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1. Multi‐Century Changes in the Ocean Carbon Cycle Controlled by the Tropical Oceans and the Southern Ocean

   https://doi.org/10.1029/2021GB007090  

   Chikamoto, Megumi O.; DiNezio, Pedro (December 2021, Global Biogeochemical Cycles)

   The oceanic absorption of anthropogenic carbon dioxide (CO2) is expected to continue in the following centuries, but the processes driving these changes remain uncertain. We studied these processes in a simulation of future changes in global climate and the carbon cycle following the RCP8.5 high emission scenario. The simulation shows increasing oceanic uptake of anthropogenic CO2 peaking towards the year 2080 and then slowing down but remaining significant in the period up to the year 2300. These multi‐century changes in uptake are dominated by changes in sea‐air CO2 fluxes in the tropical and southern oceans. In the tropics, reductions in upwelling and vertical gradients of dissolved carbon will reduce the vertical advection of carbon‐rich thermocline waters, suppressing natural outgassing of CO2. In the Southern Ocean, the upwelling of waters with relatively low dissolved carbon keeps the surface carbon relatively low, enhancing the uptake of CO2 in the next centuries. The slowdown in CO2 uptake in the subsequent centuries is caused by the decrease in CO2 solubility and storage capacity in the ocean due to ocean warming and changes in carbon chemistry. A collapse of the Atlantic Meridional Overturning Circulation (AMOC) predicted for the next century causes a substantial reduction in the uptake of anthropogenic CO2. In sum, predicting multi‐century changes in the global carbon cycle depends on future changes in carbon chemistry along with changes in oceanic and atmospheric circulations in the Southern and tropical oceans, together with a potential collapse of the AMOC. 

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2. Do Southern Ocean cloud feedbacks matter for 21st century warming?

   https://doi.org/DOI:10.1002/2017GL076339  

   Frey, W. R. (December 2017, Geophysical research letters)

   Cloud phase improvements in a state‐of‐the‐art climate model produce a large 1.5 K increase in equilibrium climate sensitivity (ECS, the surface warming in response to instantaneously doubled CO2) via extratropical shortwave cloud feedbacks. Here we show that the same model improvements produce only a small surface warming increase in a realistic 21st century emissions scenario. The small 21st century warming increase is attributed to extratropical ocean heat uptake. Southern Ocean mean‐state circulation takes up heat while a slowdown in North Atlantic circulation acts as a feedback to slow surface warming. Persistent heat uptake by extratropical oceans implies that extratropical cloud biases may not be as important to 21st century warming as biases in other regions. Observational constraints on cloud phase and shortwave radiation that produce a large ECS increase do not imply large changes in 21st century warming. 

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3. Amplifying feedback loop between drought, soil desiccation cracking, and greenhouse gas emissions

   https://doi.org/10.1088/1748-9326/ad2c23  

   Vahedifard, Farshid; Goodman, C Clay; Paul, Varun; AghaKouchak, Amir (March 2024, Environmental Research Letters)

   The continuous escalation of carbon dioxide (CO2) emissions into the atmosphere is recognized as the primary catalyst for anthropogenic climate change. In 2021, CO2 emerged as the predominant contributor to the warming effect of all human-made greenhouse gases (GHGs), accounting for two-thirds of their global heating impact. While the primary anthropogenic source of increased atmospheric CO2 concentration is the combustion of fossil fuels, the largest terrestrial source of CO2 emissions is soil where 80% of the total terrestrial carbon is stored. Approximately 62% of soil carbon is in organic form and readily released as CO2, while the remaining is made up of inorganic carbon (SIC). Here, we postulate that there is an amplifying feedback loop between drought, soil desiccation cracking, and CO2 emission in a warming climate – a critical aspect that has been overlooked in the existing literature. Further, we argue that the postulated feedback loop affects the emissions of other GHGs, such as methane (CH4) and nitrous oxide (N2O), from soils. The urgent need to recognize and characterize this exacerbating feedback loop is twofold. Firstly, it is widely acknowledged that drought accelerates the oxidation of soil organic carbon (SOC) and, thus, increases CO2 emissions into the atmosphere. Drought-induced soil moisture deficits differentially affect plant processes; while photosynthesis rates may be reduced in plants, leading to decreased carbon uptake, respiration rates can vary. Initially, drought may cause a slight increase in respiration, despite a decline in photosynthesis, leading to increased carbon emissions from the soil. These effects can differ based on ecosystem types, highlighting the complex interplay between drought, photosynthesis, and respiration. Secondly, drought triggers soil desiccation cracking, substantially increasing the permeability of the soil and the interfacial exchange area between the atmosphere and the soil, which, in turn, can considerably increase CO2 efflux in soil by exposing deeper and older stores of soil carbon. Desiccation cracking threatens earthen infrastructure systems and the natural environment. The problems associated with desiccation cracks are becoming more prevalent as anthropogenic climate change exacerbates the severity and frequency of droughts, heatwaves, and drought-heavy precipitation cycles (4). As the warming trends continue, more (and possibly older) CO2 is released from the soil, which can further contribute to global warming. Thus, a chain of events happens in a cascading manner. Failure to consider the hypothesized feedback loop can result in significant inaccuracies when modeling and predicting GHG emissions from soil. It may also lead to underestimating the overall impact of climate change on critical aspects such as soil health, crop production, and the structural integrity of earthen infrastructure. 

