Nutrient enrichment typically causes local plant diversity declines. A common but untested expectation is that nutrient enrichment also reduces variation in nutrient conditions among localities and selects for a smaller pool of species, causing greater diversity declines at larger than local scales and thus biotic homogenization. Here we apply a framework that links changes in species richness across scales to changes in the numbers of spatially restricted and widespread species for a standardized nutrient addition experiment across 72 grasslands on six continents. Overall, we find proportionally similar species loss at local and larger scales, suggesting similar declines of spatially restricted and widespread species, and no biotic homogenization after 4 years and up to 14 years of treatment. These patterns of diversity changes are generally consistent across species groups. Thus, nutrient enrichment poses threats to plant diversity, including for widespread species that are often critical for ecosystem functions.

Partitioning the terrestrial carbon sink between vegetation and soil is crucial for predicting future climate change, but the role of soils remains poorly quantified. Here, we compiled 3,099 soil organic carbon time series spanning five decades. We found a global soil organic carbon sink of 1.83 ± 0.9 (mean ± SE) petagrams per year from 1992 to 2020, driven by extratropical young forests, boreal old forests, and grasslands, while trends in tropical ecosystems remain uncertain. Our findings suggest the net land sink resides almost exclusively belowground as soil carbon, emphasizing the global opportunity of soil conservation and restoration for climate mitigation. ### Competing Interest Statement The authors have declared no competing interest. This material is based upon work supported by the National Science Foundation under Grant No. DEB-2339051. This research was supported by a seed award from the MIT Climate and Sustainability Consortium. This is a contribution of the MIT Terrer Lab. AM was supported by a Laboratory Directed Research and Development Program at PNNL. This work was generated using data from the Nutrient Network (This material is based upon work supported by the National Science Foundation under Grant No. DEB-2339051. This research was supported by a seed award from the MIT Climate and Sustainability Consortium. This is a contribution of the MIT Terrer Lab. AM was supported by a Laboratory Directed Research and Development Program at PNNL. This work was generated using data from the Nutrient Network () experiment, funded at the site-scale by individual researchers. Coordination and data management have been supported by funding to E. Borer and E. Seabloom from the National Science Foundation Research Coordination Network (NSF-DEB-1042132) and Long Term Ecological Research (NSF-DEB-1234162 and NSF-DEB-1831944 to Cedar Creek LTER) programs, and the Institute on the Environment (DG-0001-13). We also thank the Minnesota Supercomputer Institute for hosting project data and the Institute on the Environment for hosting Network meetings. Soil analyses were supported, in part, by USDA-ARS grant 58-3098-7-007 to ETB. The evaluation was based on data that was collected by partners of the official UNECE ICP Forests Network (). Part of the data was co-financed by the European Commission (Data achieved at 10/12/2023)., ,

Litter decomposition is one of the largest carbon (C) fluxes in terrestrial ecosystems and links aboveground biomass to soil C pools. In grasslands, decomposition drivers have received substantial attention but the role of grassland herbivores in influencing decay rates is often ignored despite their potentially large effects on standing biomass and nutrient cycling. Recent work has demonstrated that nutrient addition increases early-stage decay and suppresses late-stage decay. Mammalian herbivores can mediate the effects of nutrient supply on biomass, suggesting herbivores may alter the effects of nutrients on decomposition, though this is largely unknown. We examined how herbivory mediates the effects of nutrient supply on long-term decomposition across 19 grassland sites of the Nutrient Network distributed experiment. At each site, a full-factorial experiment of combined nitrogen (N), phosphorus (P), and micronutrient (K) enrichment (‘control’ or ‘ + NPK’) and mammalian herbivore (> 50 g) exclusion (‘unfenced’ or ‘fenced’) was carried out in a randomized block design. We hypothesized that nutrient effects on litter decomposition would be strongest where herbivores caused the greatest reductions in aboveground plant biomass (i.e., at sites with more intense herbivory). After accounting for wide variation in decomposition rates across sites, we found that, within sites, elevated nutrients increased early-stage decay and suppressed late-stage decay. In contrast, neither herbivore exclusion (i.e., fencing) nor site level changes in aboveground biomass due to herbivory altered the nutrient effects on decomposition rates. Across grasslands, our results indicate that elevated nutrient supply modifies litter decomposition rates independent of herbivore impacts.

