Explore the vast range of titles available.

Biomass burning is a significant source of aerosol emissions in some regions and has a considerable impact on regional climate. Earth system model simulations indicate that increased biomass burning aerosol emissions contributed to statistically significant decreases in tropical precipitation over the 20th century. In this study, we use the Community Earth System Model version 1 Large Ensemble (CESM1-LENS) experiment to evaluate the mechanisms by which biomass burning aerosol contributed to decreased tropical precipitation, with a focus on South America and Southeast Asia. We analyze the all-but-one forcing simulations in which biomass burning aerosol emissions are held constant while other forcings (e.g., greenhouse gas concentrations) vary throughout the 20th century. This allows us to isolate the influence of biomass burning aerosol on processes that contribute to decreasing precipitation, including cloud microphysics, the radiative effects of absorbing aerosol particles, and alterations in regional circulation. We also show that the 20th century reduction in precipitation identified in the CESM1-LENS historical and biomass burning experiments is consistent across Coupled Model Intercomparison Project Phase 5 models with interactive aerosol schemes and the CESM2 single-forcing experiment. Our results demonstrate that higher concentrations of biomass burning aerosol increases the quantity of cloud condensation nuclei and cloud droplets, limiting cloud droplet size and precipitation formation. Additionally, absorbing aerosols (e.g., black carbon) contribute to a warmer cloud layer, which promotes cloud evaporation, increases atmospheric stability, and alters regional circulation patterns. Corresponding convectively coupled circulation responses, particularly over the tropical Andes, contribute to further reducing the flow of moisture and moisture convergence over tropical land. These results elucidate the processes that affect the water cycle in regions prone to biomass burning and inform our understanding of how future changes in aerosol emissions may impact tropical precipitation over the 21st century.

The global monsoon is characterised by transitions between pronounced dry and wet seasons, affecting food security for two-thirds of the world’s population. Rising atmospheric CO 2 influences the terrestrial hydrological cycle through climate-radiative and vegetation-physiological forcings. How these two forcings affect the seasonal intensity and characteristics of monsoonal precipitation and runoff is poorly understood. Here we use four Earth System Models to show that in a CO 2 -enriched climate, radiative forcing changes drive annual precipitation increases for most monsoon regions. Further, vegetation feedbacks substantially affect annual precipitation in North and South America and Australia monsoon regions. In the dry season, runoff increases over most monsoon regions, due to stomatal closure-driven evapotranspiration reductions and associated atmospheric circulation change. Our results imply that flood risks may amplify in the wet season. However, the lengthening of the monsoon rainfall season and reduced evapotranspiration will shorten the water resources scarcity period for most monsoon regions. Monsoon systems have strong impacts on precipitation and food security over large areas of the world. Here, the authors show that plant responses to rising CO 2 concentrations in the atmosphere play a key role in modulating seasonal rainfall and water resources over global land monsoon regions.

The exceptional atmospheric conditions that have accelerated Greenland Ice Sheet mass loss in recent decades have been repeatedly recognized as a possible dynamical response to Arctic amplification. Here, we present evidence of two potentially synergistic mechanisms linking high-latitude warming to the observed increase in Greenland blocking. Consistent with a prominent hypothesis associating Arctic amplification and persistent weather extremes, we show that the summer atmospheric circulation over the North Atlantic has become wavier and link this wavier flow to more prevalent Greenland blocking. While a concomitant decline in terrestrial snow cover has likely contributed to this mechanism by further amplifying warming at high latitudes, we also show that there is a direct stationary Rossby wave response to low spring North American snow cover that enforces an anomalous anticyclone over Greenland, thus helping to anchor the ridge over Greenland in this wavier atmospheric state.

