CMI Science features close collaboration with Princeton’s neighbor, the Geophysical Fluid Dynamics Laboratory (GFDL) of the US Department of Commerce. Together, CMI and GFDL are improving the understanding of atmospheric, oceanic and terrestrial carbon dioxide (CO2) and other greenhouse gases. The role of oceans as carbon and heat sinks is under investigation, with an emphasis on the relatively unexplored Southern Ocean. A growing effort is focused upon developing a better understanding of past tropical cyclone activity and intensity in order to improve the ability to predict future activity in response to changing climatic conditions. The study of surface waves at the interface between the atmosphere and ocean waters has important implications for improved understanding of climate and weather. Modeling of ice flows reveals new information about ocean mixing with implications for ocean ecology. Research on the role of terrestrial vegetation in the carbon cycle continues with additional focus on the hydrological cycle. An initiative launched in late 2017 investigates the physics of soil carbon dynamics to inform practical strategies for enhancing carbon storage in soils. In addition, a new supplementary award was announced to study methane sources and sinks in the atmosphere and on land.
Research Highlights – At a Glance
Jorge Sarmiento: Previous research using ocean observations and model results has suggested that the ocean around Antarctica acts as a key sink for atmospheric CO2, mitigating global temperature increases caused by increasing anthropogenic carbon emissions. However, ship-based observations to test these findings have been scarce and mostly limited to summertime measurements. New data from robotic floats that measure biogeochemistry year-round suggest that previous studies may have missed important wintertime outgassing in certain regions, resulting in overestimates of the size of the Southern Ocean sink. The Southern Ocean Carbon and Climate Observations and Modeling (SOCCOM) researchers are working to extend such observations throughout the global ocean.
Gabriel Vecchi: Models project an increase in the rate of “rapid intensification” for tropical cyclones globally by the end of the 21st century. However, projections of changes in hurricane frequency in the Atlantic remain more uncertain, and model simulations and potential undercounts prior to available satellite data suggest observed long-term trends in hurricane counts are data artifact. The goal of this work is to reconcile these potential discrepancies and to improve the understanding of the mechanisms behind and limits to the predictability of tropical cyclone (TC) activity over the past few and next centuries. The work connects to broad questions in the climate science community, such as uncertainty over what TC changes are likely to occur over the coming century, and the extent to which intrinsic climate variability may be dominant over the impact of greenhouse forcing.
Brandon Reichl: Surface waves at the atmosphere-ocean interface have important implications for climate and weather modeling. This research focuses on two topics related to surface waves. The first is improved coupled model performance through explicit consideration of physical processes related to surface gravity waves, including upper ocean turbulent mixing and interfacial fluxes of heat, momentum, and gases. The second is the investigation of changing surface wave characteristics in an evolving climate.
François Morel: The fixation of nitrogen gas by specialized organisms such as Trichodesmium is key to controlling photosynthetic production in marine ecosystems and may be impaired by ocean acidification. Recent studies sought to untangle the separate effects of high CO2 and low pH on Trichodesmium and found that the former accelerates photosynthesis and N2-fixation whereas the latter impairs these functions. Low ambient pH results in low intracellular pH, which decreases the efficiency of the nitrogenase enzyme.
Howard Stone: Climate changes involve atmospheric motions, ocean flows, and evolution of ice on land and in the sea. These dynamics are closely interrelated; insights into individual processes can help to illuminate poorly understood aspects of global climate dynamics, such as factors affecting the maintenance of sea ice cover in the Arctic basin. Sea ice cover can impact fresh water fluxes, local ecology and ocean circulation. The Stone group is providing simplified models for understanding the movement and distribution of ice during the formation of polynyas, which refer to localized regions of water surrounded by ice, and through narrow straits, which can affect flow, mixing and ecology in the ocean. The approach seeks to draw generalizations valid for various geometric and climate conditions.
Stephen Pacala: The Pacala group’s work has continued to improve the representation of the carbon cycle in climate models, including empirical support for a new theory of evaporative water loss in plants and an explanation of tree behavior in response to drought. Additional work on the warming impacts of methane, including a collaboration with the Environmental Defense Fund, analyzed the methane budget of the US oil and gas infrastructure and provided new estimates for US emissions.
Elena Shevliakova: Human water management practices have a noticeable impact on the hydrological cycle. These include diverting water for irrigation, abstraction of groundwater, and construction of reservoirs. Hydrologic extremes, in particular, are heavily affected by water management practices, due to the existing stress on the system during droughts and floods. To prepare adaptation plans for hydrological extremes in the future, it is essential to account for water management and other human influences in state-of-the-art climate models.
Ian Bourg: An objective of the Bourg group is to resolve the physics of soil carbon storage. Field experiments indicate that the carbon storage capacity of soils increases significantly with their content in smectite clay minerals, but the cause of this relationship is unknown. The Bourg group is using atomistic-level simulations to predict the energetics of clay-organic interaction, the hydrology of clayey soils and sediments, and their dependence on aqueous chemistry. These results will enable more accurate Earth System Model predictions of soil carbon dynamics and inform practical strategies for enhancing the soil carbon sink.
François Morel (lead), Vaishali Naik, Elena Shevliakova, and Xinning Zhang: The CMI methane project, initiated in spring 2017, consists of three interconnected subprojects: an experimental project dealing with the critical issue of methane releases from wetlands, and two modeling projects aimed at quantifying the sources, sinks, and variations of methane in the atmosphere and on land. All three projects are now in full swing, following the hiring of postdoctoral researchers during the second half of 2017.
Robotic Floats Challenge Previous Estimates of the Southern Ocean Carbon Sink
Principal Investigator: Jorge Sarmiento
At a Glance
Previous research using ocean observations and model results has suggested that the ocean around Antarctica acts as a key sink for atmospheric CO2, mitigating global temperature increases caused by increasing anthropogenic carbon emissions. However, ship-based observations to test these findings have been scarce and mostly limited to summertime measurements. New data from robotic floats that measure biogeochemistry year-round suggest that previous studies may have missed important wintertime outgassing in certain regions, resulting in overestimates of the size of the Southern Ocean sink. The Southern Ocean Carbon and Climate Observations and Modeling (SOCCOM) researchers are working to extend such observations throughout the global ocean.
The Southern Ocean surrounding Antarctica plays a crucial role in the global carbon cycle and in the climate system as a whole. Observation-based estimates typically find that the Southern Ocean accounts for a significant portion of the contemporary oceanic uptake of CO2. However, measurements in this remote and inhospitable region have historically been made from ships and are severely limited in space as well as heavily biased towards spring and summer.
