[Show abstract][Hide abstract] ABSTRACT: Accurate assessment of anthropogenic carbon dioxide (CO2) emissions and their redistribution among the atmosphere, ocean, and terrestrial biosphere is important to better understand the global carbon cycle, support the development of climate policies, and project future climate change. Here we describe data sets and a methodology to quantify all major components of the global carbon budget, including their uncertainties, based on the combination of a range of data, algorithms, statistics and model estimates and their interpretation by a broad scientific community. We discuss changes compared to previous estimates, consistency within and among components, alongside methodology and data limitations. CO2 emissions from fossil-fuel combustion and cement production (EFF) are based on energy statistics, while emissions from land-use change (ELUC), mainly deforestation, are based on combined evidence from land-cover change data, fire activity associated with deforestation, and models. The global atmospheric CO2 concentration is measured directly and its rate of growth (GATM) is computed from the annual changes in concentration. The mean ocean CO2 sink (SOCEAN) is based on observations from the 1990s, while the annual anomalies and trends are estimated with ocean models. The variability in SOCEAN is evaluated for the first time in this budget with data products based on surveys of ocean CO2 measurements. The global residual terrestrial CO2 sink (SLAND) is estimated by the difference of the other terms of the global carbon budget and compared to results of independent dynamic global vegetation models forced by observed climate, CO2 and land cover change (some including nitrogen–carbon interactions). All uncertainties are reported as ±1σ, reflecting the current capacity to characterise the annual estimates of each component of the global carbon budget. For the last decade available (2003–2012), EFF was 8.6 ± 0.4 GtC yr−1, ELUC 0.9 ± 0.5 GtC yr−1, GATM 4.3 ± 0.1 GtC yr−1, SOCEAN 2.5 ± 0.5 GtC yr−1, and SLAND 2.8 ± 0.8 GtC yr−1. For year 2012 alone, EFF grew to 9.7 ± 0.5 GtC yr−1, 2.2% above 2011, reflecting a continued growing trend in these emissions, GATM was 5.1 ± 0.2 GtC yr−1, SOCEAN was 2.9 ± 0.5 GtC yr−1, and assuming an ELUC of 1.0 ± 0.5 GtC yr−1 (based on the 2001–2010 average), SLAND was 2.7 ± 0.9 GtC yr−1. GATM was high in 2012 compared to the 2003–2012 average, almost entirely reflecting the high EFF. The global atmospheric CO2 concentration reached 392.52 ± 0.10 ppm averaged over 2012. We estimate that EFF will increase by 2.1% (1.1–3.1%) to 9.9 ± 0.5 GtC in 2013, 61% above emissions in 1990, based on projections of world gross domestic product and recent changes in the carbon intensity of the economy. With this projection, cumulative emissions of CO2 will reach about 535 ± 55 GtC for 1870–2013, about 70% from EFF (390 ± 20 GtC) and 30% from ELUC (145 ± 50 GtC).
[Show abstract][Hide abstract] ABSTRACT: We investigated the millennial variability (1000 AD-2000 AD) of global Biogenic Volatile Organic Compound (BVOC) emissions by using two independent numerical models: The Model of Emissions of Gases and Aerosols from Nature (MEGAN), for isoprene, monoterpene and sesquiterpene and Lund-Potsdam-Jena General Ecosystem Simulator (LPJ-GUESS), for isoprene and monoterpenes. We found the millennial trends of global isoprene emissions to be mostly affected by land cover and atmospheric carbon dioxide changes, whereas monoterpene and sesquiterpene emission trends were dominated by temperature change. Isoprene emissions declined substantially in regions with large and rapid land cover change. In addition, isoprene emission sensitivity to drought proved to have significant short-term global effects. By the end of the past millennium MEGAN isoprene emissions were 634 TgC yr−1 (13% and 19% less than during 1750–1850 and 1000–1200, respectively) and LPJ-GUESS emissions were 323 TgC yr−1 (15% and 20% less than during 1750–1850 and 1000–1200, respectively). Monoterpene emissions were 89 TgC yr−1 (10% and 6% higher than during 1750–1850 and 1000–1200, respectively) in MEGAN, and 24 TgC yr−1 (2% higher and 5% less thanduring 1750–1850 and 1000–1200, respectively) in LPJ-GUESS. MEGAN sesquiterpene emissions were 36 TgC yr−1 (10% and 4% higher than during 1750–1850 and 1000–1200, respectively). Although both models capture similar emission trends, the magnitude of the emissions are different. This highlights the importance of building better constraints on VOC emissions from terrestrial vegetation.
