Joint optimization of land carbon uptake and albedo can help achieve moderate instantaneous and long-term cooling effects.

Both carbon dioxide uptake and albedo of the land surface affect global climate. However, climate change mitigation by increasing carbon uptake can cause a warming trade-off by decreasing albedo, with most research focusing on afforestation and its interaction with snow. Here, we present carbon uptake and albedo observations from 176 globally distributed flux stations. We demonstrate a gradual decline in maximum achievable annual albedo as carbon uptake increases, even within subgroups of non-forest and snow-free ecosystems. Based on a paired-site permutation approach, we quantify the likely impact of land use on carbon uptake and albedo. Shifting to the maximum attainable carbon uptake at each site would likely cause moderate net global warming for the first approximately 20 years, followed by a strong cooling effect. A balanced policy co-optimizing carbon uptake and albedo is possible that avoids warming on any timescale, but results in a weaker long-term cooling effect.

Incorporating the effect of heterogeneous surface heating into a semi-empirical model of the surface energy balance closure.

It was discovered several decades ago that eddy covariance measurements systematically underestimate sensible and latent heat fluxes, creating an imbalance in the surface energy budget. Since then, many studies have addressed this problem and proposed a variety of solutions to the problem, including improvements to instruments and correction methods applied during data postprocessing. However, none of these measures have led to the complete closure of the energy balance gap. The leading hypothesis is that not only surface-attached turbulent eddies but also sub-mesoscale atmospheric circulations contribute to the transport of energy in the atmospheric boundary layer, and the contribution from organized motions has been grossly neglected. The problem arises because the transport of energy through these secondary circulations cannot be captured by the standard eddy covariance method given the relatively short averaging periods of time (~30 minutes) used to compute statistics. There are various approaches to adjust the measured heat fluxes by attributing the missing energy to the sensible and latent heat flux in different proportions. However, few correction methods are based on the processes causing the energy balance gap. Several studies have shown that the magnitude of the energy balance gap depends on the atmospheric stability and the heterogeneity scale of the landscape around the measurement site. Based on this, the energy balance gap within the surface layer has already been modelled as a function of a nonlocal atmospheric stability parameter by performing a large-eddy simulation study with idealized homogeneous surfaces. We have further developed this approach by including thermal surface heterogeneity in addition to atmospheric stability in the parameterization. Specifically, we incorporated a thermal heterogeneity parameter that was shown to relate to the magnitude of the energy balance gap. For this purpose, we use a Large-Eddy Simulation dataset of 28 simulations with seven different atmospheric conditions and three heterogeneous surfaces with different heterogeneity scales as well as one homogeneous surface. The newly developed model captures very well the variability in the magnitude of the energy balance gap under different conditions. The model covers a wide range of both atmospheric stabilities and landscape heterogeneity scales and is well suited for application to eddy covariance measurements since all necessary information can be modelled or obtained from a few additional measurements.

Altered energy partitioning across terrestrial ecosystems in the European drought year 2018.

Drought and heat events, such as the 2018 European drought, interact with the exchange of energy between the land surface and the atmosphere, potentially affecting albedo, sensible and latent heat fluxes, as well as CO2 exchange. Each of these quantities may aggravate or mitigate the drought, heat, their side effects on productivity, water scarcity and global warming. We used measurements of 56 eddy covariance sites across Europe to examine the response of fluxes to extreme drought prevailing most of the year 2018 and how the response differed across various ecosystem types (forests, grasslands, croplands and peatlands). Each component of the surface radiation and energy balance observed in 2018 was compared to available data per site during a reference period 2004-2017. Based on anomalies in precipitation and reference evapotranspiration, we classified 46 sites as drought affected. These received on average 9% more solar radiation and released 32% more sensible heat to the atmosphere compared to the mean of the reference period. In general, drought decreased net CO2 uptake by 17.8%, but did not significantly change net evapotranspiration. The response of these fluxes differed characteristically between ecosystems; in particular, the general increase in the evaporative index was strongest in peatlands and weakest in croplands. This article is part of the theme issue 'Impacts of the 2018 severe drought and heatwave in Europe: from site to continental scale'.

The impact of rain events on CO2 emissions from contrasting land use systems in semi-arid West African savannas.

