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Long‐term carbon dioxide (CO2) flux measurements between the atmosphere and the ecosystem have been made since 1993 at a cool‐temperate deciduous forest site (Takayama) in Japan influenced by the Asian Monsoon, constituting the longest data set among all the AsiaFlux sites. Interannual variations (IAVs) and trends in the annual carbon budget components and related environmental factors were examined. Annual net ecosystem production (NEP) (mean ± 1σ) during the period of eddy covariance measurements in 1999–2021 was 265 ± 86 gC m−2 yr−1, and its IAV was dependent more on gross primary production (GPP) than on ecosystem respiration. The annual NEP and GPP were correlated with the monthly mean NEP, GPP and leaf area index from June to September, as well as with the length of the net carbon uptake period. Significantly increasing and decreasing trends in the annual NEP and GPP were detected during 2004–2013 and 2013–2021, respectively. The increasing trends were mainly caused by the vegetation recovery from typhoon disturbances. On the other hand, although the cause of the decreasing trends was not clearly identified, recent extreme weather events and/or forest succession might be related to the trends. Significant positive correlations between the start and the end of the net carbon uptake period, and between the leaf expansion and leaf fall were found. These may be attributed to phenological characteristics and negative correlation between air temperature in spring and solar radiation in early fall associated with ENSO events that can also influence IAVs in annual NEP and GPP. Plain Language Summary: Forest ecosystems play an important role in the global carbon cycle. However, time variations in the ecosystem carbon budget and responses of the ecosystems to climate change are not well understood. Although long‐term observational data are useful for a better understanding, >20‐year observations are limited, especially in the Asian monsoon region. In this study, long‐term measurements of carbon budget made at a cool‐temperate deciduous forest in Japan were analyzed for interannual variation (IAV), long‐term trends, and offered plausible causes for these changes. The IAV in annual net carbon uptake (NEP) was associated with the variation in summertime NEP, as well as with the variation in the length of the net carbon uptake period (NGP). The IAV in summertime NEP was associated with the variations in solar radiation (SR) and leaf density during the season, while the IAV in NGP depended on the variations in spring temperature and early‐fall SR. A decadal increasing trend in the annual NEP (2004–2013) was caused mainly by the recovery from typhoons, while a decreasing trend (2013–2021) may be related to recent extreme weather events and/or forest succession. Longer‐term ecosystem observations are certainly needed for more accurate predictions of forest ecosystems response to climate change. Key Points: Interannual variations and significant trends of carbon budget components at a forest site were detected from a long‐term observationThe annual carbon budget was governed by the summertime carbon uptake and the length of the growing seasonThe decadal‐scale trends of the carbon budget components obtained in this study could not have been found without the long‐term observation [ABSTRACT FROM AUTHOR]

Continuous observations of atmospheric δ(O2/N2) and CO 2 amount fractions have been carried out at Ryori (RYO), Japan, since August 2017. In these observations, the O 2 : CO 2 exchange ratio (ER, -Δy(O2)Δy(CO2)-1) has frequently been lower than expected from short-term variations in emissions from terrestrial biospheric activities and combustion of liquid, gas, and solid fuels. This finding suggests a substantial effect of CO 2 emissions from a cement plant located about 6 km northwest of RYO. To evaluate this effect quantitatively, we simulated CO 2 amount fractions in the area around RYO by using a fine-scale atmospheric transport model that incorporated CO 2 fluxes from terrestrial biospheric activities, fossil fuel combustion, and cement production. The simulated CO 2 amount fractions were converted to O 2 amount fractions by using the respective ER values of 1.1, 1.4, and 0 for the terrestrial biospheric activities, fossil fuel combustion, and cement production. Thus obtained O 2 and CO 2 amount fraction changes were used to derive a simulated ER for comparison with the observed ER. To extract the contribution of CO 2 emissions from the cement plant, we used y(CO2∗) as an indicator variable, where y(CO2∗) is a conservative variable for terrestrial biospheric activities and fossil fuel combustion obtained by simultaneous analysis of observed δ(O2/N2) and CO 2 amount fractions and simulated ERs. We confirmed that the observed and simulated ER values and also the y(CO2∗) values and simulated CO 2 amount fractions due only to cement production were generally consistent. These results suggest that combined measurements of δ(O2/N2) and CO 2 amount fractions will be useful for evaluating CO 2 capture from flue gas at carbon capture and storage (CCS) plants, which, similar to a cement plant, change CO 2 amount fractions without changing O 2 values, although CCS plants differ from cement plants in the direction of CO 2 exchange with the atmosphere. [ABSTRACT FROM AUTHOR]

