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Fire regimes are changing across the globe, with new wildfire behaviour phenomena and increasing impacts felt, especially in ecosystems without clear adaptations to wildfire. These trends pose significant challenges to the scientific community in understanding and communicating these changes and their implications, particularly where we lack underlying scientific evidence to inform decision-making. Here, we present a perspective on priority directions for wildfire science research—through the lens of academic and government wildfire scientists from a historically wildfire-prone (USA) and emerging wildfire-prone (UK) country. Key topic areas outlined during a series of workshops in 2023 were as follows: (A) understanding and predicting fire occurrence, fire behaviour and fire impacts; (B) increasing human and ecosystem resilience to fire; and (C) understanding the atmospheric and climate impacts of fire. Participants agreed on focused research questions that were seen as priority scientific research gaps. Fire behaviour was identified as a central connecting theme that would allow critical advances to be made across all topic areas. These findings provide one group of perspectives to feed into a more transdisciplinary outline of wildfire research priorities across the diversity of knowledge bases and perspectives that are critical in addressing wildfire research challenges under changing fire regimes. This article is part of the theme issue ‘Novel fire regimes under climate changes and human influences: impacts, ecosystem responses and feedbacks’. 

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. 

Abstract. Global fire-vegetation models are widely used to assessimpacts of environmental change on fire regimes and the carbon cycle and toinfer relationships between climate, land use and fire. However,differences in model structure and parameterizations, in both the vegetationand fire components of these models, could influence overall modelperformance, and to date there has been limited evaluation of how welldifferent models represent various aspects of fire regimes. The Fire ModelIntercomparison Project (FireMIP) is coordinating the evaluation ofstate-of-the-art global fire models, in order to improve projections of firecharacteristics and fire impacts on ecosystems and human societies in thecontext of global environmental change. Here we perform a systematicevaluation of historical simulations made by nine FireMIP models to quantifytheir ability to reproduce a range of fire and vegetation benchmarks. TheFireMIP models simulate a wide range in global annual total burnt area(39–536 Mha) and global annual fire carbon emission (0.91–4.75 Pg C yr−1) for modern conditions (2002–2012), but most of the range in burntarea is within observational uncertainty (345–468 Mha). Benchmarking scoresindicate that seven out of nine FireMIP models are able to represent thespatial pattern in burnt area. The models also reproduce the seasonality inburnt area reasonably well but struggle to simulate fire season length andare largely unable to represent interannual variations in burnt area.However, models that represent cropland fires see improved simulation offire seasonality in the Northern Hemisphere. The three FireMIP models whichexplicitly simulate individual fires are able to reproduce the spatialpattern in number of fires, but fire sizes are too small in key regions, andthis results in an underestimation of burnt area. The correct representationof spatial and seasonal patterns in vegetation appears to correlate with abetter representation of burnt area. The two older fire models included inthe FireMIP ensemble (LPJ–GUESS–GlobFIRM, MC2) clearly perform less wellglobally than other models, but it is difficult to distinguish between theremaining ensemble members; some of these models are better at representingcertain aspects of the fire regime; none clearly outperforms all othermodels across the full range of variables assessed. 

Abstract. Understanding and quantifying the global methane (CH4) budgetis important for assessing realistic pathways to mitigate climate change.Atmospheric emissions and concentrations of CH4 continue to increase,making CH4 the second most important human-influenced greenhouse gas interms of climate forcing, after carbon dioxide (CO2). The relativeimportance of CH4 compared to CO2 depends on its shorteratmospheric lifetime, stronger warming potential, and variations inatmospheric growth rate over the past decade, the causes of which are stilldebated. Two major challenges in reducing uncertainties in the atmosphericgrowth rate arise from the variety of geographically overlapping CH4sources and from the destruction of CH4 by short-lived hydroxylradicals (OH). To address these challenges, we have established aconsortium of multidisciplinary scientists under the umbrella of the GlobalCarbon Project to synthesize and stimulate new research aimed at improvingand regularly updating the global methane budget. Following Saunois et al. (2016), we present here the second version of the living review paperdedicated to the decadal methane budget, integrating results of top-downstudies (atmospheric observations within an atmospheric inverse-modellingframework) and bottom-up estimates (including process-based models forestimating land surface emissions and atmospheric chemistry, inventories ofanthropogenic emissions, and data-driven extrapolations). For the 2008–2017 decade, global methane emissions are estimated byatmospheric inversions (a top-down approach) to be 576 Tg CH4 yr−1 (range 550–594, corresponding to the minimum and maximumestimates of the model ensemble). Of this total, 359 Tg CH4 yr−1 or∼ 60 % is attributed to anthropogenic sources, that isemissions caused by direct human activity (i.e. anthropogenic emissions; range 336–376 Tg CH4 yr−1 or 50 %–65 %). The mean annual total emission for the new decade (2008–2017) is29 Tg CH4 yr−1 larger than our estimate for the previous decade (2000–2009),and 24 Tg CH4 yr−1 larger than the one reported in the previousbudget for 2003–2012 (Saunois et al., 2016). Since 2012, global CH4emissions have been tracking the warmest scenarios assessed by theIntergovernmental Panel on Climate Change. Bottom-up methods suggest almost30 % larger global emissions (737 Tg CH4 yr−1, range 594–881)than top-down inversion methods. Indeed, bottom-up estimates for naturalsources such as natural wetlands, other inland water systems, and geologicalsources are higher than top-down estimates. The atmospheric constraints onthe top-down budget suggest that at least some of these bottom-up emissionsare overestimated. The latitudinal distribution of atmosphericobservation-based emissions indicates a predominance of tropical emissions(∼ 65 % of the global budget, < 30∘ N)compared to mid-latitudes (∼ 30 %, 30–60∘ N)and high northern latitudes (∼ 4 %, 60–90∘ N). The most important source of uncertainty in the methanebudget is attributable to natural emissions, especially those from wetlandsand other inland waters. Some of our global source estimates are smaller than those in previouslypublished budgets (Saunois et al., 2016; Kirschke et al., 2013). In particular wetland emissions are about 35 Tg CH4 yr−1 lower due toimproved partition wetlands and other inland waters. Emissions fromgeological sources and wild animals are also found to be smaller by 7 Tg CH4 yr−1 by 8 Tg CH4 yr−1, respectively. However, the overalldiscrepancy between bottom-up and top-down estimates has been reduced byonly 5 % compared to Saunois et al. (2016), due to a higher estimate of emissions from inland waters, highlighting the need for more detailed research on emissions factors. Priorities for improving the methanebudget include (i) a global, high-resolution map of water-saturated soilsand inundated areas emitting methane based on a robust classification ofdifferent types of emitting habitats; (ii) further development ofprocess-based models for inland-water emissions; (iii) intensification ofmethane observations at local scales (e.g., FLUXNET-CH4 measurements)and urban-scale monitoring to constrain bottom-up land surface models, andat regional scales (surface networks and satellites) to constrainatmospheric inversions; (iv) improvements of transport models and therepresentation of photochemical sinks in top-down inversions; and (v) development of a 3D variational inversion system using isotopic and/orco-emitted species such as ethane to improve source partitioning. The data presented here can be downloaded fromhttps://doi.org/10.18160/GCP-CH4-2019 (Saunois et al., 2020) and from theGlobal Carbon Project.