Showing papers by "Kevin Schaefer published in 2012"

Terrestrial biosphere models need better representation of vegetation phenology: results from the North American Carbon Program Site Synthesis

Abstract: Phenology, by controlling the seasonal activity of vegetation on the land surface, plays a fundamental role in regulating photosynthesis and other ecosystem processes, as well as competitive interactions and feedbacks to the climate system. We conducted an analysis to evaluate the representation of phenology, and the associated seasonality of ecosystem-scale CO2 exchange, in 14 models participating in the North American Carbon Program Site Synthesis. Model predictions were evaluated using long-term measurements (emphasizing the period 2000–2006) from 10 forested sites within the AmeriFlux and Fluxnet-Canada networks. In deciduous forests, almost all models consistently predicted that the growing season started earlier, and ended later, than was actually observed; biases of 2 weeks or more were

A model-data comparison of gross primary productivity: Results from the North American Carbon Program site synthesis

Abstract: [1] Accurately simulating gross primary productivity (GPP) in terrestrial ecosystem models is critical because errors in simulated GPP propagate through the model to introduce additional errors in simulated biomass and other fluxes. We evaluated simulated, daily average GPP from 26 models against estimated GPP at 39 eddy covariance flux tower sites across the United States and Canada. None of the models in this study match estimated GPP within observed uncertainty. On average, models overestimate GPP in winter, spring, and fall, and underestimate GPP in summer. Models overpredicted GPP under dry conditions and for temperatures below 0°C. Improvements in simulated soil moisture and ecosystem response to drought or humidity stress will improve simulated GPP under dry conditions. Adding a low-temperature response to shut down GPP for temperatures below 0°C will reduce the positive bias in winter, spring, and fall and improve simulated phenology. The negative bias in summer and poor overall performance resulted from mismatches between simulated and observed light use efficiency (LUE). Improving simulated GPP requires better leaf-to-canopy scaling and better values of model parameters that control the maximum potential GPP, such asemax (LUE), Vcmax (unstressed Rubisco catalytic capacity) or Jmax (the maximum electron transport rate).

Past extreme warming events linked to massive carbon release from thawing permafrost

Abstract: Orbitally triggered decomposition of soil organic carbon in terrestrial permafrost is suggested as an explanation for a series of sudden and extreme global warming events that occurred about 55 million years ago. Earth experienced a series of sudden and extreme global warming events, or hyperthermals, about 55 million years ago. Several explanations have been proposed, but questions remain regarding the source, magnitude and timing of carbon releases associated with these successive events. Robert DeConto and colleagues propose that changes in Earth's orbit were the initial trigger, causing temperatures in the Arctic and Antarctic regions to rise. Thawing of circum-Arctic and Antarctic terrestrial permafrost could then have triggered decomposition of soil organic carbon and — through a 'greenhouse' mechanism — a hyperthermal event. Between about 55.5 and 52 million years ago, Earth experienced a series of sudden and extreme global warming events (hyperthermals) superimposed on a long-term warming trend1. The first and largest of these events, the Palaeocene–Eocene Thermal Maximum (PETM), is characterized by a massive input of carbon, ocean acidification2 and an increase in global temperature of about 5 °C within a few thousand years3. Although various explanations for the PETM have been proposed4,5,6, a satisfactory model that accounts for the source, magnitude and timing of carbon release at the PETM and successive hyperthermals remains elusive. Here we use a new astronomically calibrated cyclostratigraphic record from central Italy7 to show that the Early Eocene hyperthermals occurred during orbits with a combination of high eccentricity and high obliquity. Corresponding climate–ecosystem–soil simulations accounting for rising concentrations of background greenhouse gases8 and orbital forcing show that the magnitude and timing of the PETM and subsequent hyperthermals can be explained by the orbitally triggered decomposition of soil organic carbon in circum-Arctic and Antarctic terrestrial permafrost. This massive carbon reservoir had the potential to repeatedly release thousands of petagrams (1015 grams) of carbon to the atmosphere–ocean system, once a long-term warming threshold had been reached just before the PETM. Replenishment of permafrost soil carbon stocks following peak warming probably contributed to the rapid recovery from each event9, while providing a sensitive carbon reservoir for the next hyperthermal10. As background temperatures continued to rise following the PETM, the areal extent of permafrost steadily declined, resulting in an incrementally smaller available carbon pool and smaller hyperthermals at each successive orbital forcing maximum. A mechanism linking Earth’s orbital properties with release of soil carbon from permafrost provides a unifying model accounting for the salient features of the hyperthermals.

