Modeling past Greenhouse worlds with EC-Earth: Understanding past and predicting future responses to high greenhouse gas levels

Principal Investigator

Dr. S.L. Weber
Royal Netherlands Meteorological Institute (KNMI)
PO Box 2013730 AE
De Bilt
The Netherlands

weber@knmi.nl

Other researcher(s): Dr. S.S. Drijfhout

Project description

Paleoclimatic data indicate that the early Paleogene period (ca. 65-34 million year (Ma) Before Present) was a time of generally warm ‘greenhouse’ climate conditions. A distinct period of extreme global warmth was initiated close to the boundary between the Palaeocene and Eocene epochs, approximately 55 Ma ago (Sluijs et al., 2007). This event, termed the Palaeocene-Eocene Thermal Maximum (PETM), stands out against the background warmth as a spike in global temperatures. Both the background climate and the PETM itself were characterized by a moderate equator-to-pole temperature difference and reduced amplitude of the high-latitude seasonal cycle, particularly in the continental interiors, as compared to the modern climate. Associated with the warming is a rapid injection of CO 2 and/or CH 4 into the global carbon pool (Sluijs et al., 2007). This supports the hypothesis that increased greenhouse gas concentrations are the cause of the reconstructed warmth. The total amount of carbon input during the PETM is known to within an order of magnitude. It is about 4-8 times the anthropogenic carbon release from the start of the industrial era up to today and comparable to that expected from further anthropogenic emissions to the end of the 21 st century (IPCC 2007).

The warm and equable PETM climate constitutes a continuing challenge for the climate modeling community, as there are serious discrepancies between model results and paleoclimatic data. Simply increasing greenhouse gas levels and changing boundary conditions results in either polar regions which are too cold or tropical regions which are too warm (e.g. Huber and Sloan, 2001; Shellito et al., 2003). Many hypotheses have been proposed to explain this discrepancy. These include the extension of the Hadley circulation to high latitudes (Farrell, 1990), mixing of the ocean caused by tropical cyclones (Korty et al., 2008), stratospheric clouds during the polar night (Sloan and Pollard, 1998) and deep atmospheric convection during winter above ice-free high-latitude oceans (Abbott and Tziperman, 2008). The continued mismatch between model results and proxy-derived climate estimates suggests that some process is missing from current climate models. As similar models are used for future predictions, this issue needs to be resolved.

To simulate the reconstructed past equable climate of the PETM we will use the new comprehensive climate model EC-Earth, which is based on the ECMWF seasonal forecast model. The first version of EC-Earth (ecearth.knmi.nl) consists of the ECMWF IFS (Integrated Forecasting System) atmosphere model, the OPA ocean and sea-ice model, the TM5 atmospheric chemistry transport model, the TESSEL land component, and the OASIS coupler. The model is currently being further developed into an Earth System Model by a consortium of member states of ECMWF, lead by KNMI. Different than current climate models, the model contains highly sophisticated parameterizations developed and tested for weather forecast models. The current EC-Earth system offers excellent opportunities for studying the interaction between dynamical variability, atmospheric physics and atmospheric chemistry. At the same time, simulation of past climates poses an excellent test for models that are used to predict future climate changes (Weber et al., 2007).

The special project will improve our understanding of the dynamics of past hyperthermal events like the PETM and, at the same time, establish their relevance for understanding climate change today. Emphasis will be on the radiative budget at high latitudes, the role of polar stratospheric clouds, winter convection and atmospheric chemistry associated with increased concentrations of methane in the atmosphere. One specific question is whether the latter gives rise to enhanced water vapor levels in the stratosphere, thus increasing the likelihood of polar stratospheric clouds which warm the winter high latitudes. The project will further the development of EC-Earth as an Earth System Model, more specifically it will further the incorporation of atmospheric chemistry. This may have spin-off for the EU projects GEMS and MACC. It will also aid the development of the next generation climate scenarios.

Sensitivity studies

As a first step we propose to use the atmosphere-only version of EC-Earth, with prescribed sea surface temperatures (SSTs) which mimic reconstructions for the PETM (Huber and Sloan, 2001). We will iteratively determine greenhouse gas levels (within the range of estimates based on proxy data) to match these SSTs, so that the system can achieve thermal equilibrium. This set-up will then be applied in a suite of experiments with increasingly more realistic boundary conditions. We will change (i) land surface characteristics, replacing tundra and ice-caps at high latitudes by rain forest, (ii) global vegetation cover following available reconstructions and (iii) orography by lowering modern heights to much lower values more representative for the PETM. These studies will investigate the dynamic and thermal response of the atmosphere to the presence of a weak meridional surface temperature gradient and high greenhouse gas levels. These simulations will consist of a spin-up of a few years followed by a 10-yr run that will be used for analysis.

