A human-driven decline in global burned area Andela, N.; Morton, D. C.; Giglio, L. ...
Science (American Association for the Advancement of Science),
06/2017, Letnik:
356, Številka:
6345
Journal Article
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Fire is an essential Earth system process that alters ecosystem and atmospheric composition. Here we assessed long-term fire trends using multiple satellite data sets. We found that global burned ...area declined by 24.3 ± 8.8% over the past 18 years. The estimated decrease in burned area remained robust after adjusting for precipitation variability and was largest in savannas. Agricultural expansion and intensification were primary drivers of declining fire activity. Fewer and smaller fires reduced aerosol concentrations, modified vegetation structure, and increased the magnitude of the terrestrial carbon sink. Fire models were unable to reproduce the pattern and magnitude of observed declines, suggesting that they may overestimate fire emissions in future projections. Using economic and demographic variables, we developed a conceptual model for predicting fire in human-dominated landscapes.
Global warming is expected to considerably impact wildfire activity and aerosol emission release in the future. Due to their complexity, the future interactions between climate change, wildfire ...activity, emission release, and atmospheric aerosol processes are still uncertain. Here we use the process‐based fire model SPITFIRE within the global vegetation model JSBACH to simulate wildfire activity for present‐day climate conditions and future Representative Concentration Pathways (RCPs). The modeled fire emission fluxes and fire radiative power serve as input for the aerosol‐climate model ECHAM6‐HAM2, which has been extended by a semiempirical plume height parametrization. Our results indicate a general increase in extratropical and a decrease in tropical wildfire activity at the end of the 21st century. Changes in emission fluxes are most pronounced for the strongest warming scenario RCP8.5 (+49% in the extratropics, −37% in the tropics). Tropospheric black carbon (BC) concentrations are similarly affected by changes in emission fluxes and changes in climate conditions with regional variations of up to −50% to +100%. In the Northern Hemispheric extratropics, we attribute a mean increase in aerosol optical thickness of +0.031±0.002 to changes in wildfire emissions. Due to the compensating effects of fire intensification and more stable atmospheric conditions, global mean emission heights change by at most 0.3 km with only minor influence on BC long‐range transport. The changes in wildfire emission fluxes for the RCP8.5 scenario, however, may largely compensate the projected reduction in anthropogenic BC emissions by the end of the 21st century.
Key Points
Future wildfire activity increases in the extratropics and decreases in the tropics
Changes in wildfire activity compensate decreasing anthropogenic emissions
Future changes in emissions heights are negligible for the climate impact
Fires are a global phenomenon that impact climate and biogeochemical cycles, and interact with the biosphere, atmosphere and cryosphere. These impacts occur on a range of temporal and spatial scales ...and are difficult to quantify globally based solely on observations. Here we assess the role of fires in the climate system using model estimates of radiative forcing (RF) from global fires in pre-industrial, present day, and future time periods. Fire emissions of trace gases and aerosols are derived from Community Land Model simulations and then used in a series of Community Atmosphere Model simulations with representative emissions from the years 1850, 2000, and 2100. Additional simulations are carried out with fire emissions from the Global Fire Emission Database for a present-day comparison. These results are compared against the results of simulations with no fire emissions to compute the contribution from fires. We consider the impacts of fire on greenhouse gas concentrations, aerosol effects (including aerosol effects on biogeochemical cycles), and land and snow surface albedo. Overall, we estimate that pre-industrial fires were responsible for a RF of −1 W m−2 with respect to a pre-industrial climate without fires. The largest magnitude pre-industrial forcing from fires was the indirect aerosol effect on clouds (−1.6 W m−2). This was balanced in part by an increase in carbon dioxide concentrations due to fires (+0.83 W m−2). The RF of fires increases by 0.5 W m−2 from 1850 to 2000 and 0.2 W m−2 from 1850 to 2100 in the model representation from a combination of changes in fire activity and changes in the background environment in which fires occur, especially increases and decreases in the anthropogenic aerosol burden. Thus, fires play an important role in both the natural equilibrium climate and the climate perturbed by anthropogenic activity and need to be considered in future climate projections.