Sarmiento directs the SOCCOM project, a multi-institutional effort funded by the National Science Foundation to dramatically increase the number and variety of observations of the Southern Ocean through the world’s first large-scale deployment of biogeochemical (BGC) Argo floats. These robotic floats are equipped with newly developed biogeochemical sensors to measure pH, nitrate, and oxygen in addition to ocean temperature and salinity.
Over 100 SOCCOM BGC floats are currently operating and have collectively made nearly 5 million observations in the Southern Ocean (Figure 1.1.1) in all seasons and under ice. A subset of these floats deployed in the Southern Ocean during 2014-2017 have been used to make new estimates of air-sea fluxes of CO2 using these year-round measurements. Figure 1.1.2 shows CO2 fluxes for five regions of the Southern Ocean derived from pH measurements and alkalinity estimates based on regression algorithms. The results, while consistent with gridded CO2 flux products in the Subtropical, Subantarctic, Polar Front, and Seasonal Ice zones, are markedly different in the Antarctic zone where carbon-rich deep waters upwell to the sea surface, a region that is poorly sampled in ship-based datasets. Significant outgassing observed in this region leads to a net Southern Ocean CO2 sink of 0.03 PgC y-1 from the atmosphere to the ocean, a substantial departure from the most recent ship-based estimates of a mean sink of 1.0 PgC y-1. This finding has important implications for global climate models, which typically simulate a Southern Ocean CO2 sink of 0.5 to 1.3 PgC y-1 south of 35°S (relative to a global ocean sink of ~2 PgC y-1 and anthropogenic emissions of ~10 PgC y-1 from fossil fuels and cement). Furthermore, it suggests that our current understanding of the distribution of oceanic sources and sinks of CO2 may need revision and highlights the need for sustained year-round observations in the high-latitude Southern Ocean.
SOCCOM will continue to deploy BGC-Argo floats over the next three years, with a goal of roughly 200 floats operating in the Southern Ocean by 2020, which will give an even clearer picture of the distribution of pH, oxygen, and nutrients in the region. At the same time, high-resolution modeling studies are using the new data to better understand the current workings of the Southern Ocean and to compare and improve Earth System Models of the future of this region in a changing climate.
SOCCOM researchers are also reaching out to the oceanographic community to enable other researchers to use BGC-floats and expand the observing system worldwide. SOCCOM researchers held a town hall at this year’s American Geophysical Union Ocean Sciences meeting to inform other oceanographers about their efforts and the potential of float-based observations, and will also hold a float workshop at the University of Washington this summer to train other scientists, particularly early-career researchers, in how to use and process data from these new tools.
Figure 1.1.1. Locations and trajectories of 107 SOCCOM floats operating as of January 30, 2018. Red dots are locations of operating floats and yellow lines indicate float trajectories since deployment. (Credit: SOCCOM)
Figure 1.1.2. Annual net oceanic CO2 flux (PgC y-1) by oceanic zone estimated from float data (black bars) and from two widely used gridded estimates (red and green bars), calculated by sampling the gridded estimates in the same locations as the floats. Positive indicates net outgassing; negative indicates net ingassing. (Credit: SOCCOM)
Projecting Tropical Cyclone Intensification
Principal Investigator: Gabriel Vecchi
At a Glance
Models project an increase in the rate of “rapid intensification” for tropical cyclones globally by the end of the 21st century. However, projections of changes in hurricane frequency in the Atlantic remain more uncertain, and model simulations and potential undercounts prior to available satellite data suggest observed long-term trends in hurricane counts are data artifact. The goal of this work is to reconcile these potential discrepancies and to improve the understanding of the mechanisms behind and limits to the predictability of tropical cyclone (TC) activity over the past few and next centuries. The work connects to broad questions in the climate science community, such as uncertainty over what TC changes are likely to occur over the coming century, and the extent to which intrinsic climate variability may be dominant over the impact of greenhouse forcing.
Tropical cyclone intensification. A particular focus of Vecchi’s group has been on projections of rapid intensification of TCs, in which the wind speeds of TCs increase abruptly in less than a day, such as with hurricanes Harvey, Irma, and Maria in 2017. Rapidly intensifying TCs present a particular challenge to society as they tend to be poorly forecast on the weather scale and can leave communities with little time to prepare for impacts. The group recently submitted a manuscript describing climate model projections for a substantial increase in the rate of rapid intensification for TCs globally by the end of the 21st century, which may be evident as soon as the next few decades1.
Hurricane frequency. The team also explored the question of how TC frequency (the number of storms) may change. Modeling studies indicate the potential for both increases and decreases in regional TC frequency over the 21st century – can these diverging projections be distinguished?
Given that greenhouse warming is evident over the past century, answers to two questions can help us interpret the future: How and why has TC activity changed in the past? If a greenhouse-induced change in past TC activity is identified, one could expect it to continue into the future; if natural climate variations were dominant in the past, one may continue to experience substantial fluctuations.
The North Atlantic, where TCs are referred to as hurricanes, has the longest continuous database of TCs, with records available since the mid-1800s. These records indicate strong fluctuations in the number of hurricanes, including a nominal increase in the raw hurricane counts (see red line in Figure 1.2.1). However, potential undercounts prior to available satellite data suggest this long-term trend is a data artifact (compare black and red lines in Figure 1.2.1). Therefore, the group performed a suite of high-resolution climate model experiments (blue line and symbols in Figure 1.2.1). When constrained with estimates of past ocean temperatures, the model is able to recover many aspects of the observed Atlantic hurricane record.
This modeling study does not support the notion of a historical increase in Atlantic hurricane frequency since the late-1800s. Furthermore, these experiments support the hypothesis that official hurricane records have a substantial undercount in the pre-satellite era (Figure 1.2.1). This particular study does not address changes in the most intense hurricanes1, nor of hurricane rainfall. A warmer atmosphere is able to “hold” more moisture so hurricane rainfall is expected to increase with warming; a recent study indicates that the odds of the extreme rainfall over the Houston area during Hurricane Harvey was enhanced by global warming2, and another study indicates these extremes should increase over the coming century3.
Figure 1.2.1. Annual hurricane frequency in the Atlantic over the period 1871-2016 from: the NOAA HURDAT database (red line); the NOAA HURDAT database corrected for likely undercounts in the pre-satellite era (thick black line)4; and a suite of high-resolution climate model simulations constrained by observational estimates of ocean temperatures (blue dashed line and blue symbols; each circle represents an independent model realization to estimate the role of “weather noise”). Notice the three “active” periods (1870s-1890s, 1930s-1960s, and 1995-present) and two “inactive” eras (1900s-1920s, 1960s-1995) in the record.