Journal of Geophysical Research: Atmospheres. 05/2014;
[Show abstract][Hide abstract] ABSTRACT: This paper, developed under the framework of the RECCAP initiative, aims at providing improved estimates of the carbon and GHG (CO2, CH4 and N2O) balance of continental Africa. The various components and processes of the African carbon and GHG budget are considered, existing data reviewed, and new data from different methodologies (inventories, ecosystem flux measurements, models, and atmospheric inversions) presented. Uncertainties are quantified and current gaps and weaknesses in knowledge and monitoring systems described in order to guide future requirements. The majority of results agree that Africa is a small sink of carbon on an annual scale, with an average value of −0.61 ± 0.58 Pg C yr−1. Nevertheless, the emissions of CH4 and N2O may turn Africa into a net source of radiative forcing in CO2 equivalent terms. At sub-regional level, there is significant spatial variability in both sources and sinks, due to the diversity of biomes represented and differences in the degree of anthropic impacts. Southern Africa is the main source region; while central Africa, with its evergreen tropical forests, is the main sink. Emissions from land-use change in Africa are significant (around 0.32 ± 0.05 Pg C yr−1), even higher than the fossil fuel emissions: this is a unique feature among all the continents. There could be significant carbon losses from forest land even without deforestation, resulting from the impact of selective logging. Fires play a significant role in the African carbon cycle, with 1.03 ± 0.22 Pg C yr−1 of carbon emissions, and 90% originating in savannas and dry woodlands. A large portion of the wild fire emissions are compensated by CO2 uptake during the growing season, but an uncertain fraction of the emission from wood harvested for domestic use is not. Most of these fluxes have large interannual variability, on the order of ±0.5 Pg C yr−1 in standard deviation, accounting for around 25% of the year-to-year variation in the global carbon budget.
Despite the high uncertainty, the estimates provided in this paper show the important role that Africa plays in the global carbon cycle, both in terms of absolute contribution, and as a key source of interannual variability.
[Show abstract][Hide abstract] ABSTRACT: The LPJ-GUESS dynamic vegetation model uniquely combines an individual- and patch-based representation of vegetation dynamics with ecosystem biogeochemical cycling from regional to global scales. We present an updated version that includes plant and soil N dynamics, analysing the implications of accounting for C–N interactions on predictions and performance of the model. Stand structural dynamics and allometric scaling of tree growth suggested by global databases of forest stand structure and development were well reproduced by the model in comparison to an earlier multi-model study. Accounting for N cycle dynamics improved the goodness of fit for broadleaved forests. N limitation associated with low N-mineralisation rates reduces productivity of cold-climate and dry-climate ecosystems relative to mesic temperate and tropical ecosystems. In a model experiment emulating free-air CO2 enrichment (FACE) treatment for forests globally, N limitation associated with low N-mineralisation rates of colder soils reduces CO2 enhancement of net primary production (NPP) for boreal forests, while some temperate and tropical forests exhibit increased NPP enhancement. Under a business-as-usual future climate and emissions scenario, ecosystem C storage globally was projected to increase by ca. 10%; additional N requirements to match this increasing ecosystem C were within the high N supply limit estimated on stoichiometric grounds in an earlier study. Our results highlight the importance of accounting for C–N interactions in studies of global terrestrial N cycling, and as a basis for understanding mechanisms on local scales and in different regional contexts.