Pressure on land resources is expected to increase as global population continues to climb and the world becomes more affluent, swelling the demand for food. Changing climate may exert additional ...pressures on natural lands as present-day productive regions may shift, or soil quality may degrade, and the recent rise in demand for biofuels increases competition with edible crops for arable land. Given these projected trends there is a need to understand the global climate impacts of land use and land cover change (LULCC). Here we quantify the climate impacts of global LULCC in terms of modifications to the balance between incoming and outgoing radiation at the top of the atmosphere (radiative forcing, RF) that are caused by changes in long-lived and short-lived greenhouse gas concentrations, aerosol effects, and land surface albedo. We attribute historical changes in terrestrial carbon storage, global fire emissions, secondary organic aerosol emissions, and surface albedo to LULCC using simulations with the Community Land Model version 3.5. These LULCC emissions are combined with estimates of agricultural emissions of important trace gases and mineral dust in two sets of Community Atmosphere Model simulations to calculate the RF of changes in atmospheric chemistry and aerosol concentrations attributed to LULCC. With all forcing agents considered together, we show that 40% ( plus or minus 16%) of the present-day anthropogenic RF can be attributed to LULCC. Changes in the emission of non-CO2 greenhouse gases and aerosols from LULCC enhance the total LULCC RF by a factor of 2 to 3 with respect to the LULCC RF from CO2 alone. This enhancement factor also applies to projected LULCC RF, which we compute for four future scenarios associated with the Representative Concentration Pathways. We attribute total RFs between 0.9 and 1.9 W m-2 to LULCC for the year 2100 (relative to a pre-industrial state). To place an upper bound on the potential of LULCC to alter the global radiation budget, we include a fifth scenario in which all arable land is cultivated by 2100. This theoretical extreme case leads to a LULCC RF of 3.9 W m-2 ( plus or minus 0.9 W m-2), suggesting that not only energy policy but also land policy is necessary to minimize future increases in RF and associated climate changes.
The presence of multiple stable states has far‐reaching consequences for a system's susceptibility to disturbances, including the possibility of abrupt transitions between stable states. The ...occurrence of multiple stable states of vegetation is supported by ecological theory, models, and observations. Here we describe the occurrence of multiple stable states of tree cover in a global dynamic vegetation model and provide the first global picture on multiple stable states of tree cover due to a fire‐vegetation feedback. The multiple stable states occur in the transition zones between grasslands and forests, mainly in Africa and Asia. By sensitivity simulations and simplifying the relevant model equations we show that the occurrence of multiple states is caused by the sensitivity of the fire disturbance rate to the presence of woody plant types.
Key Points
This is the first study showing the potential of multiple stable states of vegetation globally based on a process‐based vegetation model
We identify the reason for multiple stable states as the sensitivity of fire to tree cover and illustrate it with a conceptual model
We identify the potential of multiple stable states in a region in Asia, which has not been in the focus of previous studies
We use the global circulation model ECHAM6 extended by the aerosol module HAM2 to simulate global patterns in wildfire emission heights. Prescribed plume heights in ECHAM6 are replaced by an ...implementation of a simple, semi-empirical plume height parametrization. In a first step, the global performance of the plume height parametrization is evaluated for plumes reported in the Multiangle Imaging Spectroradiometer (MISR) Plume Height Project (MPHP) data set. Our results show that the parametrization simulates a largely reasonable global distribution of plume heights. While the modeled global mean plume height (1411 plus or minus 646 m) is in good agreement with the observed mean (1382 plus or minus 702 m), the upper and lower tails of the plume height distribution tend to be slightly underrepresented. Furthermore, we compare plume heights simulated by the simple parametrization to a more complex, analytical plume model. Major differences in global plume height distributions are found for the lowest 1.5 km, but reasonable agreement is observed for higher plumes. In a second step, fire radiative power (FRP) as reported in the global fire assimilation system (GFAS) is used to simulate plume heights for observed fires globally for the period 2005-2011. The global fraction of simulated daytime plumes injecting emissions into the free troposphere (FT) ranges from 3.7 plus or minus 0.7 to 5.2 plus or minus 1.0 %. This range is comparable to results from observational studies, but it is much lower than results for prescribed plume heights in the ECHAM6-HAM2 standard setup. Nevertheless, occasionally deep emission injections exceeding 5-7 km in height are simulated for intense fires and favorable meteorological conditions. The application of a prescribed diurnal cycle in FRP turns out to be of minor importance. For a hypothetical doubling in FRP, moderate changes in plume heights of 100-400 m are simulated. These small changes indicate that a potential future increase in fire intensity will only slightly impact the emission heights on a global scale.
Prolonged sitting time has been associated with adverse health outcomes. Interventions at work may contribute to reduced sitting. The objective was to test if a multicomponent work-based intervention ...can reduce sitting time and the number of prolonged sitting periods (> 30 min), increase the number of sit-to-stand transitions and decrease waist circumference and body fat percentage among office workers. Primary outcomes were: change in sitting time, prolonged sitting periods and sit-to-stand transitions at follow-up 1 month later.
At four workplaces, 19 offices (317 workers in total) were cluster randomized for intervention or control. The intervention included the appointment of local ambassadors, management support, environmental changes, a lecture and a workshop. Sitting time was measured using an ActiGraph GT3X+ fixed on the thigh. Data were processed using Acti4 software providing data on time spent sitting, standing and doing other activities. Control participants were instructed to behave as usual. Follow-up measurements were obtained after 1 and 3 months.
At 1 and 3 months, total sitting time was 71 ( P < 0.001) and 48 min ( P < 0.001) lower per 8-h workday in the intervention group compared with the control group. At 1 month, the number of prolonged sitting periods was lower (-0.79/8-h workday, P < 0.001) and sit-to-stand transitions were higher (+14%/sitting hour, P = 0.001) in the intervention compared with the control group. After 3 months, trends persisted. The body fat percentage was lower by 0.61 percentage points ( P = 0.011) in the intervention group compared with the control group after 3 months.
