At least we in the US don’t have to deal with this:
When soggy, green Northern Ireland gets coated by red, Saharan dust, the locals get slightly perturbed.
The dust is being pumped northward into the United Kingdom as winds aloft flow from the south to southeast instead of the normal west-to-east direction, says weather.com senior meteorologist Jon Erdman. An expansive blocking area of high pressure is stretching from eastern Europe to southern Greenland, and that’s working in tandem with a strong southward dip in the jet stream centered just west of the Iberian Peninsula.
As a result, northern Europe has turned hazy with Saharan dust filling the air in some areas, according to a BBC report.
And of course, that made me think of this:
I tend to think of northern Europe and the Sahara as very, very far apart. But Europe is smaller than one might think, and the Sahara bigger. Actually, the Sahara and Europe are roughly equal in size. And winds can take stuff like sanddust really really far.
Speaking of climate change, the Sahara is a fairly recent phenomenon, a desert formed in an area that was once a larger desert but then became wetter before it became drier again:
A timeline of Sahara occupation:
22,000 to 10,500 years ago: The Sahara was devoid of any human occupation outside the Nile Valley and extended 250 miles further south than it does today.
10,500 to 9,000 years ago: Monsoon rains begin sweeping into the Sahara, transforming the region into a habitable area swiftly settled by Nile Valley dwellers.
9,000 to 7,300 years ago: Continued rains, vegetation growth, and animal migrations lead to well established human settlements, including the introduction of domesticated livestock such as sheep and goats.
7,300 to 5,500 years ago: Retreating monsoonal rains initiate desiccation in the Egyptian Sahara, prompting humans to move to remaining habitable niches in Sudanese Sahara. The end of the rains and return of desert conditions throughout the Sahara after 5,500 coincides with population return to the Nile Valley and the beginning of pharaonic society.
Although mankind has had some influence on the desert, it seems minimal compared to the ebb and flow of rain there, which appears to be connected to completely non-human factors, some of them unknown. Humans have been the beneficiaries or victims of the changes, but not usually the instigators of them.
Here is some speculation about what drove those changes:
About 12,000 years ago, slight changes in the Earth’s orbit around the sun brought the northern hemisphere into the limelight. Summers became warmer as more solar radiation hit the lands north of the Equator. Solar ‘insolation’ levels were up to 8 percent higher than today.
With insolation driving monsoonal climates like a huge heat engine, rainfall increased. One climate model estimated that the 8 percent increase in radiation in North Africa resulted in a 40 percent increase in precipitation….
We know what caused the greening of the Sahara: a complex interaction between solar insolation, vegetation cover, and ocean temperatures.
What is even more interesting for today’s civilizations is the speed at which these changes occurred.
Peter deMenocal, an expert at Columbia University’s Lamont-Doherty Earth Observatory, argues that it started and ended very abruptly—within a few decades to centuries—triggered largely by summer insolation crossing the threshold of 470 Watts per meter squared, 4.2 percent higher than today.
He speculates that there could be an insolation tipping point “whereby subtropical African climate flips abruptly between humid and arid”.
However, other evidence suggests a more gradual transition. Layers of sediment drilled from the bottom of Lake Yoa in northeastern Chad hint that the environment changed in phases from trees and bushes to shrubs and grasses and finally to nothing but sand.
It’s hard to imagine the Sahara as anything but sand. But could global warming prompt another greening of the desert? Not likely. Solar insolation is still too low, the monsoon is shifting southwards, and vegetation cover is decreasing.
So: the change was enormous, probably relatively sudden, and had much much more to do with changes in the earth’s orbit than anything else. And it drives home how little we know about the complex interplay of factors that cause climate change, which has been occurring as long as the earth has been here.