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1. Heat stored in the Earth system: where does the energy go?

   https://doi.org/10.5194/essd-12-2013-2020  

   von Schuckmann, Karina; Cheng, Lijing; Palmer, Matthew D.; Hansen, James; Tassone, Caterina; Aich, Valentin; Adusumilli, Susheel; Beltrami, Hugo; Boyer, Tim; Cuesta-Valero, Francisco José; et al (January 2020, Earth System Science Data)

   null (Ed.)

   Abstract. Human-induced atmospheric composition changes cause a radiative imbalance atthe top of the atmosphere which is driving global warming. This Earth energy imbalance (EEI) is the most critical number defining the prospects for continued global warming and climate change. Understanding the heat gain ofthe Earth system – and particularly how much and where the heat isdistributed – is fundamental to understanding how this affects warmingocean, atmosphere and land; rising surface temperature; sea level; and lossof grounded and floating ice, which are fundamental concerns for society.This study is a Global Climate Observing System (GCOS) concertedinternational effort to update the Earth heat inventory and presents anupdated assessment of ocean warming estimates as well as new and updated estimatesof heat gain in the atmosphere, cryosphere and land over the period1960–2018. The study obtains a consistent long-term Earth system heat gainover the period 1971–2018, with a total heat gain of 358±37 ZJ,which is equivalent to a global heating rate of 0.47±0.1 W m−2.Over the period 1971–2018 (2010–2018), the majority of heat gain is reportedfor the global ocean with 89 % (90 %), with 52 % for both periods inthe upper 700 m depth, 28 % (30 %) for the 700–2000 m depth layer and 9 % (8 %) below 2000 m depth. Heat gain over land amounts to 6 %(5 %) over these periods, 4 % (3 %) is available for the melting ofgrounded and floating ice, and 1 % (2 %) is available for atmospheric warming. Ourresults also show that EEI is not only continuing, but also increasing: the EEIamounts to 0.87±0.12 W m−2 during 2010–2018. Stabilization ofclimate, the goal of the universally agreed United Nations Framework Convention on ClimateChange (UNFCCC) in 1992 and the ParisAgreement in 2015, requires that EEI be reduced to approximately zero toachieve Earth's system quasi-equilibrium. The amount of CO2 in theatmosphere would need to be reduced from 410 to 353 ppm to increase heatradiation to space by 0.87 W m−2, bringing Earth back towards energybalance. This simple number, EEI, is the most fundamental metric that thescientific community and public must be aware of as the measure of how wellthe world is doing in the task of bringing climate change under control, andwe call for an implementation of the EEI into the global stocktake based onbest available science. Continued quantification and reduced uncertaintiesin the Earth heat inventory can be best achieved through the maintenance ofthe current global climate observing system, its extension into areas ofgaps in the sampling, and the establishment of an international framework forconcerted multidisciplinary research of the Earth heat inventory aspresented in this study. This Earth heat inventory is published at the German Climate Computing Centre (DKRZ, https://www.dkrz.de/, last access: 7 August 2020) under the DOIhttps://doi.org/10.26050/WDCC/GCOS_EHI_EXP_v2(von Schuckmann et al., 2020). 

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2. Snow thermal conductivity and conductive flux in the Central Arctic: Estimates from observations and implications for models

   https://doi.org/10.1525/elementa.2023.00086  

   Sledd, Anne; Shupe, Matthew D; Solomon, Amy; Cox, Christopher J; Perovich, Donald; Lei, Ruibo (January 2024, Elem Sci Anth)