The multicomponent workplace-based intervention was effective in reducing sitting time, prolonged sitting periods and body fat percentage, and in increasing the number of sit-to-stand transitions.
Global vegetation models traditionally treat anthropogenic land-use and land-cover changes (LULCCs) only as the changes in vegetation cover seen from one year to the next (net transitions). This ...approach ignores subgrid-scale processes such as shifting cultivation which do not affect the net vegetation distribution but which have an impact on the carbon budget. The differences in the carbon stocks feed back on processes like wildfires and desert formation. The simulations for the Coupled Model Intercomparison Project Phase 5 (CMIP5) all describe LULCCs using the "Land-Use Harmonization Dataset". Though this dataset describes such subgrid-scale processes (gross transitions), some of the CMIP5 models still use the traditional approach. Using JSBACH/CBALANCE - the land carbon component of the Max Planck Institute Earth System Model (MPI-ESM), this study demonstrates how this potentially leads to a severe underestimation of the carbon emissions from LULCCs Using net transitions lowers the average land-use emissions from 1.44 to 0.90 Pg C yr-1 (38%) during the historical period (1850-2005) - a total lowering by 85 Pg C. The difference between the methods is smaller in the RCP scenarios (2006-2100) but in RCP2.6 and RCP8.5 still cumulates to 30-40 Pg C (on average 0.3-0.4 Pg C yr-1 or 13-25%). In RCP4.5 essentially no difference between the methods is found. Results from models using net transitions are furthermore found to be sensitive to model resolution.
Landscape fires during the 21st century are expected to change in response to multiple agents of global change. Important controlling factors include climate controls on the length and intensity of ...the fire season, fuel availability, and fire management, which are already anthropogenically perturbed today and are predicted to change further in the future. An improved understanding of future fires will contribute to an improved ability to project future anthropogenic climate change, as changes in fire activity will in turn impact climate. In the present study we used a coupled-carbon-fire model to investigate how changes in climate, demography, and land use may alter fire emissions. We used climate projections following the SRES A1B scenario from two different climate models (ECHAM5/MPI-OM and CCSM) and changes in population. Land use and harvest rates were prescribed according to the RCP 45 scenario. In response to the combined effect of all these drivers, our model estimated, depending on our choice of climate projection, an increase in future (2075-2099) fire carbon emissions by 17 and 62% compared to present day (1985-2009). The largest increase in fire emissions was predicted for Southern Hemisphere South America for both climate projections. For Northern Hemisphere Africa, a region that contributed significantly to the global total fire carbon emissions, the response varied between a decrease and an increase depending on the climate projection. We disentangled the contribution of the single forcing factors to the overall response by conducting an additional set of simulations in which each factor was individually held constant at pre-industrial levels. The two different projections of future climate change evaluated in this study led to increases in global fire carbon emissions by 22% (CCSM) and 66% (ECHAM5/MPI-OM). The RCP 45 projection of harvest and land use led to a decrease in fire carbon emissions by -5%. The RCP 26 and RCP 60 harvest and landuse projections caused decreases around -20%. Changes in human ignition led to an increase of 20%. When we also included changes in fire management efforts to suppress fires in densely populated areas, global fire carbon emission decreased by -6% in response to changes in population density. We concluded from this study that changes in fire emissions in the future are controlled by multiple interacting factors. Although changes in climate led to an increase in future fire emissions this could be globally counterbalanced by coupled changes in land use, harvest, and demography.
The aerosol-climate model ECHAM5-HAM Stier, P.; Feichter, J.; Kinne, S. ...
Atmospheric chemistry and physics,
2005, Letnik:
5, Številka:
4
Journal Article
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The aerosol-climate modelling system ECHAM5-HAM is introduced. It is based on a flexible microphysical approach and, as the number of externally imposed parameters is minimised, allows the ...application in a wide range of climate regimes. ECHAM5-HAM predicts the evolution of an ensemble of microphysically interacting internally- and externally-mixed aerosol populations as well as their size-distribution and composition. The size-distribution is represented by a superposition of log-normal modes. In the current setup, the major global aerosol compounds sulfate (SU), black carbon (BC), particulate organic matter (POM), sea salt (SS), and mineral dust (DU) are included. The simulated global annual mean aerosol burdens (lifetimes) for the year 2000 are for SU: 0.80 Tg(S) (3.9 days), for BC: 0.11 Tg (5.4 days), for POM: 0.99 Tg (5.4 days), for SS: 10.5 Tg (0.8 days), and for DU: 8.28 Tg (4.6 days). An extensive evaluation with in-situ and remote sensing measurements underscores that the model results are generally in good agreement with observations of the global aerosol system. The simulated global annual mean aerosol optical depth (AOD) is with 0.14 in excellent agreement with an estimate derived from AERONET measurements (0.14) and a composite derived from MODIS-MISR satellite retrievals (0.16). Regionally, the deviations are not negligible. However, the main patterns of AOD attributable to anthropogenic activity are reproduced.
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