Future prospects - Ice Ages and Interglacials: Measurements, Interpretation, and Models - page 357

''The 'Next Ice Age' conference theme often manifested itself in animated
discussions based on widely varying interpretations of observational data, its
meaning, and future implications. One interpretation is that a natural pattern of
low orbital obliquity exists where the dark tropical oceans warm at the expense of
the polar regions, thereby increasing meridional vapor transport and glaciations.
Based on an interglacial period 400,000 years ago, another interpretation
estimated that the current interglacial period will persist for another 14,000 years
in the absence of anthropogenic forcing.''
In order to predict the onset of the next ice age, we need to know why and
how ice ages begin and evolve. Since we only have incomplete inferences as to
their causes, it is dicult (if not impossible) to predict whether and when the next
ice age will begin. It would be dicult enough to predict the putative emergence of
the next ice age were the Earth unperturbed by large-scale intervention by
humans. But, with the great increase in CO 2 concentration in the past 100 years
or so, and the possibility that this has contributed to the global warming we have
recently observed, the problem is made even more complex in trying to understand
the impact of putative human-induced global warming on the natural evolution of
the climate.
As usual, there are many blogs on both sides of the question. There are blogs
that purport to show that the climate is actually cooling in preparation for the
next ice age. The cold fluctuation of 2007-2008 was a great boon to these
enthusiasts. 16 There are also blogs that deny the next ice age will ever come.
Basically, there are three considerations regarding the next ice age. One is the
natural turn of events which assumes that ice ages are driven by variations in the
Earth's orbit. As we show below, the expectation of low eccentricity in the future
suggests that, in the natural order of things, solar oscillations will have low
amplitude implying that another ice age may be likely. A second consideration is
that warming may interfere with thermohaline circulation leading to a new ice age
as a consequence of the reduced delivery of heat to higher latitudes. 17 A third
consideration is the longevity of excess CO 2 in the Earth's atmosphere and the
role it may play in heating the Earth, thereby preventing future ice ages.
Berger and Loutre (2002) showed that the Earth's eccentricity, presently
around 0.016, will decrease in the future, bottoming out 25,000 years from now
at around 0.004 and remaining at 0.015 or less over the next 100,000 years. As a
consequence, the amplitude of oscillations in solar input to high latitudes will
remain extremely low for the next 25,000 years and will remain moderately low
over 100,000 years. These are historically low values; eccentricity has reached the
range 0.04 to 0.05 in the past. As Berger and Loutre (2002) emphasized, ''The
small amplitude of future insolation variations is exceptional.'' They pointed out
16 For example, http://www.iceagenow.com/Record_Lows_2008.htm and http://www.citeuli-
ke.org/tag/ice-age
17 However, as pointed out earlier, Wunsch (2002) insists (correctly) ''The upper layers of the
ocean are clearly wind-driven.''