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4. Riverine photosynthesis influences the carbon sequestration potential of enhanced rock weathering

   https://doi.org/10.3389/fclim.2025.1582786  

   Neumann, Rebecca B; Kukla, Tyler; Zhang, Shuang; Butman, David E (April 2025, Frontiers in Climate)

   As climate mitigation efforts lag, dependence on anthropogenic CO₂removal increases. Enhanced rock weathering (ERW) is a rapidly growing CO₂removal approach. In terrestrial ERW, crushed rocks are spread on land where they react with CO₂and water, forming dissolved inorganic carbon (DIC) and alkalinity. For long-term sequestration, these products must travel through rivers to oceans, where carbon remains stored for over 10,000 years. Carbon and alkalinity can be lost during river transport, reducing ERW efficacy. However, the ability of biological processes, such as aquatic photosynthesis, to affect the fate of DIC and alkalinity within rivers has been overlooked. Our analysis indicates that within a stream-order segment, aquatic photosynthesis uptakes 1%–30% of DIC delivered by flow for most locations. The effect of this uptake on ERW efficacy, however, depends on the cell-membrane transport mechanism and the fate of photosynthetic carbon. Different pathways can decrease just DIC, DIC and alkalinity, or just alkalinity, and the relative importance of each is unknown. Further, data show that expected river chemistry changes from ERW may stimulate photosynthesis, amplifying the importance of these biological processes. We argue that estimating ERW’s carbon sequestration potential requires consideration and better understanding of biological processes in rivers. 

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5. Reduced global fire activity due to human demography slows global warming by enhanced land carbon uptake

   https://doi.org/10.1073/pnas.2101186119  

   Wu, Chao; Sitch, Stephen; Huntingford, Chris; Mercado, Lina M.; Venevsky, Sergey; Lasslop, Gitta; Archibald, Sally; Staver, A. Carla (May 2022, Proceedings of the National Academy of Sciences)

   Fire is an important climate-driven disturbance in terrestrial ecosystems, also modulated by human ignitions or fire suppression. Changes in fire emissions can feed back on the global carbon cycle, but whether the trajectories of changing fire activity will exacerbate or attenuate climate change is poorly understood. Here, we quantify fire dynamics under historical and future climate and human demography using a coupled global climate–fire–carbon cycle model that emulates 34 individual Earth system models (ESMs). Results are compared with counterfactual worlds, one with a constant preindustrial fire regime and another without fire. Although uncertainty in projected fire effects is large and depends on ESM, socioeconomic trajectory, and emissions scenario, we find that changes in human demography tend to suppress global fire activity, keeping more carbon within terrestrial ecosystems and attenuating warming. Globally, changes in fire have acted to warm climate throughout most of the 20th century. However, recent and predicted future reductions in fire activity may reverse this, enhancing land carbon uptake and corresponding to offsetting ∼5 to 10 y of global CO 2 emissions at today’s levels. This potentially reduces warming by up to 0.11 °C by 2100. We show that climate–carbon cycle feedbacks, as caused by changing fire regimes, are most effective at slowing global warming under lower emission scenarios. Our study highlights that ignitions and active and passive fire suppression can be as important in driving future fire regimes as changes in climate, although with some risk of more extreme fires regionally and with implications for other ecosystem functions in fire-dependent ecosystems. 

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