Michael T. Coe

Testing the performance of a dynamic global ecosystem model: Water balance, carbon balance, and vegetation structure

While a new class of Dynamic Global Ecosystem Models (DGEMs) has emerged in the past few years as an important tool for describing global biogeochemical cycles and atmosphere-biosphere interactions, these models are still largely untested. Here we analyze the behavior of a new DGEM and compare the results to global-scale observations of water balance, carbon balance, and vegetation structure. In this study, we use version 2 of the Integrated Biosphere Simulator (IBIS), which includes several major improvements and additions to the prototype model developed by Foley et al. [1996]. IBIS is designed to be a comprehensive model of the terrestrial biosphere; the model represents a wide range of processes, including land surface physics, canopy physiology, plant phenology, vegetation dynamics and competition, and carbon and nutrient cycling. The model generates global simulations of the surface water balance (e.g., runoff), the terrestrial carbon balance (e.g., net primary production, net ecosystem exchange, soil carbon, aboveground and belowground litter, and soil CO2 fluxes), and vegetation structure (e.g., biomass, leaf area index, and vegetation composition). In order to test the performance of the model, we have assembled a wide range of continental and global-scale data, including measurements of river discharge, net primary production, vegetation structure, root biomass, soil carbon, litter carbon, and soil CO2 flux. Using these field data and model results for the contemporary biosphere (1965–1994), our evaluation shows that simulated patterns of runoff, NPP, biomass, leaf area index, soil carbon, and total soil CO2 flux agree reasonably well with measurements that have been compiled from numerous ecosystems. These results also compare favorably to other global model results.

The Amazon basin in transition

Agricultural expansion and climate variability have become important agents of disturbance in the Amazon basin. Recent studies have demonstrated considerable resilience of Amazonian forests to moderate annual drought, but they also show that interactions between deforestation, fire and drought potentially lead to losses of carbon storage and changes in regional precipitation patterns and river discharge. Although the basin-wide impacts of land use and drought may not yet surpass the magnitude of natural variability of hydrologic and biogeochemical cycles, there are some signs of a transition to a disturbance-dominated regime. These signs include changing energy and water cycles in the southern and eastern portions of the Amazon basin.

Impact of vegetation and preferential source areas on global dust aerosol: Results from a model study

[1] We present a model of the dust cycle that successfully predicts dust emissions as determined by land surface properties, monthly vegetation and snow cover, and 6-hourly surface wind speeds for the years 1982–1993. The model takes account of the role of dry lake beds as preferential source areas for dust emission. The occurrence of these preferential sources is determined by a water routing and storage model. The dust source scheme also explicitly takes into account the role of vegetation type as well as monthly vegetation cover. Dust transport is computed using assimilated winds for the years 1987–1990. Deposition of dust occurs through dry and wet deposition, where subcloud scavenging is calculated using assimilated precipitation fields. Comparison of simulated patterns of atmospheric dust loading with the Total Ozone Mapping Spectrometer satellite absorbing aerosol index shows that the model produces realistic results from daily to interannual timescales. The magnitude of dust deposition agrees well with sediment flux data from marine sites. Emission of submicron dust from preferential source areas are required for the computation of a realistic dust optical thickness. Sensitivity studies show that Asian dust source strengths are particularly sensitive to the seasonality of vegetation cover.

Cracking Brazil's Forest Code

Brazils controversial new Forest Code grants amnesty to illegal deforesters, but creates new mechanisms for forest conservation. Roughly 53% of Brazils native vegetation occurs on private properties. Native forests and savannahs on these lands store 105 ± 21 GtCO2e (billion tons of CO2 equivalents) and play a vital role in maintaining a broad range of ecosystem services (1). Sound management of these private landscapes is critical if global efforts to mitigate climate change are to succeed. Recent approval of controversial revisions to Brazils Forest Code (FC)—the central piece of legislation regulating land use and management on private properties—may therefore have global consequences. Here, we quantify changes resulting from the FC revisions in terms of environmental obligations and rights granted to land-owners. We then discuss conservation opportunities arising from new policy mechanisms in the FC and challenges for its implementation.