In the coming year, the results from the simulations over the period 1871-2017 will be further analyzed, with a focus on hurricane-induced and regional rainfall. Further, the sensitivity to uncertainties in the knowledge of past ocean temperatures, and the sensitivity of the results to climate model formulation, will be explored by repeating the simulations with different estimates of historical temperature and a different high-resolution climate model. Of particular interest will be the potential discrepancy between model estimates of mid-20th century TC activity and those in historical reconstructions (Figure 1.2.2).
Figure 1.2.2. (a) Satellite era hurricane tracks, and (b-d) ship tracks density in the pre-satellite era indicates the potential for “missed” storms prior to satellite records. The pre-satellite era is divided into: (b) the pre-Panama Canal era, (c) post-Panama Canal through WWII era, and (d) post-WWII era5.
Volcanic activity. The response of TCs to volcanic forcing will also be explored through targeted climate model experiments in order to test the hypothesis that TC sensitivity to volcanoes depends fundamentally on the hemisphere in which the volcanic plume is most pronounced. Preliminary results indicate that El Chichón (1982), whose plume had a northern hemisphere maximum, increased TC activity in the southern hemisphere, and Agung (1963), which had its plume primarily in the southern hemisphere, acted to increase TC activity in the northern hemisphere. Emissions from explosive volcanoes can remain in the stratosphere for years and reduce the amount of sunlight reaching the surface, leading to a cooling of the planet. If the volcanic plume is primarily in one hemisphere, one would expect the cooling to be concentrated in that hemisphere, and the hypothesis of “relative warming” control on TC activity and rainfall6,7 suggests a more favorable environment for TCs and rainfall in the opposite hemisphere.
These experiments will also be used to understand the sensitivity of rainfall (and its extremes) beyond volcanic forcing, including the extent to which the impact of volcanic forcing serves as a useful analogue to the impact of surface warming due to greenhouse gas buildups on regional climate changes and extremes.
These projects are part of a broader effort to understand how TCs and tropical climate have changed, and the sensitivity of TCs to a range of factors, from the intrinsic variability of the atmosphere to the possible range of ocean temperature changes over the coming century, and the extent to which one can make predictions at regional and local scales.
1 Bhatia, K., G.A. Vecchi, H. Murakami, S. Underwood, and J. Kossin, 2018. Projected Response of Tropical Cyclone Intensity and Intensification in a Global Climate Model. J. Climate, submitted.
2 Van Oldenborgh, G.J., K. van der Wiel, A. Sebastian, R. Singh, J. Arrighi, F. Otto, K. Haustein, S. Li, G. Vecchi, and H. Cullen, 2017. Attribution of Extreme Rainfall from Hurricane Harvey, August 2017. Env. Res. Lett., 12(12): 124009. doi: 10.1088/1748-9326/aa9ef2.
3 Emanuel, K.A., 2017. Assessing the present and future probability of Hurricane Harvey’s rainfall. Proc. Nat. Acad. Sci. 114(28): 12681-12684. doi: 10.1073/pnas.1716222114.
4 Vecchi, G.A. and T.R. Knutson, 2011. Estimating annual numbers of Atlantic hurricanes missing from the HURDAT database (1878-1965) using ship track density. J. Climate. 24(6): 1736-1746. doi: 10.1175/2010JCLI3810.1.
5 Vecchi, G.A. and T.R. Knutson, 2008. On Estimates of Historical North Atlantic Topical Cyclone Activity J. Climate. 21(14): 3580-3600. doi: 10.1175/2008JCLI2178.1.
6 Vecchi, G.A. and B.J. Soden, 2007. Effect of remote sea surface temperature change on tropical cyclone potential intensity. Nature. 450: 1066-1070. doi:10.1038/nature06423.
7 Xie, S.P., C. Deser, G.A. Vecchi, J. Ma, H. Teng, and A.T. Wittenberg, 2010. Global Warming Pattern Formation: Sea Surface Temperature and Rainfall. J. Climate. 23: 966-986. doi: 10.1175/2009JCLI3329.1
Surface Gravity Waves in Climate Modeling
Principal Investigator: Brandon Reichl
At a Glance
Surface waves at the atmosphere-ocean interface have important implications for climate and weather modeling. This research focuses on two topics related to surface waves. The first is improved coupled model performance through explicit consideration of physical processes related to surface gravity waves, including upper ocean turbulent mixing and interfacial fluxes of heat, momentum, and gases. The second is the investigation of changing surface wave characteristics in an evolving climate.
A present research focus within the first topic is the role of surface waves in upper ocean mixing. Earth Systems Models have historically suffered from biases in under-predicting upper-ocean mixing. One region where this bias is particularly notable is in the Southern Ocean, where simulated mean mixed layer depth using GFDL’s MOM6 (Figure 1.3b) average approximately 50 m, approximately 30 m shallower than the observations of Hosoda et al., 2010 (Figure 1.3a).
This discrepancy motivated a high-resolution process model investigation to understand and parameterize the role of surface waves in ocean mixing via a wave-turbulence interaction mechanism called Langmuir turbulence (LT). The result was a modified parameterization for upper ocean mixing that significantly improves the simulated mixed layer depth in the Southern Ocean relative to observations (Figure 1.3c). The implications of this are significant for improving climate models through simulating atmosphere-ocean exchange and ocean uptake of properties such as heat and carbon.
Figure 1.3. Summer mixed layer depths from [a] observed climatology by Hosoda et al., 2010, [b] GFDL CM4/MOM6 prototype model with no wave-driven mixing (original) and [c] the same model with the new wave-driven mixing parameterization (Langmuir Turbulence, LT). The mixed layer depth is determined by a threshold vertical density change of 0.03 kg/m3.
Another finding of this study is the failure of the wave-driven parameterization to improve mixed layer depth biases in the Equatorial region (Figure 1.3a-c). The model bias in this region is therefore likely unrelated to missing wave processes, motivating future work to improve parameterized turbulent mixing driven by other sources (e.g., due to vertical current shear).
The second topic has driven a separate research focus on trends in surface wave properties over the historical record and projections for the future. The importance of this work lies largely in societal impacts. Marine development and offshore operations rely on accurately knowing the local ocean environment and how the environment will evolve over several decades. Possibly the most relevant environmental parameter for these concerns is the ocean wave height, both on average and during extreme weather events such as hurricanes. Trends in wave statistics can be investigated over the historical record to understand how conditions may change in the future. Furthermore, using a wave model coupled to a climate model designed for projection allows another method for predicting how waves will respond to evolving environmental conditions.
