winton, and J. L. Sarmiento, 2014. Continued global warming after CO2 emissions stoppage. Nat. Clim. Chang., 4(1): 40-44. doi:10.1038/nclimate2060.
  3. Levitus, S., J. I. Antonov, T. P. Boyer, O. K. Baranova, H. E. Garcia, R. A. Locarnini, A. V. Mishonov, J. R. Reagan, D. Seidov, E. S. Yarosh, and M. M. Zweng, 2012. World ocean heat content and thermosteric sea level change (0–2000 m), 1955–2010. Geophysical Research Letters, 39: L10603. doi:10.1029/2012GL051106.
  4. Sarmiento, J.L, Gruber, N., Brzezinski, M.A. and J.P. Dunne, 2004. High-latitude controls of thermocline nutrients and low latitude biological productivity, Nature 427, 56-60 (1 January 2004), http://dx.doi.org/10.1038/nature02127.
  5. Marinov, I., A. Gnanadesikan, J. Toggweiler, and J. L. Sarmiento, 2006, The Southern Ocean biogeochemical divide, Nature, 441, 964–967, doi:10.1038/nature04883.
  6. McNeil, B.I., and R. J. Matear, 2008. Southern Ocean acidification: A tipping point at 450-ppm atmospheric CO2. Proc. Natl. Acad. Sci., 105(48): 18860–18864. doi:10.1073/pnas.0806318105.
  7. Feely, R.A., J. Orr, V.J. Fabry, J.A. Kleypas, C.L. Sabine, and C. Landgon, 2009. Present and future changes in sea-water chemistry due to ocean acidification. In AGU Monograph on Carbon Sequestration and Its Role in the Global Carbon Cycle. Eds. B.J. McPherson, E.T. Sundquist. Am. Geophys. Union, 183, 175-188.

High Productivity of Antarctic Ecosystems and their Response to Global Change
Principal Investigator: François M.M. Morel

At a Glance

Field studies in Antarctica and lab experiments with cold-adapted microalgae yield new insight into the efficient sequestration of CO2 by high latitude oceanic ecosystems and their response to a global climate.

Research Highlight

High latitude oceans are major contributors to global primary phytoplankton production with the Southern Ocean alone accounting for around 20% of annual global biomass. A large proportion of this production is confined to continental shelf regions such as the Western Antarctic Peninsula, where intense phytoplankton blooms, common in the spring, provide the basis for a short food web, transferring algal biomass to krill, seals and whales with high efficiency and resulting in a high degree of carbon dioxide (CO2) sequestration. The physiological and biochemical adaptations that allow Antarctic phytoplankton to grow rapidly and sustain high productivity in very cold water are not well understood.

Field data from a six-month deployment at Palmer station (see Figure 1.5) in the Western Antarctic Peninsula, complemented with laboratory experiments on model phytoplankton species, has provided the Morel group with new insights into the adaptations responsible for high biological productivity in cold waters. These results also provide a basis for predicting the most likely responses of Antarctic flora to changing environmental conditions.

In high latitude ecosystems, enzymatic cellular processes for organisms fundamentally slow down due to low temperature. However, cold-adapted microalgae resolve this problem by

  • having many of their key enzymes exhibit small structural variations that allow relatively rapid turnover rates at low temperatures (biochemical adaptation); and
  • synthesizing far larger cellular concentrations of the essential enzymes for which no coldadapted form exist (physiological adaptation).

The net result is a biochemical composition of high latitude phytoplankton different from that of temperate species, that includes an unusually high protein content. In addition, high latitude marine ecosystems exhibit relatively low rates of respiration. This is due to the synthesis of high energy compounds (ATP) via photochemical rather than respiratory processes and the long duration of the light period in the austral summer. High latitude oceans and the Western Antarctic Peninsula are now experiencing some of the most extreme warming on the planet; the low buffering capacity of high latitude seawater will result in a particularly intense degree of acidification in response to rising anthropogenic CO2. The Morel Science group’s studies of the mechanisms controlling the rate of photosynthetic biomass production in high latitude marine ecosystems will help ascertain how Antarctic (and Artic) marine ecosystems respond to ongoing warming and acidification. These experimental results provide a basis for collaboration with colleagues at the Geophysical Fluid Dynamics Laboratory to develop Earth System Models with mechanistic representation of biological processes suited to high latitude oceans. The ultimate goal is to more robustly assess the consequences of climate warming and acidification to carbon uptake and biogeochemistry in the Arctic and Antarctic oceans.

Figure 1.5
Figure 1.5. Postdoctoral Associates from the Morel Group arriving at Palmer station by zodiac after gathering samples and field measurements. (Photo courtesy of Jodi Young.)

Studies of Greenhouse Gases and Antarctic Climate 1,000,000 Years Ago
Principal Investigator: Michael Bender

At a Glance

Million-year old ice has been retrieved from Antarctic glaciers. Analyses of this ice and its trapped air shows that climate was warmer, and CO2 concentrations higher, than in more recent times.