Accurately predicting weather and climate in cities is critical for safeguarding human health and strengthening urban resilience. Multimodel evaluations can lead to model improvements; however, there have been no major intercomparisons of urban‐focussed land surface models in over a decade. Here, in Phase 1 of the Urban‐PLUMBER project, we evaluate the ability of 30 land surface models to simulate surface energy fluxes critical to atmospheric meteorological and air quality simulations. We establish minimum and upper performance expectations for participating models using simple information‐limited models as benchmarks. Compared with the last major model intercomparison at the same site, we find broad improvement in the current cohort's predictions of short‐wave radiation, sensible and latent heat fluxes, but little or no improvement in long‐wave radiation and momentum fluxes. Models with a simple urban representation (e.g., 'slab' schemes) generally perform well, particularly when combined with sophisticated hydrological/vegetation models. Some mid‐complexity models (e.g., 'canyon' schemes) also perform well, indicating efforts to integrate vegetation and hydrology processes have paid dividends. The most complex models that resolve three‐dimensional interactions between buildings in general did not perform as well as other categories. However, these models also tended to have the simplest representations of hydrology and vegetation. Models without any urban representation (i.e., vegetation‐only land surface models) performed poorly for latent heat fluxes, and reasonably for other energy fluxes at this suburban site. Our analysis identified widespread human errors in initial submissions that substantially affected model performances. Although significant efforts are applied to correct these errors, we conclude that human factors are likely to influence results in this (or any) model intercomparison, particularly where participating scientists have varying experience and first languages. These initial results are for one suburban site, and future phases of Urban‐PLUMBER will evaluate models across 20 sites in different urban and regional climate zones. [ABSTRACT FROM AUTHOR]

Tropical vegetation is a major source of global land surface evapotranspiration, and can thus play a major role in global hydrological cycles and global atmospheric circulation. Accurate prediction of tropical evapotranspiration is critical to our understanding of these processes under changing climate. We examined the controls on evapotranspiration in tropical vegetation at 21 pan-tropical eddy covariance sites, conducted a comprehensive and systematic evaluation of 13 evapotranspiration models at these sites, and assessed the ability to scale up model estimates of evapotranspiration for the test region of Amazonia. Net radiation was the strongest determinant of evapotranspiration (mean evaporative fraction was 0.72) and explained 87% of the variance in monthly evapotranspiration across the sites. Vapor pressure deficit was the strongest residual predictor (14%), followed by normalized difference vegetation index (9%), precipitation (6%) and wind speed (4%). The radiation-based evapotranspiration models performed best overall for three reasons: (1) the vegetation was largely decoupled from atmospheric turbulent transfer (calculated from Ω decoupling factor), especially at the wetter sites; (2) the resistance-based models were hindered by difficulty in consistently characterizing canopy (and stomatal) resistance in the highly diverse vegetation; (3) the temperature-based models inadequately captured the variability in tropical evapotranspiration. We evaluated the potential to predict regional evapotranspiration for one test region: Amazonia. We estimated an Amazonia-wide evapotranspiration of 1370 mm yr⁻¹, but this value is dependent on assumptions about energy balance closure for the tropical eddy covariance sites; a lower value (1096 mm yr⁻¹) is considered in discussion on the use of flux data to validate and interpolate models.