   During the Arctic winter, the conductive heat flux through the sea ice and snow balances the radiative and turbulent heat fluxes at the surface. Snow on sea ice is a thermal insulator that reduces the magnitude of the conductive flux. The thermal conductivity of snow, that is, how readily energy is conducted, is known to vary significantly in time and space from observations, but most forecast and climate models use a constant value. This work begins with a demonstration of the importance of snow thermal conductivity in a regional coupled forecast model. Varying snow thermal conductivity impacts the magnitudes of all surface fluxes, not just conduction, and their responses to atmospheric forcing. Given the importance of snow thermal conductivity in models, we use observations from sea ice mass balance buoys installed during the Multidisciplinary drifting Observatory for the Study of Arctic Climate expedition to derive the profiles of thermal conductivity, density, and conductive flux. From 13 sites, median snow thermal conductivity ranges from 0.33 W m−1 K−1 to 0.47 W m−1 K−1 with a median from all data of 0.39 W m−1 K−1 from October to February. In terms of surface energy budget closure, estimated conductive fluxes are generally smaller than the net atmospheric flux by as much as 20 W m−2, but the average residual during winter is −6 W m−2, which is within the uncertainties. The spatial variability of conductive heat flux is highest during clear and cold time periods. Higher surface temperature, which often occurs during cloudy conditions, and thicker snowpacks reduce temporal and spatial variability. These relationships are compared between observations and the coupled forecast model, emphasizing both the importance and challenge of describing thermodynamic parameters of snow cover for modeling the Arctic as a coupled system. 

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3. Frequent burning causes large losses of carbon from deep soil layers in a temperate savanna

   https://doi.org/10.1111/1365-2745.13351  

   Pellegrini, Adam F. A.; McLauchlan, Kendra K.; Hobbie, Sarah E.; Mack, Michelle C.; Marcotte, Abbey L.; Nelson, David M.; Perakis, Steven S.; Reich, Peter B.; Whittinghill, Kyle; Lamb, ed., Eric (February 2020, Journal of Ecology)

   Abstract Fire activity is changing dramatically across the globe, with uncertain effects on ecosystem processes, especially below‐ground. Fire‐driven losses of soil carbon (C) are often assumed to occur primarily in the upper soil layers because the repeated combustion of above‐ground biomass limits organic matter inputs into surface soil. However, C losses from deeper soil may occur if frequent burning reduces root biomass inputs of C into deep soil layers or stimulates losses of C via leaching and priming.To assess the effects of fire on soil C, we sampled 12 plots in a 51‐year‐long fire frequency manipulation experiment in a temperate oak savanna, where variation in prescribed burning frequency has created a gradient in vegetation structure from closed‐canopy forest in unburned plots to open‐canopy savanna in frequently burned plots.Soil C stocks were nonlinearly related to fire frequency, with soil C peaking in savanna plots burned at an intermediate fire frequency and declining in the most frequently burned plots. Losses from deep soil pools were significant, with the absolute difference between intermediately burned plots versus most frequently burned plots more than doubling when the full 1 m sample was considered rather than the top 0–20 cm alone (losses of 98.5 Mg C/ha [−76%] and 42.3 Mg C/ha [−68%] in the full 1 m and 0–20 cm layers respectively). Compared to unburned forested plots, the most frequently burned plots had 65.8 Mg C/ha (−58%) less C in the full 1 m sample. Root biomass below the top 20 cm also declined by 39% with more frequent burning. Concurrent fire‐driven losses of nitrogen and gains in calcium and phosphorus suggest that burning may increase nitrogen limitation and play a key role in the calcium and phosphorus cycles in temperate savannas.Synthesis. Our results illustrate that fire‐driven losses in soil C and root biomass in deep soil layers may be critical factors regulating the net effect of shifting fire regimes on ecosystem C in forest‐savanna transitions. Projected changes in soil C with shifting fire frequencies in savannas may be 50% too low if they only consider changes in the topsoil. 

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4. Distinct surface response to black carbon aerosols

   https://doi.org/10.5194/acp-21-13797-2021  

   Tang, Tao; Shindell, Drew; Zhang, Yuqiang; Voulgarakis, Apostolos; Lamarque, Jean-Francois; Myhre, Gunnar; Faluvegi, Gregory; Samset, Bjørn H.; Andrews, Timothy; Olivié, Dirk; et al (January 2021, Atmospheric Chemistry and Physics)

