17 research outputs found
Effect of (quasi-)optimum model parameter sets and model characteristics on future discharge projection of two basins from Europe and Asia
Impact of deforestation and climate on the Amazon Basinâs above-ground biomass during 1993-2012
Abstract Since the 1960s, large-scale deforestation in the Amazon Basin has contributed to rising global CO2 concentrations and to climate change. Recent advances in satellite observations enable estimates of gross losses of above-ground biomass (AGB) stocks due to deforestation. However, because of simultaneous regrowth, the net contribution of deforestation emissions to rising atmospheric CO2 concentrations is poorly quantified. Climate change may also reduce the potential for forest regeneration in previously disturbed regions. Here, we address these points of uncertainty with a machine-learning approach that combines satellite observations of AGB with climate data across the Amazon Basin to reconstruct annual maps of potential AGB during 1993â2012, the above-ground C storage potential of the undisturbed landscape. We derive a 2.2 Pg C loss of AGB over the study period, and, for the regions where these losses occur, we estimate a 0.7 Pg C reduction in potential AGB. Thus, climate change has led to a decline of ~1/3 in the capacity of these disturbed forests to recover and recapture the C lost in disturbances during 1993â2012. Our approach further shows that annual variations in land use change mask the natural relationship between the El Niño/Southern Oscillation and AGB stocks in disturbed regions
Response of microbial decomposition to spin-up explains CMIP5 soil carbon range until 2100
Soil carbon storage simulated by the Coupled Model Intercomparison Project
(CMIP5) models varies 6-fold for the present day. Here, we confirm earlier
work showing that this range already exists at the beginning of the CMIP5 historical
simulations. We additionally show that this range is largely determined by
the response of microbial decomposition during each model's spin-up procedure
from initialization to equilibration. The 6-fold range in soil carbon, once
established prior to the beginning of the historical period (and prior to the
beginning of a CMIP5 simulation), is then maintained through the present and
to 2100 almost unchanged even under a strong business-as-usual emissions
scenario. We therefore highlight that a commonly ignored part of CMIP5
analyses â the land surface state achieved through the spin-up procedure â
can be important for determining future carbon storage and land surface
fluxes. We identify the need to better constrain the outcome of the spin-up
procedure as an important step in reducing uncertainty in both projected soil
carbon and land surface fluxes in CMIP5 transient simulations
Disentangling residence time and temperature sensitivity of microbial decomposition in a global soil carbon model
Recent studies have identified the first-order parameterization of microbial decomposition as
a major source of uncertainty in simulations and projections of the terrestrial carbon
balance. Here, we use a reduced complexity model representative of the current state-of-the-art
parameterization of soil organic carbon decomposition. We undertake a systematic sensitivity
analysis to disentangle the effect of the time-invariant baseline residence time (<i>k</i>) and the
sensitvity of microbial decomposition to temperature (<i>Q</i><sub>10</sub>) on soil carbon dynamics at
regional and global scales. Our simulations produce a range in total soil carbon at equilibrium of
~ 592 to 2745 Pg C which is similar to the ~ 561 to 2938 Pg C range in
pre-industrial soil carbon in models used in the fifth phase of the Coupled Model Intercomparison
Project. This range depends primarily on the value of <i>k</i>, although the impact of <i>Q</i><sub>10</sub> is not
trivial at regional scales. As climate changes through the historical period, and into the future,
<i>k</i> is primarily responsible for the magnitude of the response in soil carbon, whereas <i>Q</i><sub>10</sub>
determines whether the soil remains a sink, or becomes a source in the future mostly by its effect
on mid-latitudes carbon balance. If we restrict our simulations to those simulating total soil
carbon stocks consistent with observations of current stocks, the projected range in total soil
carbon change is reduced by 42% for the historical simulations and 45% for the future
projections. However, while this observation-based selection dismisses outliers it does not
increase confidence in the future sign of the soil carbon feedback. We conclude that despite this
