Future prospects - Ice Ages and Interglacials: Measurements, Interpretation, and Models - page 360

in response to increases in greenhouse-gas concentrations has been demonstrated
in a number of simulations,

it remains dicult to assess the likelihood of
future changes in the thermohaline circulation, mainly owing to poorly con-
strained model parameterizations and uncertainties in the response of the climate
system to greenhouse warming.''

...

Archer (2005) used a climate model to study the long-term fate of CO 2 in the
atmosphere amounting to anywhere from 300 to 5,000 Gt of carbon as a result of
emissions over the next 150 years. In each case, CO 2 concentration spikes after
150 years with a peak concentration ranging as high as 1,700 ppm for 5,000Gt of
emissions and 800 ppm for the (more likely) 2,000 Gt emissions of carbon. As the
ocean absorbs ever more CO 2 from the atmosphere, it becomes more acid and so
dissolves more calcium carbonate from the shells of marine organisms. This in
turn reduces the oceans' ability to absorb more CO 2 , leaving more greenhouse gas
in the atmosphere. At first, the CO 2 concentration rapidly declines from the peak
(dropping to less than half the peak value in about a century or two), but then it
declines slowly over several tens of thousands of years, returning ultimately to pre-
industrial levels. Archer made the point that this slow decline would result in
elevated CO 2 levels (albeit well below the peak) for a very long time. Tyrrell et al.
(2007) carried out a similar study.
Whether such enhanced levels of CO 2 influence the probability of occurrence
of the next ice age remains uncertain.
Crowley and Hyde (2008) hypothesized that the Earth's climate has been
transitioning from non-glacial to glacial over the past 50 million years. Notably,
the transition to longer period higher amplitude fluctuations about a million years
ago was viewed as a ''climate bifurcation point'' leading to a transition period
over the past million years to a second climate bifurcation point that will lead the
Earth into permanent deep glaciation in the next 20,000 years or so. They
employed a coupled energy balance/ice sheet model to support this interpretation.
This paper was criticized by a number of notable and prominent climatologists. 20
Some of the critics argued that such untestable predictions based on simplistic
models are not science. Crowley responded: ''this is science—you might not like it,
but it is science.'' However, in Section 11.2.1 we emphasize that the scientific
method requires (1) that hypotheses can be proven wrong (if they are wrong),
(2) that they are based on objective tests, and (3) that the results must be repeat-
able. The Crowley and Hyde (2008) model (like most climate models) fails this
test.

Another factor that may affect climate is the activity of the Sun. As we
pointed out in Section 8.7.2, there is some evidence to suggest that when the Sun
is inactive (e.g., as evidenced by a lower-than-normal incidence of sunspots)
cosmic ray flux into the atmosphere will increase, leading to greater cloud forma-
tion and potential climate cooling. The transition from sunspot cycle 23 to sunspot

20 ''More on whether a big chill is nigh,'' http://dotearth.blogs.nytimes.com/2008/11/13/more-
on-whether-a-big-chill-is-nigh/