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Ciais P.,CEA Saclay Nuclear Research Center | Tagliabue A.,CEA Saclay Nuclear Research Center | Tagliabue A.,South African Council for Scientific and Industrial Research | Cuntz M.,Helmholtz Center for Environmental Research | And 13 more authors.
Nature Geoscience | Year: 2012

During each of the late Pleistocene glacial- interglacial transitions, atmospheric carbon dioxide concentrations rose by almost 100ppm. The sources of this carbon are unclear, and efforts to identify them are hampered by uncertainties in the magnitude of carbon reservoirs and fluxes under glacial conditions. Here we use oxygen isotope measurements from air trapped in ice cores and ocean carbon-cycle modelling to estimate terrestrial and oceanic gross primary productivity during the Last Glacial Maximum. We find that the rate of gross terrestrial primary production during the Last Glacial Maximum was about 40 ± 10 Pg C yr -1, half that of the pre-industrial Holocene. Despite the low levels of photosynthesis, we estimate that the late glacial terrestrial biosphere contained only 330 Pg less carbon than pre-industrial levels. We infer that the area covered by carbon-rich but unproductive biomes such as tundra and cold steppes was significantly larger during the Last Glacial Maximum, consistent with palaeoecological data. Our data also indicate the presence of an inert carbon pool of 2,300 Pg C, about 700 Pg larger than the inert carbon locked in permafrost today. We suggest that the disappearance of this carbon pool at the end of the Last Glacial Maximum may have contributed to the deglacial rise in atmospheric carbon dioxide concentrations.

Li G.,Macquarie University | Harrison S.P.,Macquarie University | Harrison S.P.,University of Reading | Bartlein P.J.,University of Oregon | And 3 more authors.
Geophysical Research Letters | Year: 2013

We investigate the scaling between precipitation and temperature changes in warm and cold climates using six models that have simulated the response to both increased CO2 and Last Glacial Maximum (LGM) boundary conditions. Globally, precipitation increases in warm climates and decreases in cold climates by between 1.5%/°C and 3%/°C. Precipitation sensitivity to temperature changes is lower over the land than over the ocean and lower over the tropical land than over the extratropical land, reflecting the constraint of water availability. The wet tropics get wetter in warm climates and drier in cold climates, but the changes in dry areas differ among models. Seasonal changes of tropical precipitation in a warmer world also reflect this "rich get richer" syndrome. Precipitation seasonality is decreased in the cold-climate state. The simulated changes in precipitation per degree temperature change are comparable to the observed changes in both the historical period and the LGM. © 2013. American Geophysical Union. All Rights Reserved.

Prentice I.C.,University of Bristol | Prentice I.C.,Macquarie University | Prentice I.C.,Grantham Institute for Climate Change | Kelley D.I.,Macquarie University | And 8 more authors.
Global Biogeochemical Cycles | Year: 2011

Four CO 2 concentration inversions and the Global Fire Emissions Database (GFED) versions 2.1 and 3 are used to provide benchmarks for climate-driven modeling of the global land-atmosphere CO 2 flux and the contribution of wildfire to this flux. The Land surface Processes and exchanges (LPX) model is introduced. LPX is based on the Lund-Potsdam-Jena Spread and Intensity of FIRE (LPJ-SPITFIRE) model with amended fire probability calculations. LPX omits human ignition sources yet simulates many aspects of global fire adequately. It captures the major features of observed geographic pattern in burnt area and its seasonal timing and the unimodal relationship of burnt area to precipitation. It simulates features of geographic variation in the sign of the interannual correlations of burnt area with antecedent dryness and precipitation. It simulates well the interannual variability of the global total land-atmosphere CO 2 flux. There are differences among the global burnt area time series from GFED2.1, GFED3 and LPX, but some features are common to all. GFED3 fire CO 2 fluxes account for only about 1/3 of the variation in total CO 2 flux during 1997-2005. This relationship appears to be dominated by the strong climatic dependence of deforestation fires. The relationship of LPX-modeled fire CO 2 fluxes to total CO 2 fluxes is weak. Observed and modeled total CO 2 fluxes track the El Nio-Southern Oscillation (ENSO) closely; GFED3 burnt area and global fire CO 2 flux track the ENSO much less so. The GFED3 fire CO 2 flux-ENSO connection is most prominent for the El Nio of 1997-1998, which produced exceptional burning conditions in several regions, especially equatorial Asia. The sign of the observed relationship between ENSO and fire varies regionally, and LPX captures the broad features of this variation. These complexities underscore the need for process-based modeling to assess the consequences of global change for fire and its implications for the carbon cycle. Copyright 2011 by the American Geophysical Union.