[Show abstract][Hide abstract] ABSTRACT: Recently a considerable amount of effort has been put into quantifying how interactions of the carbon and nitrogen cycle affect future terrestrial carbon sinks. Dynamic vegetation models, representing the nitrogen cycle with varying degree of complexity, have shown diverging constraints of nitrogen dynamics on future carbon sequestration. In this study, we use the dynamic vegetation model LPJ-GUESS to evaluate how population dynamics and resource competition between plant functional types, combined with nitrogen dynamics, have influenced the terrestrial carbon storage in the past and to investigate how terrestrial carbon and nitrogen dynamics might change in the future (1850 to 2100; one exemplary "business-as-usual" climate scenario). Single factor model experiments of CO2 fertilisation and climate change show generally similar directions of the responses of C-N interactions, compared to the C-only version of the model, as documented in previous studies. Under a RCP 8.5 scenario, nitrogen limitation suppresses potential CO2 fertilisation, reducing the cumulative net ecosystem carbon uptake between 1850 and 2100 by 61%, and soil warming-induced increase in nitrogen mineralisation reduces terrestrial carbon loss by 31%. When environmental changes are considered conjointly, carbon sequestration is limited by nitrogen dynamics until present. However, during the 21st century nitrogen dynamics induce a net increase in carbon sequestration, resulting in an overall larger carbon uptake of 17% over the full period. This contradicts earlier model results that showed an 8 to 37% decrease in carbon uptake, questioning the often stated assumption that projections of future terrestrial C dynamics from C-only models are too optimistic.
[Show abstract][Hide abstract] ABSTRACT: This chapter provides an overview of the current knowledge on aerosols in
the marine atmosphere and the effects of aerosols on climate and on processes
in the oceanic surface layer. Aerosol particles in the marine atmosphere originate
predominantly from direct production at the sea surface due to the interaction
between wind and waves (sea spray aerosol, or SSA) and indirect production
by gas to particle conversion. These aerosols are supplemented by aerosols
produced over the continents, as well as aerosols emitted by volcanoes and ship
traffic, a large part of it being deposited to the ocean surface by dry and wet
deposition. The SSA sources, chemical composition and ensuing physical and
optical effects, are discussed. An overview is presented of continental sources and
their ageing and mixing processes during transport. The current status of our
knowledge on effects of marine aerosols on the Earth radiative balance, both
direct by their interaction with solar radiation and indirect through their effects on
cloud properties, is discussed. The deposition on the ocean surface of some key
species, such as nutrients, their bioavailability and how they impact biogeochemical
cycles are shown and discussed through different time and space scales
[Show abstract][Hide abstract] ABSTRACT: Here we present the results from an intercomparison of multiple global gridded crop models (GGCMs) within the framework of the Agricultural Model Intercomparison and Improvement Project and the Inter-Sectoral Impacts Model Intercomparison Project. Results indicate strong negative effects of climate change, especially at higher levels of warming and at low latitudes; models that include explicit nitrogen stress project more severe impacts. Across seven GGCMs, five global climate models, and four representative concentration pathways, model agreement on direction of yield changes is found in many major agricultural regions at both low and high latitudes; however, reducing uncertainty in sign of response in mid-latitude regions remains a challenge. Uncertainties related to the representation of carbon dioxide, nitrogen, and high temperature effects demonstrated here show that further research is urgently needed to better understand effects of climate change on agricultural production and to devise targeted adaptation strategies.
Proceedings of the National Academy of Sciences 12/2013; · 9.81 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We investigated atmospheric aerosol particle dynamics in a boreal forest
zone in Northern Scandinavia. We used aerosol size distribution data
measured with either a Differential Mobility Particle Sizer (DMPS) or
Scanning Mobility Particle Sizer (SMPS) at three stations
(Värriö, Pallas and Abisko), and combined these data with the
HYSPLIT air mass trajectory analysis. We compared three approaches:
analysis of new particle formation events, investigation of air masses
transport from the ocean to individual stations with different over-land
transport times, and analysis of changes in aerosol particle size
distributions during the air masses transport from one measurement
station to another. Aitken mode particles were found to have an apparent
average growth rate of 0.6-0.7 nm h-1 when the air masses
travelled over land. Particle growth rates during the NPF events were
3-6 times higher than the apparent particle growth. When comparing
aerosol dynamics between the different stations for different over-land
transport times, no major differences were found except that in Abisko
the new particle formation events were observed to take place in air
masses having shorter over-land times than at the other stations. We
speculate that this is related to the meteorological differences along
the paths of air masses caused by the land surface topology. When
comparing between air masses travelling the east-to-west direction to
those traveling the west-to-east directions, clear differences in the
aerosol dynamics were seen. Our results suggest that the condensation
growth has an important role in aerosol dynamics also when new particle
formation is not evident.