Forbs ("wildflowers") are important contributors to grassland biodiversity but are vulnerable to environmental changes. In a factorial experiment at 94 sites on 6 continents, we test the global generality of several broad predictions: (1) Forb cover and richness decline under nutrient enrichment, particularly nitrogen enrichment. (2) Forb cover and richness increase under herbivory by large mammals. (3) Forb richness and cover are less affected by nutrient enrichment and herbivory in more arid climates, because water limitation reduces the impacts of competition with grasses. (4) Forb families will respond differently to nutrient enrichment and mammalian herbivory due to differences in nutrient requirements. We find strong evidence for the first, partial support for the second, no support for the third, and support for the fourth prediction. Our results underscore that anthropogenic nitrogen addition is a major threat to grassland forbs, but grazing under high herbivore intensity can offset these nutrient effects.

Aims: The community composition of native and alien plant species is influenced by the environment (e.g., nutrient addition and changes in temperature or precipitation). A key objective of our study is to understand how differences in the traits of alien and native species vary across diverse environmental conditions. For example, the study examines how changes in nutrient availability affect community composition and functional traits, such as specific leaf area and plant height. Additionally, it seeks to assess the vulnerability of high-nutrient environments, such as grasslands, to alien species colonization and the potential for alien species to surpass natives in abundance. Finally, the study explores how climatic factors, including temperature and precipitation, modulate the relationship between traits and environmental conditions, shaping species success. Location: In our study, we used data from a globally distributed experiment manipulating nutrient supplies in grasslands worldwide (NutNet). Methods: We investigate how temporal shifts in the abundance of native and alien species are influenced by species-specific functional traits, including specific leaf area (SLA) and leaf nutrient concentrations, as well as by environmental conditions such as climate and nutrient treatments, across 17 study sites. Mixed-effects models were used to assess these relationships. Results: Alien and native species increasing in their abundance did not differ in their leaf traits. We found significantly lower specific leaf area (SLA) with an increase in mean annual temperature and lower leaf Potassium with mean annual precipitation. For trait-environment relationships, when compared to native species, successful aliens exhibited an increase in leaf Phosphorus and a decrease in leaf Potassium with an increase in mean annual precipitation. Finally, aliens' SLA decreased in plots with higher mean annual temperatures. Conclusions: Therefore, studying the relationship between environment and functional traits may portray grasslands' dynamics better than focusing exclusively on traits of successful species, per se.

The bidirectional relationship between plant species richness and community biomass is often variable and poorly resolved in natural grassland ecosystems, impeding progress in predicting impacts of environmental changes. Most biological communities have long-tailed species abundance distributions (for example, biomass, cover, number of individuals), a general property that may provide predictive power for species richness and community biomass. Here we show mathematical relationships between community characteristics and the abundance of dominant species arising from long-tailed distributions and test these predictions using observational and experimental data from 76 grassland sites across 6 continents. We find that community biomass provides little predictive ability for community richness, consistent with previous findings. By contrast, the relative abundance of dominant species quantitatively predicts species richness, whereas their absolute abundance quantitatively predicts community biomass under both ambient and altered environmental conditions, as expected mathematically. These results are robust to the type of abundance measure used. Three types of simulated data further show the generality of these results. Our integrative framework, arising from a few dominant species and mathematical properties of species abundance distributions, fills a persistent gap in our ability to predict community richness and biomass under ambient and anthropogenically altered conditions.