A critical next step for both research paths will be explicitly coupling a surface wave model into NOAA/GFDL’s climate modeling framework to improve model capabilities and better understand impacts in a changing climate. Presently, the uncoupled simulations with prescribed (observed) wind fields allow the role of waves to be understood only in a historical context. Predicting the role of surface waves in a changing climate requires the ability to model how the waves will respond to changing forcing, and thus clearly merges both research branches. Furthermore, research has recently begun on improved model performance considering other aspects of wave effects, such as air entrainment, which will allow for better estimates of heat, gas, and momentum flux within the model.
Reichl, B.G. and R. Hallberg. An Energetically Constrained Planetary Boundary Layer (ePBL) Approach for Ocean Climate Simulation. In revision.
Reichl, B.G., A. Adcroft, S.M. Griffies, R.W. Hallberg, Q. Li, and B. Fox-Kemper. Impact of Langmuir Turbulence on Energetic Constraints of the Ocean Surface Boundary Layer. In prep.
Hosoda, S., T. Ohira, K. Sato, and T. Suga, 2010. Improved description of global mixed-layer depth using Argo profiling floats. J. Oceanogr. 66(6): 773-787. doi:10.1007/s10872-010-0063-3.
Effects of Ocean Acidification on N2-Fixing Cyanobacteria
Principal Investigator: François Morel
At a Glance
The fixation of nitrogen gas by specialized organisms such as Trichodesmium is key to controlling photosynthetic production in marine ecosystems and may be impaired by ocean acidification. Recent studies sought to untangle the separate effects of high CO2 and low pH on Trichodesmium and found that the former accelerates photosynthesis and N2-fixation whereas the latter impairs these functions. Low ambient pH results in low intracellular pH, which decreases the efficiency of the nitrogenase enzyme.
The photosynthetic production of organic carbon that supports marine ecosystems is limited by the bioavailability of nitrogen in large regions of the oceans. As a result, the fixation of nitrogen gas (N2) by specialized organisms (diazotrophs, which synthesize the nitrogenase enzyme) is a key process controlling marine productivity. There have been numerous recent publications on the effect of ocean acidification, caused by the CO2 emitted by fossil fuel burning, on N2-fixation by marine organisms. In particular, the cyanobacterium Trichodesmium, which is ubiquitous in low latitude oceans, has been reported to increase N2-fixation under acidic conditions.
A fundamental complication in assessing biological responses to ocean acidification is the need to unravel the differential effects of the simultaneous decrease in pH and increase in CO2 concentration. Experiments in which only one of these two parameters is varied at a time demonstrate that increasing CO2 while maintaining pH constant accelerates the rates of photosynthesis and N2-fixation by Trichodesmium. In contrast, decreasing pH at constant CO2 impairs photosynthesis and N2-fixation.
Figure 1.4. The cyanobacterium Trichodesmium, ubiquitous in low latitude oceans, decreases N2-fixation under the acidic conditions caused by high CO2. Image credit: Sven Kranz.
The negative effect of low pH, which overwhelms the positive effect of high CO2 when both co-vary, is enhanced under conditions of low iron availability, which are prevalent in large regions of the open ocean. Notably, the detrimental effect of low pH can be masked under experimental conditions if the culture medium is contaminated by traces of ammonium as acidification decreases the concentration of the uncharged and toxic ammonia (lower NH3/NH4+ ratio).
Mechanistic explanations for the positive effect of high CO2 and negative effect of low pH on Trichodesmium are provided by physiological measurements and proteomic analyses of the organism under various conditions. In particular, low ambient pH results in a decreased intracellular pH despite increased energy devoted to proton (H+) export. In turn, the low intracellular pH impairs the efficiency of the nitrogenase enzyme and leads to a decrease in N2-fixation rate despite an increase in enzyme concentration. Results of acidification experiments with field populations of Trichodesmium in the South China Sea are wholly consistent with laboratory results.
This study reconciles previous studies that gave conflicting results on the response of Trichodesmium to acidification. Extrapolation to the conditions expected for year 2100 suggests a potentially significant decline in the supply of new nitrogen to oceanic ecosystems.
Modeling the Formation and Extent of Coastal Polynyas
Principal Investigator: Howard A. Stone
At a Glance
Climate changes involve atmospheric motions, ocean flows, and evolution of ice on land and in the sea. These dynamics are closely interrelated; insights into individual processes can help to illuminate poorly understood aspects of global climate dynamics, such as factors affecting the maintenance of sea ice cover in the Arctic basin. Sea ice cover can impact fresh water fluxes, local ecology and ocean circulation. The Stone group is providing simplified models for understanding the movement and distribution of ice during the formation of polynyas, which refer to localized regions of water surrounded by ice, and through narrow straits, which can affect flow, mixing and ecology in the ocean. The approach seeks to draw generalizations valid for various geometric and climate conditions.
A polynya is a region of persistent open ocean water surrounded by sea ice (Figures 1.5a and 1.5b). Polynyas remain open from a regional balance between the rate of ice production (due to freezing seawater) and the rate of ice depletion, for example, due to flow. Such polynyas may exist either in the open ocean or close to coastal boundaries. The latter are formed when winds sweep ice away from the coast, exposing open sea water that freezes to form new ice. Thus, these coastal polynyas, especially along the Antarctic coast and some regions of the Arctic, are an important source of new sea ice, are crucial to ocean-atmosphere energy exchange, and are thought to regulate thermohaline circulation, i.e., the circulation of temperature and salinity, in the ocean. In addition, phytoplankton and other marine life thrive in polynyas, especially in summer months, and so these water-ice structures are important to ecology.
Although some qualitative mechanisms of polynya formation have been identified, modeling their extent precisely has proved challenging. In particular, previous attempts have predicted unrealistically large or unstable polynyas. Alternatively, they have relied on high-resolution numerical simulations, where a clear connection between polynya formation dynamics and the mechanical stresses due to the ice motion is lacking. Recently, the Stone group has succeeded in developing simplified descriptions of ice motion due to wind in the context of ice bridge formation in straits, taking into account the frictional stresses in ice. These simplified models accurately represent the mechanical behavior of ice and ice flows, agreeing both with measurements and with numerical simulations. This approach forms the basis for our more recent investigation of coastal polynyas.
Figure 1.5. (a) Map of the Arctic indicating coastal polynyas in orange and sea ice in blue. Adapted from Barber and Massom, 2007. Elsevier Oceanography Series. 74: 1-54. (b) Aerial view of the Weddell polynya (dark blue) off the Antarctic coast (white) surrounded by ice on all sides; image from ACE CRC, Australia. (c) Results of a preliminary numerical simulation of a wind-driven coastal polynya downstream of an island (white), showing a pileup of ice (red) upstream of the island with open water (dark blue) downstream. The arrows indicate the velocity of the ice, shading indicates ice thickness, and the contours indicate the ice area fraction.
