I’m on holiday. I’m on vacation from my day job of thinking about smoking bans and the EU and Donald Trump and all the rest of it. Instead I’m in Cuzco. Or Luxor. Or maybe Gunung Padang. Think of me as exploring ancient megalithic sites, pushing uphill through dense forest, assaulted by mosquitoes. Or more likely sitting in a bar somewhere, with a beer and a cigarette, thinking about pushing uphill through dense forests…
Yesterday I arrived at a world in which almost all plant and animal (and human) life was constrained to the tropics. I was considering a similar idea over 15 years ago, where I imagined stone age humans were similarly constrained, before they became technologically innovative enough to live at higher latitudes,
My current idea is a bit different: I think humans were constrained to the tropics by ice sheets that extended all the way from the poles, and on which life was impossible. They could only advance north as the ice melted. But what caused the ice to melt?
I was interested by an article about ice ages on Judith Curry’s blog a month ago. The author thought that dust deposition on the ice sheets were what caused them to melt:
Eventually, the Ice Age reached its ultimate depth of cold, CO2was depleted to less that 200 ppm, desertification generated large sources of dust, and winds transferred the dust to the ice sheets. This tipped the scales of the energy balance, and now the ice sheets rapidly ablated as the albedo decreased.
His idea was that as CO2 levels dropped, plants died, and the land they had covered became a dustbowl. But wouldn’t the same thing have happened as the climate became drier, and plants got insufficient water?
In my simulation model, the ice had advanced south in a chain reaction of falling air temperatures bringing more snow cover, and lower Earth albedo, and yet lower temperatures. So now I added a steady sprinkling of windblown dust onto the snow, that would slowly reduce the albedo (reflectivity) of the ice from a high value of 0.9 down to a low value of 0.3.
And when this happened, the ice would begin to slowly retreat northward, and the mean planet air temperature would begin to rise. And that is exactly what happened, with the Earth’s mean temperature slowly rising back to nearly its initial temperature. The ice first advanced, and then retreated. The only surprise was that when it had advanced and retreated, it then advanced again. But this was because the old, dust-covered ice had eventually more or less all melted, and had begun to be replaced by fresh new ice, repeating the original cycle, and then gradually stabilizing.
But I was using a constant dust deposition rate at all latitudes. If the amount of dust in the atmosphere had risen as the planet got colder, then the dust deposition rate would have slowly risen as the ice extended southwards, reducing the available unglaciated areas where water could evaporate. And since this area was mostly in the tropics, dust blown up from there was more likely to be deposited across the ice nearest the tropics than the ice nearest the poles. So that’s another option to explore.
But the outline mechanism is fairly simple. First the ice advances, and its bright white snow caused the planet’s temperature to drop. On this colder, drier planet, dust levels rise, and dust deposited on the ice causes the ice albedo to fall, and air temperatures to rise, and the ice to start melting. The ice then retreats until most of it has melted, whereupon the cycle repeats.
The only problem with this cycle is that it looks at the moment like the ice advances much faster than it retreats. And in the last ice age, it was actually the other way around, with the ice advancing slowly, and finally rapidly retreating.
Are there other possible sources of dust than windblown dust from deserts? Increased vulcanism is one possibility. And ice ages might well lead to an increase in vulcanism. And this might happen because the blanket of ice on the surface of the Earth acts to slowly raise its surface temperature, causing magma to rise, and become more likely to erupt.
And as the ice retreated, and humans moved north, there would appear a series of new civilisations, first in Egypt (24º N), then Mesopotamia (30º N), Crete (35º N), Greece (38º N), Rome (42º N), as these new lands were colonised.