In the future the Sudanian savanna - one of West Africa's high-potential "bread baskets" - will likely face shorter rainy seasons with more extreme rains and droughts. That could have serious impacts on the vegetation and its carbon dioxide (CO2) exchange with potentially increasing CO2 emissions accelerating climate warming. Understanding how the CO2 fluxes in this area respond to environmental variables, in particular rain events, is essential, but available data are scarce. In this study, we monitored net ecosystem exchange (NEE) of CO2, rainfall and other environmental parameters during four years at three savannas. Savannas were characterized by different vegetation due to different land use: i) woody and nearly pristine, ii) mixture of cropland and grassland and iii) intensive grazing. The impact of rain events on CO2 exchange for these contrasting ecosystems were analyzed for single rain events (short-term) and on a yearly time scale (long-term) using three eddy covariance towers. We found that the woody pristine savanna site was a prominent sink of CO2 (-864 to -1299 g CO2 m-2 y-1) while the degraded sites were net CO2 sources (118 to 605 g CO2 m-2 y-1) with a complicated relation with annual rainfall amounts. The NEE responses to single rain events revealed that daytime rain systematically decreased the sink strengths at all sites, which might be associated with decreased light availability. At the degraded sites, additional factors increasing CO2 losses were rain duration and dry spell length. The observed patterns of immediate CO2 flux responses to rainfall at differently used savannas indicate strong internal feedbacks between vegetation and land use changes and raise the question whether the CO2 sink strengths might be overestimated with possible implications for global CO2 budgets. Sustainable adaptation strategies need to be developed for West Africa.

A semi-empirical model of the energy balance closure in the surface layer.

It has been hypothesized that the energy balance closure problem of single-tower eddy-covariance measurements is linked to large-scale turbulent transport. In order to shed light on this problem, we investigate the functional dependence of the normalized residual for the potential temperature and humidity conservation equations, i.e. the imbalance ratio for the fluxes of latent and sensible heat. We set up a suite of simulations consisting of cases with different stability and surface Bowen ratio. We employ a nesting approach in the lower part of the atmospheric boundary-layer to achieve higher spatial resolution near the surface. Our simulations reproduce earlier simulation results for the mixed layer and also mimic the saw-blade pattern of real flux measurements. Focusing on homogeneous terrain, we derive a parameterization for the spatially averaged flux imbalance ratios of latent and sensible heat in the surface layer. We also investigate how the remaining imbalance for a given point measurement is related to the local turbulence, by deriving a statistical model based on turbulence characteristics that are related to large-scale turbulence. The average imbalance ratio scales well with friction velocity, especially for sensible heat. For the latent heat flux, our results show that the Bowen ratio also influences the underestimation. Furthermore, in the surface layer the residual has a linear dependence on the absolute height divided by the boundary-layer height. Our parameterization allows us to deduce an expression for the residual in the energy budget for a particular measurement half hour, based on the measurement height and stability.

Carbon dioxide fluxes from contrasting ecosystems in the Sudanian Savanna in West Africa.

BACKGROUND: The terrestrial land surface in West Africa is made up of several types of savanna ecosystems differing in land use changes which modulate gas exchanges between their vegetation and the overlying atmosphere. This study compares diurnal and seasonal estimates of CO2 fluxes from three contrasting ecosystems, a grassland, a mixture of fallow and cropland, and nature reserve in the Sudanian Savanna and relate them to water availability and land use characteristics. RESULTS: Over the study period, and for the three study sites, low soil moisture availability, high vapour pressure deficit and low ecosystem respiration were prevalent during the dry season (November to March), but the contrary occurred during the rainy season (May to October). Carbon uptake predominantly took place in the rainy season, while net carbon efflux occurred in the dry season as well as the dry to wet and wet to dry transition periods (AM and ND) respectively. Carbon uptake decreased in the order of the nature reserve, a mixture of fallow and cropland, and grassland. Only the nature reserve ecosystem at the Nazinga Park served as a net sink of CO2, mostly by virtue of a several times larger carbon uptake and ecosystem water use efficiency during the rainy season than at the other sites. These differences were influenced by albedo, LAI, EWUE, PPFD and climatology during the period of study. CONCLUSION: These results suggest that land use characteristics affect plant physiological processes that lead to flux exchanges over the Sudanian Savanna ecosystems. It affects the diurnal, seasonal and annual changes in NEE and its composite signals, GPP and RE. GPP and NEE were generally related as NEE scaled with photosynthesis with higher CO2 assimilation leading to higher GPP. However, CO2 effluxes over the study period suggest that besides biomass regrowth, other processes, most likely from the soil might have also contributed to the enhancement of ecosystem respiration.