Goal

The aim of the PBM project is to develop the existing LPJmL Dynamic Global Vegetation Model towards a new-generation biosphere model that is suited as a fast and effective component of an Earth System Model. Therefore, the structure of LPJ is fundamentally revised and the model developed further both in terms of process representations and in terms of proven ability to reproduce observed biogeochemical and hydrological dynamics.

Background

The role of the land biosphere in the Earth system – both as a substrate for human activity and as a significant mechanism producing potentially nonlinearities in the carbon cycle – has been demonstrated by numerous studies. Modelling these dynamic processes at a level of complexity comparable to present-day climate models has been achieved by Dynamic Global Vegetation Models (DGVMs) such as LPJmL, but their interface to climate models has not received sufficient technical nor scientific attention. As a consequence, climate models are using outdated representations of land surface processes (if any at all), and DGVMs cannot use climate model outputs directly.

This dilemma originates not only in different scientific communities, but also in the different scales of the processes receiving most attention in these communities. Climate models typically are concerned with quick, large-scale flows of the atmosphere, while land surface processes are slow, fine-scale processes such as the growth of plants or the dynamics of soil carbon pools.

This situation calls for a new-generation numerical simulation model – PBM – which is capable of representing the main physiological and biophysical processes in the land biosphere, as a function of the main driver complexes ‘climate’ and ‘land use’. PBM is named a ‘module’, because its design is oriented towards the coupling with other components within a global integrated assessment model. It will simultaneously also be suited for use in classical offline mode.

Conceptual and technical development

PBM inherits all scientific knowledge encoded in the most recent version of LPJmL, but its structure is fundamentally revised to be computationally more efficient and more flexible. The structural revision includes e.g. the following steps:

• Simpler extension for additional plant functional types, such as arable crops and managed forests

• Option for spatial rather than temporal looping so as to enable lateral exchange between grid cells (e.g. along river networks, or for migration of plants)

• Recoding of model in C language

• Modular and object-based model structure

• Restart option (e.g. after spinup)

• Temporal synchronisation of represented processes

Model evaluation

PBM’s process representations and simulation quality are continuously improved in the following ways:

• Multi-parameter validation against a set of biogeochemical, biophysical, and hydrological data, at scales from local to global

• Sensitivity analyses for many parameters

• Uncertainty analyses

• Continuous improvements in terms of process representations (e.g. inclusion of permafrost) and parameterisations (of e.g. rooting depths and soil hydrology)

Some model applications

In a series of offline simulation studies, LPJmL/PBM was employed for assessing transient biosphere responses to climate changes with a focus on climate effects on the coupled carbon and water cycle.

In an offline simulation study, the potential future evolution of the terres­trial carbon cycle under different climate change scenar­ios was investigated. It was found that response patterns differ among scenarios, due primarily to differences in precipitation projections. Yet, there is a general tendency of the biosphere to shift from a net carbon sink to a carbon source from the midst of this century, implying a positive feedback to the at­mosphere that will lead to further warming.

An associated study showed for the same scenarios that although water availability will decrease in many regions during this century, the world’s ecosystems will experience unchanged, or even less, water limitation. This hydrologic resilience of the biosphere is due to a fragile balance between climate effects, direct CO₂ effects upon plants, and (regionally drastic) ecosystem structural changes.

In a model intercomparison study that investigated individual and joint effects of precipitation, temperature and CO₂ changes on carbon and water fluxes in (in the frame of the EPRECOT project), the model’s ability to reproduce observed fluxes and to simulate plausibly complex ecosystem changes was demonstrated for a suite of terrestrial ecosystems located in different climate zones.

Project team

Wolfgang Cramer, Dieter Gerten, Wolfgang Lucht, Sibyll Schaphoff, Werner von Bloh

Publications

Beer, C., Lucht, W., Gerten, D., Thonicke, K., Schmullius, C. 2007. Effects of freeze-thaw processes on biomass in Siberia. Global Biogeochemical Cycles 21, GB1012, dx.doi.org/10.1029/2006GB002760.

Bondeau, A., Smith, P., Zaehle, S., Schaphoff, S., Lucht, W., Cramer, W., Gerten, D., Lotze-Campen, H., Müller, C., Reichstein, M., Smith, B. 2007. Modelling the role of agriculture for the 20th century global terrestrial carbon balance. Global Change Biology 13, OnlineEarly.

Cramer, W., Bondeau, A., Schaphoff, S., Lucht, W., Smith, B., Sitch, S. 2004. Tropical forests and the global carbon cycle: Impacts of atmospheric CO₂, climate change and rate of deforestation. Phil Trans Roy Soc B 359:331-343.

Gerten, D., Lucht, W., Schaphoff, S., Cramer, W., Hickler, T., Wagner, W. 2005. Hydrologic resilience of the terrestrial biosphere. Geophysical Research Letters 32, L21408, doi: 10.1029/2005GL024247.

Gerten, D., Haberlandt, U., Cramer, W., Erhard, M. 2005. Terrestrial carbon and water fluxes. In: Hantel, M. (Ed.), Observed Global Climate, Landolt-Börnstein New Series, Group V: Geophysics, Vol. 6, Springer, Berlin, 12-1–12-17.

Gerten, D., Schaphoff, S., Lucht, W. 2007. Potential future changes in water limitations of the terrestrial biosphere. Climatic Change 80, 277–299.

Gerten, D., Luo, Y., Le Maire, G., Parton, W.J., Keough, C., Weng, E., Beier, C., Ciais, P., Cramer, W., Dukes, J.S., Emmett, B., Hanson, P.J., Knapp, A., Linder, S., Nepstad, D., Rustad, L. Modelled effects of multiple global change factors on ecosystem carbon and water dynamics in different climatic zones. Part I: Precipitation-only effects. Global Change Biology (submitted).

Leipprand, A., Gerten, D. 2006. Global effects of doubled atmospheric CO₂ content on evapotranspiration, soil moisture and runoff under potential natural vegetation. Hydrological Sciences Journal 51, 171–185.

Lucht, W., Schaphoff, S., Erbrecht, T., Heyder, U., Cramer, W. 2006. Terrestrial vegetation redistribution and carbon balance under climate change. Carbon Balance Management 1:6, doi: 10.1186/1750-0680-1-6 (open access)

Luo, Y., Gerten, D., Le Maire, G., Parton, W.J., Weng, E., Keough, C., Zhou, X., Beier, C., Ciais, P., Cramer, W., Dukes, J.S., Emmett, B., Hanson, P.J., Knapp, A., Linder, S., Nepstad, D., Rustad, L. Modelled effects of multiple global change factors on ecosystem carbon and water dynamics in different climatic zones. Part II: Interactive effects of precipitation, temperature, and CO₂. Global Change Biology (submitted).

Schaphoff, S., Lucht, W., Gerten, D., Sitch, S., Cramer, W., Prentice, I.C. 2006. Terrestrial biosphere carbon storage under alternative climate projections. Climatic Change 74, 97–122.

Zaehle, S., Sitch, S., Prentice, I.C., Liski, J., Cramer, W., Erhard, M., Hickler, T., Smith, B. 2006. The importance of age-related decline in forest NPP for modeling regional carbon balances. Ecological Applications 16, 1555-1574