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Societal Impact Statement The terrestrial carbon sink currently consumes about a third of the CO2 released to the atmosphere by fossil fuel emissions and land‐use change. In light of future carbon emissions, it is important to understand how that carbon sink will change in order to better determine how much CO2 remains in the atmosphere. This study uses a modeling approach to compare a middle‐of‐the‐road (SSP245) versus rocky road (SSP370) scenario for the conterminous United States. While both scenarios result in a carbon sink by the end of the century, it is larger in SSP245, due to less deforestation and more moderate climate. Summary This modeling study explores how elevated atmosphere CO2, climate warming, and land use and land cover change (LULCC) will affect the direction and magnitude of the terrestrial carbon sink during the 21st century for the conterminous United States. Initial conditions are averaged and condensed for each plant functional type (PFT) from a historical run from 1750 to 2014 with the Terrestrial Ecosystems Model (TEM). Future experiments consider the effect of climate change, LULCC, and the CO2 fertilization effect (CFE) from 2015 to 2099 using the SSP245 and SSP370 scenarios from the National Center for Atmospheric Research Community Earth System Model (NCAR CESM2) model. The resultant effect on accumulated net carbon exchange (NCE) is 12.2 PgC for SSP245 versus 2.4 PgC for SSP370. Separating out the effects of climate, LULCC, and CFE results in a negative effect from both climate (−1.6 vs. −14.6 PgC) and LULCC (−5.6 vs. −15.4 PgC) and a positive effect from CFE (8 vs. 17.1 PgC) for SSP245 versus SSP370. The effect on water dynamics is that the SSP370 climate results in more evapotranspiration (ET) with less soil moisture and runoff, while the LULCC effect of SSP370 results in lower ET with more soil moisture and runoff. Both scenarios produce lower ET with elevated CO2, and more soil moisture and runoff. The SSP245 scenario has more afforestation and less deforestation than the SSP370 scenario, providing more potential carbon offsets for the voluntary carbon market. This study highlights the carbon and water benefits of a more sustainable LULCC scenario. The terrestrial carbon sink currently consumes about a third of the CO2 released to the atmosphere by fossil fuel emissions and land‐use change. In light of future carbon emissions, it is important to understand how that carbon sink will change in order to better determine how much CO2 remains in the atmosphere. This study uses a modeling approach to compare a middle‐of‐the‐road (SSP245) versus rocky road (SSP370) scenario for the conterminous United States. While both scenarios result in a carbon sink by the end of the century, it is larger in SSP245, due to less deforestation and more moderate climate.

Climate change is expected to impact vegetation in the western United States, leading to shifts in dominant Plant Functional Types and carbon storage. Here, we used a biogeographic model integrated with a biogeochemical model to predict changes in dominant Plant Functional Type by 2070−2100. Results show that under the Representative Concentration Pathway 4.5 scenario, 40% of the originally forested areas will transition to shrubland (7%) or grassland (32%), while under the Representative Concentration Pathway 8.5 scenario, 58% of forested areas shift to shrubland (18%) or grassland (40%). These shifts in Plant Functional Types result in a net overall loss in carbon storage equal to −60 gigagram of carbon and −82 gigagram of carbon under Representative Concentration Pathway 4.5 and 8.5, respectively. Our findings highlight the need for urgent action to mitigate the effects of climate change on vegetation and carbon storage in the region.

A global biofuels program will lead to intense pressures on land supply and can increase greenhouse gas emissions from land-use changes. Using linked economic and terrestrial biogeochemistry models, we examined direct and indirect effects of possible land-use changes from an expanded global cellulosic bioenergy program on greenhouse gas emissions over the 21st century. Our model predicts that indirect land use will be responsible for substantially more carbon loss (up to twice as much) than direct land use; however, because of predicted increases in fertilizer use, nitrous oxide emissions will be more important than carbon losses themselves in terms of warming potential. A global greenhouse gas emissions policy that protects forests and encourages best practices for nitrogen fertilizer use can dramatically reduce emissions associated with biofuels production.