Anthropogenic impacts increasingly involve multi‐factor changes to resource pools and flows, whose outcomes on ecosystem function are difficult to predict. Here, we factorially manipulated drought with eutrophication (nitrogen fertilization) in C 3 and C 4 grassland, testing for impacts on aboveground community‐ and species‐level plant biomass. Drought and eutrophication may be especially detrimental to plant production, with elevated water demand by drought‐stressed and fertilized plants hypothesized to limit growth while potentially triggering insect outbreaks given changes to foliar‐nitrogen (N). Despite eliminating precipitation for two months, biomass was stabilized by treatment‐induced biomass increases in some species offsetting declines by others (i.e. ‘niche complementarity'), even in drought‐sensitive C 3 grassland: community‐level plant biomass remained stable via significant species‐level biomass shifts among the 61 taxa in both communities. These compensatory responses occurred despite increased water demands by fertilized plants, greater drought‐tolerance by C 4 grasses compared to C 3 taxa, elevated plant mortality, and the reduction of soil moisture below the field‐calculated plant wilting point by mid‐summer. A key mechanism for resilience appeared to be variation in rooting depth, given that soil moisture reductions were greatest within the top 30 cm. There was no detectable change in foliar‐N with drought nor any increase in insect abundance or herbivore damage. We imposed one of the lowest rainfall totals of the past three decades in our study region yet observed resilience via species‐specific compensatory responses. Our results confirm short‐term drought‐tolerance in grasslands, with co‐occurring rainfall reductions and eutrophication unable to alter community‐level production of standing biomass.

The proportionality between global mean temperature and cumulative emissions of CO2 predicted in Earth system models (ESMs) is the foundation of carbon budgeting frameworks. Deviations from this behavior could impact estimates of required net-zero timings and negative emissions requirements to meet the Paris Agreement climate targets. However, existing ESM diagnostic experiments do not allow for direct estimation of these deviations as a function of defined emissions pathways. Here, we perform a set of climate model diagnostic experiments for the assessment of transient climate response to cumulative CO2 emissions (TCRE), the Zero Emissions Commitment (ZEC), and climate reversibility metrics in an emissions-driven framework. The emissions-driven experiments provide consistent independent variables simplifying simulation, analysis and interpretation, with emissions rates more comparable to recent levels than existing protocols using model-specific compatible emissions from the CMIP DECK 1pctCO2 experiment, where emissions rates tend to increase during the experiment, such that at the time of CO2 doubling in year 70, emissions are much greater than present-day values. A base experiment, "esm-flat10", has constant emissions of CO2 of 10 GtC per year (near-present-day values), and initial results show that the TCRE estimated in this experiment is about 0.1 K less than that obtained using 1pctCO2. A subset of ESMs exhibit land carbon sinks that saturate during this experiment. A branch experiment, esm-flat10-zec, illustrates that both positive and negative ZEC effects are less pronounced under esm-flat10 than under 1pctCO2 - the magnitude of ZEC50 in ESMs is, on average, reduced by 30 % compared with 1pctCO2 branch experiments. A final experiment, esm-flat10-cdr, assesses climate reversibility under negative emissions, where we find that peak warming may occur before or after net zero and that the asymmetry in temperature at a given level of cumulative emissions between the positive and negative emissions phases is well described by ZEC in most models. Further, we find that existing probabilistic simple climate model (SCM) ensembles tend to overestimate temperature reversibility compared with ESMs, highlighting the need for additional constraints. We propose a set of climate diagnostic indicators to quantify various aspects of climate reversibility. These experiments were suggested as potential candidates in CMIP7 and have since been adopted as "fast track" simulations.

To assess the effectiveness of wetland restoration in reversing soil organic carbon (SOC) loss from degradation. Global. 1996–2023. Wetland. We conducted a global meta-analysis to compare SOC levels in restored, degraded, and natural wetlands across different restoration approaches and wetland types and to examine the dynamic trajectories of SOC recovery and the influence of climatic and edaphic factors. We found that passive restoration increased SOC in degraded sites by 141%, compared to an 8% increase from active restoration. Restored inland wetlands showed an increase in SOC of 118%, while coastal wetlands showed a limited improvement of 5%, in comparison with degraded wetlands. Increases in SOC primarily occurred within the first 10 years after restoration and then levelled off. That being said, SOC accumulation in restored wetlands rarely approached the levels found in natural wetlands, highlighting the importance of protecting wetlands from degradation for SOC targets. Key factors for wetland SOC restoration were total soil nitrogen and mean annual temperature. We conclude that wetland restoration is effective but insufficient to compensate for SOC losses from degradation. This study provides valuable insights for climate change mitigation through wetland restoration, supporting the goals of the United Nations Decade on Ecosystem Restoration and the Paris Agreement.