The Stone group’s current efforts are focused on modeling polynya formation in coastal regions, including islands and fjords. As with our previous studies of ice flows, we have interacted closely with Michael Winton at GFDL as part of regular discussions we have had with GFDL colleagues. These studies of polynyas quantify the formation of new ice by freezing seawater, while incorporating findings from the group’s previous work to quantify the stresses and motion of the formed ice in response to wind. The study will develop a fully resolved numerical model, as well as a simplified model to predict the roles of freezing (ice production), flow, and ice accumulation in determining the extent of coastal polynyas. The combination of modeling approaches will provide clear connections between the mechanics of sea ice motion and the thermodynamics of sea ice production. Thus, our modeling efforts not only explain a complex geophysical phenomenon but also provide a means to refine the modeling of sea ice in the more general context of Arctic and Antarctic ice flows near land boundaries.
A related theme that is being pursued by the Stone group is to understand the mechanics of ice more broadly in the context of granular flows. Although the qualitative similarities of ice motion to flowing grains has been recognized in the ice-modeling community, no systematic quantification of these similarities has been attempted previously. This quantification will be particularly important as the structure of sea ice changes with climate change, requiring a reworking of current ice models, and the insights may also be useful to the community focused on granular mechanics.
Representing Plant Behavior in Climate Models
Principal Investigator: Stephen Pacala
At a Glance
The Pacala group’s work has continued to improve the representation of the carbon cycle in climate models, including empirical support for a new theory of evaporative water loss in plants and an explanation of tree behavior in response to drought. Additional work on the warming impacts of methane, including a collaboration with the Environmental Defense Fund, analyzed the methane budget of the US oil and gas infrastructure and provided new estimates for US emissions.
The Pacala group’s CMI research continues to be focused on the carbon cycle and improving its representation in climate models. One of the most important factors that could limit the natural carbon sink is water limitation. Because evaporative water loss increases with temperature, global warming will water-stress plants even in locations with no decrease in rainfall. Water escapes plants from small valves called stomates. The group published a paper in 2016 that established a new theory of stomatal regulation1. This year, the team authored two papers showing that empirical data strongly support the new theory over the 40-year-old classical theory2,3.
The Pacala group also cleared up an important mystery impacting the carbon cycle. When drought kills trees, the trees that will eventually die usually grow quickly for up to a decade and then drop dead. No one could explain this behavior, even though drought-kill has the potential for large positive feedback in the carbon cycle. Drought from climate change kills trees, releasing carbon to the atmosphere, causing further climate change, and so on. The group’s new paper offers a simple explanation consistent with all available data.
Figure 1.6. The expected effects of warmer springs and drier summers on species composition of boreal forests and their role in the carbon cycle4.
The new and advanced model of the terrestrial biosphere and carbon cycle that the group has been working on for many years was implemented this year in GFDL’s next-generation Earth System Models, which will be used in the next Intergovernmental Panel on Climate Change report. This model contains all of the team’s recent findings about stomatal regulation and, as a result, has enhanced ability to model water limitation of the terrestrial carbon sink. The model also contains the results of past CMI-funded research on fire and many other topics.
Finally, Pacala continued his work with the Environmental Defense Fund methane project. He and collaborators published one paper in Science calling for a new standard for reporting warming impacts of methane. He also completed analysis of the synthesis of the past five years of work: a methane budget of the US oil and gas infrastructure, from well to end use. This work shows that US emissions are 60% higher than the Environmental Protection Agency’s estimates because of upstream emission. The paper is currently in review at Science.
1 Wolf, A., W.R.L. Anderegg, & S.W. Pacala (2016). Optimal stomatal behavior with competition for water and risk of hydraulic impairment. PNAS. doi: 10.1111/pce.12852.
2 Anderegg, W.R.L., A. Wolf, A. Arango-Velez, B. Choat, D.J. Chmura, S. Jansen, T. Kolb, S. Li, F. Meinzer, P. Pita, V. Resco de Dios, J.S. Sperry, B.T. Wolfe, & S.W. Pacala (2017). Plant water potential improves prediction of stomatal models. PLOS One, 12(10), doi: 10.1371/journal.pone.0185481.
3 Anderegg, W.R.L., A. Wolf, A. Arango-Velez, B. Choat, D.J. Chmura, S. Jansen, T. Kolb, S. Li, F. Meinzer, P. Pita, V. Resco de Dios, J.S. Sperry, B.T. Wolfe, and S.W. Pacala, 2018. Woody plants optimize stomatal behavior relative to hydraulic risk. Ecology Letters, in press.
4 Trugman, A.T., D. Medvigy, W.R.L. Anderegg, and S.W. Pacala, 2017. Differential declines in Alaskan boreal forest vitality related to climate and competition. Glob. Change Biol. 24: 1097-1107. doi: 10.1111/gcb.13952.
Simulating Human Water Management Practices in a Climate Model
Principal Investigator: Elena Shevliakova
At a Glance
Human water management practices have a noticeable impact on the hydrological cycle. These include diverting water for irrigation, abstraction of groundwater, and construction of reservoirs. Hydrologic extremes, in particular, are heavily affected by water management practices, due to the existing stress on the system during droughts and floods. To prepare adaptation plans for hydrological extremes in the future, it is essential to account for water management and other human influences in state-of-the-art climate models.
This project focuses on the implementation of irrigation practices and reservoirs in the LM3 model, which is the land component of GFDL’s Earth System Model. Irrigation accounts for 70% of global freshwater abstractions, while reservoirs significantly change the hydrological regime of major rivers. It is therefore crucial to include these processes in the current model to improve our ability to accurately simulate the hydrological cycle.
Irrigation is estimated as the difference between the amount of water that plants need for optimal growth and the actual available water in the soil. This method was used to dynamically estimate irrigation demand for croplands across the continental US (Figure 1.7.1). The modeled estimates of total irrigation demand show a similar spatial pattern to reported irrigation water withdrawals, although there is a slight underestimation in some regions. Reservoirs in the model change the flow towards the rivers, where the reservoir release is dependent on the downstream irrigation demand and minimum flow requirements. Most water is released in summer, while in winter a net increase in reservoir storage is observed.
Figure 1.7.1. Estimated irrigation demand across the continental US.
Including these irrigation processes led to changes in simulated vegetation characteristics and hydrological variables, such as increased vegetation growth over the croplands (Figure 1.7.2), and increased transpiration (i.e., evaporation by plants) and runoff. This led to a decrease in canopy temperature, with some locations showing a decrease of more than 8°C.
