F. S. Chapin

Methane bubbling from Siberian thaw lakes as a positive feedback to climate warming.

Large uncertainties in the budget of atmospheric methane, an important greenhouse gas, limit the accuracy of climate change projections. Thaw lakes in North Siberia are known to emit methane, but the magnitude of these emissions remains uncertain because most methane is released through ebullition (bubbling), which is spatially and temporally variable. Here we report a new method of measuring ebullition and use it to quantify methane emissions from two thaw lakes in North Siberia. We show that ebullition accounts for 95 per cent of methane emissions from these lakes, and that methane flux from thaw lakes in our study region may be five times higher than previously estimated. Extrapolation of these fluxes indicates that thaw lakes in North Siberia emit 3.8 teragrams of methane per year, which increases present estimates of methane emissions from northern wetlands (< 6–40 teragrams per year; refs 1, 2, 4–6) by between 10 and 63 per cent. We find that thawing permafrost along lake margins accounts for most of the methane released from the lakes, and estimate that an expansion of thaw lakes between 1974 and 2000, which was concurrent with regional warming, increased methane emissions in our study region by 58 per cent. Furthermore, the Pleistocene age (35,260–42,900 years) of methane emitted from hotspots along thawing lake margins indicates that this positive feedback to climate warming has led to the release of old carbon stocks previously stored in permafrost.

Reconciling carbon-cycle concepts, terminology, and methods

Recent projections of climatic change have focused a great deal of scientific and public attention on patterns of carbon (C) cycling as well as its controls, particularly the factors that determine whether an ecosystem is a net source or sink of atmospheric carbon dioxide (CO2). Net ecosystem production (NEP), a central concept in C-cycling research, has been used by scientists to represent two different concepts. We propose that NEP be restricted to just one of its two original definitions—the imbalance between gross primary production (GPP) and ecosystem respiration (ER). We further propose that a new term—net ecosystem carbon balance (NECB)—be applied to the net rate of C accumulation in (or loss from [negative sign]) ecosystems. Net ecosystem carbon balance differs from NEP when C fluxes other than C fixation and respiration occur, or when inorganic C enters or leaves in dissolved form. These fluxes include the leaching loss or lateral transfer of C from the ecosystem; the emission of volatile organic C, methane, and carbon monoxide; and the release of soot and CO2 from fire. Carbon fluxes in addition to NEP are particularly important determinants of NECB over long time scales. However, even over short time scales, they are important in ecosystems such as streams, estuaries, wetlands, and cities. Recent technological advances have led to a diversity of approaches to the measurement of C fluxes at different temporal and spatial scales. These approaches frequently capture different components of NEP or NECB and can therefore be compared across scales only by carefully specifying the fluxes included in the measurements. By explicitly identifying the fluxes that comprise NECB and other components of the C cycle, such as net ecosystem exchange (NEE) and net biome production (NBP), we can provide a less ambiguous framework for understanding and communicating recent changes in the global C cycle.

Global Warming and Terrestrial Ecosystems: A Conceptual Framework for Analysis

raise global mean temperature over the next century by 1.0–3.5 °C (Houghton et al. 1995, 1996). Ecologists from around the world have begun experiments to investigate the effects of global warming on terrestrial ecosystems, the aspect of global climate change that attracts the most public attention (Woodwell and McKenzie 1995, Walker and Steffen 1999). The effort to understand response to warming builds on a history of investigations of the effects of elevated CO 2 on plants and ecosystems (Koch and Mooney 1996, Schulze et al. 1999). There are important differences, however, between increases in atmospheric CO 2 and temperature change, both in the temporal and spatial patterns of change and in how they affect ecosystems. The scientists involved in temperature change research have had to face new technical and conceptual challenges in designing and interpreting their experiments (Schulze et al. 1999). In this paper we describe these challenges and present a conceptual framework for interpreting experimental results and predicting effects of warming on ecosystems.

The Impact of Boreal Forest Fire on Climate Warming

We report measurements and analysis of a boreal forest fire, integrating the effects of greenhouse gases, aerosols, black carbon deposition on snow and sea ice, and postfire changes in surface albedo. The net effect of all agents was to increase radiative forcing during the first year (34 ± 31 Watts per square meter of burned area), but to decrease radiative forcing when averaged over an 80-year fire cycle (–2.3 ± 2.2 Watts per square meter) because multidecadal increases in surface albedo had a larger impact than fire-emitted greenhouse gases. This result implies that future increases in boreal fire may not accelerate climate warming.