result, future estimates of soil carbon, and how soil carbon responds to climate change should be
constrained by available observational data sets
Alternate trait-based leaf respiration schemes evaluated at ecosystem-scale through carbon optimization modeling and canopy property data
Leaf maintenance respiration (Rleaf,m) is a major but poorly understood component of the terrestrial carbon cycle (C). Earth systems models (ESMs) use simple subâmodels relating Rleaf,m to leaf traits, applied at canopy scale. Rleaf,m models vary depending on which leaf N traits they incorporate (e.g., mass or area based) and the form of relationship (linear or nonlinear). To simulate vegetation responses to global change, some ESMs include ecological optimization to identify canopy structures that maximize net C accumulation. However, the implications for optimization of using alternate leafâscale empirical Rleaf,m models are undetermined. Here we combine alternate wellâknown empirical models of Rleaf,m with a process model of canopy photosynthesis. We quantify how net canopy exports of C vary with leaf area index (LAI) and total canopy N (TCN). Using data from tropical and arctic canopies, we show that estimates of canopy Rleaf,m vary widely among the three models. Using an optimization framework, we show that the LAI and TCN values maximizing C export depends strongly on the Rleaf,m model used. No single model could match observed arctic and tropical LAIâTCN patterns with predictions of optimal LAIâTCN. We recommend caution in using leafâscale empirical models for components of ESMs at canopyâscale. Rleaf,m models may produce reasonable results for a specified LAI, but, due to their varied representations of Rleaf,mfoliar N sensitivity, are associated with different and potentially unrealistic optimization dynamics at canopy scale. We recommend ESMs to be evaluated using response surfaces of canopy C export in LAIâTCN space to understand and mitigate these risks
Examining soil carbon uncertainty in a global model:response of microbial decomposition to temperature, moisture and nutrient limitation
Reliable projections of future climate require landâatmosphere carbon (C)
fluxes to be represented realistically in Earth system models (ESMs). There are
several sources of uncertainty in how carbon is parameterised in these
models. First, while interactions between the C, nitrogen (N) and phosphorus
(P) cycles have been implemented in some models, these lead to diverse
changes in landâatmosphere fluxes. Second, while the first-order
parameterisation of soil organic matter decomposition is similar between
models, formulations of the control of the soil physical state on microbial
activity vary widely. For the first time, we address these sources of
uncertainty simultaneously by implementing three soil moisture and three soil
temperature respiration functions in an ESM that can be run
with three degrees of biogeochemical nutrient limitation (C-only, C and N,
and C and N and P). All 27 possible combinations of response functions and
biogeochemical mode are equilibrated before transient historical (1850â2005)
simulations are performed. As expected, implementing N and P limitation
reduces the land carbon sink, transforming some regional sinks into net
sources over the historical period. Meanwhile, regardless of which nutrient
mode is used, various combinations of response functions imply a two-fold
difference in the net ecosystem accumulation and a four-fold difference in
equilibrated total soil C. We further show that regions with initially larger
pools are more likely to become carbon sources, especially when nutrient
availability limits the response of primary production to increasing
atmospheric CO<sub>2</sub>. Simulating changes in soil C content therefore
critically depends on both nutrient limitation and the choice of respiration
functions
Assimilation of repeated woody biomass observations constrains decadal ecosystem carbon cycle uncertainty in aggrading forests
Influence of Leaf Area Index Prescriptions on Simulations of Heat, Moisture, and Carbon Fluxes