Kelley D.I.,Macquarie University | Prentice I.C.,Macquarie University | Prentice I.C.,Grantham Institute for Climate Change | Harrison S.P.,Macquarie University | And 6 more authors.
Biogeosciences | Year: 2013

We present a benchmark system for global vegetation models. This system provides a quantitative evaluation of multiple simulated vegetation properties, including primary production; seasonal net ecosystem production; vegetation cover; composition and height; fire regime; and runoff. The benchmarks are derived from remotely sensed gridded datasets and site-based observations. The datasets allow comparisons of annual average conditions and seasonal and inter-annual variability, and they allow the impact of spatial and temporal biases in means and variability to be assessed separately. Specifically designed metrics quantify model performance for each process, and are compared to scores based on the temporal or spatial mean value of the observations and a "random" model produced by bootstrap resampling of the observations. The benchmark system is applied to three models: a simple light-use efficiency and waterbalance model (the Simple Diagnostic Biosphere Model: SDBM), the Lund-Potsdam-Jena (LPJ) and Land Processes and eXchanges (LPX) dynamic global vegetation models (DGVMs). In general, the SDBM performs better than either of the DGVMs. It reproduces independent measurements of net primary production (NPP) but underestimates the amplitude of the observed CO2 seasonal cycle. The two DGVMs show little difference for most benchmarks (including the inter-annual variability in the growth rate and seasonal cycle of atmospheric CO2), but LPX represents burnt fraction demonstrably more accurately. Benchmarking also identified several weaknesses common to both DGVMs. The benchmarking system provides a quantitative approach for evaluating how adequately processes are represented in a model, identifying errors and biases, tracking improvements in performance through model development, and discriminating among models. Adoption of such a system would do much to improve confidence in terrestrial model predictions of climate change impacts and feedbacks. © Author(s) 2013.

Ukkola A.M.,Macquarie University | Ukkola A.M.,CSIRO | Prentice I.C.,Macquarie University | Prentice I.C.,Grantham Institute for Climate Change
Hydrology and Earth System Sciences | Year: 2013

Climate change is expected to alter the global hydrological cycle, with inevitable consequences for freshwater availability to people and ecosystems. But the attribution of recent trends in the terrestrial water balance remains disputed. This study attempts to account statistically for both trends and interannual variability in water-balance evapotranspiration (ET), estimated from the annual observed streamflow in 109 river basins during "water years" 1961-1999 and two gridded precipitation data sets. The basins were chosen based on the availability of streamflow time-series data in the Dai et al. (2009) synthesis. They were divided into water-limited "dry" and energy-limited "wet" basins following the Budyko framework. We investigated the potential roles of precipitation, aerosol-corrected solar radiation, land use change, wind speed, air temperature, and atmospheric CO2. Both trends and variability in ET show strong control by precipitation. There is some additional control of ET trends by vegetation processes, but little evidence for control by other factors. Interannual variability in ET was overwhelmingly dominated by precipitation, which accounted on average for 54-55% of the variation in wet basins (ranging from 0 to 100%) and 94-95% in dry basins (ranging from 69 to 100%). Precipitation accounted for 45-46% of ET trends in wet basins and 80-84% in dry basins. Net atmospheric CO2 effects on transpiration, estimated using the Land-surface Processes and eXchanges (LPX) model, did not contribute to observed trends in ET because declining stomatal conductance was counteracted by slightly but significantly increasing foliage cover. © Author(s) 2013.

Gallego-Sala A.V.,University of Bristol | Gallego-Sala A.V.,Lund University | Gallego-Sala A.V.,University of Exeter | Colin Prentice I.,University of Bristol | And 2 more authors.
Nature Climate Change | Year: 2013

Blanket bog is a highly distinctive biome restricted to disjunct hyperoceanic regions. It is characterized by a landscape covering of peat broken only by the steepest slopes. Plant and microbial life are adapted to anoxia, low pH and low nutrient availability. Plant productivity exceeds soil organic matter decomposition, so carbon is sequestered over time. Unique climatic requirements, including high year-round rainfall and low summer temperatures, make this biome amenable to bioclimatic modelling. However, projections of the fate of peatlands in general, and blanket bogs in particular, under climate change have been contradictory. Here we use a simple, well-founded global bioclimatic model, with climate-change projections from seven climate models, to indicate this biome's fate. We show marked shrinkage of its present bioclimatic space with only a few, restricted areas of persistence. Many blanket bog regions are thus at risk of progressive peat erosion and vegetation changes as a direct consequence of climate change. New areas suitable for blanket bog are also projected, but these are often disjunct from present areas and their location is inconsistently predicted by different climate models. © 2013 Macmillan Publishers Limited. All rights reserved.

Mackey B.,Griffith University | Prentice I.C.,Macquarie University | Prentice I.C.,Grantham Institute for Climate Change | Steffen W.,Australian National University | And 4 more authors.
Nature Climate Change | Year: 2013

Depletion of ecosystem carbon stocks is a significant source of atmospheric CO 2 and reducing land-based emissions and maintaining land carbon stocks contributes to climate change mitigation. We summarize current understanding about human perturbation of the global carbon cycle, examine three scientific issues and consider implications for the interpretation of international climate change policy decisions, concluding that considering carbon storage on land as a means to 'offset' CO 2 emissions from burning fossil fuels (an idea with wide currency) is scientifically flawed. The capacity of terrestrial ecosystems to store carbon is finite and the current sequestration potential primarily reflects depletion due to past land use. Avoiding emissions from land carbon stocks and refilling depleted stocks reduces atmospheric CO 2 concentration, but the maximum amount of this reduction is equivalent to only a small fraction of potential fossil fuel emissions. © 2013 Macmillan Publishers Limited. All rights reserved.

