During interglacial periods, global temperatures and sea levels rise due to naturally occurring Milankovitch cycles.
this has always occurred without the presence of the current
unnaturally high levels of CO2 caused by the release of greenhouse gases
by mankind. Now, in the current interglacial period, we have suddenly
doubled the level of CO2 in the atmosphere and oceans (and will be quadrupled at current
overall rates of release). This has already caused a perceptible
increase in global temperatures. When the Milankovitch warming influence
strengthens as it has in previous interglacials, it will be amplified
by the already increasing Greenhouse Effect.
Instead of hippos
merely finding hospitable conditions in England like during the last
interglacial, they will be able to bask in the fjords of Norway. Polar
bears, however, will not survive this interglacial because conditions
will be too severe. The human populations in the lower latitudes will be
decimated. And if global temperatures rise to the point that 'methane
hydrate gasification' becomes a self sustaining cycle, then the world
will become more than just "ice free".
Why? Because greenhouse
gases will remain largely unsequestered for millennia and sustain higher temperatures long after the
Milankovitch cycle has swung back the other way. Homo sapiens will then
perhaps evolve into a new species that will gradually repopulate the
planet during the next ice age. If and when mankind regains ascendancy
in the next interglacial, they will not be able to easily make the same
fossil fuel error that we did because most of the easy access coal and
oil will be gone. They'll have to develop the use of wind power, solar
power, cellulosic ethanol production, etcetera that we failed to take
advantage of in time to do ourselves any good.