Publication Date:
2015-12-21
Description:
It is still an open question how equilibrium warming
in response to increasing radiative forcing – the specific
equilibrium climate sensitivity S – depends on background
climate. We here present palaeodata-based evidence
on the state dependency of S, by using CO2 proxy data together
with a 3-D ice-sheet-model-based reconstruction of
land ice albedo over the last 5 million years (Myr). We find
that the land ice albedo forcing depends non-linearly on the
background climate, while any non-linearity of CO2 radiative
forcing depends on the CO2 data set used. This nonlinearity
has not, so far, been accounted for in similar approaches
due to previously more simplistic approximations,
in which land ice albedo radiative forcing was a linear function
of sea level change. The latitudinal dependency of icesheet
area changes is important for the non-linearity between
land ice albedo and sea level. In our set-up, in which the
radiative forcing of CO2 and of the land ice albedo (LI) is
combined, we find a state dependence in the calculated specific
equilibrium climate sensitivity, STCO2,LIU, for most of the
Pleistocene (last 2.1 Myr). During Pleistocene intermediate
glaciated climates and interglacial periods, STCO2,LIU is on average
� 45% larger than during Pleistocene full glacial conditions.
In the Pliocene part of our analysis (2.6–5 MyrBP)
the CO2 data uncertainties prevent a well-supported calculation
for STCO2,LIU, but our analysis suggests that during times
without a large land ice area in the Northern Hemisphere
(e.g. before 2.82 MyrBP), the specific equilibrium climate
sensitivity, STCO2,LIU, was smaller than during interglacials of
the Pleistocene. We thus find support for a previously proposed
state change in the climate system with the widespread
appearance of northern hemispheric ice sheets. This study
points for the first time to a so far overlooked non-linearity
in the land ice albedo radiative forcing, which is important
for similar palaeodata-based approaches to calculate climate
sensitivity. However, the implications of this study for a suggested
warming under CO2 doubling are not yet entirely clear
since the details of necessary corrections for other slow feedbacks
are not fully known and the uncertainties that exist in
the ice-sheet simulations and global temperature reconstructions
are large.
Repository Name:
EPIC Alfred Wegener Institut
Type:
Article
,
isiRev
Format:
application/pdf
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