Impacts of large-scale climatic disturbances on the terrestrial carbon cycle
© Erbrecht and Lucht; licensee BioMed Central Ltd. 2006
Received: 11 July 2006
Accepted: 27 July 2006
Published: 27 July 2006
The amount of carbon dioxide in the atmosphere steadily increases as a consequence of anthropogenic emissions but with large interannual variability caused by the terrestrial biosphere. These variations in the CO2 growth rate are caused by large-scale climate anomalies but the relative contributions of vegetation growth and soil decomposition is uncertain. We use a biogeochemical model of the terrestrial biosphere to differentiate the effects of temperature and precipitation on net primary production (NPP) and heterotrophic respiration (Rh) during the two largest anomalies in atmospheric CO2 increase during the last 25 years. One of these, the smallest atmospheric year-to-year increase (largest land carbon uptake) in that period, was caused by global cooling in 1992/93 after the Pinatubo volcanic eruption. The other, the largest atmospheric increase on record (largest land carbon release), was caused by the strong El Niño event of 1997/98.
We find that the LPJ model correctly simulates the magnitude of terrestrial modulation of atmospheric carbon anomalies for these two extreme disturbances. The response of soil respiration to changes in temperature and precipitation explains most of the modelled anomalous CO2 flux.
Observed and modelled NEE anomalies are in good agreement, therefore we suggest that the temporal variability of heterotrophic respiration produced by our model is reasonably realistic. We therefore conclude that during the last 25 years the two largest disturbances of the global carbon cycle were strongly controlled by soil processes rather then the response of vegetation to these large-scale climatic events.
Anthropogenic emissions continuously add about 7000–8000 million metric tons of carbon to the atmosphere per annum . Atmospheric carbon dioxide measurements show that the rate of increase of atmospheric CO2 varies substantially from year to year . It is widely accepted that these variations are caused by the terrestrial biosphere through the processes of carbon uptake during photosynthesis and carbon release during soil respiration . Additionally, strong disturbances such as large-scale fires can significantly alter the exchange of carbon between terrestrial ecosystems and the atmosphere. For example, up to 65 % of the observed CO2 growth rate in 1998 was attributed to burnt biomass in tropical and boreal regions . In comparison, variations in the oceans , deforestation, and land use change are much smaller .
Uncertainty remains, however, regarding the relative influence of the driving climatic anomalies (temperature and precipitation anomalies) on the most prominent terrestrial carbon processes, namely vegetation growth (NPP) and soil decomposition (Rh). Numerical models of the land carbon cycle allow investigations of these relationships.
The two largest anomalies of atmospheric CO2 growth rate during the last 25 years are related to two large climatic disturbances – the increased planetary albedo after the eruption of Mount Pinatubo in 1991 and the strong El Niño event of 1997/98. The Pinatubo eruption was an extraordinary event because of the large amount of aerosols that were injected into the lower stratosphere where they were distributed around the globe, leading to a world-wide cooling of about 0.5°C . The 1997/98 El Niño event was unusual in that it was extremely strong .
We use the LPJ model of terrestrial carbon and water cycles [9, 10] to explore the covariability of climatic forcings and physiological responses (namely_NPP and Rh) of the terrestrial biosphere on a global scale as well as for selected latitudinal regions. Results show that a large fraction of the observed CO2 growth rate variability is controlled by varying soil organic matter decomposition rather than changing plant productivity. This sheds additional light on previous studies that highlighted connections between the interannual variability of atmospheric CO2 growth and net primary productivity [11–13].
The 1992/93 sink event
What was the cause for this large anomalous sink – changes in NPP or Rh? The model shows that the pronounced reduction of atmospheric CO2 increase results from reduced soil respiration whereas NPP did not change (cf. [15–17]).
The 1998 source event
The physiological response of the land biosphere during the strong 1997/98 El Niño event was diametrically opposite to that of the 1992/93 period. A large anomalous flux of 2.82 GtC into the atmosphere was measured in 1998 of which 1.62 GtC can be assigned to vegetation growth and soil decomposition processes of the terrestrial biosphere (fig. 1). LPJ simulates a value of 1.85 GtC due to stronger soil respiration that was partly counteracted by increased photosynthetic activity (fig. 2). The remaining 1.2 GtC of the anomalous land source consists of a small contribution from anthropogenic emissions (about 0.2 GtC) while the rest is attributable to carbon emissions from extensive fires in tropical, subtropical and boreal regions [4, 18](fig. 1).