ATMOSPHERIC CHEMISTRY AND PHYSICS 12/2013; 13:11887–11903. · 5.51 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We describe the implementation of a biochemical model of isoprene
emission that depends on the electron requirement for isoprene synthesis
into the Farquhar-Ball-Berry leaf model of photosynthesis and stomatal
conductance that is embedded within a global chemistry-climate
simulation framework. The isoprene production is calculated as a
function of electron transport-limited photosynthesis, intercellular and
atmospheric carbon dioxide concentration, and canopy temperature. The
vegetation biophysics module computes the photosynthetic uptake of
carbon dioxide coupled with the transpiration of water vapor and the
isoprene emission rate at the 30 min physical integration time step of
the global chemistry-climate model. In the model, the rate of carbon
assimilation provides the dominant control on isoprene emission
variability over canopy temperature. A control simulation representative
of the present-day climatic state that uses 8 plant functional types
(PFTs), prescribed phenology and generic PFT-specific isoprene emission
potentials (fraction of electrons available for isoprene synthesis)
reproduces 50% of the variability across different ecosystems and
seasons in a global database of 28 measured campaign-average fluxes.
Compared to time-varying isoprene flux measurements at 9 select sites,
the model authentically captures the observed variability in the 30 min
average diurnal cycle (R2 = 64-96%) and simulates the flux
magnitude to within a factor of 2. The control run yields a global
isoprene source strength of 451 TgC yr-1 that increases by
30% in the artificial absence of plant water stress and by 55% for
potential natural vegetation.
ATMOSPHERIC CHEMISTRY AND PHYSICS 10/2013; 13(20):10243-10269. · 5.51 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Aim: To reconstruct spatial and temporal patterns of European fire activity during the Holocene and to explore their potential drivers, by relating biomass burning to simulated biotic and abiotic parameters.
Methods: Holocene fire activity was investigated based on 156 sedimentary charcoal records from lakes and peat bogs across Europe. Charcoal data covering the last 9000 years were statistically compared with palaeoclimate data derived from the Max Planck Institute for Meteorology/University of Wisconsin-Madison Earth System Model, with vegetation and fire indices simulated with the dynamic vegetation model LPJ-GUESS
and with two independent scenarios of past anthropogenic land-cover change.
Results: The combined sedimentary charcoal records suggest that there was little fire activity during the early and the middle Holocene compared with recent millennia. A progressive increase in fire frequency began around 3500 cal. yr BP. and continues into the late Holocene. Biomass burning rose sharply from 250 cal. yr
BP onwards, reaching a maximum during the early Industrial Era and then declining abruptly. When considering the whole Holocene, the long-term control of fire is best explained by anthropogenic land-cover change, litter availability and temperature-related parameters.
Main conclusions: While the general patterns found across Europe suggest the primary role of vegetation, precipitation and temperature-related parameters in explaining fire dynamics during the early Holocene, the increase in fire activity observed in the mid–late Holocene is mainly related to anthropogenic land-cover changes, followed by vegetation and temperature-related parameters. The 20th-century decline in biomass burning seems to be due to increased landscape fragmentation and active fire suppression policies. Our hypothesis that human activities played a primary role in Holocene biomass burning across Europe could
be tested by improved palaeoclimate reconstructions and more refined representations of anthropogenic fires in climate and vegetation models.