Stocker B.D.,University of Bern | Roth R.,University of Bern | Joos F.,University of Bern | Spahni R.,University of Bern | And 7 more authors.
Nature Climate Change | Year: 2013

Atmospheric concentrations of the three important greenhouse gases (GHGs) CO2, CH4 and N2 O are mediated by processes in the terrestrial biosphere that are sensitive to climate and CO2. This leads to feedbacks between climate and land and has contributed to the sharp rise in atmospheric GHG concentrations since pre-industrial times. Here, we apply a process-based model to reproduce the historical atmospheric N 2 O and CH4 budgets within their uncertainties and apply future scenarios for climate, land-use change and reactive nitrogen (Nr) inputs to investigate future GHG emissions and their feedbacks with climate in a consistent and comprehensive framework. Results suggest that in a business-as-usual scenario, terrestrial N2 O and CH4 emissions increase by 80 and 45%, respectively, and the land becomes a net source of C by AD 2100. N2 O and CH4 feedbacks imply an additional warming of 0.4-0.5C by AD 2300; on top of 0.8-1.0C caused by terrestrial carbon cycle and Albedo feedbacks. The land biosphere represents an increasingly positive feedback to anthropogenic climate change and amplifies equilibrium climate sensitivity by 22-27%. Strong mitigation limits the increase of terrestrial GHG emissions and prevents the land biosphere from acting as an increasingly strong amplifier to anthropogenic climate change. © 2013 Macmillan Publishers Limited. All rights reserved.

Kelley D.I.,Macquarie University | Harrison S.P.,Macquarie University | Prentice I.C.,Macquarie University | Prentice I.C.,Grantham Institute for Climate Change
Geoscientific Model Development | Year: 2014

The Land surface Processes and eXchanges (LPX) model is a fire-enabled dynamic global vegetation model that performs well globally but has problems representing fire regimes and vegetative mix in savannas. Here we focus on improving the fire module. To improve the representation of ignitions, we introduced a reatment of lightning that allows the fraction of ground strikes to vary spatially and seasonally, realistically partitions strike distribution between wet and dry days, and varies the number of dry days with strikes. Fuel availability and moisture content were improved by implementing decomposition rates specific to individual plant functional types and litter classes, and litter drying rates driven by atmospheric water content. To improve water extraction by grasses, we use realistic plant-specific treatments of deep roots. To improve fire responses, we introduced adaptive bark thickness and post-fire resprouting for tropical and temperate broadleaf trees. All improvements are based on extensive analyses of relevant observational data sets. We test model performance for Australia, first evaluating parameterisations separately and then measuring overall behaviour against standard benchmarks. Changes to the lightning parameterisation produce a more realistic simulation of fires in southeastern and central Australia. Implementation of PFT-specific decomposition rates enhances performance in central Australia. Changes in fuel drying improve fire in northern Australia, while changes in rooting depth produce a more realistic simulation of fuel availability and structure in central and northern Australia. The introduction of adaptive bark thickness and resprouting produces more realistic fire regimes in Australian savannas. We also show that the model simulates biomass recovery rates consistent with observations from several different regions of the world characterised by resprouting vegetation. The new model (LPX-Mv1) produces an improved simulation of observed vegetation composition and mean annual burnt area, by 33 and 18% respectively compared to LPX. © 2014 Author(s).

Xu-Ri,CAS Institute of Tibetan Plateau Research | Prentice I.C.,Macquarie University | Prentice I.C.,Grantham Institute for Climate Change | Spahni R.,University of Bern | Niu H.S.,University of Chinese Academy of Sciences
New Phytologist | Year: 2012

Ecosystem nitrous oxide (N2O) emissions respond to changes in climate and CO2 concentration as well as anthropogenic nitrogen (N) enhancements. Here, we aimed to quantify the responses of natural ecosystem N2O emissions to multiple environmental drivers using a process-based global vegetation model (DyN-LPJ). We checked that modelled annual N2O emissions from nonagricultural ecosystems could reproduce field measurements worldwide, and experimentally observed responses to step changes in environmental factors. We then simulated global N2O emissions throughout the 20th century and analysed the effects of environmental changes. The model reproduced well the global pattern of N2O emissions and the observed responses of N cycle components to changes in environmental factors. Simulated 20th century global decadal-average soil emissions were c. 8.2-9.5 Tg N yr-1 (or 8.3-10.3 Tg N yr-1 with N deposition). Warming and N deposition contributed 0.85 ± 0.41 and 0.80 ± 0.14 Tg N yr-1, respectively, to an overall upward trend. Rising CO2 also contributed, in part, through a positive interaction with warming. The modelled temperature dependence of N2O emission (c. 1 Tg N yr-1 K-1) implies a positive climate feedback which, over the lifetime of N2O (114 yr), could become as important as the climate-carbon cycle feedback caused by soil CO2 release. See also the Commentary by Del Grosso and Parton. © 2012 New Phytologist Trust.

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