The 1992/93 sink event
The quality of modelled soil carbon decomposition is less certain than that of NPP. Studies show, however, that the temporal variability of modelled vegetation activity is in good agreement with independent satellite data [17, 19], indicating that the temporal variations of the associated simulated NPP are likely reliable. Because measured and modelled NEE, i.e. the difference of Rh and NPP, are found to be very similar in magnitude, we suggest that our simulation of the magnitude of anomalous soil respiration is plausible.
The 1998 source event
Removing precipitation anomalies from the climate data results in a small terrestrial carbon sink that is triggered by increasing NPP in the tropics while reducing the carbon flux from soils (fig. 4, P const). The influence of the temperature anomalies is more important outside the low latitudes. Eliminating the positive temperature anomaly of 1998 reduces Rh significantly in the mid-latitudes whereas the carbon fluxes in and out of tropical ecosystems become balanced (fig. 4, T const). Thus the 1998 carbon source resulted from two mechanisms, a limitation of vegetation activity and a stimulation of soil decomposition in the tropics as well as temperature-driven acceleration (possibly via teleconnections) of heterotrophic respiration relative to NPP in northern temperate regions.
We conclude that the two largest variations of the global carbon cycle observed during the last 25 years were predominantly controlled by soil processes rather than by vegetation activity. This adds a different perspective to previous analysis [11–13] that concentrated on contributions of global NPP anomalies to atmospheric CO2 growth rate anomalies. While NPP indeed plays an important role, analysis shows that in some periods changes in NPP alone do not explain the observed excursions of atmospheric carbon dioxide accumulation. This applies particularly to the two large events analysed in this paper, and in both cases changes in soil respiration explain the observed variability. Considering only the relationship between climate and NPP underestimates the true variability of the global carbon cycle. Vegetation growth and soil decomposition react differentially to anomalies in temperature and precipitation.
Observed and modelled NEE anomalies agree surprisingly well, suggesting that the LPJ model simulates the temporal variability of soil organic matter decomposition sufficiently. The model can hence be applied to the analysis of the land biosphere's modulation of atmospheric CO2 concentration and the biogeochemical effects of large-scale climate variations.
There are several implication for policy. First, long-term climate protection strategies aimed at full accounting of terrestrial carbon sources and sinks should focus on soil and vegetation processes equally. Currently, much more is known about vegetation responses to climate change than about soil processes . Second, the terrestrial carbon cycle varies strongly in space and time. A monitoring regime therefore should take into account the characteristics of the dynamics observed and modelled for different global regions and temporal periods. Climatic events may strongly alter the short-term balance, rendering them untypical of the average behaviour.
The LPJ DGVM
The LPJ Dynamic Global Vegetation Model [9, 10] is a biogeochemical model of fluxes of carbon and water in terrestrial vegetation and soils. Carbon uptake during photosynthesis is estimated using the Farquhar-Collatz scheme which is coupled to two soil layers [24, 25]. Assimilated carbon is allocated to four pools (leaves, sapwood, heartwood and fine roots) following allometric and functional relationships . Carbon from dead biomass enters above- and belowground litter pools and is then transferred to a fast and slow decomposing soil carbon pool. Soil organic matter decomposition is calculated using a modified Arrhenius formulation  which implies a decline in apparent Q10 with temperature, as well as an empirical soil moisture relationship .
We performed simulations with 0.5 degrees spatial resolution (59199 grid cells) using an interpolated climatology of monthly values of temperature, precipitation and radiation [28, 29]. A land cover data set produced at the University of Maryland provided a realistic distribution of global crop lands . The LPJ-DGVM has been extensively validated using various data from atmospheric measurements, active and passive remote sensing data, and flux measurements. It was shown that the model is capable of simulating large-scale structure, distribution and phenology of global vegetation [9, 17] as well as the inferred seasonal cycles of soil moisture , evapotranspiration and runoff .
We acknowledge project funding from the State of Brandenburg (TE) and the International Max Planck Research School on Earth System Modelling. We thank Sibyll Schaphoff, Dieter Gerten, and Wolfgang Cramer for valuable contributions and continued support. We would like to express our thanks to Will Steffen for having been the Handling Editor and to three anonymous reviewers for their very helpful comments.