Rising atmospheric CO2 concentrations enhance greenhouse warming and drive changes to plant physiology, leading to innumerable climate impacts. This study explores the impacts of plant responses on hydrological cycling at 2x preindustrial CO2 concentrations by analyzing simulations that isolate plant physiological effects using the Community Earth System Model versions 1 and 2. We find that leaf area growth increases canopy evaporation, which offsets transpiration declines, and dampens changes in global mean evapotranspiration, precipitation, and runoff in a CESM2 experiment with dynamic leaf area. These leaf area impacts are also evident in the differences between CESM1 and CESM2, with CESM2 better capturing observed leaf area magnitudes but potentially overestimating leaf area‐CO2 sensitivity, highlighting the importance of plant CO2 physiology on hydrological cycle changes and the need to improve its representation in climate models. Plain Language Summary Atmospheric CO2 concentrations are expected to continue rising through the 21st century due to fossil fuel emissions and impacting many parts of Earth's climate, including the water cycle. These impacts are largely associated with the enhanced greenhouse effect, but recent work highlights that plant responses can also influence the climate. By analyzing several climate model simulations, we investigate the role of leaf area responses to elevated CO2 concentrations. We find that leaf area growth leads to greater canopy evaporation (i.e., water that collects and evaporates from the surface of leaves). This offsets transpiration declines (i.e., leaf stomata—tiny pores in leaves that control gas exchange—do not open as widely at high CO2 concentrations) and leads to smaller changes in global mean evapotranspiration, precipitation, and runoff compared to simulations with smaller leaf area changes. When compared to leaf area derived from satellite observations, the later version of a climate model more closely captures the observed leaf area values but potentially overestimates leaf area responses to CO2 changes. Our findings highlight the importance of plant responses on water cycle changes and the need to improve their representation in climate models. Key Points Leaf area growth influences surface water fluxes via canopy evaporation, which can offset transpiration declines due to rising CO2 Impacts of leaf area growth on the water cycle reduce plant CO2 physiology driven changes in precipitation, total evaporation, and runoff Leaf area impacts in a controlled leaf area experiment are also evident in Community Earth System Model v1 versus v2 differences, with stronger CO2 sensitivity in v2

We investigate 21st-century hydroclimate changes over the United States (US) during winter and the sources of projection uncertainty under three emission scenarios (SSP2–4.5, SSP3–7.0, and SSP5–8.5) using CMIP6 models. Our study reveals a robust intensification of winter precipitation across the US, except in the Southern Great Plains, where changes are very small. By the end of the 21st century, winter precipitation is projected to increase by about 2–5% K −1 over most of the US. The frequency of very wet winters is also expected to increase, with 6–7 out of 30 winters exceeding the very wet threshold under the different scenarios. Our results suggest that the enhancement of future winter precipitation is modulated largely by coupled dynamic and thermodynamic responses, though partly offset by thermodynamic responses. Overall, our results highlight a high likelihood of increasing impacts from winter precipitation due to climate change.

Natural modes of variability on many timescales influence aerosol particle distributions and cloud properties such that isolating statistically significant differences in cloud radiative forcing due to anthropogenic aerosol perturbations (indirect effects) typically requires integrating over long simulations. For state‐of‐the‐art global climate models (GCM), especially those in which embedded cloud‐resolving models replace conventional statistical parameterizations (i.e., multiscale modeling framework, MMF), the required long integrations can be prohibitively expensive. Here an alternative approach is explored, which implements Newtonian relaxation (nudging) to constrain simulations with both pre‐industrial and present‐day aerosol emissions toward identical meteorological conditions, thus reducing differences in natural variability and dampening feedback responses in order to isolate radiative forcing. Ten‐year GCM simulations with nudging provide a more stable estimate of the global‐annual mean net aerosol indirect radiative forcing than do conventional free‐running simulations. The estimates have mean values and 95% confidence intervals of −1.19 ± 0.02 W/m2 and −1.37 ± 0.13 W/m2for nudged and free‐running simulations, respectively. Nudging also substantially increases the fraction of the world's area in which a statistically significant aerosol indirect effect can be detected (66% and 28% of the Earth's surface for nudged and free‐running simulations, respectively). One‐year MMF simulations with and without nudging provide global‐annual mean net aerosol indirect radiative forcing estimates of −0.81 W/m2 and −0.82 W/m2, respectively. These results compare well with previous estimates from three‐year free‐running MMF simulations (−0.83 W/m2), which showed the aerosol‐cloud relationship to be in better agreement with observations and high‐resolution models than in the results obtained with conventional cloud parameterizations. Key Points Nudged simulations provide more stable estimates of aerosol indirect effects Nudging increases the area a statistically significant signal can be detected Nudging enables computation‐expensive GCMs to estimate aerosol indirect effects