Feedbacks related to atmospheric chemistry

As the next step we plan to apply the TM5 atmospheric chemistry transport model driven by the ‘best guess’ for the PETM climate which can be derived from the sensitivity experiments described above. Here we will need to test various scenario’s for methane emissions, like an increased source from large wetlands (Sloan et al., 1992) or a sudden release of methane clathrates (Bralower et al., 1997). In addition, we need to make assumptions on other emissions. Focus of these studies will be on the methane budget, associated changes in the amount of stratospheric water vapour and radiative and dynamic feedbacks on climate.

The coupled chemistry-climate system

Finally, we propose a simulation with the full EC-Earth system where we will apply the forcings and boundary conditions as they are derived from the sensitivity runs and off-line runs with TM5.

Timetable

2009: Perform test simulations and sensitivity runs with atmosphere-only version of EC-Earth

2010: Perform simulations with TM5, driven by modelled PETM climate

2011: Perform coupled chemistry-climate simulations with EC-Earth

Explanation of the requested budget

A 10-year run of EC-Earth without atmospheric chemistry costs about 20.000 units requiring 20 Gbyte for output data storage. A similar amount is needed for the atmospheric chemistry module at the present default resolution. Thus the requested budget allows for ca. 10 sensitivity experiments with the atmosphere-only EC-Earth and off-line TM5 configurations in the first two years, together with a reasonable amount of runs required for code development and testing. In the final year we carry out somewhat longer (10-50 yr) runs with the coupled system, but need fewer of them.

References

Abbott, D.S., and E. Tziperman, 2008. Sea ice, high-latitude convection and equable climates. Geophys. Res. Letters, 35, L03702, doi: 10.1029/2007GL032286.

Bralower, T., D.J. Thomas, J.C. Zachos, M.M. Hirschmann, U. Rohl, H. Sigurdsson, E. Thomas, and D.L. Whitney, 1997. High-resolution records of the late Paloecene thermal maximum and circum-Caribbean volcanis: is there a causal link? Geology, 25, 963-966.

Farrell, B.F., 1990. Equable climate dynamics, J. Atmos Sci, 47 (24), 2986-2995.

Huber, M., and L.C. Sloan, 2001. Heat transport, deep waters and thermal gradients: coupled simulation of an Eocene greenhouse climate. Geophys. Res. Letters, 28, 3481-3484.

Korty, R.L., K.A. Emanuel and J.R. Scott, 2007. Tropical cyclone mixing during equable climates: could enhanced ocean mixing weaken meridional temperature gradients? J. Clim, in press.

Shellito, C.J., L.C. Sloan, and M. Huber, 2003. Climate model sensitivity to atmospheric CO2 levels in the early-middle Paleogene. Pelogeography, Paleoclimatology, Paleoecology, 93, 183-202.

Sloan, L.C., J.C.G. Walker, T.C. Moore, D.K. Rea, and J.C. Zachos, 1992. Possible methane-induced polar warming in the early Eocene, Nature, 357, 320-322.

Sloan, L. Cirbus and D. Pollard, 1998. Polar stratospheric clouds: a high latitude warming mechanism in an ancient greenhouse world, Geophys. Res. Letters, 25, 3517-3520.

Sluijs, A., G.J. Bowen, H. Brinkhuis, L.J. Lourens and E. Thomas, 2007. The PETM super greenhouse: biotic and geochemical signatures, age models and mechanisms of global change. In: Deep time perspectives on Climate Change: marrying the signal from computer models and biological proxies. Eds M. Williams et al. The Geological Society London, 323-349.

Weber, S.L., S.S. Drijfhout, A. Abe-Ouchi, M. Crucifix, M. Eby, A. Ganopolski, S. Murakami, B. Otto-Bliesner and W.R. Peltier, 2007. The modern and glacial overturning circulation in the Atlantic ocean in PMIP coupled model simulations, Climate of the Past, 3, 51-64.

Additional information

New Project for 2009

Would not accept support for 1 year only.