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1. Quantifying Key Mechanisms That Contribute to the Deviation of the Tropical Warming Profile From a Moist Adiabat

   https://doi.org/10.1029/2020GL089136  

   Miyawaki, Osamu; Tan, Zhihong; Shaw, Tiffany A.; Jansen, Malte F. (October 2020, Geophysical Research Letters)

   Abstract Climate models project tropical warming is amplified aloft relative to the surface in response to increased CO₂. Here we show moist adiabatic adjustment overpredicts the multimodel mean 300 hPa temperature response by 16.6–25.3% across the CMIP5 model hierarchy. We show three mechanisms influence overprediction: climatological large‐scale circulation, direct effect of increased CO₂, and convective entrainment. Accounting for the presence of a climatological large‐scale circulation and the direct effect of CO₂reduces the CMIP5 multimodel mean overprediction by 0.7–7.2% and 2.8–3.9%, respectively, but does not eliminate it. To quantify the influence of entrainment, we vary the Tokioka parameter in aquaplanet simulations. When entrainment is decreased by decreasing the Tokioka parameter from 0.1 to 0, overprediction decreases by 9.6% and 10.4% with and without a large‐scale circulation, respectively. The sensitivity of overprediction to climatological entrainment rate in the aquaplanet mostly follows the predictions of zero‐buoyancy bulk‐plume and spectral‐plume models. 

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2. Changes to the Madden‐Julian Oscillation in Coupled and Uncoupled Aquaplanet Simulations With 4xCO ₂

   https://doi.org/10.1029/2020MS002179  

   Bui, Hien X.; Maloney, Eric D. (August 2020, Journal of Advances in Modeling Earth Systems)

   Abstract The impacts of rising carbon dioxide (CO₂) concentration and ocean feedbacks on the Madden‐Julian Oscillation (MJO) are investigated with the Community Atmospheric Model Version 5 (CAM5) in an idealized aquaplanet configuration. The climate response associated with quadrupled CO₂concentrations and sea surface temperature (SST) warming are examined in both the uncoupled CAM5 and a version coupled to a slab ocean model. Increasing CO₂concentrations while holding SST fixed produces only small impacts to MJO characteristics, while the SST change resulting from increased CO₂concentrations produces a significant increase in MJO precipitation anomaly amplitude but smaller increase in MJO circulation anomaly amplitude, consistent with previous studies. MJO propagation speed increases in both coupled simulations with quadrupling of CO₂and uncoupled simulations with the same climatological surface temperature warming imposed, although propagation speed is increased more with coupling. While climatological SST changes are identical between coupled and uncoupled runs, other aspects of the basic state such as zonal winds do not change identically. For example, climate warming produces stronger superrotation and weaker mean lower tropospheric easterlies in the coupled run, which contributes to greater increases in MJO eastward propagation speed with warming through its effect on moisture advection. The column process, representing the sum of vertical moist static energy (MSE) advection and radiative heating anomalies, also supports faster eastward propagation with warming in the coupled run. How differing basic states between coupled and uncoupled runs contribute to this behavior is discussed in more detail. 

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3. Quantitative critique of leaf‐based paleo‐CO ₂ proxies: Consequences for their reliability and applicability

   https://doi.org/10.1002/gj.3807  

   Konrad, Wilfried; Royer, Dana L.; Franks, Peter J.; Roth‐Nebelsick, Anita (March 2020, Geological Journal)

   A variety of proxies have been developed to reconstruct paleo‐CO₂from fossil leaves. These proxies rely on some combination of stomatal morphology, leafδ¹³C, and leaf gas exchange. A common conceptual framework for evaluating these proxies is lacking, which has hampered efforts for inter‐comparison. Here we develop such a framework, based on the underlying physics and biochemistry. From this conceptual framework, we find that the more extensively parameterised proxies, such as the optimisation model, are likely to be the most robust. The simpler proxies, such as the stomatal ratio model, tend to under‐predict CO₂, especially in warm (>15^°C) and moist (>50%humidity) environments. This identification of a structural under‐prediction may help to explain the common observation that the simpler proxies often produce estimates of paleo‐CO₂that are lower than those from the more complex proxies and other, non‐leaf‐based CO₂proxies. The use of extensively parameterised models is not always possible, depending on the preservation state of the fossils and the state of knowledge about the fossil's nearest living relative. With this caveat in mind, our analysis highlights the value of using the most complex leaf‐based model as possible. 

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4. Contribution and consequences of xylem‐transported CO ₂ assimilation for C ₃ plants

   https://doi.org/10.1111/nph.15907  

   Stutz, Samantha S.; Hanson, David T. (June 2019, New Phytologist)

   Summary Traditionally, leaves were thought to be supplied withCO₂for photosynthesis by the atmosphere and respiration. Recent studies, however, have shown that the xylem also transports a significant amount of inorganic carbon into leaves through the bulk flow of water. However, little is known about the dynamics and proportion of xylem‐transportedCO₂that is assimilated, vs simply lost to transpiration.Cut leaves ofPopulus deltoidesandBrassica napuswere placed in eitherKCl or one of three [NaH¹³CO₃] solutions dissolved in water to simultaneously measure the assimilation and the efflux of xylem‐transportedCO₂exiting the leaf across light andCO₂response curves in real‐time using a tunable diode laser absorption spectroscope.The rates of assimilation and efflux of xylem‐transportedCO₂increased with increasing xylem [¹³CO₂*] and transpiration. Under saturating irradiance, rates of assimilation using xylem‐transportedCO₂accounted forc.2.5% of the total assimilation in both species in the highest [¹³CO₂*].The majority of xylem‐transportedCO₂is assimilated, and efflux is small compared to respiration. Assimilation of xylem‐transportedCO₂comprises a small portion of total photosynthesis, but may be more important whenCO₂is limiting. 

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5. Using multirate rapid A / C _i curves as a tool to explore new questions in the photosynthetic physiology of plants

   https://doi.org/10.1111/nph.15657  

   Stinziano, Joseph R.; Adamson, Rachael K.; Hanson, David T. (January 2019, New Phytologist)

   Summary Steady‐state photosyntheticCO₂responses (A/C_icurves) are used to assess environmental responses of photosynthetic traits and to predict future vegetative carbon uptake through modeling. The recent development of rapidA/C_icurves (RACiRs) permits faster assessment of these traits by continuously changing [CO₂] around the leaf, and may reveal additional photosynthetic properties beyond what is practical or possible with steady‐state methods.Gas exchange necessarily incorporates photosynthesis and (photo)respiration. Each process was expected to respond on different timescales due to differences in metabolite compartmentation, biochemistry and diffusive pathways. We hypothesized that metabolic lags in photorespiration relative to photosynthesis/respiration andCO₂diffusional limitations can be detected by varying the rate of change in [CO₂] duringRACiR assays. We tested these hypotheses through modeling and experiments at ambient and 2% oxygen.Our data show that photorespiratory delays cause offsets in predictedCO₂compensation points that are dependent on the rate of change in [CO₂]. Diffusional limitations may reduce the rate of change in chloroplastic [CO₂], causing a reduction in apparentRACiR slopes under highCO₂ramp rates.MultirateRACiRs may prove useful in assessing diffusional limitations to gas exchange and photorespiratory rates. 

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