Reduction in carbon uptake during turn of the century drought in western North America

Abstract: The severity and incidence of climatic extremes, including drought, have increased as a result of climate warming. Analyses of observational and reanalysis data suggest that the strength of the western North American carbon sink declined by 30–298 Tg carbon per year during the drought at the turn of the century. Fossil fuel emissions aside, temperate North America is a net sink of carbon dioxide at present1,2,3. Year-to-year variations in this carbon sink are linked to variations in hydroclimate that affect net ecosystem productivity3,4. The severity and incidence of climatic extremes, including drought, have increased as a result of climate warming5,6,7,8. Here, we examine the effect of the turn of the century drought in western North America on carbon uptake in the region, using reanalysis data, remote sensing observations and data from global monitoring networks. We show that the area-integrated strength of the western North American carbon sink declined by 30–298 Tg C yr−1 during the 2000–2004 drought. We further document a pronounced drying of the terrestrial biosphere during this period, together with a reduction in river discharge and a loss of cropland productivity. We compare our findings with previous palaeoclimate reconstructions7 and show that the last drought of this magnitude occurred more than 800 years ago. Based on projected changes in precipitation and drought severity, we estimate that the present mid-latitude carbon sink of 177–623 Tg C yr−1 in western North America could disappear by the end of the century.

Climate and vegetation controls on the surface water balance: Synthesis of evapotranspiration measured across a global network of flux towers

Abstract: [1] The Budyko framework elegantly reduces the complex spatial patterns of actual evapotranspiration and runoff to a general function of two variables: mean annual precipitation (MAP) and net radiation. While the methodology has first-order skill, departures from a globally averaged curve can be significant and may be usefully attributed to additional controls such as vegetation type. This paper explores the magnitude of such departures as detected from flux tower measurements of ecosystem-scale evapotranspiration, and investigates their attribution to site characteristics (biome, seasonal rainfall distribution, and frozen precipitation). The global synthesis (based on 167 sites with 764 tower-years) shows smooth transition from water-limited to energy-limited control, broadly consistent with catchment-scale relations and explaining 62% of the across site variation in evaporative index (the fraction of MAP consumed by evapotranspiration). Climate and vegetation types act as additional controls, combining to explain an additional 13% of the variation in evaporative index. Warm temperate winter wet sites (Mediterranean) exhibit a reduced evaporative index, 9% lower than the average value expected based on dryness index, implying elevated runoff. Seasonal hydrologic surplus explains a small but significant fraction of variance in departures of evaporative index from that expected for a given dryness index. Surprisingly, grasslands on average have a higher evaporative index than forested landscapes, with 9% more annual precipitation consumed by annual evapotranspiration compared to forests. In sum, the simple framework of supply- or demand-limited evapotranspiration is supported by global FLUXNET observations but climate type and vegetation type are seen to exert sizeable additional controls.