Amazonia revealed: forest degradation and loss of ecosystem goods and services in the Amazon Basin

The Amazon Basin is one of the worlds most important bioregions, harboring a rich array of plant and animal species and offering a wealth of goods and services to society. For years, ecological science has shown how large-scale forest clearings cause declines in biodiversity and the availability of forest products. Yet some important changes in the rainforests, and in the ecosystem services they provide, have been underappreciated until recently. Emerging research indicates that land use in the Amazon goes far beyond clearing large areas of forest; selective logging and other canopy damage is much more pervasive than once believed. Deforestation causes collateral damage to the surrounding forests – through enhanced drying of the forest floor, increased frequency of fires, and lowered productivity. The loss of healthy forests can degrade key ecosystem services, such as carbon storage in biomass and soils, the regulation of water balance and river flow, the modulation of regional climate patterns, and the ...

Abrupt increases in Amazonian tree mortality due to drought-fire interactions

Significance Climate change alone is unlikely to drive severe tropical forest degradation in the next few decades, but an alternative process associated with severe weather and forest fires is already operating in southeastern Amazonia. Recent droughts caused greatly elevated fire-induced tree mortality in a fire experiment and widespread regional forest fires that burned 5–12% of southeastern Amazon forests. These results suggest that feedbacks between fires and extreme climatic conditions could increase the likelihood of an Amazon forest “dieback” in the near-term. To secure the integrity of seasonally dry Amazon forests, efforts to end deforestation must be accompanied by initiatives that reduce the accidental spread of land management fires into neighboring forest reserves and effectively suppress forest fires when they start. Interactions between climate and land-use change may drive widespread degradation of Amazonian forests. High-intensity fires associated with extreme weather events could accelerate this degradation by abruptly increasing tree mortality, but this process remains poorly understood. Here we present, to our knowledge, the first field-based evidence of a tipping point in Amazon forests due to altered fire regimes. Based on results of a large-scale, long-term experiment with annual and triennial burn regimes (B1yr and B3yr, respectively) in the Amazon, we found abrupt increases in fire-induced tree mortality (226 and 462%) during a severe drought event, when fuel loads and air temperatures were substantially higher and relative humidity was lower than long-term averages. This threshold mortality response had a cascading effect, causing sharp declines in canopy cover (23 and 31%) and aboveground live biomass (12 and 30%) and favoring widespread invasion by flammable grasses across the forest edge area (80 and 63%), where fires were most intense (e.g., 220 and 820 kW⋅m−1). During the droughts of 2007 and 2010, regional forest fires burned 12 and 5% of southeastern Amazon forests, respectively, compared with <1% in nondrought years. These results show that a few extreme drought events, coupled with forest fragmentation and anthropogenic ignition sources, are already causing widespread fire-induced tree mortality and forest degradation across southeastern Amazon forests. Future projections of vegetation responses to climate change across drier portions of the Amazon require more than simulation of global climate forcing alone and must also include interactions of extreme weather events, fire, and land-use change.

Regime Shifts in the Sahara and Sahel: Interactions between Ecological and Climatic Systems in Northern Africa

The Sahara and Sahel regions of northern Africa have complex environmental histories punctuated by sudden and dramatic “regime shifts” in climate and ecological conditions. Here we review the current understanding of the causes and consequences of two environmental regime shifts in the Sahara and Sahel. The first regime shift is the sudden transition from vegetated to desert conditions in the Sahara about 5500 years ago. Geologic data show that wet environmental conditions in this region—giving rise to extensive vegetation, lakes, and wetlands—came to an abrupt end about 5500 years ago. Explanations for climatic changes in northern Africa during the Holocene have suggested that millennial-scale changes in the Earth’s orbit could have caused the wet conditions that prevailed in the early Holocene and the dry conditions prevalent today. However, the orbital hypothesis, by itself, does not explain the sudden regime shift 5500 years ago. Several modeling studies have proposed that strong, nonlinear feedbacks between vegetation and the atmosphere could amplify the effects of orbital variations and create two alternative stable states (or “regimes”) in the climate and ecosystems of the Sahara: a “green Sahara” and a “desert Sahara.” A recent coupled atmosphere-ocean-land model confirmed that there was a sudden shift from the “green Sahara” to the “desert Sahara” regime approximately 5500 years ago. The second regime shift is the onset of a major 30-year drought over the Sahel around 1969. Several lines of evidence have suggested that the interactions between atmosphere and vegetation act to reinforce either a “wet Sahel” or a “dry Sahel” climatic regime, which may persist for decades at a time. Recent modeling studies have indicated that the shift from a “wet Sahel” to a “dry Sahel” regime was caused by strong feedbacks between the climate and vegetation cover and may have been triggered by slow changes in either land degradation or sea-surface temperatures. Taken together, we conclude that the existence of alternative stable states (or regimes) in the climate and ecosystems of the Sahara and Sahel may be the result of strong, nonlinear interactions between vegetation and the atmosphere. Although the shifts between these regimes occur rapidly, they are made possible by slow, subtle changes in underlying environmental conditions, including slow changes in incoming solar radiation, sea-surface temperatures, or the degree of land degradation.

