Biochemical feedbacks in the climate system

Principal Investigator

Dr Wolfgang Knorr
Max-Planck-Institut für Biochemie
Max-Planck-Institute for Meteorology
Bundesstr. 55
D-20146 Hamburg
Germany

knorr@dkrz.de

Project description

The rising level of atmospheric carbon dioxide is one of the most concerning issues in the new century: Fossil fuel combustion and land use change lead to the well known CO2-accumulation in the atmosphere. About half of the current anthropogenic emissions of carbon dioxide (7 Gt/a) are being absorbed by the ocean (2 Gt/a) and by land ecosystems (2 Gt/a). About 3 Gt CO2 per year remain in the atmosphere (Prentice et al., in press). The cumulative increase, together with other green house gases, present a total radiative forcing of about 2.8 W/m2 to the climate system compared to pre-industrial times, resulting in significant global warming. Both, radiatively induced climate change and changes in land use and land cover lead to certain response pattern of the soils and vegetation. These in return alter atmospheric CO2 levels and the land surface structure feeding back on the regional and global climate system. In this way, increasing atmospheric CO2 concentration is expected to lead to a reduced net uptake rate of CO2 by land vegetation. It is well possible that the positive feedback effects may eventually prevail, turning the land vegetation-soil system from a current net sink into a net source for CO2. This has been demonstrated by Cox et al. (2000) with a coupled carbon-climate system including a dynamic terrestrial, albeit largely simplified, model component.

Current efforts at the MPI-BGC include the coupling of a new, sophisticated land surface scheme to the climate model ECHAM. This project (so-called 'JSBACH' - Joint Simulation of Biosphere Atmosphere Coupling in Hamburg) provides the necessary tools for investigating all relevant feedbacks between the physical climate system and land surface processes. Relevance is here defined as helping to understand any process that leads to major changes of either global or regional climates, starting from the climate system of today, or form a past one that is similar to today's and may therefore serve as test case for our understanding of the coupled climate system.

Project Description

This project addresses various feedback mechanism between the physical climate system and land surface processes including biogeochemical cycles. A completely innovative approach will be the inclusion of the process-based modelling of vegetation dynamics, including the explicit representation of processes like disturbance, mortality, establishment, seed dispersal, and land use change. Also, both fast and slow process modes will provide a broad range of validation facilities, made easy through the implementation of an off-line model setup. For example, fast processes may thus be validated with local measurements or remote sensing of elements of the energy balance, such as skin temperatures, or slow processes by remotely sensed greenness and vegetation maps, based e.g. on proxy data based on pollen analysis for paleoclimate studies. Therefore, validation of the coupled JSBACH-ECHAM5 scheme will first be done by modelling the atmospheric CO2 concentration time series of the last 100 years. This will also provide knowledge of the relevant biosphere processes which contribute to CO2 concentration variation in the atmosphere. We will be able to trace the important feedback mechanism and determine their importance and their strength.

During the Holocene, i.e. for the last 11,000 years, the distribution of carbon between the components of the ocean-atmosphere-biosphere system remained remarkably stable, with atmospheric CO2 levels remaining between 260 and 280 ppmv (Indermühle et al. 1999). Therefore one of the key applications of the ECHAM5/JSBACH system will be the simulation of paleo time slices to obtain a deeper understanding of the carbon cycle of the past.

For the time period 2001-2003, we plan the following work for which we request computing resources on the VPP5000 at the ECMWF:

1. Climate effect of reduced stomatal densitity at increased atmospheric CO2 levels (Sellers, P.J., et al., 1996). Test different assumptions about phenological climate adaptation and shift in community composition (C3/C4, woody/herbaceous).
   Tool: ECHAM5/JSBACH. Time slices: pre-industrial, 2xCO2, 6k, 21k.
   Stomatal models: BETHY, GIWACOM
   Vegetation dynamics: LPJ
   Number of runs: 12, 10 years each (10.800 billing units) , plus 4 of 100 years to obtain vegetation distribution in climate equilibrium (possibly with asynchronous coupling reduction to 30 years each). (36.000 billing units)
   Sum: approx. 50.000
   
   
2. Bifurcation and stability of the North African climate system: sensitivity of atmosphere/land vegetation system to small changes in external forcing through Holocene period (Kutzbach & Liu, 1997; Texier et al., 1997; Broström et al., 1998; Ganopolski et al., 1998; Doherty et al., 2000)
   Time slices: every 1000 years from 11k to 4k
   External forcing: orbital, SST anomalies (can be obtained from Paul Valdes, Department of Meteorology, University of Reading)
   Tool: ECHAM5/JSBACH
   Innovation: new albedo data set, dynamic vegetation model, better precipitation simulation than other models.
   Simulations: 9 time slices of 100 years each. (81.000 billing units)
   Sum: approx. 85.000
   
   
3. Boreal and subtropical feedbacks in a coupled atmosphere/vegetation model for the last glacial maximum. Study strength of boreal forest/snow albedo feedback and impact of reduced vegetation cover on generally reduced hydrological cycle during LGM. Do the two effects reinforce cooling/drying compared to climate system without vegetation changes? (Ganopolski et al., 1998; Kubatzki & Claussen, 1998; Levis et al., 1999)
   Tools: ECHAM5/JSBACH.
   Data: updated Peltier ice sheet, CLIMAP SST or other update, when becoming available; Pollen data for validation.
   Simulations: 6 climatological runs 30 years each (17.000 billing units)
   Sum: approx. 20.000

Model Description

The JSBACH framework is based upon the MPI-Hamburg GCM ECHAM (Roeckner et al. 1996). This climate model has already been run at ECMWF in several special projects.