- Marland G, Boden TA, Andres RJ: Global, Regional, and National CO 2 Emissions. In Trends: A Compendium of Data on Global Change. Carbon Dioxide Information Analysis Center, Oak Ridge National Laboratory, U.S. Department of Energy, Oak Ridge, Tenn., USA; 2005.Google Scholar
- Peylin P, Bousquet P, Quere CL, Sitch S, Friedlingstein P, McKinley G, Gruber N, Rayner P, Ciais P: Multiple constraints on regional CO 2 flux variations over land and oceans. Global Biogeochemical Cycles 2005, 19.Google Scholar
- Houghton JT: Climate Change 2001: The Scientific Basis. Cambridge University Press, Cambridge; 2001.Google Scholar
- Van der Werf G, Randerson J, Collatz G, Giglio L, Kasibhatla P, Arellano A Jr, Olsen S, Kasischke E: Continental-Scale Partitioning of Fire Emissions During the 1997 to 2001 El Niño/La Niña Period. Science 2004, 303: 73–76. 10.1126/science.1090753View ArticleGoogle Scholar
- Le Quéré C, Aumont OL, Bopp L, Bousquet P, Ciais P, Francey R, Heimann M, Keeling CD, Keeling RF, Kheshgi H, Peylin P, Piper SC, Prentice IC, Rayner PJ: Two decades of ocean CO 2 sink and variability. Tellus 2003, 55B: 649–656.View ArticleGoogle Scholar
- Houghton RA, Hackler JL: Carbon Flux to the Atmosphere from Land-Use Changes. In Trends: A Compendium of Data on Global Change. Carbon Dioxide Information Analysis Center, Oak Ridge National Laboratory, U.S. Department of Energy, Oak Ridge, Tenn., USA; 2002.Google Scholar
- Hansen J, Ruedy R, Sato M, Reynolds R: Global surface air temperature in 1995: Return to pre-Pinatubo level. Geophysical Research Letters 1996, 23: 1665–1668. 10.1029/96GL01040View ArticleGoogle Scholar
- McPhaden J: Genesis and Evolution of the 1997–98 El Niño. Science 1999, 283: 950–954. 10.1126/science.283.5404.950View ArticleGoogle Scholar
- Sitch S, Smith B, Prentice IC, Arneth A, Bondeau A, Cramer W, Kaplan JO, Levis S, Lucht W, Sykes MT, Thonicke K, Venevsky S: Evaluation of ecosystem dynamics, plant geography and terrestial carbon cycling in the LPJ dynamic global vegetation model. Global Change Biology 2003, 9: 161–185. 10.1046/j.1365-2486.2003.00569.xView ArticleGoogle Scholar
- Gerten D, Schaphoff S, Haberlandt U, Lucht W, Sitch S: Terrestrial vegetation and water balance – hydrological evaluation of a dynamic global vegetation model. Journal of Hydrology 2004, 286: 249–270. 10.1016/j.jhydrol.2003.09.029View ArticleGoogle Scholar
- Hicke JA, Asner GP, Randerson JT, Tucker C, Los S, Birdsey R, Jenkins JC, Field C, Holland E: Satellite-derived increases in net primary productivity across North America, 1982–1998. Geophysical Research Letters 2002, 29: 69–73. 10.1029/2001GL013578View ArticleGoogle Scholar
- Nemani RR, Keeling CD, Hashimoto H, Jolly WM, Piper SC, Tucker CJ, Myneni RB, Running SR: Climate-Driven Increases in Global Terrestial Net Primary Production from 1982 to 1999. Science 2003, 300: 1560–1563. 10.1126/science.1082750View ArticleGoogle Scholar
- Potter CS, Klooster S, Myneni RB, Genovese V, Tan P, Kumar V: Continental-scale comparisons of terrestial carbon sinks estimated from satellite data and ecosystem modeling 1982–1998. Global and Planetary Change 2003, 39: 201–213. 10.1016/j.gloplacha.2003.07.001View ArticleGoogle Scholar
- Goldammer JG, Mutch RW, (Eds): Global forest fire assessment 1990–2000. FAO Forestry Department; 2001.Google Scholar
- Angert A, Biraud S, Bonfils C, Buermann W, Fung I: CO 2 seasonality indicates origins of post-Pinatubo sink. Geophysical Research Letters 2004, 31.Google Scholar
- Gu L, Baldocchi DD, Wofsy SC, Munger JW, Michalsky JJ, Urbanski SP, Boden TA: Response of a Deciduous Forest to the Mount Pinatubo Eruption: Enhanced Photosynthesis. Science 2003, 299: 2035–2038. 10.1126/science.1078366View ArticleGoogle Scholar