Arctic ecosystems in a changing climate: An ecophysiological perspective

F.S. Chapin III, R.L. Jefferies, J.F. Reynolds, G.R. Shaver, and J. Svoboda, Arctic Plant Physiological Ecology: A Challenge for the Future. The Arctic System: B. Maxwell, Arctic Climate: Potential for Change under Global Warming. D.L. Kane, L.D. Hinzman, M. Woo, and K.R. Everett, Arctic Hydrology and Climate Change. L.C. Bliss and N.V. Matveyeva, Circumpolar Arctic Vegetation. W.D. Billings, Phytogeographic and Evolutionary Potential for the Arctic Flora and Vegetation in a Changing Climate. L.C. Bliss and K.M. Peterson, Plant Succession, Competition, and the Physiological Constraints of Species in the Arctic. Carbon Balance: W.C. Oechel and W.D. Billings, Effects of Global Change on the Carbon Balance of Arctic Plants and Ecosystems. O.A. Semikhatova, T.V. Gerasimenko, and T.I. Ivanova, Photosynthesis, Respiration, and Growth of Plants in the Soviet Arctic. G.R. Shaver and J. Kummerow, Phenology, Resource Allocation, and Growth of Arctic Vascular Plants. J.D. Tenhunen, O.L. Lange, S. Hahn, R. Siegwolf, and S.F. Oberbauer, The Ecosystem Role of Poikilohydric Tundra Plants. B. Sveinbj~adornsson, Arctic Tree Line in a Changing Climate. Water and Nutrient Balance: S.F. Oberbauer and T.E. Dawson, Water Relations of Arctic Vascular Plants. K.J. Nadelhoffer, A.E. Giblin, G.R. Shaver, and A.E. Linkins, Microbial Processes and Plant Nutrient Availability in Arctic Soils. D.M. Chapin and C.S. Bledsoe, Nitrogen Fixation in Arctic Plant Communities. K. Kielland and F.S. Chapin III, Nutrient Absorption and Accumulation in Arctic Plants. F. Berendse and S. Jonasson, Nutrient Use and Nutrient Cycling in Northern Ecosystems. Interactions: J.B. McGraw and N. Fetcher, Response of Tundra Plant Populations to Climatic Change. J.P. Bryant and P.B. Reichardt, Controls over Secondary Metabolite Production by Arctic Woody Plants. R.L. Jefferies, J. Svoboda, G. Henry, M. Raillard, and R. Ruess, Tundra Grazing Systems and Climatic Change. J.F. Reynolds and P.W. Leadley, Modeling the Response of Arctic Plants to Changing Climate. F.S. Chapin III, R.L. Jefferies, J.F. Reynolds, G.R. Shaver, and J. Svoboda, Arctic Plant Physiological Ecology in an Ecosystem Context. Index.

Temperature and vegetation seasonality diminishment over northern lands

Pronounced increases in winter temperature result in lower seasonal temperature differences, with implications for vegetation seasonality and productivity. Research now indicates that temperature and vegetation seasonality in northern ecosystems have diminished to an extent equivalent to a southerly shift of 4°– 7° in latitude, and may reach the equivalent of up to 20° over the twenty-first century.

Paying for ecosystem services - Promise and peril

Payment mechanisms designed without regard to the properties of the services they cover may be environmentally harmful. The Millennium Ecosystem Assessment concluded that over the past 50 years, 60% of all ecosystem services (ES) had declined as a direct result of the growth of agriculture, forestry, fisheries, industries, and urban areas (1). This is not surprising: We get what we pay for. Markets exist for the products of agriculture, aquaculture, and forestry. But the benefits of watershed protection (2), habitat provision (3), pest and disease regulation (4), climatic regulation (5), and hazard protection (6) are largely unpriced. Because existing markets seldom reflect the full social cost of production, we have incorrect measures of the scarcity of some ES and no measures for the rest.

Element Cycling in Taiga Forests: State-Factor Control

ronment characterized by drastic seasonal fluctuations in day length and temperature, a short growing season, low soil temperatures, and permafrost (Van Cleve and Alexander 1981). The taiga is part of the circumpolar forest region near the latitudinal limit of tree growth. The taiga occupies large areas of Alaska, Canada, Scandinavia, and the Soviet Union (Van Cleve and Alexander 1981). With changing climate, ecological relationships within the taiga could assume global importance, because this region contains 20% of the worlds stored carbon and is a large but unexplored source of methane and carbon dioxide, two gases implicated in causing climate change (Billings 1987, McBeath 1984, Reeburgh 1990). Flux rates of these gases are expected to change An understanding of taiga ecosystem controls is important for predicting global responses to climate change

Taiga Ecosystems in Interior Alaska

For several years the University of Alaska and the Institute of Northern Forestry (USDA Forest Service) have conducted a multidisciplinary study of interior-Alaska forest ecosystems, especially the black spruce type. Black spruce forests are widespread in interior Alaska and are the most fire-prone forest type. They are also the most nutrient-limited and least productive forest type, especially in the late stages of succession. Ecosystem differences in productivity and degree of nutrient limitation are controlled mainly by soil and forest-floor temperatures. (Accepted for publication 3 August 1982)

NET ECOSYSTEM PRODUCTION: A COMPREHENSIVE MEASURE OF NET CARBON ACCUMULATION BY ECOSYSTEMS

The conceptual framework used by ecologists and biogeochemists must allow for accurate and clearly defined comparisons of carbon fluxes made with disparate techniques across a spectrum of temporal and spatial scales. Consistent with usage over the past four decades, we define “net ecosystem production” (NEP) as the net carbon accumulation by ecosystems. Past use of this term has been ambiguous, because it has been used conceptually as a measure of carbon accumulation by ecosystems, but it has often been calculated considering only the balance between gross primary production (GPP) and ecosystem respiration. This calculation ignores other carbon fluxes from ecosystems (e.g., leaching of dissolved carbon and losses associated with disturbance). To avoid conceptual ambiguities, we argue that NEP be defined, as in the past, as the net carbon accumulation by ecosystems and that it explicitly incorporate all the carbon fluxes from an ecosystem, including autotrophic respiration, heterotrophic respiration, losses associated with disturbance, dissolved and particulate carbon losses, volatile organic compound emissions, and lateral transfers among ecosystems. Net biome productivity (NBP), which has been proposed to account for carbon loss during episodic disturbance, is equivalent to NEP at regional or global scales. The multi-scale conceptual framework we describe provides continuity between flux measurements made at the scale of soil profiles and chambers, forest inventories, eddy covariance towers, aircraft, and inversions of remote atmospheric flask samples, allowing a direct comparison of NEP estimates made at all temporal and spatial scales.