In most coupled climate models, effective climate sensitivity increases for a few decades following an abrupt CO2 increase. The change in the climate feedback parameter between the first 20 years and the subsequent 130 years is highly model dependent. In this study, we suggest that the intermodel spread of changes in climate feedback can be partially traced to the evolution of the Atlantic Meridional Overturning Circulation. Models with stronger Atlantic Meridional Overturning Circulation recovery tend to project more amplified warming in the Northern Hemisphere a few decades after a quadrupling of CO2. Tropospheric stability then decreases as the Northern Hemisphere gets warmer, which leads to an increase in both the lapse‐rate and shortwave cloud feedbacks. Our results suggest that constraining future ocean circulation changes will be necessary for accurate climate sensitivity projections.
Mud volcanoes provide an accessible channel through which deep subsurface environments can be observed. The manner in which deeply sourced materials shape biogeochemical processes and microbial communities in such geological features remains largely unknown. This study characterized redox transitions, biogeochemical fluxes and microbial communities for samples collected from a methane-rich mud volcano in southwestern Taiwan. Our results indicated that oxygen penetration was confined within the upper 4 mm of fluids/muds and counteracted by the oxidation of pyrite, dissolved sulfide, methane and organic matter at various degrees. Beneath the oxic zone, anaerobic sulfur oxidation, sulfate reduction, anaerobic methanotrophy and methanogenesis were compartmentalized into different depths in the pool periphery, forming a metabolic network that efficiently cycles methane and sulfur. Community members affiliated with various Proteobacteria capable of aerobic oxidation of sulfur, methane and methyl compounds were more abundant in the anoxic zone with diminished sulfate and high methane. These findings suggest either the requirement of alternative electron acceptors or a persistent population that once flourished in the oxic zone. Overall, this study demonstrates the distribution pattern for a suite of oxidative and reductive metabolic reactions along a steep redox gradient imposed by deep fluids in a mud volcano ecosystem.
While most models agree that the Atlantic Meridional Overturning Circulation (AMOC) becomes weaker under greenhouse gas emission and is likely to weaken over the 21st century, they disagree on the projected magnitudes of AMOC weakening. In this work, CMIP6 models with stronger climatological AMOC are shown to project stronger AMOC weakening in both 1 percent ramping CO2 and abrupt CO2 quadrupling simulations. A physical interpretation of this result is developed. For models with stronger mean state AMOC, stratification in the upper Labrador Sea is weaker, allowing for stronger mixing of the surface buoyancy flux. In response to CO2 increase, surface warming is mixed to the deeper Labrador Sea in models with stronger upper ocean mixing. This subsurface warming and corresponding density decrease drives AMOC weakening through advection from the Labrador Sea to the subtropics via the Deep Western Boundary Current. Time series analysis shows that most CMIP6 models agree that the decrease in subsurface Labrador Sea density leads AMOC weakening in the subtropics by several years. Also, idealized experiments conducted in an ocean-only model show that the subsurface warming over 500-1500 m in the Labrador Sea leads to stronger AMOC weakening several years later, while the warming that is too shallow (<500 m) or too deep (>1500 m) in Labrador Sea causes little AMOC weakening. These results suggest that a better representation of mean state AMOC is necessary for narrowing the inter-model uncertainty of AMOC weakening to greenhouse gas emission and its corresponding impacts on future warming projections.
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