Earth’s atmospheric methane (CH4) concentration has risen more than 162% since pre-industrial levels in the mid-18th century, and about 30% of the rise in global temperatures since the pre-industrial era is due to CH4 The build-up of methane in the atmosphere in 2020–2022 was the largest since systematic measurements started in 1983, more than double the average yearly growth rate measured over the previous 17 years (15.2 ppb yr−1 vs. 5.71 ppb yr−1, respectively). During 2020, with a growth rate of 14.81 ppb yr−1, the level of atmospheric CH4 broke the previous record (which was set in 1991), and it was broken again immediately the following year, with an increase of 17.64 ppb yr−1 in 2021. For 2022, the final estimate is 13.25 ppb yr−1, the fourth largest annual growth rate. The most recent explanations for this surge in tropospheric CH4 include increased emissions from tropical wetlands, more floods, and increased temperatures. For 2020 and part of 2021, a reduction in the oxidative capacity of the atmosphere due to COVID-19 lockdowns was also proposed. Our main hypothesis is that this CH4 surge in 2020–2021 may also be caused by reduced sulfate emissions, which have been shown to decrease methanotrophy and increase methanogenesis rates in wetlands. Then, for the CH4 slowdown in 2022–2024, our hypotheses are that the emissions from wetlands remained high, but that there was an even higher increase in the oxidative capacity of the atmosphere due to multiple other parameters that are detailed in this article. This perspective review paper is mainly qualitative; it demonstrates that coupled climate–chemistry models will also need to integrate biochemistry, as the evolution of the atmospheric composition is multifactorial and non-linear.

The impact of carbon–nitrogen dynamics in terrestrial ecosystems on the interaction between the carbon cycle and climate is studied using an earth system model of intermediate complexity, the MIT Integrated Global Systems Model (IGSM). Numerical simulations were carried out with two versions of the IGSM’s Terrestrial Ecosystems Model, one with and one without carbon–nitrogen dynamics. Simulations show that consideration of carbon–nitrogen interactions not only limits the effect of CO₂ fertilization but also changes the sign of the feedback between the climate and terrestrial carbon cycle. In the absence of carbon–nitrogen interactions, surface warming significantly reduces carbon sequestration in both vegetation and soil by increasing respiration and decomposition (a positive feedback). If plant carbon uptake, however, is assumed to be nitrogen limited, an increase in decomposition leads to an increase in nitrogen availability stimulating plant growth. The resulting increase in carbon uptake by vegetation exceeds carbon loss from the soil, leading to enhanced carbon sequestration (a negative feedback). Under very strong surface warming, however, terrestrial ecosystems become a carbon source whether or not carbon–nitrogen interactions are considered. Overall, for small or moderate increases in surface temperatures, consideration of carbon–nitrogen interactions result in a larger increase in atmospheric CO₂ concentration in the simulations with prescribed carbon emissions. This suggests that models that ignore terrestrial carbon–nitrogen dynamics will underestimate reductions in carbon emissions required to achieve atmospheric CO₂ stabilization at a given level. At the same time, compensation between climate-related changes in the terrestrial and oceanic carbon uptakes significantly reduces uncertainty in projected CO₂ concentration.

An important goal of macrosystems ecology (MSE) research is to advance understanding of ecological systems at both fine and broad temporal and spatial scales. Our premise in this paper is that MSE projects require integrated information management at their inception. Such efforts will lead to improved communication and sharing of knowledge among diverse project participants, better science outcomes, and more transparent and accessible (ie \"open\") science. We encourage researchers to \"complete the data life cycle\" by publishing well-documented datasets, thereby facilitating re-use of the data to answer new and different questions from the ones conceived by those involved in the original projects. The practice of documenting and submitting datasets to data repositories that are publicly accessible ensures that research results and data are available to and use-able by other researchers, thus fostering open science. However, ecologists are often unfamiliar with the requirements and information management tools for effectively preserving data and receive little institutional or professional incentive to do so. Here, we provide recommendations for achieving these ends and give examples from current MSE projects to demonstrate why information management is critical for ensuring that scientific results can be reproduced and that data can be shared for future use.