In this study, we compare the Energy Exascale Earth Systems Model (E3SM) multiscale modeling framework (MMF) with the cloud resolving model (CRM) configured in two (2dMMF) and three (3dMMF) dimensions. We explore how CRM dimensionality impacts the representation of mean and extreme precipitation characteristics. Our results show that tropical mean precipitation patterns are better represented in 3dMMF compared to observations (Integrated Multi‐satellitE Retrivals for GPM and Global Precipitation Climatology Project One Degree Daily products), while 2dMMF better captures extreme precipitation intensity, with systematic land‐ocean differences in precipitation and cloud‐associated variables. These differences are attributed to the co‐occurrence of CRM throttling (i.e., suppressed convection in due to smaller numbers of CRM columns and domain size) and dilution (i.e., 3‐D cloud circulations with increased entrainment and lower precipitation efficiency) effects. Overall, throttling results in more low‐level humidity in 2dMMF and dilution contributes to more high clouds with less precipitation efficiency in 3dMMF. Since throttling occurs more strongly over the ocean than land, the 3dMMF tends to have less cloud liquid and precipitation over the ocean and more cloud ice and precipitation over land. These results may serve as a guide for choosing the CRM structure to reduce precipitation and cloud‐related biases. Plain Language Summary Global cloud‐resolving models (CRMs) are compulationally prohibitive for climate length simulations, but an alternate approach that embeds independent kilometer‐scale CRMs in each column of a low‐resolution (∼100 km) global grid can permit convection with lower computational expanse. Such an approach allows cloud‐scale motions to be represented in multi‐year global climate simulations, though at the expense of a disconnection between the global model and CRM grids. In this study we compare two different ways of configuring the embedded CRMs: two‐dimensions (2‐D) aligned in north‐south direction versus three‐dimensions (3‐D) including both north‐south and west‐east directions. The results demonstrate a strong land‐ocean contrast in precipitation, cloud properties, and radiation in the difference between the 2‐D and 3‐D CRM simulations. And the differences are generated by the co‐occurrence of a throttling effect associated with a smaller number of CRM columns in 2‐D, which constrains deep convection, and a dilution effect associated with 3‐D cloud circulations, which enhances mixing and reduces precipitation efficiency. While the dilution effect impacts most of the tropics, the throttling effect is more influential over the ocean. This information can be used to inform the best configuration of the CRM approach for simulating precipitation and related processes in a global climate model. Key Points E3SM MMF with a 3‐D CRM reduces mean precipitation pattern biases relative to IMERG, but weakens overall intensity compared to a 2‐D CRM Weaker throttling with dilution effects in 3dMMF result in less low‐level humidity, more high clouds, and lower precipitation efficiency The impacts of dilution and throttling differ over land and ocean, which leads to an overall shift of precipitation toward land in 3dMMF

Biomass and fossil fuel burning impact air quality by injecting fine particulate matter (PM2.5) and its precursors into the atmosphere, which poses serious threats to human health. However, the surface concentration of PM2.5 depends not only on the magnitude of emissions, but also secondary production, transport, and removal. For example, in response to greenhouse gas driven warming, meteorological conditions that govern aerosol removal, primarily through rainfall and wet deposition, could shift in pattern, frequency, and intensity. This climate change driven process can impact air quality even without changes in aerosol emissions. In this experiment, we conduct new simulations by fixing aerosol emissions at present‐day levels in the Community Earth System Model Version 2, but increasing greenhouse gases through the 21st century. In our results, the changes in patterns and intensity of PM2.5 are found to be associated with precipitation (via aerosol removal), temperature (via secondary organic aerosol (SOA) formation), and moisture and clouds (via sulfate production). A decrease in wet day frequency (∼1.2% global mean) contributes to increases in the surface concentrations of black carbon, primary organic matter, and sulfate in many regions. This is offset in some regions by an upward vertical shift in the level where SOA forms, which contributes to higher column burden but lower surface concentration. These results highlight a need, using a variety of modeling tools, to continually reassess aerosol emissions regulations in response to anticipated climate changes. Key Points In 21st century simulations with fixed present‐day aerosol emissions, air quality worsens in many parts of South America, Africa and Asia Surface concentrations of sulfate, black carbon and primary organic matter increase globally, while secondary organic matter declines Greenhouse gas driven changes in rainfall frequency, temperature, and moisture availability impact the production and removal of aerosols