Figure 1.7.2. Changes in simulated net primary production.
Apart from causing these changes, the water management practices also had a significant influence on drought across the US. The simulations indicated that irrigation aggravated drought conditions in rivers, because water was taken from the water ways to stimulate plant evaporation. In contrast, the inclusion of reservoirs increased the river outflow in large parts of the US. However, the effect of reservoirs is very much dependent on the operation rules of each specific reservoir. It is assumed that these rules are aimed at preventing and alleviating drought.
This new modeling framework provides a unique opportunity to study impacts of human water management at high resolutions, aiding a better understanding of anthropogenic impacts on vegetation growth, the hydrological cycle, and drought. In the future, the modeling framework will be elaborated with improved methods to estimate water management practices. Finally, the framework will be used for simulations at the global scale to be able to investigate human impacts across different climates.
Resolving the Physics of Soil Carbon Storage
Principal Investigator: Ian Bourg
At a Glance
An objective of the Bourg group is to resolve the physics of soil carbon storage. Field experiments indicate that the carbon storage capacity of soils increases significantly with their content in smectite clay minerals, but the cause of this relationship is unknown. The Bourg group is using atomistic-level simulations to predict the energetics of clay-organic interaction, the hydrology of clayey soils and sediments, and their dependence on aqueous chemistry. These results will enable more accurate Earth System Model predictions of soil carbon dynamics and inform practical strategies for enhancing the soil carbon sink.
Soil carbon is the largest pool of carbon near the Earth’s surface, about as large as the atmosphere, biosphere, and surface ocean combined1. This soil carbon pool has important implications for the Earth’s carbon budget. For example, a minor annual increase in soil carbon content (0.2% per year) would effectively balance global CO2 sources and sinks as required by the Paris Agreement. Conversely, increased soil carbon respiration rates in a warming climate could greatly magnify anthropogenic CO2 emissions2.
Efforts to predict, and eventually control, soil carbon content are challenged by a lack of understanding of the relevant physics. An important clue into the fundamental mechanisms is that soil carbon content correlates strongly with the content of certain aluminosilicate clay minerals (smectite, imogolite, allophane). Possible mechanisms for this relation include carbon protection by adsorption on clay surfaces and the distinct hydrology (strong water retention, low permeability) of clay-rich regions within soils. At present, little is known of these fundamental processes and their dependence on temperature, aqueous chemistry, and the structure of the organic compounds. This information is challenging to extract from field studies because of the inherent complexity of soils, but is critical to the development of a predictive understanding of soil carbon.
The Bourg group is using all-atom molecular dynamics (MD) simulations of simple clay-water-organic systems to gain insight into the fundamental controls on soil carbon storage. The research focuses on two major questions: first, how organic molecules interact with clay surfaces and, second, how clay-water and clay-water-organic interactions control the hydrologic properties of soils. The first effort is carried out under the auspices of CMI along with the PEI Grand Challenges initiative. The second effort is supported since 2017 by grants secured by the Bourg group from the US Department of Energy and the US National Science Foundation. The two research efforts are independent, but their results strongly inform each other, and both may prove crucial to understanding the role of fine-grained minerals in soil carbon storage.
A key breakthrough in the first part of this initiative in 2017 is the demonstration that the atomistic-level simulation methodologies developed by the Bourg group accurately predict the adsorption of dissolved gases3 and organic compounds4. The simulations provide detailed information on the energetics of organic adsorption on clay surfaces and intercalation in clay interlayer nanopores (Figure 1.8.1), properties that are thought to enable protection from microbial degradation. The simulations, which solve Newton’s equations of motion for systems of about 105 atoms using semi-empirical models of all relevant interatomic interactions, require about one month of time on hundreds of parallel processors. The main challenge is to develop models of these interatomic interactions that accurately predict the properties of real clay-water-organic systems, a research area in which the Bourg group is actively involved5.
Figure 1.8.1. Snapshot of a simulation cell containing two Ca-smectite clay nanoparticles (1 nm thick particles with 0.6 nm thick interlayer nanopores) in contact with bulk-liquid-like water (0.1 M CaCl2 solution). Clay structural atoms are shown as red, yellow, pink, green, and white spheres (O, Si, Al, Mg, and H, respectively); water molecules are shown as red and white sticks. In the lower half of the figure, the overlain color map shows the free energy landscape as seen by a single molecule of di-ethyl phthalate (DEP). Dark blue regions near the clay basal surfaces indicate free energy wells where DEP is adsorbed by a combination of polar, Van der Waals, and hydrophobic interactions.
The results show that the affinities of phthalates (compounds selected to mimic the average stoichiometry and aromaticity of soil organic matter) for smectite clay surfaces are commensurate with the reported affinity of soil organic leachate for soil clay minerals. The results reveal that polar interactions associated primarily with the carbonyl groups (>C=O) contribute roughly half of the free energy of adsorption of phthalates on clay surfaces. Further simulations and complementary wet-chemical experiments are underway to characterize the influence of organic structure and aqueous chemistry on adsorption for a broader range of organic compounds.
An important breakthrough in the second part of this initiative is the demonstration that methodologies developed in the Bourg group allow simulating systems of tens of clay particles suspended in liquid water. Simulations are underway to predict the hydrology of clay-water mixtures as a function of compaction and aqueous chemistry (Figure 1.8.2). The results are likely to provide insight into the transport properties of clay-rich regions in soils and sediments6.
In addition to providing an advanced understanding of carbon cycling in soils, this initiative will enable more accurate representations of the migration of water and organics in sediments and sedimentary rocks, for example, in basin modeling and CO2-enhanced oil recovery.
Figure 1.8.2. Snapshots of a simulation cell containing 10 Na-smectite particles (10-nm diameter hexagons) suspended in liquid water. Clay particles are shown as red, yellow, pink, green, and white spheres. Exchangeable Na ions are shown as blue spheres. Water molecules are not shown. The system, initially cubic and containing > 1 million atoms, is progressively dehydrated while applying a constant pressure in the vertical direction in a manner designed to mimic the drying of a clay suspension or the burial of clay-rich sediment. Simulation trajectories will be analyzed to determine how microstructure and transport properties vary with water content and aqueous chemistry in the clayey regions of soils and sediments.
1 Lehmann, J. and M. Kleber, 2015. The contentious nature of soil organic matter. Nature. 528(60): 60-68. doi:10.1038/nature16069.