Leaf-area index (LAI), the total one-sided surface area of leaf per ground surface area, is a key component of land surface models. We investigate the influence of differing, plausible LAI prescriptions on heat, moisture, and carbon fluxes simulated by the Community Atmosphere Biosphere Land Exchange (CABLEv1.4b) model over the Australian continent. A 15-member ensemble monthly LAI data-set is generated using the MODIS LAI product and gridded observations of temperature and precipitation. Offline simulations lasting 29 years (1980-2008) are carried out at 25 km resolution with the composite monthly means from the MODIS LAI product (control simulation) and compared with simulations using each of the 15-member ensemble monthly-varying LAI data-sets generated. The imposed changes in LAI did not strongly influence the sensible and latent fluxes but the carbon fluxes were more strongly affected. Croplands showed the largest sensitivity in gross primary production with differences ranging from -90 to 60 %. PFTs with high absolute LAI and low inter-annual variability, such as evergreen broadleaf trees, showed the least response to the different LAI prescriptions, whilst those with lower absolute LAI and higher inter-annual variability, such as croplands, were more sensitive. We show that reliance on a single LAI prescription may not accurately reflect the uncertainty in the simulation of the terrestrial carbon fluxes, especially for PFTs with high inter-annual variability. Our study highlights that the accurate representation of LAI in land surface models is key to the simulation of the terrestrial carbon cycle. Hence this will become critical in quantifying the uncertainty in future changes in primary production
Reduction of predictive uncertainty in estimating irrigation water requirement through multi-model ensembles and ensemble averaging
Irrigation agriculture plays an increasingly important role in food supply. Many evapotranspiration models are used today to estimate the water demand for irrigation. They consider different stages of crop growth by empirical crop coefficients to adapt evapotranspiration throughout the vegetation period. We investigate the importance of the model structural versus model parametric uncertainty for irrigation simulations by considering six evapotranspiration models and five crop coefficient sets to estimate irrigation water requirements for growing wheat in the MurrayÂDarling Basin, Australia. The study is carried out using the spatial decision support system SPARE:WATER. We find that structural model uncertainty among reference ET is far more important than model parametric uncertainty introduced by crop coefficients. These crop coefficients are used to estimate irrigation water requirement following the single crop coefficient approach. Using the reliability ensemble averaging (REA) technique, we are able to reduce the overall predictive model uncertainty by more than 10%. The exceedance probability curve of irrigation water requirements shows that a certain threshold, e.g. an irrigation water limit due to water right of 400 mm, would be less frequently exceeded in case of the REA ensemble average (45%) in comparison to the equally weighted ensemble average (66%). We conclude that multi-model ensemble predictions and sophisticated model averaging techniques are helpful in predicting irrigation demand and provide relevant information for decision making
Reliability Ensemble Averaging of 21st century projections of terrestrial net primary productivity reduces global and regional uncertainties
Multi-model averaging techniques provide opportunities to
extract
additional information from large ensembles of simulations. In
particular, present-day model skill can be used to evaluate their
potential performance in future climate simulations. Multi-model
averaging methods have been used extensively in climate and
hydrological sciences, but they have not been used to constrain
projected plant productivity responses to climate change, which is
a major uncertainty in Earth system modelling. Here, we use three
global observationally orientated estimates of current net primary
productivity (NPP) to perform a reliability ensemble averaging (REA) method
using 30 global simulations of the 21st century change in NPP based
on the Inter-Sectoral Impact Model Intercomparison Project (ISIMIP)
<q>business as usual</q> emissions scenario. We find that the three
REA methods support an increase in global NPP by the end of the 21st
century (2095â2099) compared to 2001â2005, which is 2â3âŻ%
stronger than the ensemble ISIMIP mean value of
24.2âŻPgâCây<sup>â1</sup>. Using REA also leads to a 45â68âŻ%
reduction in the global uncertainty of 21st century NPP projection,
which strengthens confidence in the resilience of the
CO<sub>2</sub> fertilization effect to climate change. This reduction
in uncertainty is especially clear for boreal ecosystems although it
may be an artefact due to the lack of representation of nutrient
limitations on NPP in most models. Conversely, the large uncertainty
that remains on the sign of the response of NPP in semi-arid regions
points to the need for better observations and model development in
these regions