[jofer]

It's also worth putting this in a bit of geologic perspective. The article mentions this, but doesn't explain it.

The Eocene is just before the earth went into a period of "icehouse" climate (that's still going on). During the part of the Eocene this article refers to, the earth was in a "greenhouse" phase, and global temperatures were much warmer than they've been any time since.

At that time (and throughout the majority of Earth's history), there likely weren't any significant ice caps on the planet.

On very long timescales, the Earth goes through periods of icehouse and greenhouse climates. We're currently in an "interglacial" within a period of icehouse climate that has persisted since the late Eocene. Most of Earth's history is dominated by long phases of greenhouse climate. During these, the global climate is more stable and much warmer. We don't have ice ages, and there probably aren't any ice caps at the poles.

The last period of icehouse climate began just after this, in the late Eocene. (Again, the "ice ages" you hear about are glacials and interglacials within a period of icehouse climate.)

Also, as other folks have mentioned, the Eocene is relatively recent, and Antarctica was essentially in its present-day position (the coals and other "warm-weather" rocks in Antarctica date from much earlier when it was near the equator).

Regardless, it is quite interesting that sea surface temperatures were as warm as this evidence shows! I just wanted to put this into a bit of perspective.

*Caveat to all of this: I'm a geologist and not a climatologist.

[sologoub]

That's exactly the detail missing from this article!

The other bit of information that I would really find useful as a lay person is how temperature distribution varied during the "greenhouse" climates. For example, was it linear to todays differential, implying that if I can directly apply an increase of 20C to today's temp and say that the avg. CA temp would be 34C from the 14C referenced currently or is it more even and the jump at extremes will be significantly larger or smaller than at equator?

[jofer]

It's probably very spatially variable, but beyond that, I'm out of my depth.

Ocean circulation plays a large role in regional (and global) variations in climate. If nothing else, the changes in the shape of the ocean basins over time would result in very different regional climate patterns, even if the average global temperature stayed the same.

On top of that, changes in average global temperature tend to vary strongly by latitude, if I remember correctly.

[dragop]

In the current ice age, the cycle between glacials and inter-glacials is primarily driven by variations in the Earth's orbit, and therefore the distribution of incoming solar radiation over the Earth's surface by season and latitude. Obviously, as you mention, this then interacts with global climate through atmospheric and ocean circulation. They're called Milankovitch cycles after Milutin Milanković, who calculated them all (by hand!) at the beginning of the 20th century.

Obviously orbital forcing would exist in "greenhouse" times, to answer the parent comment.

If you are interested, there are three variations:

- eccentricity, change in the ellipticity of the orbit around the Sun, ie how close to a circular orbit it is. So if it's close to circular, temperature variations by season will be low. If it's more eccentric, you will get greater variation by season.

- tilt of the axis of rotation, which varies around 22-24 degrees. So when the Earth is tilted over more, at the poles it will be hotter when facing the Sun, colder when facing away.

- axial precession, which is the wobble of the axis of rotation, a bit like if you put a toy gyroscope on a table. This has a latitudinal and seasonal effect, particularly at the poles.

These cycles are approximately 100, 41 an 25 thousand years respectively. Put them together and the periods very closely follow the current ice age climate variations.

[argumentum]

> Most of Earth's history is dominated by long phases of greenhouse climate. During these, the global climate is more stable and much warmer.

Shouldn't we welcome this? Lost in the debate about climate change is the question of whether or not the earth would be better off much warmer. Obviously there are costs to such change, but there are clearly benefits.

A lot of earth's land is tied up in uninhabitable frigid places. The idea that we should not slow or even intentionally accelerate global warming is not a completely nonsensical one.

[dragop]

It's a problem of how fast. If the Earth's climate slowly changed over thousands of years, we could probably adapt. Fast climatic changes would be a problem as the biosphere would take time to adapt. For example, remove all the ice from Antarctica (and ignoring most of it is currently below sea level and needs to rebound). You'd have a rocky landscape with no soil. Not very useful for growing crops.