Human and natural impacts on the water resources of the Lake Chad basin

An integrated biosphere model (IBIS) and a hydrological routing algorithm (HYDRA) are used in conjunction with long time-series climate data to investigate the response of the Lake Chad drainage basin of northern Africa to climate variability and water use practices over the last 43 years. The simulated discharge, lake level, and lake area of the drainage basin for the period 1953–1979 are in good agreement with the observations. For example, the correlation coefficient (r2) between the simulated and the observed level of Lake Chad for the 288 months of available observations is 0.93. Although irrigation is only a modest portion of the hydrology in the period 1953–1979; representing only 5 of the 30% decrease in simulated lake area for the decade 1966–1975, the simulated lake level and area are in better agreement with the observations when irrigation is included. For the period 1983–1994 the observed water use for irrigation increased fourfold compared to 1953–1979. A comparison of the simulated surface water area, with and without irrigation, suggests that climate variability still controls the interannual fluctuations of the water inflow but that human water use accounts for roughly 50% of the observed decrease in lake area since the 1960s and 1970s.

Modeling Terrestrial Hydrological Systems at the Continental Scale: Testing the Accuracy of an Atmospheric GCM

Abstract A global hydrological routing algorithm (HYDRA) that simulates seasonal river discharge and changes in surface water level on a spatial resolution of 5′ long × 5′ lat is presented. The model is based on previous work by M. T. Coe and incorporates major improvements from that work including 1) the ability to simulate monthly and seasonal variations in discharge and lake and wetland level, and 2) direct representation of man-made dams and reservoirs. HYDRA requires as input daily or monthly mean averages of runoff, precipitation, and evaporation either from GCM output or observations. As an example of the utility of HYDRA in evaluating GCM simulations, the model is forced with monthly mean estimates of runoff from the National Centers for Environmental Prediction (NCEP) reanalysis dataset. The simulated river discharge clearly shows that although the NCEP runoff captures the large-scale features of the observed terrestrial hydrology, there are numerous differences in detail from observations. The s...

A linked global model of terrestrial hydrologic processes: Simulation of modern rivers, lakes, and wetlands

A terrain-based hydrologic model is developed to simulate rivers, lakes, and wetlands on the continental scale as a linked dynamical system. This surface water area model (SWAM) is an extension of earlier work [Coe, 1997] and is based on a linear reservoir model. The model requires, as input, estimates of runoff, precipitation, and evaporation from either observations or climate simulations. The model develops its own river transport directions based on elevation. The river discharge is calculated at each grid cell as the accumulated flow of water across the land surface. Lake and wetland area and volume are calculated as a function of the local precipitation minus evaporation, the streamflow into and out of the grid cells, and the potential for storage of water on the surface as derived from 5′ × 5′ resolution digital terrain models. SWAM is applied on a 5′ × 5′ spatial resolution to simulate present-day surface waters and river transport for all continents (except Antarctica and Greenland). The model simulates the discharge of the 27 major rivers of the world in fair agreement with the observations; 12 of the rivers have simulated discharge within ±20% of the observational estimates. The discharge from rivers in arid and semi-arid climates is generally overestimated probably due to underestimated evaporation from wetlands, irrigated croplands, and reservoirs within the river basins. The modern simulated lake area (about 3% of the land area) is larger than the observed area (about 2% of the land area). Wetlands are poorly simulated by the model due to the coarse vertical and horizontal resolution of the digital terrain models. The model simulates the potential for increased surface water (up to 7.5% of the global land area) and river basin area in semi-arid and arid regions where closed basins exist in the present day climate which illustrates the utility of SWAM for climate change experiments. While higher resolution and more accurate digital terrain models are needed to improve these surface water simulations (particularly for wetlands), these preliminary results indicate that current digital terrain models are adequate for including lakes and rivers as interactive components in the surface hydrology parameterizations of climate models and for studying feedbacks between lakes and climate.