Global Ecology and Biogeography 07/2013; 22(12):1248–1260. · 7.22 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Atmospheric aerosol particles influence the climate system directly by
scattering and absorbing solar radiation, and indirectly by acting as
cloud condensation nuclei. Apart from black carbon aerosol, aerosols
cause a negative radiative forcing at the top of the atmosphere and
substantially mitigate the warming caused by greenhouse gases. In the
future, tightening of controls on anthropogenic aerosol and precursor
vapour emissions to achieve higher air quality may weaken this
beneficial effect. Natural aerosols, too, might affect future warming.
Here we analyse long-term observations of concentrations and
compositions of aerosol particles and their biogenic precursor vapours
in continental mid- and high-latitude environments. We use measurements
of particle number size distribution together with boundary layer
heights derived from reanalysis data to show that the boundary layer
burden of cloud condensation nuclei increases exponentially with
temperature. Our results confirm a negative feedback mechanism between
the continental biosphere, aerosols and climate: aerosol cooling effects
are strengthened by rising biogenic organic vapour emissions in response
to warming, which in turn enhance condensation on particles and their
growth to the size of cloud condensation nuclei. This natural growth
mechanism produces roughly 50% of particles at the size of cloud
condensation nuclei across Europe. We conclude that biosphere-atmosphere
interactions are crucial for aerosol climate effects and can
significantly influence the effects of anthropogenic aerosol emission
controls, both on climate and air quality.
[Show abstract][Hide abstract] ABSTRACT: We use formaldehyde (HCHO) vertical column measurements from the Scanning
Imaging Absorption spectrometer for Atmospheric Chartography (SCIAMACHY) and
Ozone Monitoring Instrument (OMI), and a nested-grid version of the GEOS-Chem
chemistry transport model, to infer an ensemble of top-down isoprene emission estimates from tropical South America during 2006, using different model configurations and assumptions in the HCHO air-mass factor (AMF) calculation. Scenes affected by biomass burning are removed on a daily basis using fire count observations, and we use the local model sensitivity to identify locations where the impact of spatial smearing is small, though this comprises spatial coverage over the region. We find that the use of the HCHO column data more tightly constrains the ensemble isoprene emission range from
27–61 Tg C to 31–38 Tg C for SCIAMACHY, and 45–104 Tg C to 28–38 Tg C for OMI.
Median uncertainties of the top-down emissions are about 60–260% for SCIAMACHY,
and 10–90% for OMI. We find that the inferred emissions are most sensitive to uncertainties in cloud fraction and cloud top pressure (differences of ˙10%), the a priori
isoprene emissions (˙20%), and the HCHO vertical column retrieval (˙30%).
Construction of continuous top-down emission maps generally improves GEOS-Chem’s
simulation of HCHO columns over the region, with respect to both the SCIAMACHY
and OMI data. However, if local time top-down emissions are scaled to monthly mean
values, the annual emission inferred from SCIAMACHY are nearly twice those from
OMI. This difference cannot be explained by the different sampling of the sensors or
uncertainties in the AMF calculation.
Journal of Geophysical Research: Atmospheres. 06/2013; 118:n/a-n/a.
[Show abstract][Hide abstract] ABSTRACT: This paper, developed under the framework of the RECCAP initiative, aims at providing improved estimates of the carbon and GHG (CO2, CH4 and N2O) balance of continental Africa. The various components and processes of the African carbon and GHG budget were considered, and new and available data derived by different methodologies (based on inventories, ecosystem fluxes, models, and atmospheric inversions) were integrated. The related uncertainties were quantified and current gaps and weakness in knowledge and in the monitoring systems were also considered in order to provide indications on the future requirements. The vast majority of the results seem to agree that Africa is probably a small sink of carbon on an annual scale, with an average value of −0.61 ± 0.58 Pg C yr−1. Nevertheless the emissions of CH4 and N2O may turn Africa into a source in terms of CO2 equivalents. At sub-regional level there is a significant spatial variability in both sources and sinks, mainly due to the biome's differences and the different anthropic impacts, with southern Africa as the main source and central Africa, with its evergreen tropical forests, as the main sink. Emissions from land use change in Africa are significant (around 0.32 ± 0.05 Pg C yr−1) and even higher than the fossil fuel ones; this is a unique feature among all the continents. In addition there can be significant carbon losses from land even without changes in the land use (forest), as results from the impact of selective logging. Fires also play a significant role, with 1.03 ± 0.22 Pg C yr−1 of carbon emissions, mainly (90%) originated by savanna and woodland burning. But whether fire carbon emissions are compensated by CO2 uptake during the growing season, or are a non-reversible loss of CO2, remains unclear. Most of these figures are subjected to a significant interannual variability, on the order of ± 0.5 Pg C yr−1 in standard deviation, accounting for around 25% of the year-to-year variation in the global carbon budget.