   Abstract. For the radiative impact of individual climate forcings,most previous studies focused on the global mean values at the top of theatmosphere (TOA), and less attention has been paid to surface processes,especially for black carbon (BC) aerosols. In this study, the surface radiativeresponses to five different forcing agents were analyzed by using idealizedmodel simulations. Our analyses reveal that for greenhouse gases, solarirradiance, and scattering aerosols, the surface temperature changes aremainly dictated by the changes of surface radiative heating, but for BC,surface energy redistribution between different components plays a morecrucial role. Globally, when a unit BC forcing is imposed at TOA, the netshortwave radiation at the surface decreases by -5.87±0.67 W m−2 (W m−2)−1 (averaged over global land without Antarctica), which ispartially offset by increased downward longwave radiation (2.32±0.38 W m−2 (W m−2)−1 from the warmer atmosphere, causing a netdecrease in the incoming downward surface radiation of -3.56±0.60 W m−2 (W m−2)−1. Despite a reduction in the downward radiationenergy, the surface air temperature still increases by 0.25±0.08 Kbecause of less efficient energy dissipation, manifested by reduced surfacesensible (-2.88±0.43 W m−2 (W m−2)−1) and latent heat flux(-1.54±0.27 W m−2 (W m−2)−1), as well as a decrease inBowen ratio (-0.20±0.07 (W m−2)−1). Such reductions of turbulentfluxes can be largely explained by enhanced air stability (0.07±0.02 K (W m−2)−1), measured as the difference of the potential temperaturebetween 925 hPa and surface, and reduced surface wind speed (-0.05±0.01 m s−1 (W m−2)−1). The enhanced stability is due to the fasteratmospheric warming relative to the surface, whereas the reduced wind speedcan be partially explained by enhanced stability and reduced Equator-to-poleatmospheric temperature gradient. These rapid adjustments under BC forcingoccur in the lower atmosphere and propagate downward to influence thesurface energy redistribution and thus surface temperature response, whichis not observed under greenhouse gases or scattering aerosols. Our studyprovides new insights into the impact of absorbing aerosols on surfaceenergy balance and surface temperature response. 

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5. Modeling long-term fire impact on ecosystem characteristics and surface energy using a process-based vegetation–fire model SSiB4/TRIFFID-Fire v1.0

   https://doi.org/10.5194/gmd-13-6029-2020  

   Huang, Huilin; Xue, Yongkang; Li, Fang; Liu, Ye (January 2020, Geoscientific Model Development)

   null (Ed.)

   Abstract. Fire is one of the primary disturbances to the distribution and ecologicalproperties of the world's major biomes and can influence the surface fluxesand climate through vegetation–climate interactions. This study incorporatesa fire model of intermediate complexity to a biophysical model with dynamicvegetation, SSiB4/TRIFFID (The Simplified Simple Biosphere Model coupledwith the Top-down Representation of Interactive Foliage and Flora IncludingDynamics Model). This new model, SSiB4/TRIFFID-Fire, updating fire impact onthe terrestrial carbon cycle every 10 d, is then used to simulate theburned area during 1948–2014. The simulated global burned area in 2000–2014is 471.9 Mha yr−1, close to the estimate of 478.1 Mha yr−1 inGlobal Fire Emission Database v4s (GFED4s), with a spatial correlation of0.8. The SSiB4/TRIFFID-Fire reproduces temporal variations of the burnedarea at monthly to interannual scales. Specifically, it captures theobserved decline trend in northern African savanna fire and accuratelysimulates the fire seasonality in most major fire regions. The simulatedfire carbon emission is 2.19 Pg yr−1, slightly higher than the GFED4s(2.07 Pg yr−1). The SSiB4/TRIFFID-Fire is applied to assess the long-term fire impact onecosystem characteristics and surface energy budget by comparing model runswith and without fire (FIRE-ON minus FIRE-OFF). The FIRE-ON simulationreduces tree cover over 4.5 % of the global land surface, accompanied bya decrease in leaf area index and vegetation height by 0.10 m2 m−2and 1.24 m, respectively. The surface albedo and sensible heat are reducedthroughout the year, while latent heat flux decreases in the fire season butincreases in the rainy season. Fire results in an increase in surfacetemperature over most fire regions. 

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