[alancalverd (OP)]

We have great ice core records of global temperature and CO2 concentration, going back over a million years. Lots of peole argue over their interpretation, but what about their existence?

The fact that we have a sample of snow that fell a million years ago means that it didn't evaporate. And the fact that it is umpteen meters below the surface means that neither did the subsequent snowfalls.  In other words, Antarctica is - or at least has been for the last million years -  an unlimited sink for water. For as long as the ice caps have been accruing, there has been a net transfer to the polar regions: some layers are thinner than others but AFAIK there are no actual gaps - accretion is continuous and inexorable. So eventually, all the water in the atmosphere will be frozen at the poles, gradually replaced by evaporation from the oceans and then frozen, until the planet consists of two ice caps with no water liquid or gas anywhere else - a bit like Mars?

Fortunately the planet seems to go through occasional warming cycles, and since life depends on the availability of water, global warming appears to be essential to the maintenance of life.

[evan_au]

Antarctica is - or at least has been for the last million years -  an unlimited sink for water

Antarctica is also a desert - the low temperatures mean that there is very little evaporation from the ocean, and very little water in the atmosphere. Precipitation is so low that it is classified as a desert.
So while water does accumulate there, it accumulates very slowly.
And the snow compacts into ice, which flows down into the sea, and breaks off as icebergs.

While periodic warmings triggered by the slow Milankovitch cycles do change things around, currently the climate is a lot more temperate than during the last ice age, when all of Canada and part of USA & Europe were covered by ice sheets.

[jeffreyH]

Yes. It would be exactly like mars. Now that is an interesting take on things.

[jeffreyH]

Now interestingly the amount of deuterium varies in the ice cores. Which means that over time there will be slightly less deuterium in the general climatic environment. The ratio of the isotopes will change. It is likely to be an insignificant change in the short term. What about the long term? It this depletion of deuterium also beneficial?

[Tim the Plumber]

The amount of water on the earth's surface would make that extreme impossible because the thickness would be so much that the top of the ice sheets would be above the active weather layer. It would stop snowing as the altitude got to be 10km above the ocean. Long before even.

The time this would take is also beyond human scales. It is also probably longer than the time it would take for the continents to drift away from the poles.

[evan_au]

Is global warming essential for the continuation of terrestrial life?

We could say that global warming was important for the origin of human life (plus whales & dolphins).

Biologists tell is that both groups originated during the Paleocene-Eocene Thermal Maximum (PETM), when a sharp temperature spike increased temperatures by 5C. The exact causes are unknown, but temperatures increased rapidly, and resulted in significant ocean acidification (pointing to higher CO₂). It took about 100,000 years to get the temperatures back to their values before the spike.

Of course, the ice-age world in which we find ourselves today (with ice at both North and South poles) is a very different world than that which existed before, during and after the PETM, with crocodiles reported in Antarctica! (There was also a land bridge from South America to Antarctica, so they could have waddled there...)

Read: https://en.wikipedia.org/wiki/Paleocene%E2%80%93Eocene_Thermal_Maximum
Listen (43 minutes): http://www.bbc.co.uk/programmes/b08hpmmf

[jeffreyH]

We appear to have a seesaw effect in operation. One extreme is like mars with all the water frozen at the poles and the other like Venus where a runaway greenhouse effect makes the planet too hot to be inhabitable. Is this a realistic viewpoint?

Would this then put a limit on the number of habitable planets in the goldilocks zone?

[Tim the Plumber]

We appear to have a seesaw effect in operation. One extreme is like mars with all the water frozen at the poles and the other like Venus where a runaway greenhouse effect makes the planet too hot to be inhabitable. Is this a realistic viewpoint?

Would this then put a limit on the number of habitable planets in the goldilocks zone?

We also appear to have a stability zone where natural feedback effects make a habitable (for us) climate a dependable result.

The climate of earth has been very stable for a long time with ice ages happening when there is land at the poles or a land locked sea. Other wise the normal state is a very good for life hot tropical climate over most of the earth.

[jeffreyH]

We appear to have a seesaw effect in operation. One extreme is like mars with all the water frozen at the poles and the other like Venus where a runaway greenhouse effect makes the planet too hot to be inhabitable. Is this a realistic viewpoint?

Would this then put a limit on the number of habitable planets in the goldilocks zone?

We also appear to have a stability zone where natural feedback effects make a habitable (for us) climate a dependable result.

The climate of earth has been very stable for a long time with ice ages happening when there is land at the poles or a land locked sea. Other wise the normal state is a very good for life hot tropical climate over most of the earth.

It is likely that mars was on the edge of any such stable zone. Which may be why all the water ended up at the poles.

[puppypower]

It is not so much global warming that is essential to life, but the cycling between global warming and global cooling and then from global cooling back to global warming. What this does is create change and necessity for life, with this necessary the mother of genetic and conscious invention.

As things cool from the poles toward the equator, some animals will migrate. The forced change in the environment changes the parameters of selection. The animals that do not migrate, also see a change in the climate parameters of selection, allowing new animals to be selected.

For example, if the coast was to flood due to warming, some animals will migrate further inland, where the terrain is different. Selective advantage may now go to those members who function better on the hills. Animals that remain closer to the ocean see more water, so now selection may go to the swimmers. The result is a changing of the guard adding new blood to the species.