These results, even if still highly uncertain, show the important role that Africa plays in the carbon cycle at global level, both in terms of absolute values and variability.
[Show abstract][Hide abstract] ABSTRACT: Accurate estimation of gross primary production (GPP) of savanna woodlands is needed for evaluating the
terrestrial carbon cycle at various spatial and temporal scales. The eddy covariance (EC) technique provides
continuous measurements of net CO2 exchange (NEE) between terrestrial ecosystems and the atmosphere.
Only a few flux tower sites were run in Africa and very limited observational data of savanna woodlands in
Africa are available. Although several publications have reported on the seasonal dynamics and interannual
variation of GPP of savanna vegetation through partitioning the measured NEE data, current knowledge
about GPP and phenology of savanna ecosystems is still limited. This study focused on two savanna woodland
flux tower sites in Botswana and Zambia, representing two dominant savanna woodlands (mopane and
miombo) and climate patterns (semi-arid and semi-humid) in Southern Africa. Phenology of these savanna
woodlands was delineated from three vegetation indices derived from Moderate Resolution Imaging
Spectroradiometer (MODIS) and GPP estimated from eddy covariance measurements at flux tower sites
(GPPEC). The Vegetation Photosynthesis Model (VPM), which is driven by satellite images and meteorological
data, was also evaluated, and the results showed that the VPM-based GPP estimates (GPPVPM) were able to
track the seasonal dynamics of GPPEC. The total GPPVPM and GPPEC within the plant growing season defined
by a water-related vegetation index differed within the range of ±6%. This study suggests that the VPM is
a valuable tool for estimating GPP of semi-arid and semi-humid savanna woodland ecosystems in Southern
Remote Sensing of Environment 05/2013; 135:189–201. · 5.10 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Measured aerosol size distributions from three measurement stations and
modeled air mass trajectory data were combined to study aerosol dynamics
in the boreal forest zone in Northern Scandinavia. Three approaches were
used: investigation of new particle formation events, analysis of air
masses arriving from ocean to continent, and study of changes in the
aerosol size distributions when air masses travel from one measurement
site to another. The statistical analysis of air masses travelling
either from the Atlantic Ocean to measurement sites or from one site to
another showed that on average the condensational growth was present
during the summer season, and it was not restricted only to the days
when evident new particle formation was observed. The rate of this
average apparent growth of particle diameter was 3-7 times smaller than
the growth rate of nucleation mode particles during the new particle
[Show abstract][Hide abstract] ABSTRACT: In this study we report on water and carbon dioxide fluxes, measured
using the eddy covariance (EC) technology, from different ecosystems in
Sub-Saharan Africa. These sites differed in ecosystem type (C3 plant
dominated woodlands to C4 plant dominated grass savannas) and covered
the very dry regions of the Sahel (250 mm rainfall, Sudan), the tropical
areas in Central Africa (1650 mm in Uganda) further south to the
subtropical areas in Botswana, Zambia and South Africa (400-900 mm in
precipitation). The link between water and carbon dioxide fluxes were
evaluated for time periods (see also the corresponding abstract by
Bruemmer et al.) without water limitation during the peak growing
season. Our results show that plant stomata control ecosystem scale
water and carbon dioxide fluxes and mediate between plant growth and
plant survival. On continental scale, this switch between maximizing
carbon uptake and minimizing water losses, from here on called the
"Carbon-Water-Tipping Point" was positively correlated to the mean
annual growing season temperature at each site. Even though similar
responses of plants were shown at the individual leaf-level scale this
has to our knowledge not yet been shown at the ecosystem scale further
suggesting a long-term adaptation of the complete ecosystems to certain
climatic regions. It remains unclear how this adaption will influence
the ecosystem response to ongoing climate change and according
temperature increases and changes in precipitation.