Terrestrial biosphere model performance for inter‐annual variability of land‐atmosphere CO2 exchange

Abstract: Interannual variability in biosphere-atmosphere exchange of CO2 is driven by a diverse range of biotic and abiotic factors. Replicating this variability thus represents the ‘acid test’ for terrestrial biosphere models. Although such models are commonly used to project responses to both normal and anomalous variability in climate, they are rarely tested explicitly against inter-annual variability in observations. Herein, using standardized data from the North American Carbon Program, we assess the performance of 16 terrestrial biosphere models and 3 remote sensing products against long-term measurements of biosphere-atmosphere CO2 exchange made with eddy-covariance flux towers at 11 forested sites in North America. Instead of focusing on model-data agreement we take a systematic, variability-oriented approach and show that although the models tend to reproduce the mean magnitude of the observed annual flux variability, they fail to reproduce the timing. Large biases in modeled annual means are evident for all models. Observed interannual variability is found to commonly be on the order of magnitude of the mean fluxes. None of the models consistently reproduce observed interannual variability within measurement uncertainty. Underrepresentation of variability in spring phenology, soil thaw and snowpack melting, and difficulties in reproducing the lagged response to extreme climatic events are identified as systematic errors, common to all models included in this study.

Estimating 1992-2000 average active layer thickness on the Alaskan North Slope from remotely sensed surface subsidence

Abstract: [1] The measurement of temporal changes in active layer thickness (ALT) is crucial to monitoring permafrost degradation in the Arctic. We develop a retrieval algorithm to estimate long-term average ALT using thaw-season surface subsidence derived from spaceborne interferometric synthetic aperture radar (InSAR) measurements. Our algorithm uses a model of vertical distribution of water content within the active layer accounting for soil texture, organic matter, and moisture. We determine the 1992–2000 average ALT for an 80 × 100 km study area of continuous permafrost on the North Slope of Alaska near Prudhoe Bay. We obtain an ALT of 30–50 cm over moist tundra areas, and a larger ALT of 50–80 cm over wet tundra areas. Our estimated ALT values match in situ measurements at Circumpolar Active Layer Monitoring (CALM) sites within uncertainties. Our results demonstrate that InSAR can provide ALT estimates over large areas at high spatial resolution.

Impact of hydrological variations on modeling of peatland CO2 fluxes: Results from the North American Carbon Program site synthesis

Abstract: [1] Northern peatlands are likely to be important in future carbon cycle-climate feedbacks due to their large carbon pools and vulnerability to hydrological change. Use of non-peatland-specific models could lead to bias in modeling studies of peatland-rich regions. Here, seven ecosystem models were used to simulate CO2fluxes at three wetland sites in Canada and the northern United States, including two nutrient-rich fens and one nutrient-poor,sphagnum-dominated bog, over periods between 1999 and 2007. Models consistently overestimated mean annual gross ecosystem production (GEP) and ecosystem respiration (ER) at all three sites. Monthly flux residuals (simulated – observed) were correlated with measured water table for GEP and ER at the two fen sites, but were not consistently correlated with water table at the bog site. Models that inhibited soil respiration under saturated conditions had less mean bias than models that did not. Modeled diurnal cycles agreed well with eddy covariance measurements at fen sites, but overestimated fluxes at the bog site. Eddy covariance GEP and ER at fens were higher during dry periods than during wet periods, while models predicted either the opposite relationship or no significant difference. At the bog site, eddy covariance GEP did not depend on water table, while simulated GEP was higher during wet periods. Carbon cycle modeling in peatland-rich regions could be improved by incorporating wetland-specific hydrology and by inhibiting GEP and ER under saturated conditions. Bogs and fens likely require distinct plant and soil parameterizations in ecosystem models due to differences in nutrients, peat properties, and plant communities.

Policy Implications of Warming Permafrost

Abstract: This report seeks to inform a broad audience about permafrost and communicate to decision-makers and the general public the implications of changing permafrost in a warming climate. It defines basic terminology and describes fundamental physical and biological processes that shape the permafrost landscape using the best scientific information available from published literature. The report discusses the impacts of a changing climate on ecosystems and human infrastructure in regions with significant presence of permafrost, as well as the impacts of thawing permafrost on global climate. Graphics, illustrations and photographs help explain complicated concepts and ideas in a way that is easily understood and visualized by a non-scientific audience.

Corrigendum: Past extreme warming events linked to massive carbon release from thawing permafrost

Abstract: Corrigendum: Past extreme warming events linked to massive carbon release from thawing permafrost