Modeling the effects of the terrestrial carbon sink in the future depends upon not just current-day land use and land cover (LULC) but also the legacy of past LULC change (LULCC), which is often not considered. The age distribution of trees in the forest depends upon the history of past disturbances, while the nutrients in the soil depend upon past LULC. Thus, establishing the correct initial state of the vegetation and soil is crucial to model accurately the effect of biogeochemical cycling with environmental change in the future. This study models the effects of LULCC from 1750 to 2014 using the land-use harmonization dataset (LUH2) of land-use transitions with the terrestrial ecosystems model (TEM) for the conterminous US. Modeled LULC include plant functional types (PFTs) of potential vegetation, as well as managed cropland, pastureland, and urban areas. LULCC is treated using a cohort approach, in which a separate cohort occurs every year there is a land-use transition, thereby ensuring proper age structure of forests and regrowth with the correct soil nutrients. From 2000–2014 the modeled net ecosystem productivity (NEP) is 989 TgC yr−1 for the conterminous US but only −15 TgC yr−1 if accounting for carbon lost from land-use transitions and management. The hypothesis is that the initial state of the vegetation and soils significantly affects the future state of the terrestrial carbon sink. In this study, LULC remains constant in the future, with the NCAR CCSM4 RCP8.5 climate used to force the TEM-Hydro model. The following experiments are run from 2015 to 2100, including (a) restarting from existing cohorts in 2014 (RESTART), (b) reinitializing in 2015 based on condensing the cohorts for each PFT into a single cohort (CONDENSED), and (c) restarting from average cohort conditions for each PFT (AVERAGE). The NEP is too low when using condensed cohorts without reinitializing due to a larger increase in heterotrophic respiration (Rh) resulting from the assumption of mature forests. The carbon stocks are larger than using all the cohorts if condensed cohorts are reinitialized due to the assumption of mature, equilibrated forests. Where nitrogen-limited, forest regrowth is enhanced if regrowth starts from more nutrient-rich conditions. Water fluxes are dominated by environmental factors but can be slightly dependent upon the underlying carbon dynamics. It is therefore necessary to account for past disturbances when modeling future changes in carbon dynamics.

Observed intensification of precipitation extremes, responsible for extensive societal impacts, are widely attributed to anthropogenic sources, which may include indirect effects of agricultural irrigation. However quantifying the effects of irrigation on far-downstream climate remains a challenge. We use three paired Community Earth System Model simulations to assess mechanisms of irrigation-induced precipitation trends and extremes in the conterminous US and the effect on the terrestrial carbon sink. Results suggest precipitation enhancement in the central US reduced drought conditions and increased regional carbon uptake, while further downstream, the heaviest precipitation events were more frequent and intense. Specifically, moisture advection from irrigation in the western U.S. and recycling of enhanced local convective precipitation produced very-heavy storm events that were 11% more intense and occurred 23% more frequently in the densely populated greater New York City region.

Ground-level ozone is an air pollutant that is harmful to human health, as well as to plants, trees and crops. New analyses based on Earth system modelling show that reducing ozone from the energy, industrial and transportation sectors could mitigate climate change by enhancing the ability of vegetation to remove carbon dioxide from the atmosphere through photosynthesis.

Characterizing precipitation seasonality and variability in the face of future uncertainty is important for a well-informed climate change adaptation strategy. Using the Colwell index of predictability and monthly normalized precipitation data from the Coupled Model Intercomparison Project Phase 5 (CMIP5) multi-model ensembles, this study identifies spatial hotspots of changes in precipitation predictability in the United States under various climate scenarios. Over the historic period (1950–2005), the recurrent pattern of precipitation is highly predictable in the East and along the coastal Northwest, and is less so in the arid Southwest. Comparing the future (2040–2095) to the historic period, larger changes in precipitation predictability are observed under Representative Concentration Pathways (RCP) 8.5 than those under RCP 4.5. Finally, there are region-specific hotspots of future changes in precipitation predictability, and these hotspots often coincide with regions of little projected change in total precipitation, with exceptions along the wetter East and parts of the drier central West. Therefore, decision-makers are advised to not rely on future total precipitation as an indicator of water resources. Changes in precipitation predictability and the subsequent changes on seasonality and variability are equally, if not more, important factors to be included in future regional environmental assessment.