- Lucht W, Prentice IC, Myneni RB, Sitch S, Friedlingstein P, Cramer W, Bousquet P, Buermann W, Smith B: Climate Control of the High-Latitude Vegetation Greening Trend and Pinatubo Effect. Science 2002, 296: 1687–1689. 10.1126/science.1071828View ArticleGoogle Scholar
- Randerson JT, Van der Werf GR, Collatz GJ, Giglio L, Still CJ, Kasibhatla P, Miller JB, White JWC, DeFries RS, Kasischke ES: Fire emissions from C 3 and C 4 vegetation and their influence on interannual variability of atmospheric CO 2 and δ 13 CO 2 . Global Biogeochemical Cycles 2005, 19.Google Scholar
- Hickler T, Smith B, Sykes MT, Davis M, Sugita S, Walker K: Using a generalized vegetation model to simulate vegetation dynamics in northeastern USA. Ecology 2004, 85: 519–530.View ArticleGoogle Scholar
- Saleska SR, Miller SD, Matross DM, Goulden ML, Wofsy SC, da Rocha HR, de Camargo PB, Crill P, Daube BC, de Freitas HC, Hutyra L, Keller M, Kirchhoff V, Menton M, Munger JW, Pyle EH, Rice AH, Silva H: Carbon in Amazon Forests: Unexpected Seasonal Fluxes and Disturbance-Induced Losses. Science 2003, 302: 1554–1557. 10.1126/science.1091165View ArticleGoogle Scholar
- Buermann W, Anderson B, Tucker CJ, Dickinson RE, Lucht W, Potter CS, Myneni RB: Interannual covariability in Northern Hemisphere air temperetures and greenness associated with El Nino-Southern Oscillation and the Arctic Oscillation. Journal of Geophysical Research 2003, 108: 4396–441. 10.1029/2002JD002630View ArticleGoogle Scholar
- Zeng N, Mariotti A, Wetzel P: Terrestrial mechanisms of interannual CO 2 variability. Global Biogeochemical Cycles 2005, 19.Google Scholar
- Davidson EA, Janssens IA: Temperature sensitivity of soil carbon decomposition and feedbacks to climate change. Nature 2006, 440: 165–173. 10.1038/nature04514View ArticleGoogle Scholar
- Farquhar GD, von Caemmerer S, Berry JA: A biochemical model of photosynthetic CO 2 assimilation in leaves of C3 species. Planta 1980, 149: 78–90. 10.1007/BF00386231View ArticleGoogle Scholar
- Collatz GJ, Ribas-Carbo M, Berry JA: Coupled photosynthesis-stomatal conductancemodel for leaves of C4 plants. Australian Journal of Plant Physiology 1992, 19: 519–538.View ArticleGoogle Scholar
- Lloyd J, Taylor JA: On the temperature dependence of soil respiration. Functional Ecology 1994, 8: 315–323. 10.2307/2389824View ArticleGoogle Scholar
- Foley JA: An equilibrium model of the terrestrial carbon budget. Tellus 2005, 47B: 310–319.Google Scholar
- New MG, Hulme M, Jones PD: Representing twentieth-century space-time climate variability, Part II, Development of 1901–1996 monthly grids of terrestrial surface climate. Journal of Climate 2000, 13: 2217–2238. 10.1175/1520-0442(2000)013<2217:RTCSTC>2.0.CO;2View ArticleGoogle Scholar
- Oesterle H, Gerstengarbe FW, Werner P: Homogenisierung und Aktualisierung des Klimadatensatzes der Climate Research Unit der University of East Anglia, Norwich. In Terra Nostra 6. Deutsche Klimatagung Potsdam, Germany; 2003.Google Scholar
- Hansen M, DeFries R, Townshend JRG, Sohlberg R: Global land cover classification at 1 km spatial resolution using a classification tree approach. International Journal of Remote Sensing 2000, 21: 1331–1364. 10.1080/014311600210209View ArticleGoogle Scholar
- Wagner W, Scipal K, Pathe C, Gerten D, Lucht W, Rudolf B: Evaluation of the agreement between the first global remotely sensed soil moisture data with model and precipitation data. Journal of Geophysical Research 2003, 108: 1–10. 10.1029/2003JD003663View ArticleGoogle Scholar
This article is published under license to BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.