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Abstract
Geological data for the Early Eocene (56–47.8 Ma) indicate extensive
global warming, with very warm temperatures at both poles. However,
despite numerous attempts to simulate this warmth, there are remarkable
data–model differences in the prediction of these polar surface
temperatures, resulting in the so-called ‘equable climate problem’. In
this paper, for the first time an ensemble with a perturbed
climate-sensitive model parameters approach has been applied to
modelling the Early Eocene climate. We performed more than 100
simulations with perturbed physics parameters, and identified two
simulations that have an optimal fit with the proxy data. We have
simulated the warmth of the Early Eocene at 560 ppmv CO2, which is a much lower CO2
level than many other models. We investigate the changes in atmospheric
circulation, cloud properties and ocean circulation that are common to
these simulations and how they differ from the remaining simulations in
order to understand what mechanisms contribute to the polar warming. The
parameter set from one of the optimal Early Eocene simulations also
produces a favourable fit for the last glacial maximum boundary climate
and outperforms the control parameter set for the present day. Although
this does not ‘prove’ that this model is correct, it is very encouraging
that there is a parameter set that creates a climate model able to
simulate well very different palaeoclimates and the present-day climate.
Interestingly, to achieve the great warmth of the Early Eocene this
version of the model does not have a strong future climate change
Charney climate sensitivity. It produces a Charney climate sensitivity
of 2.7°C, whereas the mean value of the 18 models in the IPCC Fourth Assessment Report (AR4) is 3.26°C±0.69°C. Thus, this value is within the range and below the mean of the models included in the AR4.
Original language | English |
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Article number | 20130123 |
Number of pages | 25 |
Journal | Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences |
Volume | 317 |
Early online date | 16 Sept 2013 |
DOIs | |
Publication status | Published - 28 Oct 2013 |
Keywords
- perturbed physics ensemble Eocene Eocene model equable data/model
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