Research Highlight

Ice below the surface of glaciers contains air that was trapped in earlier times, during the compaction of ancient snow. Scientists have extracted this ancient air and analyzed it to determine how the atmospheric carbon dioxide (CO2) concentration changed continuously with time over the past 800,000 years. The Bender group’s research program documents climate history from the record of ancient ice sampled at the Allan Hills in Antarctica. Sample cores were easily acquired of “chunks” of ancient ice, driven to the surface as the glacier flowed over subglacial mountains (see Figure 1.6). The ice dates back to 1,000,000 years, representing the planet in a different climate configuration from that of the last 800,000 years. This work reveals that atmospheric CO2 concentrations were tightly linked to global temperature and the size of the great glaciers during this earlier regime. The data also show that average CO2 concentrations dropped by 10’s of parts per million (pre-industrial CO2=280 ppm) 800,000 years ago, when glacial cycles became much more intense.

This research program was undertaken in collaboration with John Higgins at Princeton University and researchers at the University of Maine and Oregon State University. Studies of greenhouse gases in these samples show the close link between CO2 and climate extends back beyond 800,000 years, to the earlier world of shorter glacial-interglacial cycles with smaller amplitudes. In general, this work extends the physical evidence for a close link between greenhouse gas concentrations and climate.

This work strengthens evidence for a close link between CO2 and climate. It adds to the broad body of evidence supporting climate models that simulate a key role for CO2 in global warming. Finally, it provides additional constraints on models of Earth’s climate and how climate will change due to natural and anthropogenic perturbations.

The Bender group will return to the Allan Hills, for additional studies with two objectives. The first is to obtain a broader picture of climate properties around 1,000,000 years ago. The second is to extend the ice core record of climate and greenhouse gas concentrations further back in time.

Figure 1.6
Figure 1.6. Drilling for old ice in the Allan Hills, Antarctica. A tube of drilled ice is being retrieved. The drill is lowered on a wire coiled around the winch behind the core tube. The curtain in the background is a wind-screen. (Photo courtesy of the Bender group.)

Science Publications

Batterman, S.A., L.O. Hedin, M. van Breugel, J. Ransijn, D.J. Craven, and J.S. Hall, 2013. Key role of symbiotic dinitrogen fixation in tropical forest secondary succession. Nature, 502: 224- 227. doi:10.1038/nature12525. doi:10.1038/ nature12525.

Bernardello, R., I. Marinov, J. B. Palter, J. L. Sarmiento, E. D. Galbraith, and R. D. Slater, 2014. Response of the ocean natural carbon storage to projected twenty-first-century climate change. J. Climate, 27: 2033-2053. doi: 10.1175/JCLI-D-13-00343.1.

Bernadello, R., I. Marinov, J. Palter, and J. L. Sarmiento, 2014. Impact of Weddell Sea deep convection on natural and anthropogenic carbon in a climate model. Geophys. Res. Lett., 41: 7262–7269. doi: 10.1002/2014GL061313.

Carter, B. R., J. R. Toggweiler, R. M. Key, and J. L. Sarmiento, 2014. Processes determining the marine alkalinity and carbonate mineral saturation distributions. Biogeosci. Discuss., 11: 7349-7362. doi:10.5194/bg-11-7349-2014.

Chen, A.P., J.W. Lichstein, J.L.D. Osnas, and S.W. Pacala, 2014. Species-independent down-regulation of leaf photosynthesis and respiration in response to shading: evidence from six temperate forest tree species. PLoS ONE, 9(4): e91798.

Christensen, V., M. Coll, J. Buszowski, W. Cheung, T. L. Frölicher, J. Steenbeek, C. Stock, R. Watson, and C. J. Walters, 2014. The global ocean is an ecosystem: Simulating marine life and fisheries. Glob. Ecol. Biogeogr., 2 February, 2015. doi:10.1111/geb.12281.

de Souza, G. F., R. D. Slater, J. P. Dunne, and J. L. Sarmiento, 2014. Deconvolving the controls on the deep ocean’s silicon stable isotope distribution. Earth Planet. Sci. Lett., 398: 66-76. doi:10.1016/j.epsl.2014.04.040.

Dybzinski, R., C.E. Farrior, and S.W. Pacala, 2015. Increased forest carbon storage with increased atmospheric CO2 despite nitrogen limitation: A game-theoretic allocation model for trees in competition for nitrogen and light. Glob. Chang. Biol., 21(3):1182-96. doi:10.1111/gcb.12783.

Frölicher, T. L., M. Winton, and J. L. Sarmiento, 2014. Continued global warming after CO2 emissions stoppage. Nat. Clim. Chang., 4: 40-44. doi:10.1038/ nclimate2060.

Frölicher, T. L., J. L. Sarmiento, D. J. Paynter, J. P. Dunne, and M. Winton, 2014. Dominance of the Southern Ocean in anthropogenic carbon and heat uptake in CMIP5 models. J. Climate, 28(2): 862-886. doi:10.1175/JCLI-D-14-00117.1.

Goldman J.A.L., S.A. Kranz, J.N. Young, P.D. Tortell, R.H.R. Stanley, M.L. Bender, and F.M.M. Morel, 2015. Gross and net production during the spring bloom along the Western Antarctic Peninsula. New Phytologist., 205: 182-191. doi:10.1111/ nph.13125.