Ecosystems are experiencing changing global patterns of mean annual precipitation (MAP) and enrichment with multiple nutrients that potentially colimit plant biomass production. In grasslands, mean aboveground plant biomass is closely related to MAP, but how this relationship changes after enrichment with multiple nutrients remains unclear. We hypothesized the global biomass-MAP relationship becomes steeper with an increasing number of added nutrients, with increases in steepness corresponding to the form of interaction among added nutrients and with increased mediation by changes in plant community diversity. We measured aboveground plant biomass production and species diversity in 71 grasslands on six continents representing the global span of grassland MAP, diversity, management, and soils. We fertilized all sites with nitrogen, phosphorus, and potassium with micronutrients in all combinations to identify which nutrients limited biomass at each site. As hypothesized, fertilizing with one, two, or three nutrients progressively steepened the global biomass-MAP relationship. The magnitude of the increase in steepness corresponded to whether sites were not limited by nitrogen or phosphorus, were limited by either one, or were colimited by both in additive, or synergistic forms. Unexpectedly, we found only weak evidence for mediation of biomass-MAP relationships by plant community diversity because relationships of species richness, evenness, and beta diversity to MAP and to biomass were weak or opposing. Site-level properties including baseline biomass production, soils, and management explained little variation in biomass-MAP relationships. These findings reveal multiple nutrient colimitation as a defining feature of the global grassland biomass-MAP relationship.

Humans have increased atmospheric carbon dioxide (CO2), causing major changes in global climate while concurrently increasing the supply of biologically limiting nutrients especially nitrogen (N). Despite myriad negative effects on ecosystems and human health, nutrient pollution can increase the storage of soil carbon (C) in grasslands and retired farmland, potentially reducing atmospheric CO2. However, the persistence of nutrient-induced soil carbon gains remains a knowledge gap at the heart of a potential policy dilemma: whether reducing nutrient pollution could lead to the release of soil carbon that accumulated under high nutrient supply. Here we use a four-decade experiment conducted on retired, marginal cropland to demonstrate that nutrient addition increased soil C storage after intensive tilling, and that these soil C gains persisted for at least three decades following fertilizer and tilling cessation. This occurred despite plant biomass rapidly returning to pre-fertilization levels and plant community composition recovering from the effects of fertilization. These results demonstrate that nutrient-induced increases in soil C can persist for decades following reduced nutrient pollution as long as the soil remains untilled. Nutrient-induced soil carbon gains in grasslands can persist for decades after fertilization stops as long as the soil remains untilled, according to a multidecadal field experiment on retired agricultural fields with sandy soils.

Implementing precision fertilization to maximize crop yield while minimizing economic and environmental impacts has become critical for agriculture. Variability in biomass response to fertilization within fields, among regions, and over time creates simultaneous risks of under-yielding and overfertilization. We quantify factors determining fertilization responsiveness (i.e., biomass increases with fertilization) up to 15 years in 61 unfertilized rangelands on six continents. We demonstrate widespread multi-year variability in responsiveness, with fertilization increasing average yield by 43% but failing to improve biomass 26% of the time. All sites were responsive at least once, but only four of 61 responded in all plots and years. Modelled management scenarios highlighted that fertilizer cessation is likely to generate sizable economic savings but always reduces yield because of the difficulty in predicting when and where biomass will be unresponsive. This work reveals substantial scale-dependent variability in fertilization responsiveness globally, while clarifying the prospects and pitfalls of managing more spatially and temporally precise nutrient application.