Additional components are representing the new surface scheme and are implemented and adapted to the ECHAM structure from existing models:

• Land surface scheme: the model to be implemented is based on the VIC (Variable Infiltration Capacity) model (Liang et al, 1994), developed by the Universities of Washington (Dennis Lettenmeier) and Princeton (Eric Wood). It offers a flexible vertical structure, needed for the description of competition between rooting strategies of different plants, and a complete description of soil heat and moisture interactions, including frozen soils.
  
  
• Fast vegetation processes: this model part is based on the BETHY scheme (Knorr, 2000), which already contains a description of photosynthesis embedded in the full land surface energy balance, plant respiration, and a cold and dry phenology scheme. In contrast to most terrestrial ecosystem models (e.g. LPJ, below), it computes fluxes in a diurnal cycle on basis of the GCM-time step.
  
  
• Slow vegetation processes: these include vegetation dynamics (dispersal, establishment, growth, dieback), soil carbon turnover, carbon-nutrient interactions and nutrient cycling, fire and other disturbances, and land-use impacts. The model implemented will be based on the LPJ (Lund-Potsdam-Jena) model (Sitch et al. 2000), which is being adopted as the standard vegetation dynamics model of MPI Jena, Lund University (Sweden), and the Potsdam Institute for Climate Impact Research (PIK). This will ensure that further developments of LPJ will directly flow into JSBACH.

As JSBACH is based upon ECHAM it is highly optimised for vector machines. It uses about 500 MB of memory in T42 resolution with 19 levels and takes about 30000 CPU seconds on a VPP5000 machine per simulated year (one processor). Disk space requirements are 1.0 GB for the intermediate storage of model output, and about 1.0 GB on the mass storage system for accumulating output per year before it can be transferred to Hamburg.

References

Broström, A., et al., 1998. Land surface feedbacks and palaeomonsoons in Northern Africa. Geophysical Research Letters 25, 3615-3618.

Cox, P.M., et al., 2000. Acceleration of global warming due to carbon-cycle feedbacks in a coupled climate model. Nature 408, 184-187.

Doherty, R., et al., 2000. Fully coupled climate/dynamical vegetation model simulations over Northern Africa during the mid-Holocene. Climate Dynamics 16, 561-573.

Ganopolski, A., et al., 1998. The influence of vegetation-atmosphere-ocean interaction on climate during the mid-Holocene. Science 280, 1916-1919.

Ganopolski, A., et al., 1998. Simulation of modern and glacial climates with a coupled global model of intermediate complexity. Nature 391, 351-356.

Indermühle, A. et al., 1998. Holocene carbon-cycle dynamics based on CO2 trapped in ice at Talyer Dome, Antarctica. Nature 398, 121-126.

Knorr, W., 2000. Annual and interannual CO2 exchanges of the terrestrial biosphere: process based simulations and uncertainties, Global Ecology and Biogeography 9, 225-252.

Kubatzki, C. and M. Claussen, 1998. Simulation of the global bio-geophysical interactions during the Last Glacial Maximum. Climate Dynamics 14, 461-471.

Kutzbach, J.E. and Z. Liu, 1997. Response of the African Monsoon to orbital forcing and ocean feedbacks in the middle Holocene. Science 278, 440-443.

Liang, X. et al., 1994. A simple hydrologically based model of land surface water and energy fluxes for GCMs, Journal of Geophysical Research 99, 14415-14428.

Levis, S. et al., 1999. CO2, climate, and vegetation feedbacks at the Last Glacial Maximum. Journal of Geophysical Research Atmospheres 104, 31191-31198.

Prentice, I. C. et al. The carbon cycle and atmospheric CO2, in: IPCC Third Assessment Report, Intergovernmental Panel on Climate Change, in press.

Roeckner, E. et al., 1996. The atmospheric general circulation model ECHAM4: Model description and simulation of the present-day climate, Report of the Max-Planck-Institute for Meteorology, 218, Hamburg.

Sellers, P.J., et al., 1996. Comparison of radiative and physiological effects of doubled
atmospheric CO2 on climate. Science 271, 1402-1406.

Sitch, S. et al., 2000. LPJ- A coupled model of vegetation dynamics and the terrestrial carbon cycle, in: Stephen Sitch, The role of vegetation dynamics in the control of atmospheric CO2 content, Ph.D.Thesis, Lund Universty, Sweden.

Texier, D., et al., 1997. Quantifying the role of biosphere-atmosphere feedbacks in climate change - coupled model simulations for 6000 years BP and comparison with palaeodata for Northern Eurasia and Northern Africa. Climate Dynamics 13, 865-882.

Final report.

Additional information

Project started in 2001 - finished 2003.