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[en] Megacities are major sources of anthropogenic fossil fuel CO2 (FFCO2) emissions. The spatial extents of these large urban systems cover areas of 10 000 km2 or more with complex topography and changing landscapes. We present a high-resolution land–atmosphere modelling system for urban CO2 emissions over the Los Angeles (LA) megacity area. The Weather Research and Forecasting (WRF)-Chem model was coupled to a very high-resolution FFCO2 emission product, Hestia-LA, to simulate atmospheric CO2 concentrations across the LA megacity at spatial resolutions as fine as ~1 km. We evaluated multiple WRF configurations, selecting one that minimized errors in wind speed, wind direction, and boundary layer height as evaluated by its performance against meteorological data collected during the CalNex-LA campaign (May–June 2010). Our results show no significant difference between moderate-resolution (4 km) and high-resolution (1.3 km) simulations when evaluated against surface meteorological data, but the high-resolution configurations better resolved planetary boundary layer heights and vertical gradients in the horizontal mean winds. We coupled our WRF configuration with the Vulcan 2.2 (10 km resolution) and Hestia-LA (1.3 km resolution) fossil fuel CO2 emission products to evaluate the impact of the spatial resolution of the CO2 emission products and the meteorological transport model on the representation of spatiotemporal variability in simulated atmospheric CO2 concentrations. We find that high spatial resolution in the fossil fuel CO2 emissions is more important than in the atmospheric model to capture CO2 concentration variability across the LA megacity. Finally, we present a novel approach that employs simultaneous correlations of the simulated atmospheric CO2 fields to qualitatively evaluate the greenhouse gas measurement network over the LA megacity. Spatial correlations in the atmospheric CO2 fields reflect the coverage of individual measurement sites when a statistically significant number of sites observe emissions from a specific source or location. We conclude that elevated atmospheric CO2 concentrations over the LA megacity are composed of multiple fine-scale plumes rather than a single homogenous urban dome. Furthermore, we conclude that FFCO2 emissions monitoring in the LA megacity requires FFCO2 emissions modelling with ~1 km resolution because coarser-resolution emissions modelling tends to overestimate the observational constraints on the emissions estimates.
[en] Verifying national greenhouse gas (GHG) emissions inventories is a critical step to ensure that reported emissions data to the United Nations Framework Convention on Climate Change (UNFCCC) are accurate and representative of a country’s contribution to GHG concentrations in the atmosphere. Furthermore, verifying biogenic fluxes provides a check on estimated emissions associated with managing lands for carbon sequestration and other activities, which often have large uncertainties. We report here on the challenges and results associated with a case study using atmospheric measurements of CO_2 concentrations and inverse modeling to verify nationally-reported biogenic CO_2 emissions. The biogenic CO_2 emissions inventory was compiled for the Mid-Continent region of United States based on methods and data used by the US government for reporting to the UNFCCC, along with additional sources and sinks to produce a full carbon balance. The biogenic emissions inventory produced an estimated flux of −408 ± 136 Tg CO_2 for the entire study region, which was not statistically different from the biogenic flux of −478 ± 146 Tg CO_2 that was estimated using the atmospheric CO_2 concentration data. At sub-regional scales, the spatial density of atmospheric observations did not appear sufficient to verify emissions in general. However, a difference between the inventory and inversion results was found in one isolated area of West-central Wisconsin. This part of the region is dominated by forestlands, suggesting that further investigation may be warranted into the forest C stock or harvested wood product data from this portion of the study area. The results suggest that observations of atmospheric CO_2 concentration data and inverse modeling could be used to verify biogenic emissions, and provide more confidence in biogenic GHG emissions reporting to the UNFCCC. (letter)
[en] Reporting on the climate action of cities and regions in the context of the pandemic and the renewal of national contributions to the Paris Agreement. Each year, the Climate Chance Observatory proposes a summary of the progress made in terms of climate action and published by cities and regions around the world. Although the absence of consolidated and comparable data remains a challenge, this does not mean that there is no action or mobilisation. The analysis of the remarkable evolution of emissions at the local level, the monitoring of the development of the main international initiatives led by networks of local authorities, and publications of academic and specialised literature, make it possible to draw global trends. The formulation, implementation and monitoring-evaluation of local climate actions is a complex process that requires both the support of States and a proper consideration of the inhabitants' needs. This is why our monitoring is accompanied by analyses of multi-level governance and the localisation of Sustainable Development Goals. The reduction of GHG emissions by European cities is encouraging. However, in a context of mass adoption of carbon neutrality objectives, the monitoring of the impact of local climate policies remains scattered and poorly consolidated, even at the national level. The mobilisation of local governments and the structuring of their climate action is continuing. Although international initiatives show a certain dynamism in Latin America, Europe and North Africa, they do not account for the action of Asian cities and regions. Even in times of Covid-19, local governments remain places of innovation and experimentation for climate policies. At the city level, the densification of services is now seen as the remedy to the health and climate crises. Few of the renewed national contributions to the Paris Agreement mention governance mechanisms that integrate local and sub-national governments, except in Latin America. Their sectoral approach to tackling local emissions reduction masks the potential of spatial planning and local governance. Multi-level governance in G20 countries: our first case studies (Germany, Canada, France, Brazil) show that few cities are subject to climate obligations, whose action relies on the disparate support of federal and federated states. The lack of harmonisation of monitoring methods makes it difficult to integrate the potential of cities into national strategies. Agenda 2030: after a few years in the adoption phase, local governments are embracing the Sustainable Development Goals (SDGs) to cushion the socio-economic shocks of climate policies. Despite the lack of funding, driven by the dynamic exchanges between scientists and decision-makers, adaptation to climate change is accelerating within regions and cities.