Climate and Ocean Circulation in the Aftermath of a Marinoan Snowball Earth

When a snowball Earth deglaciates through a very high atmospheric CO2 concentration, the resulting inflow of freshwater leads to a stably stratified ocean, and the strong greenhouse conditions drive the climate into a very warm state. Here, we use a coupled atmosphere-ocean general circulation model, applying different scenarios for the evolution of atmospheric CO2, to conduct the first simulation of the climate and the three-dimensional ocean circulation in the aftermath of the Marinoan snowball Earth. The simulations show that the strong freshwater stratification breaks up on a timescale in the order of 103 years, mostly independent of the applied CO2 scenario. This is driven by the upwelling of salty waters in high latitudes, mainly the northern hemisphere, where a strong circumpolar current dominates the circulation. In the warmest CO2 scenario, the simulated Marinoan supergreenhouse climate reaches a global mean surface temperature of about 30 °C under an atmospheric CO2 concentration of 15 × 103 parts per million by volume, which is a moderate temperature compared to previous estimates. Consequently, the thermal expansion of seawater causes a sea-level rise of only 8 m, with most of it occurring during the first 3000 years. Our results imply that the surface temperatures of that time were potentially not as threatening for early metazoa as previously assumed. Furthermore, the short destratification timescale found in this study implies a very rapid accumulation of Marinoan cap dolostones, given that they were deposited in a freshwater environment.

The 3-machines energy transition model: exploring the energy frontiers for restoring a habitable climate

Earth's atmospheric CO2 concentration is on the rise, currently exceeding 420ppm. This is far above the 180ppm to 280ppm range of the past one million years and the anticipated safe limit of 350ppm. Consequently, halting fossil carbon emissions is necessary but insufficient to navigate to a safe climate future - massive and permanent removal of CO2 is inevitable. Humanity needs to do both: transit from the current fossil to a solar energy supply system and clean-up excess CO2 emissions from the atmosphere. The required global-scale transformation is ultimately limited by the availability of energy, beyond political ambitions and economic considerations. In this paper, the 3-machines energy transition model, a global system dynamics model based on energy balances, is presented and used to explore the energy frontiers for stabilizing the Earth's climate. The model comprises a hypothetical fossil engine, a solar engine including energy storage, and a carbon scrubber. These machines interact with Earth's carbon cycle and satisfy humanity’s energy demand. In 25 simulation experiments, shaped by a set of parameters regarding e.g. energy demand, energy storage and progression of the machines, the dynamics of the transformation and the effect on cumulative CO2 emissions were analysed. The resulting pathways reveal that, theoretically, atmospheric CO2 concentration can be reduced to 350ppm well before the end of this century while staying below 1.5°C with more than 50% probability. However, this requires the fastest possible energy transition, a massive and lasting carbon removal from atmosphere and hydrosphere, minimization of energy storage and a reduction of energy demand per capita.

Effects of Elevated Atmospheric CO2 Concentration on Phragmites australis and Wastewater Treatment Efficiency in Constructed Wetlands

Elevated atmospheric CO2 concentration (e[CO2]) has been predicted to rise to more than 400 ppm by the end of this century. It has received extensive attention with regard to the pros and cons of e[CO2] effects in terrestrial and marine ecosystems, while the effects of e[CO2] on wastewater treatment efficiency in constructed wetlands (CWs) are rarely known. In this study, the atmospheric CO2 concentration was set as 400 ppm (that is, ambient [CO2]) and 800 ppm (that is, e[CO2]). The physiological performance of Phragmites australis and microbial enzyme activities in constructed wetlands in response to e[CO2] were tested. Significantly higher net photosynthetic rate and plant growth were found under e[CO2]. The concentrations of nitrate, total anions, and total ions in the xylem sap of Phragmites australis were reduced, while the uptake of N and P in plants were not affected under e[CO2] condition. In addition, the ammonia monooxygenase activity was reduced, while the phosphatase activity was enhanced by e[CO2]. The increased removal efficiency of chemical oxygen demand and total nitrogen in CWs could be ascribed to the changes in physiological performance of Phragmites australis and activities of microbial enzymes under e[CO2]. These results suggested that the future atmospheric CO2 concentration could affect the wastewater treatment efficiency in CWs, due to the direct effects on plants and microorganisms.

Effects of Elevated Atmospheric CO2 Concentration and Water Regime on Rice Yield, Water Use Efficiency, and Arsenic and Cadmium Accumulation in Grain

(1) Background: Elevated atmospheric CO2 concentration affects the growth and development of the rice crop. In Southern Brazil, rice is traditionally produced with continuous irrigation, implying a significant amount of water used. Besides, continuous flooding favors the uptake of toxic elements such as arsenic (As) and cadmium (Cd). In this work, one Brazilian rice cultivar (IRGA 424) was tested for the effects of elevated CO2 concentration and different water regimes on rice yield, and As and Cd accumulation in grain. (2) Methods: Rice was grown in two CO2 concentrations (400 and 700 µmol mol−1) and two irrigation regimes (continuous and intermittent). It was evaluated the number of tillers, plant height, aboveground dry weight (ADW), water use efficiency (WUE), rice yield components, and As and Cd concentration in rice grain. (3) Results: Rice plants were taller and had a higher WUE when cultivated at e[CO2]. The ADW and the rice yield component were not affected by CO2 levels nor water regimes. Intermittent flood regimes had a lower average As concentration. The Cd concentration in the samples in both growing seasons and all treatments was below the limit of quantitation (8.76 μg kg−1). (4) Conclusions: Enhanced CO2 concentration did not affect rice yield, increased the WUE, and reduced As concentration in grains. Regarding water management, the intermittent regime enhanced WUE and promoted a reduction in As concentration in grains.