Signatories to the Paris Agreement have pledged to keep global warming to well below 2 °C above pre-industrial levels and preferably below 1.5 °C above pre-industrial levels. Beyond over-shooting Paris Agreement warming levels followed by net negative emissions, achieving a state of net zero carbon dioxide emissions is required to satisfy Paris Agreement warming goals. Research on climate changes under net zero CO _2 emissions is very limited to date with no comprehensive analysis of changes in extremes. In this study, we use results from Earth System Models in the zero emissions commitment model intercomparison project to understand regional mean-state climate change patterns during a 100 year period following carbon dioxide emissions cessation. We also perform an initial study of the evolution of hot and cold monthly temperature extremes after net zero CO _2 emissions, including an assessment of how the change in frequency of temperature extremes affects areas of different levels of socioeconomic development based on regional Human Development Index (HDI). The results show that most land regions experience a fast and continuous cooling response following emissions cessation, with large areas of significant model agreement. In contrast, the Southern Ocean continues warming over the century after emissions cessation. The frequency of land-based local monthly high temperature extremes generally stays constant or decreases during the century after emissions cessation, however, decreases in heat extreme frequencies are generally less for locations with lower modern HDI than areas with higher HDI which suggests that inequality of climate change will remain an issue even after net zero CO _2 emissions. There is an evident emergence of local monthly cold extremes following emissions cessation with most significant impact over high HDI mid- and high-latitude land regions.

Predicting how biological communities assemble in restored ecosystems can assist in conservation efforts, but most research has focused on plants, with relatively little attention paid to soil microbial organisms that plants interact with. Arbuscular mycorrhizal (AM) fungi are an ecologically significant functional group of soil microbes that form mutualistic symbioses with plants and could therefore respond positively to plant community restoration. To evaluate the effects of plant community restoration on AM fungi, we compared AM fungal abundance, species richness, and community composition of five annually cultivated, conventionally managed agricultural fields with paired adjacent retired agricultural fields that had undergone prairie restoration 5-9 years prior to sampling. We hypothesized that restoration stimulates AM fungal abundance and species richness, particularly for disturbance-sensitive taxa, and that gains of new taxa would not displace AM fungal species present prior to restoration due to legacy effects. AM fungal abundance was quantified by measuring soil spore density and root colonization. AM fungal species richness and community composition were determined in soils and plant roots using DNA high-throughput sequencing. Soil spore density was 2.3 times higher in restored prairies compared to agricultural fields, but AM fungal root colonization did not differ between land use types. AM fungal species richness was 2.7 and 1.4 times higher in restored prairies versus agricultural fields for soil and roots, respectively. The abundance of Glomeraceae, a disturbance-tolerant family, decreased by 25% from agricultural to restored prairie soils but did not differ in plant roots. The abundance of Claroideoglomeraceae and Diversisporaceae, both disturbance-sensitive families, was 4.6 and 3.2 times higher in restored prairie versus agricultural soils, respectively. Species turnover was higher than expected relative to a null model, indicating that AM fungal species were gained by replacement. Our findings demonstrate that restoration can promote a relatively rapid increase in the abundance and diversity of soil microbial communities that had been degraded by decades of intensive land use, and community compositional change can be predicted by the disturbance tolerance of soil microbial taxonomic and functional groups.

Global change is associated with variable shifts in the annual production of aboveground plant biomass, suggesting localized sensitivities with unclear causal origins. Combining remotely sensed normalized difference vegetation index data since the 1980s with contemporary field data from 84 grasslands on 6 continents, we show a widening divergence in site-level biomass ranging from +51% to -34% globally. Biomass generally increased in warmer, wetter and species-rich sites with longer growing seasons and declined in species-poor arid areas. Phenological changes were widespread, revealing substantive transitions in grassland seasonal cycling. Grazing, nitrogen deposition and plant invasion were prevalent in some regions but did not predict overall trends. Grasslands are undergoing sizable changes in production, with implications for food security, biodiversity and carbon storage especially in arid regions where declines are accelerating. Remotely sensed NDVI data and contemporary field data from 84 grasslands on 6 continents show increasing divergence in aboveground plant biomass between sites in different bioclimatic regions.