2 Hicks Pries, C.E., C. Castanha, R. Porras, and M.S. Torn, 2017. The whole-soil carbon flux in response to warming. Science, 355(6332): 1420-1423. doi: 10.1126/science.aal1319.
3 Gadikota, G., B. Dazas, G. Rother, M.C. Cheshire, and I.C. Bourg, 2017. Hydrophobic solvation of gases (CO2, CH4, H2, noble gases) in clay interlayer nanopores. J. Phys. Chem. C. 121(47): 26539-26550. doi: 10.1021/acs.jpcc.7b09768.
4 Willemsen, J. and I.C. Bourg, 2017. Molecular dynamics simulation and experimental study of the adsorption of dimethyl phthalate on clay surfaces. Paper presented at the Goldschmidt Conference, Paris, August 2017.
5 Bourg, I.C., S.S. Lee, P. Fenter, and C. Tournassat, 2017. Structure and energetics of the Stern layer at mica-water interfaces. J. Phys. Chem. C. 121(17): 9402-9412. doi: 10.1021/acs.jpcc.7b01828.
6 Bourg, I.C. and J.B. Ajo-Franklin, 2017. Clay, water, and salt: Controls on the permeability of fine-grained sedimentary rocks. Accounts of Chemical Research. 50(9): 2067-2074. doi: 10.1021/acs.accounts.7b00261.
The CMI Methane Project: Wetland, Atmospheric, and Terrestrial Methane Cycles
Principal Investigators: François Morel (lead), Vaishali Naik, Elena Shevliakova, and Xinning Zhang
At a Glance
The CMI methane project, initiated in spring 2017, consists of three interconnected subprojects: an experimental project dealing with the critical issue of methane releases from wetlands, and two modeling projects aimed at quantifying the sources, sinks, and variations of methane in the atmosphere and on land. All three projects are now in full swing, following the hiring of postdoctoral researchers during the second half of 2017.
1) The controls on methane release from wetlands
Wetlands are large and highly variable sources of the potent greenhouse gas methane. In these systems, methane emission to the atmosphere is shaped by a complex interplay of microbial, hydrological, and plant-associated processes, which vary in time and space. Contrary to the paradigm that microbial methane production is confined to oxygen-free habitats, recent research suggests that unsaturated soils and peats, containing significant levels of oxygen, may be important sources of methane. To understand the mechanisms underpinning this phenomenon, a group led by Xinning Zhang is studying how oxygen variability shapes carbon and nutrient transformation, community composition, and the activity of microbes important for methane cycling (Figure 1.9). Preliminary results indicate a potentially large enhancement of methane formation in wetlands subjected to cyclic variations in oxygen concentrations. Results will better constrain predictions of methane from high latitude ecosystems, where permafrost thaw is increasing the extent of wetlands and our understanding of methane cycling remains highly uncertain.
2) A global model of the atmospheric methane cycle.
Global atmospheric concentrations of methane started increasing in 2007 after a period of stabilization from 1999 to 2006. It has been challenging to attribute the changes in methane growth rate to specific sources or sinks of methane. A group led by Vaishali Naik is involved in developing and applying bottom-up global-scale chemistry climate models to better understand the processes that control the variability of atmospheric methane at decadal to centennial time scales. Initial tests show that the next-generation GFDL model, driven by historical emissions from the inventory developed for the Intercontinental Panel on Climate Change’s Sixth Assessment Report, is able to capture the observed variability and trends of the past 20 years. To facilitate the characterization of the drivers of methane variability and trends, the model is currently being advanced to include the representation of carbon and hydrogen isotopes of methane. Together with observations and model results, the group will be able to better quantify the roles of individual sources and sinks in driving methane variability.
3) A global model of the terrestrial methane cycle.
Quantification of past and future terrestrial sources and sinks of carbon requires a global comprehensive and high-resolution land model with enhanced ecological, biogeochemical, and hydrological capabilities, including prognostic methane emissions from natural and managed systems. The terrestrial component of GFDL’s new Earth System Model, LM4, includes a number of new capabilities and improvements, such as dynamic vegetation and carbon cycling, a representation of changing land-use practices, frozen soil dynamics, and a new vertically resolved soil biogeochemistry for carbon and nitrogen cycling. GFDL scientists, led by Elena Shevliakova, in collaboration with Princeton Environmental Institute researchers are implementing a new component characterizing explicitly soil microbes, which shape most soil biogeochemical cycles and control releases of the most potent greenhouses gases, carbon dioxide, and methane. The model captures wetland soil microbial processes, including growth and decomposition of microbes involved in methane production and oxidation. Methane is transported through aerobic layers of the soil column, where methanotrophic microbes oxidize part of the methane, and the rest escapes to the atmosphere.
Figure 1.9. CMI Researchers sampling sphagnum peat from a Northeastern wetland in June 2017 (top left panel). Peat samples (top right panel) were mixed with pore water, slurried, and exposed to different amounts of oxygen over the course of several months prior to the onset of an anoxic period. Samples of gas headspace and slurry material were taken from incubations (bottom right panel) over the course of the treatment to characterize the chemistry and microbiology of decaying, methanogenic peat.
Anderegg, W.R.L., A. Wolf, A. Arango-Velez, B. Choat, D.J. Chmura, S. Jansen, T. Kolb, S. Li, F. Meinzer, P. Pita, V. Resco de Dios, J.S. Sperry, B.T. Wolfe, and S.W. Pacala, 2017. Plant water potential improves prediction of stomatal models. PLOS One. 12(10): e0185481. doi: 10.1371/journal.pone.0185481.
Ballantyne, A., W. Smith, W. Anderegg, P. Kauppi, J. L. Sarmiento, P. Tans, E. Shevliakova, Y. Pan, B. Poulter, A. Anav, P. Friedlingstein, R. Houghton, and S. Running, 2017. Accelerating net terrestrial carbon uptake during the warming hiatus due to reduced respiration. Nat. Clim. Change. 7: 148-152. doi: 10.1038/nclimate3204.
Bourg, I.C. and J.B. Ajo-Franklin, 2017. Clay, water, and salt: Controls on the permeability of fine-grained sedimentary rocks. Accounts of Chemical Research. 50(9): 2067-2074. doi: 10.1021/acs.accounts.7b00261.
Bourg, I.C., S.S. Lee, P. Fenter, and C. Tournassat, 2017. Structure and energetics of the Stern layer at mica-water interfaces. J. Phys. Chem. C. 121(17): 9402-9412. doi: 10.1021/acs.jpcc.7b01828.
Bushinsky, S., A.R. Gray, K.S. Johnson, J.L. Sarmiento, 2017. Oxygen in the Southern Ocean from Argo floats: determination of processes driving air-sea fluxes. J. Geophys. Res.- Oceans. 122(11): 8661-8682. doi: 10.1002/2017JC012923.
