Comparison of seed and ovule development in representative taxa of the tribe Cercideae (Caesalpinioideae, Leguminosae)

A scanning electron microscopy survey of mature Cercideae seed anatomy, both internal and external, was combined with a developmental study of Cercideae ovules and immature seeds. Such a survey has not been previously undertaken although several features of Cercideae seed anatomy are cited as being unique among legumes; nor have there been any published studies of megagametogenesis or embryogenesis in Bauhinia or in Cercis canadensis;Daily collections of flowering and fruiting material of Cercis canadensis, collections of six Bauhinia taxa at several stages of flowering and fruiting, and mature seeds from 53 of about 200 Bauhinia species, one of two species of Adenolobus, one of four species of Griffonia, and five of six species of Cercis provided material for this survey which yielded new data on ovular configuration and development, differentiation of ovular vasculature and of macrosclereids, derivation of hilar structures and the radicular lobe or micropylar lens, and on epidermal testa patterns in Cercideae. For example, the deeply pseudocrassinucellate Cercideae ovule has both an epistase and a hypostase. The latter has not been previously reported in this tribe nor has the former been noted in the subtribe Bauhiniinae. Also, comparative anatomical studies of ovule and seed demonstrated that one of the structural features often cited as unique to Cercideae, the micropylar lens, is derived from an intercalary meristem located in the mesotesta contiguous to the radicle and may be equivalent in derivation and location, if not in function, to the papilionoid legume radicular lobe. Anatomical observations and a hypothetical model are presented in support of the proposition that the parenchymatous bridge of tissue extending through the exotesta, at the hilar groove, functions in conjunction with the exotesta as a mechanism for seed desiccation;Common testa epidermal patterns on mature Cercideae seeds are reticulate and foveolate in the subtribe Bauhiniinae and papillose in Cercidinae. These epidermal patterns, and hilar configuration, distinguish between the two Cercideae subtribes and are valuable taxonomic characters

Three flavors of radiative feedbacks and their implications for estimating equilibrium climate sensitivity

Abstract The realization that atmospheric radiative feedbacks depend on the underlying patterns of surface warming and global temperature, and thus, change over time has lead to a proliferation of feedback definitions and methods to estimate equilibrium climate sensitivity (ECS). We contrast three flavors of radiative feedbacks ? equilibrium, effective, and differential feedback ? and discuss their physical interpretations and applications. We show that their values at any given time can differ more than 1  and their implied equilibrium or effective climate sensitivity can differ several degrees. With ten (quasi) equilibrated climate models, we show that 400 years might be enough to estimate the true ECS within a 5% error using a simple regression method utilizing the differential feedback parameter. We argue that a community-wide agreement on the interpretation of the different feedback definitions would advance the quest to narrow the estimate of climate sensitivity

Snowfall-albedo feedbacks could have led to deglaciation of Snowball Earth starting from mid-latitudes

Simple and complex climate models suggest a hard snowball – a completely ice-covered planet – is one of the steady-states of Earth’s climate. However, a seemingly insurmountable challenge to the hard-snowball hypothesis lies in the difficulty in explaining how the planet could have exited the glaciated state within a realistic range of atmospheric carbon dioxide concentrations. Here, we use simulations with the Earth system model MPI-ESM to demonstrate that terminal deglaciation could have been triggered by high dust deposition fluxes. In these simulations, deglaciation is not initiated in the tropics, where a strong hydrological cycle constantly regenerates fresh snow at the surface, which limits the dust accumulation and snow aging, resulting in a high surface albedo. Instead, comparatively low precipitation rates in the mid-latitudes in combination with high maximum temperatures facilitate lower albedos and snow dynamics that – for extreme dust fluxes – trigger deglaciation even at present-day carbon dioxide level