Nutrient enrichment impacts grassland plant diversity such as species richness, functional trait composition and diversity, but whether and how these changes affect ecosystem stability in the face of increasing climate extremes remains largely unknown. We quantified the direct and diversity-mediated effects of nutrient addition (by nitrogen, phosphorus, and potassium) on the stability of above-ground biomass production in 10 long-term grassland experimental sites. We measured five facets of stability as the temporal invariability, resistance during and recovery after extreme dry and wet growing seasons. Leaf traits (leaf carbon, nitrogen, phosphorus, potassium, and specific leaf area) were measured under ambient and nutrient addition conditions in the field and were used to construct the leaf economic spectrum (LES). We calculated functional trait composition and diversity of LES and of single leaf traits. We quantified the contribution of intraspecific trait shifts and species replacement to change in functional trait composition as responses to nutrient addition and its implications for ecosystem stability. Nutrient addition decreased functional trait diversity and drove grassland communities to the faster end of the LES primarily through intraspecific trait shifts, suggesting that intraspecific trait shifts should be included for accurately predicting ecosystem stability. Moreover, the change in functional trait diversity of the LES in turn influenced different facets of stability. That said, these diversity-mediated effects were overall weak and/or overwhelmed by the direct effects of nutrient addition on stability. As a result, nutrient addition did not strongly impact any of the stability facets. These results were generally consistent using individual leaf traits but the dominant pathways differed. Importantly, major influencing pathways differed using average trait values extracted from global trait databases (e.g. TRY). Synthesis. Investigating changes in multiple facets of plant diversity and their impacts on multidimensional stability under global changes such as nutrient enrichment can improve our understanding of the processes and mechanisms maintaining ecosystem stability. Nutrient addition decreased species richness, functional trait diversity (FD), and drove grassland communities to the faster end of the leaf economic spectrum (increasing CWM of LES) primarily through intraspecific trait shifts. Change in FD of LES influenced multidimensional stability. But these diversity-mediated effects were weak and/or overwhelmed by the direct effects of nutrient addition on stability facets.image

Theory suggests that eutrophication impacts plant community biodiversity by constraining niche differences and increasing competitive inequalities among species, leading to the exclusion of weaker competitors. However, explicit tests of nutrient effects on the strength and direction of inter- and intraspecific plant-plant interactions that dictate coexistence are lacking, especially in complex field settings where multiple processes can simultaneously affect plant growth and reproduction. We conducted a field experiment in southeastern Vancouver Island, Canada, using four annual grassland plant species to test how species interactions responded to eutrophication and the covariation of herbivore pressure and neighbour species light- and moisture-use. We found that focal species reproduction was limited by nutrient availability and herbivory, such that the nature of species interactions was context-dependent. Competitive interactions failed to predict species persistence under these conditions. Instead, facilitation was critical-phytometers grown without neighbours consistently failed to produce seed under herbivore pressure meaning that fecundity significantly increased with neighbour density. Synthesis. Our work demonstrates the importance of species interactions in plant community responses to eutrophication but emphasizes that 'indirect' drivers of plant performance may influence or overwhelm nutrient effects on plant-plant competition. The widespread occurrence of mortality overcome primarily by facilitation highlights the importance of positive density dependence. Overall, our results suggest that knowledge of resource niche overlap may be insufficient to explain plant community responses to eutrophication. This highlights the necessity of considering the broader environmental context when leveraging ecological theory to understand global change effects in empirical settings. Theory based on resource competition was insufficient to explain eutrophication-driven changes in grassland plant-plant interactions as multiple processes, especially herbivory, overwhelmed nutrient effects and increased the importance of facilitation.image

AimEnvironmental change affects metacommunity structure both directly-via abiotic factors and dispersal that affect species occurrence-and indirectly-via complex interactions among co-occurring species. We examined how the three main metacommunity factors-environmental conditions, spatial processes and species associations-affect metacommunity structure and whether responses are predictable in real-world systems by using novel methods to disentangle the drivers.LocationEastern Asia, northern Europe and central North America.Time periodContemporary.Major taxa studiedFreshwater fish.MethodsWe used a dataset of freshwater fish species occurrences in temperate lakes in three countries in different biogeographic regions. We analysed co-occurrence patterns by using a joint species distribution model.ResultsWe demonstrated that environmental processes are the main drivers of species' distribution and diversity, suggesting that future climate change (anthropogenic alteration of abiotic factors) will heavily influence the structure of metacommunities. We also showed that spatial processes and species interactions mediated the influence of environmental processes, especially at the lake level.Main conclusionsOur results indicate that ongoing changes in metacommunity structure are modulated not only by the direct impacts of shifting abiotic factors but also by indirect effects of species interactions. Our global analysis indicates that even under the current high rate of environmental change, an identifiable set of underlying processes can be used to predict impacts of this change on metacommunity structure.