Stomata mediate fluxes of carbon and water between terrestrial plants and the atmosphere. These fluxes are governed by stomatal function and can be modulated in many Earth system models by an empirical parameter within the calculation of stomatal conductance, the stomatal slope g1M$\\left({g}_{1M}\\right)$ . Intuitively, g1M${g}_{1M}$represents the marginal water cost of carbon, relating it to the emergent plant property of water use efficiency. Observations show that g1M${g}_{1M}$can range widely across and within plant types in varying environments, and this distribution of g1M${g}_{1M}$is not captured within Earth system models which represent each plant type with a single g1M${g}_{1M}$value. Here we examine how g1M${g}_{1M}$influences photosynthesis using coupled Earth system model simulations by perturbing g1M${g}_{1M}$to observed 5th$5\\mathrm{t}\\mathrm{h}$and 95th$95\\mathrm{t}\\mathrm{h}$percentiles for each plant type. We find that high g1M${g}_{1M}$reduces photosynthesis nearly everywhere, while low g1M${g}_{1M}$has regionally dependent responses. Under fixed atmospheric conditions, low g1M${g}_{1M}$increases photosynthesis in the Amazon and central North America but decreases photosynthesis in boreal Canada. These responses reverse when the atmosphere responds interactively due to spatially differing sensitivity to increases in temperature and vapor pressure deficit. Choice of g1M${g}_{1M}$also influences photosynthetic response to changes in atmospheric carbon dioxide (CO2${\\text{CO}}_{2}$ ), with lower and higher g1M${g}_{1M}$modifying total global response to elevated 2x preindustrial CO2${\\text{CO}}_{2}$by +6.4% and −9.6%, respectively. Our work demonstrates that atmospheric feedbacks are critical for determining the photosynthetic response to g1M${g}_{1M}$assumptions and some regions are particularly sensitive to choice of g1M${g}_{1M}$ . Plain Language Summary Plants affect the Earth system's carbon, water, and energy fluxes through photosynthesis and transpiration, regulated by stomata that control gas exchange. Stomatal function controls the water cost per carbon gain for photosynthesis, where lower water cost means less water lost per carbon gain and higher water cost means more water lost. Observations show a range of stomatal function across and within plant types in varying environments which are not captured in Earth system models. In our study, we explored how changes in stomatal function impact photosynthesis using an Earth system model. We find higher water cost generally decreases photosynthesis everywhere while lower water cost has mixed effects, increasing photosynthesis in the Amazon and central North America but decreasing it in boreal Canada. These responses change when we allow the atmosphere to respond to changes on land, mainly due to spatially varying sensitivity to warmer temperature and drier air. Additionally, changes in stomatal function alter photosynthetic response to higher atmospheric carbon dioxide concentrations, with lower and higher water cost changing global photosynthesis by +6.4% and −9.6%, respectively. Our study demonstrates that accounting for atmospheric responses to land changes is critical for understanding the sensitivity of photosynthesis to stomatal function. Key Points Atmospheric feedbacks reverse the direction of photosynthesis sensitivity to stomatal function in the tropics and high latitudes Stomatal function with higher water cost per carbon gain leads to substantially reduced photosynthesis, especially in the tropics The inclusion of atmospheric feedbacks is critical for evaluating stomatal function in land surface models

River flow statistics are expected to change as a result of increasing atmospheric CO2 but uncertainty in Earth system model projections is high. While this is partly driven by changing precipitation, with well-known Earth system model uncertainties, here we show that the influence of plant stomatal conductance feedbacks can cause equally large changes in regional flood extremes and even act as the main control on future low latitude streamflow. Over most tropical land masses, modern climate predictions suggest that plant physiological effects will boost streamflow, overwhelming opposing effects of soil drying driven by the effects of CO2 on atmospheric radiation, warming and rainfall redistribution. The relatively unknown uncertainties in representing eco-physiological processes must therefore be better constrained in land-surface models. To this end, we identify a distinct plant physiological fingerprint on annual peak, low and mean discharge throughout the tropics and identify river basins where physiological responses dominate radiative responses to rising CO2 in modern climate projections.