[Show abstract][Hide abstract] ABSTRACT: Fires are expected to change under future climate change, climatic fire
is is increasing due to increase in droughts and heat waves affecting
vegetation productivity and ecosystem function. Vegetation productivity
influences fuel production, but can also limit fire spread.
Vegetation-fire models allow investigating the interaction between
wildfires and vegetation dynamics, thus non-linear effects between
changes in fuel composition and production on fire as well as changes in
fire regimes on fire-related plant mortality and fuel combustion. Here
we present results from simulation experiments, where the
vegetation-fire models LPJmL-SPITFIRE and LPJ-GUESS are applied to
future climate change scenarios from regional climate models in Europe
and Northern Africa. Climate change impacts on fire regimes, vegetation
dynamics and carbon fluxes are quantified and presented. New fire-prone
regions are mapped and changes in fire regimes of ecosystems with a
long-fire history are analyzed. Fuel limitation is likely to increase
in Mediterranean-type ecosystems, indicating non-linear connection
between increasing fire risk and fuel production. Increased warming in
temperate ecosystems in Eastern Europe and continued fuel production
leads to increases not only in climatic fire risk, but also area burnt
and biomass burnt. This has implications for fire management, where
adaptive capacity to this new vulnerability might be limited.
[Show abstract][Hide abstract] ABSTRACT: The effects of climatic factors and vegetation type on
evapotranspiration (E) and water use efficiency (WUE) were analyzed
using tower-based eddy-covariance (EC) data of eleven African sites (22
site years) located across a continental-scale transect. The seasonal
pattern of E was closely linked to growing-season length and rainfall
distribution. Although annual precipitation (P) was highly variable
among sites (290 to 1650 mm), minimum annual E was not less than 250 mm
and reached a maximum of 900 mm where annual P exceeded 1200 mm.
Site-specific interannual variability in E could be explained by either
changes in total P or variations in solar irradiance. At some sites, a
highly positive linear correlation was found between monthly sums of E
and net radiation (Rn), whereas a hysteretic relationship at other sites
indicated that E lagged behind the typical seasonal progression of Rn.
Results of a cross-correlation analysis between daily (24-h) E and Rn
revealed that site-specific lag times were between 0 days and up to a
few weeks depending on the lag of vapor pressure deficit (D) behind Rn
and vegetation type. Physiological parameters (e.g. mean dry-foliage
Priestley-Taylor alpha) implied that stomatal limitation to
transpiration prevailed. During the rainy season, a strong linear
correlation between monthly mean values of gross primary production
(GPP) and E resulted in water use efficiency being constant with lower
values at grass-dominated sites (~2 to ~3.5 g C kg-1 H2O) than at
natural woodland sites and plantations (~4.5 to ~6 g C kg-1 H2O).
[Show abstract][Hide abstract] ABSTRACT: Widlfires are a major component of most terrestrial ecosystems, and
constitute a major hazard for humans. However, the way people influence
fire occurrence remains poorly understood, in particular at the global
scale. While local studies have found increasing numbers of fires near
settlements and roads, a few regional studies for Africa have found that
fire frequency (fractional area burned) generally decreases with
increasing population density. Charcoal records have also been cited as
evidence for a decline in fire occurrence during the last 100 years,
which coincided with rapid population growth. Here, we present a global
non-linear parameter optimisation complete with posterior uncertainty
estimates based on three global burned area data sets. We find that only
for sparsely populated areas, fire frequency increases by between 10 and
20% for an increase in population density from 0 to 0.1 people per km2.
Including all areas not dominated by agriculture results in a decline of
fire frequency with population density independent of the burned area
data used, with fires essentially suppressed for more than 100 people
per km2. After applying the results to historical population data and
observed burned area, we infer a decline of global burned area by
between 4 and 16% (95% confidence interval) since 1800, with most of the
decline happening after 1950.