The carbon cycle plays a foundational role in the estimation of the remaining carbon budget. It is intrinsic for the determination of the transient climate response to cumulative CO2 emissions and the zero-emissions commitment. For the terrestrial carbon cycle, nutrient limitation is a core regulation on the amount of carbon fixed by terrestrial vegetation. Hence, the addition of nutrients such as nitrogen and phosphorus in land model structures in Earth system models is essential for an accurate representation of the carbon cycle feedback in future climate projections. Therefore, the estimation of the remaining carbon budget is impacted by the representation of nutrient limitation in modelled terrestrial ecosystems; however, it is rarely accounted for. Here, we estimate the carbon budget and remaining carbon budget of a nutrient-limited Earth system model, using nitrogen and phosphorus cycles to limit vegetation productivity and biomass. We use eight Shared Socioeconomic Pathways (hereafter SSP) scenarios and idealized experiments with three distinct model structures: (1) carbon cycle without nutrient limitation, (2) carbon cycle with terrestrial nitrogen limitation, and (3) carbon cycle with terrestrial nitrogen and phosphorus limitation. To capture the uncertainty in the remaining carbon budget, three different climate sensitives were tuned for each model version. Our results show that, overall, nutrient limitation reduced the remaining carbon budget for all simulations in comparison with the carbon cycle without nutrient limitation. Between nitrogen and nitrogen-phosphorus limitation, the latter had the lowest remaining carbon budget. The mean remaining carbon budgets obtained from the SSP scenario simulations for the 1.5 degrees C target in the non-nutrient-limited, nitrogen-limited, and nitrogen-phosphorus-limited models were 228, 185, and 175 PgC, respectively, relative to the year 2020. For the 2 degrees C target, the mean remaining carbon budget values were 471, 373, and 351 PgC for the non-nutrient-limited, nitrogen-limited, and nitrogen-phosphorus-limited models, respectively, relative to the year 2020. This represents a reduction of 19 % and 24 % for the 1.5 degrees C target and 21 % and 26 % for the 2 degrees C target for the respective nitrogen- and nitrogen-phosphorus-limited simulations compared with the non-nutrient-limited model. These results show that terrestrial nutrient limitation constitutes an important factor to be considered when estimating or interpreting remaining carbon budgets and that it is an essential uncertainty in the remaining carbon budgets from Earth system model simulations.

The era of anthropogenic climate change can be described by defined climate milestones. These milestones mark changes in the historic trajectory of change, and include peak greenhouse gas emissions, peak greenhouse gas concentration, deceleration of warming, net-zero emissions, and a transition to global cooling. However, given internal variability in the Earth system and measurement uncertainty, definitively saying that a milestone has passed requires rigour. Here CMIP6 simulations of peak-and-decline scenarios are used to examine the time needed to robustly detect three climate milestones: (1) the slowdown of global warming; (2) the end of global surface temperature increase; and (3) peak concentration of CO2. It is estimated that it will take 40 to 60 years after a simulated slowdown in warming rate, to robustly detect ( >95% change) the signal in the global average temperature record. Detecting when warming has stopped will also be difficult and it takes until the mid 22nd century to have enough data to conclude warming has stopped. Detecting that CO2 concentration has peaked is far easier and a drop in CO2 concentration of 3 ppm is consistent with a greater than 99% chance that CO2 has peaked in all scenarios examined. Thus it is likely that as the rate of CO2 emissions is reduced, and net-zero emissions is approached, interpreting the global temperature record will become difficult-with a high potential to create confusion amongst policy makers and the general public.