HR: 1340h
AN: U43A-0864    [Abstracts]
TI: On the Linearity of Climate Sensitivity on Geological Timescales: A Case Study for the Modern and the Eocene
AU: Valdes, P
EM: p.j.valdes@bristol.ac.uk
AF: University of Bristol, School of Geographical Sciences, University of Bristol, University Road, Bristol, BS8 1SS, United Kingdom
AU: * Lunt, D J
EM: d.j.lunt@bristol.ac.uk
AF: University of Bristol, School of Geographical Sciences, University of Bristol, University Road, Bristol, BS8 1SS, United Kingdom
AU: Ridgwell, A
EM: andy@seao2.org
AF: University of Bristol, School of Geographical Sciences, University of Bristol, University Road, Bristol, BS8 1SS, United Kingdom
AB: Climate Sensitivity is a key parameter for quantifying the response of the entire Earth System to a perturbation in radiative forcing, via a doubling of atmospheric CO₂ concentration. However, Climate Sensitivity is not a constant, in that it depends on the background state of the system. For example, due to the non-linear nature of climatic feedbacks, it is expected to have a different value if doubling CO₂ from a preindustrial level (×1 to ×2), to doubling CO₂ from an already elevated level (e.g. ×2 to ×4). Climate Sensitivity is also expected to have varied in Earth's history, where potential feedback mechanisms, in particular ice-albedo, may have been considerably different to modern (e.g. at the Last Glacial Maximum, 21,000 years ago, or in the icesheet-free world of the Eocene, 50 million years ago). On geological timescales, different configurations of ocean basins could also have led to different circulation states, with potential impacts on the linearity of the response of the system to increasing CO₂ concentrations. In this paper, we investigate Climate Sensitivity on geological timescales. In particular, we investigate the non- linearity of Climate Sensitivity under modern-day and Eocene boundary conditions. Using the UK Met Office fully coupled GCM (HadCM3L), we carry out simulations at ×1, ×2, ×4, ×8, and ×16 CO₂ concentrations, relative to pre-industrial. We then carry out an identical suite of CO₂ simulations but under Eocene boundary conditions (i.e. solar constant, paleotopography and paleobathymetry appropriate for 50 million years ago). For the modern, we find that the climate sensitivity of the model is remarkably linear, with a value of 2.5°C over the range from ×1 to ×16 CO₂ concentration. Perhaps counterintuitively, the Eocene Climate Sensitivity is greater than at modern - 3.5°C, despite there being less ice in the system. Additionally, the Eocene Climate Sensitivity is non-linear over the range considered. This is due to changes in the ocean circulation which take place at elevated CO₂. At low CO₂, the Eocene ocean circulation is in a state in which deep water formation occurs round Antarctica. However, at higher CO₂, the deep water formation switches off, and the THC collapses, resulting in a state in which the deep ocean heats up only diffusively, the surface waters becoming relatively warm.
DE: 1626 Global climate models (3337, 4928)
DE: 3309 Climatology (1616, 1620, 3305, 4215, 8408)
DE: 3344 Paleoclimatology (0473, 4900)
SC: Union [U]
MN: 2007 Fall Meeting