Griffies, S. M., M. Winton, W. G. Anderson, R. Benson, T. L. Delworth, C. O. Dufour, J. P. Dunne, P. Goddard, A. K. Morrison, A. Rosati, A. Wittenberg, J. Yin, and R. Zhang, 2014. Impacts on ocean heat from transient mesoscale eddies in a hierarchy of climate models. J. Climate, 28(3): 952-977. doi:10.1175/JCLIM-D-14-00353.1.

Kranz, S. A., J.N. Young, B.M. Hopkinson, J.A.L. Goldman, P.D. Tortell, and F.M.M. ScienceMorel, 2015. Low temperature reduces the energetic requirement for the CO2 concentrating mechanism in diatoms. New Phytologist., 205: 192-201. doi:10.1111/ nph.12976.

Levy, J., D. Medvigy, S.A. Batterman, X. Xu, and L.O. Hedin. Individual-based dinitrogen fixation and biodiversity interact to determine tropical forest carbon uptake. To be submitted to Nat. Clim. Chang., spring 2015.

Lichstein, J. W., N.-Z. Golaz, S. Malyshev, E. Shevliakova, T. Zhang, J. Sheffield, R. A. Birdsey, J. L. Sarmiento, and S. W. Pacala, 2014. Confronting terrestrial biosphere models with forest inventory data. Ecol. App., 24: 699-715. doi:10.1890/13-0600.1.

Majkut, J. D., B. Carter, C. O. Dufour, T. L. Frölicher, K. Rodgers, and J. L. Sarmiento, 2014. An observing system simulation for Southern Ocean CO2 uptake. Phil. Trans. R. Soc. A., 312: 20130046. doi:10.1098/ rsta.2013.0046.

Majkut, J. D., J. L. Sarmiento, and K. B. Rodgers, 2014. A growing oceanic carbon uptake: results from an inversion study of surface pCO2 data. Glob. Biogeochem. Cycles, 28: 335-351. doi:10.1002/2013GB004585.

Mislan, K.A.S., C. A. Stock, J. P. Dunne, and J. L. Sarmiento, 2014. Group behavior among model bacteria influences particulate carbon remineralization depths. J. Mar. Res., 72: 1-36.

Morrison, A. K., T. L. Frölicher, and J. L. Sarmiento, 2015. Upwelling in the Southern Ocean. Phys. Today, 68: 27-32. doi:10.1063/ PT.3.2654.

Ogle, K., S. Pathikonda, K. Sartor, J.W. Lichstein, J. Osnas, and S.W. Pacala, 2014. A model-based meta-analysis for estimating species specific wood density and identifying potential sources of variation. J. Ecology, 102(1): 194-208. doi:10.1111/1365- 2745.12178.

Raupach, M. R., M. Gloor, J. L. Sarmiento, J. G. Canadell, T. Gasser, R. A. Houghton, C. Le Quéré, and C. M. Trudinger, 2014. The declining uptake rate of atmospheric CO2 by land and ocean sinks. Biogeosciences, 11: 3453-3475. doi:10.5194/bg-11-3453-2014.

Sulman, B.N., R.P. Phillips, A.C. Oishi, E. Shevliakova, and S.W. Pacala, 2014. Microbe-driven turnover offsets mineralmediated storage of soil carbon under elevated CO2. Nat. Clim. Change, 4: 1099- 1102. doi:10.1038/nclimate2436.

Tortell, P.D., E.C. Asher, H.W. Ducklow, J.A.L. Goldman, J.. H. Dacey, J.J. Grzymski, J.N. Young, S.A. Kranz, K. S. Bernard, and F.M.M. Morel, 2014. Metabolic balance of coastal Antarctic waters revealed by autonomous pCO2 and ΔO2 /Ar measurements. Geophysical Research Letters, 41(19): 6803-6810. doi:10.1002/2014GL061266.

Violle, C., P.B. Reich, S.W. Pacala, B.J. Enquist, and J. Kattge, 2014. The emergence and promise of functional biogeography. Proc. Natl. Acad. Sci., 111(38): 13690-13696. doi: 10.1073/pnas.1415442111.

Weng, E., S. Malyshev, J. W. Lichstein, C.E. Farrior, R. Dybzinski, T. Zhang, E. Shevliakova, and S.W. Pacala, 2014. Scaling from individuals to ecosystems in an Earth System Model using a mathematically tractable model of height-structured competition for light. Biogeosciences Discuss., 11: 17757-17860, in review. doi:10.5194/bgd-11-17757-2014.

Young J.N., J.A.L. Goldman, S.A. Kranz, P.D. Tortell, and F.M.M. Morel, 2015. Slow carboxylation of Rubisco constrains the rate of carbon fixation during Antarctic phytoplankton blooms. New Phytologist., 205: 172-181. doi: 10.1111/nph.13021.

Young, J.N., S. Kranz, J. Goldman, P.D. Tortell, and F.M.M. Morel, 2015. Under high CO2, Antarctic phytoplankton down-regulate their carbon concentrating mechanisms with no change in growth rates. MEPS, in review.

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