Chou, C., L.O. Hedin, and S.W. Pacala, 2017. Functional groups, species, and light interact with nutrient limitation during tropical rainforest sapling bottleneck. J. Ecology. 106(1): 157-167. doi: 10.1111/1365-2745.12823.
Gadikota, G., B. Dazas, G. Rother, M.C. Cheshire, and I.C. Bourg, 2017. Hydrophobic solvation of gases (CO2, CH4, H2, noble gases) in clay interlayer nanopores. J. Phys. Chem. C. 121(47): 26539-26550. doi: 10.1021/acs.jpcc.7b09768.
Hèbert-Dufresne, L., A.F.A. Pellegrini, U. Bhat, S. Redner, S.W. Pacala, and A. Berdahl, 2018. Edge fires drive the shape and stability of tropical forests. Ecology Letters. doi: 10.1111/ele.12942.
Henson, S., C. Beaulieu, T. Ilyina, J.G. John, M. Long. R. Seferian, J. Tjiputra, J.L. Sarmiento, 2017. Rapid emergence of climate change in environmental drivers of marine ecosystems. Nat. Commun. 8:14862. doi: 10.1038/ncomms14682.
Hong, H., R. Shen, F. Zhang, Z. Wen, S. Chang, W.F. Lin, S.A. Kranz, Y.W. Luo, S.J. Kao, F.M.M. Morel, and D. Shi. The complex effects of ocean acidification on the dominant N2-fixing cyanobacterium Trichodesmium. Science. 356(6337): 527-530. doi: 10.1126/science.aal2981.
Johnson, K.S., J.N. Plant, L.J. Coletti, H.W. Jannasch, C.M. Sakamoto, S.C. Riser, D.D. Swift, N.L. Williams, E.Boss, N. Haëntjens, L.D. Talley, and J.L. Sarmiento, 2017. Biogeochemical sensor performance in the SOCCOM profiling float array. J. Geophys. Res.- Oceans. 122(8): 6416-6436. doi: 10. 1002/2017JC012838.
Johnson, K.S., J.N. Plant, J.P. Dunne, L.D. Talley, and J.L. Sarmiento, 2017. Annual nitrate drawdown observed by SOCCOM profiling floats and the relationship to annual net community production. J. Geophys. Res.- Oceans. 122(8): 6668-6683. doi: 10.1002/2017JC012839.
Menge, D., S. Batterman, L. Hedin, W. Liao, S.W. Pacala, and B. Taylor, 2017. Why are nitrogen-fixing trees rare at higher compared to lower latitudes? Ecology. 98(12): 3127-3140. doi: 10.1002/ecy.2034.
Ocko, I.B., S.P. Hamburg, D.J. Jacob, D.W. Keith, N.O. Keohane, M. Oppenheimer, J.D. Roy-Mayhew, D.P. Schrag, and S.W. Pacala, 2017. Unmask temporal trade-offs in climate policy debates. Science, 356(6337): 492-493. doi: 10.1126/science.aaj2350.
Pellegrini, A.F.A., W.R.L. Anderegg, C.E.T. Paine, W.A. Hoffmann, T. Kartzinel, S. Rabin, D. Sheil, A.C. Franco, and S.W. Pacala, 2017. Convergence of bark investment according to fire and climate structures ecosystem vulnerability to future change. Ecology Letters. 20: 307–316. doi: 10.1111/ele.12725.
Rabin, S., S. Malyshev, B. Magi, E. Shevliakova, and S.W. Pacala, 2018. A fire model with distinct crop, pasture, and non-agricultural burning: Use of new data and a model-fitting algorithm for FINALv1. Geoscientific Model Development. 11: 815-842. doi: 10.5194/gmd-11-815-2018.
Rallabandi, B., Z. Zheng, M. Winton, and H.A. Stone, 2017. Formation of ice bridges in narrow straits as a response to wind and water stresses. J. Geophys. Res.- Oceans. 122(7): 5588-5610. doi: 10.1002/2017JC012822.
Rallabandi, B., Z. Zheng, M. Winton, and H.A. Stone, 2017. Wind-driven formation of ice bridges in straits. Phys. Rev. Lett. 118 (12): 128701. doi: 10.1103/PhysRevLett.118.128701.
Tamsitt, V., H.F. Drake, A.K. Morrison, L.D. Talley, C.O. Dufour, A.R. Gray, S. M. Griffies, M.R. Mazloff, J.L. Sarmiento, J. Wang, and W. Weijer, 2017. Spiraling pathways of global deep waters to the surface of the Southern Ocean. Nat. Commun. 8(1): 172. doi: 10.1038/s41467-017-00197-0.
Toyama, K., K.B. Rodgers, B. Blanke, D. Iudicone, M. Ishii, O. Aumont, and J.L. Sarmiento, 2017. Large re-emergence of anthropogenic carbon into the ocean’s surface mixed layer sustained by the ocean’s overturning circulation. J. Clim. 30: 8615-8631. doi: 10.1175/JCL1-D-16-0725.1.
Trugman, A.T., D. Medvigy, W.R.L. Anderegg, and S.W. Pacala, 2017. Differential declines in Alaskan boreal forest vitality related to climate and competition. Glob. Change Biol. 24: 1097-1107. doi: 10.1111/gcb.13952.
Uyehara, I.K.U. and S.W. Pacala, 2018. The role of succession in the evolution of flammability. Theor. Ecol. 1-13. doi: 10.1007/s12080-018-0366-3.
Willemsen, J. and I.C. Bourg, 2017. Molecular dynamics simulation and experimental study of the adsorption of dimethyl phthalate on clay surfaces. Paper presented at the Goldschmidt Conference, Paris, August 2017.
Williams, N.L., L.W. Juranek, R.A Feely, K.S. Johnson, J.L. Sarmiento, L.D. Talley, A.G. Dickson, A.R. Gray, R. Wanninkhof, J.L. Russell, and S.C. Riser, 2017. Calculating surface ocean pCO2 from biogeochemical Argo floats equipped with pH: an uncertainty analysis. Global Biogeochem. Cycles. 31(3): 591-604. doi: 10.1002/2016GB005541.
Xu, X., D. Medvigy, S.J. Wright, K. Kitajima, J. Wu, L. Albert, G. Martins, S. Saleska, and S.W. Pacala, 2017. Variations of leaf longevity in tropical moist forests predicted by a trait-driven carbon optimality model. Ecology Letters. 20: 1097-1106. doi: 10.1111/ele.12804.