Multi-annual ocean-atmosphere adjustments to radiative forcing

In radiative forcing and climate feedback frameworks, the initial stratospheric and tropospheric adjustments to a forcing agent can be treated as part of the forcing and not as a feedback, as long as the average global surface temperature response is negligible. Here, a very large initial condition ensemble of the Community Earth System Model is used to analyze how the ocean shapes the fast response to radiative forcing. It is shown that not only the stratosphere and troposphere but also the ocean adjusts. This oceanic adjustment includes meridional ocean heat transport convergence anomalies, which are locally as large as the surface heat flux anomalies, and an increase of the Atlantic meridional overturning circulation. These oceanic adjustments set the lower boundary condition for the atmospheric response of the first few years, in particular, the shortwave cloud radiative effect. This cloud adjustment causes a nonlinear relationship between global energy imbalance and temperature. It proceeds with a characteristic time scale of a few years in response to the forcing rather than scaling nonlinearly with global mean temperature anomaly. It is proposed that even very short time scales are treated as a fully coupled problem and encourage other modeling groups to investigate whether our description also suits their models’ behavior. A definition of the forcing term (“virtual forcing”) including oceanic adjustment processes is introduced and serves as an interpretive idea for longer time scales

All aboard! Earth system investigations with the CH2O-CHOO TRAIN v1.0

Models of the carbon cycle and climate on geologic (&gt;104-year) timescales have improved tremendously in the last 50 years due to parallel advances in our understanding of the Earth system and the increase in computing power to simulate its key processes. Still, balancing the Earth system's complexity with a model's computational expense is a primary challenge in model development. Simulations spanning hundreds of thousands of years or more generally require a reduction in the complexity of the climate system, omitting features such as radiative feedbacks, shifts in atmospheric circulation, and the expansion and decay of ice sheets, which can have profound effects on the long-term carbon cycle. Here, we present a model for climate and the long-term carbon cycle that captures many fundamental features of global climate while retaining the computational efficiency needed to simulate millions of years of time. The Carbon–H2O Coupled HydrOlOgical model with Terrestrial Runoff And INsolation, or CH2O-CHOO TRAIN, couples a one-dimensional (latitudinal) moist static energy balance model of climate with a model for rock weathering and the long-term carbon cycle. The CH2O-CHOO TRAIN is capable of running million-year-long simulations in about 30 min on a laptop PC. The key advantages of this framework are (1) it simulates fundamental climate forcings and feedbacks; (2) it accounts for geographic configuration; and (3) it is flexible, equipped to easily add features, change the strength of feedbacks, and prescribe conditions that are often hard-coded or emergent properties of more complex models, such as climate sensitivity and the strength of meridional heat transport. We show how climate variables governing temperature and the water cycle can impact long-term carbon cycling and climate, and we discuss how the magnitude and direction of this impact can depend on boundary conditions like continental geography. This paper outlines the model equations, presents a sensitivity analysis of the climate responses to varied climatic and carbon cycle perturbations, and discusses potential applications and next stops for the CH2O-CHOO TRAIN.</p

21st Century Scenario Forcing Increases More for CMIP6 Than CMIP5 Models

Although the Coupled Model Intercomparison Project 6 (CMIP6) protocol provides an experiment to estimate effective radiative forcing (ERF), it is only quantified for few models. We present new estimates of ERF for models participating in CMIP6 by applying the method developed in Fredriksen et al. (2021, https://doi.org/10.1029/2020JD034145), and validate our approach with available fixed-SST forcing estimates. We estimate ERF for experiments with abrupt changes of CO2, 1% increase of CO2, historical forcings, and future scenarios, and demonstrate that CMIP6 ERF is lower than CMIP5 ERF at the end of the historical period, but grows faster than CMIP5 in the future scenarios, ending up at higher levels than CMIP5 at the end of the 21st century. The simulated radiative efficiency of CO2 has not changed much, suggesting that the larger future increase in CO2 concentrations in CMIP6 compared to CMIP5 is important for explaining the forcing difference