How Pressure Systems Control Climate Part 3:
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Welcome back and best wishes for the new year
Today in part 3 of how pressure systems control the climate, I look at how the sun controls the location of the intertropical convergence zone or ITCZ and how a shifting ITCZ and its linked high-pressure systems have led to the rise and fall of many civilizations.
To ensure we properly adapt to future devastating weather events, we must correctly understand natural climate change. Natural weather disasters, much worse than those in recent times, have happened throughout history and will continue to happen regardless of any changes in human CO2 emissions.
Warming from the sun, affects climate very differently than warming from a CO2 greenhouse effect.
Solar radiation can penetrate the ocean as deeply as 200 meters compared to CO2's infrared radiation that barely penetrates a micron of the ocean surface.
Due to changes in the earth's orbit, the direct rays of the sun which can heat the earth's surface up to 1000 w/m2 at noon on a cloudless day. The sun's orbit shifts the location of that warming by 5000 kilometers in a year. By shifting the ITCZ, atmospheric and ocean circulations are also altered.
In contrast well-mixed CO2 warms the earth's surfaces equally.
So, let’s examine the sun's relationship to climate
The Hadley circulation discussed in part 2, links the rainy ITCZ and dry high-pressure systems, and provides the general framework and background for all the more transitory weather events.
The sun's orbital cycles (the Milankovitch cycles) shift the ITCZ and Hadley circulation. Here I focus on the ITCZ shifts since the end of the ice age's last glacial maximum, a time when the ITCZ had been shifted further southward than it is today.
As the ITCZ moved northward, driven by changes in solar heating, the glaciers began melting and the northern hemisphere experienced what scientists call the Holocene Thermal Maximum, roughly lasting between 6 and 10 thousand years ago. Temperatures rose about 3 degrees Celsius warmer than today. Subsequently, the ITCZ and its tropical rainfall shifted much further north than today
Driven by the sun's Milankovitch cycles, a steady 6000-year migration of the ITCZ towards it last glacial maximum southern extent then ensued, coinciding with a cooling trend known as the neo-glaciation. That ITCZ migration and its linked high-pressure systems also changed the locations of the earth's deserts and droughts and civilization collapses.
As the ITCZ migrated southward it also caused greater climate variability.
The ITCZ’s southward migration increased the number of El Nino events, and those events altered the earth's temperature balance, which in turn created feedbacks that further altered the ITCZ 's location.
How evolving El Ninos and other ocean oscillations altered pressure systems and generated the greater weather variability seen today will be discussed in part 4.
Milankovitch's obliquity cycle refers to the changes in the tilt of the earth's axis. If the axis was perpendicular to the sun's rays, there would be no seasons and the arctic would be in a perpetual twilight.
However, currently the earth's axis is tilted 23.5 degrees which causes more sunlight and warmer temperatures to move northward, producing summer conditions when the northern axis is pointed towards the sun. Over the course of about 41,000 years the earth's axis oscillates between 24.5 degrees, which will cause the warmest artic summers, then shifts to 22 degrees causing the coolest arctic summers
While keeping the same tilt, the axis also wobbles. This wobble is Milankovitch's precession cycle causing the axis to go from pointing at the north star, Polaris, as it does today, then circling to point at other stars over a period of about 26,000 years
In June the earth's axis points towards the sun causing our northern hemisphere's summer. As the earth revolves around the sun the northern axis continues to point towards the north star but by December it points away from the sun causing the northern hemisphere's winter while summer conditions shift to the southern hemisphere.
Surprisingly, the northern hemisphere experiences winter even though the earth's orbit is closer to the sun than at any other time.
Due to precession, 13,000 years into the future, as well as 13,000 years ago, the north axis was pointed towards the sun at the same time it was closest to the sun. It is believed that such an alignment of obliquity and precession maximized the Arctic's solar heating and triggered the melting of the ice age glaciers.
But as the orbital factors transitioned to their cooler phases, arctic sea ice and glaciers began to return and the earth entered its recent 6,000-year cooling trend, the neoglaciation, and the ITCZ migrated southwards
The extent of the location of the sun's hottest direct rays defines the tropics and the tilt of the earth's axis determines how far poleward that maximum solar radiation, as well as the ITCZ, can migrate.
The current 23.5-degree tilt of the axis causes the sun to be directly over the Tropic of Cancer, 23.5 degrees north of the equator, during the northern hemisphere's summer.
Due to ocean circulation effects, the ITCZ does not reach the Tropic of Cancer over the ocean as it does over land. During the southern hemisphere's summer, the direct rays reach the tropic of Capricorn 23.5 degrees south of the equator
Due to high obliquity, 7000 years ago the ITCZ and the north Atlantic subtropical high-pressure system referred to here as the NASH, was located much further north than today. The clockwise circulation of the NASH forces the westerly winds northward, shown here as the dashed line. Without rains from the ITCZ or westerlies, the Iberian Peninsula was extremely arid. Elsewhere, the northerly migration of the summer ITCZ and strengthened NASH also moved the rainy westerlies away from Scandinavia and towards Iceland
By 5000 years ago, Iberia's Mediterranean climate began to evolve as the ITCZ, and NASH migrated further southward. When ITCZ and NASH moved further south over Africa in the winter, the westerly winds could bring rains to Iberia
However, when the ITCZ and NASH moved northward in summer, the westerly winds were pushed northward causing Iberia to experience dry summers So, like California, as discussed in part 2, Iberia similarly evolved into a Mediterranean climate with hot dry summers and cool wet winters.
But the ITCZ 's southward migration now devastated northern Africa
When the ITCZ was centered closer to northern Africa the Sahara Desert was a lush grassland with large lakes. Cave drawings from southern Algiers depict abundant grazing antelope and cattle and giraffes. But driven by decreasing obliquity and precession, the steady southward shift of the ITCZ and NASH, initiated the earth's greatest drought known to humans, converting a humid African savannah
Into the greatest desert on earth --the Sahara Desert
It should be noted, because the ITCZ today is not as far south as it was during the ice age's glacial maximum, today's Sahara is not quite as extensive as it was during the ice age. If obliquity is indeed the primary controller of the ITCZ, expanding desertification can be expected over the next 10,000 years
As the rich grasslands of northern Africa converted to desert, the large human populations it supported were forced to migrate. The genetics of Mediterranean people suggest many Africans moved into southern Europe.
The increasing dryness forced other people to settle in the major river valleys where reliable water could be obtained such as the Nile Valley.
This great drying happened at similar latitudes, and other great river civilizations developed, in Mesopotamia, the Indus River valley, and yellow river valley.
The once lush region just south of the Sahara, known as the Sahel, seen here in light orange, did not turn to desert, but became increasingly vulnerable to small migrations of the ITCZ and the increasing climate variability.
That forced the Bantu speaking people of northwest Africa to migrate southward Either conquering or integrating with existing tribes throughout southern Africa
The summer warmth currently moves the ITCZ far enough to the north, that rains from the summer monsoons can reach the Sahel. But in the winter the ITCZ moves south again. While gifting southern Africa with rain, the Sahel experiences a cool season drought. And whenever the ITCZ remains too far south, either driven by ocean oscillations or changes in sunspots, it has brought major devastating droughts to the Sahel every century since the 1600s.
The tragic Sahel droughts of the 1960s to 1980s required massive world-wide relief funds to minimize the starvation experienced by people of the Sahel who depended on rainfall for farming and grazing.
Why did the ITCZ move further south?
One explanation is the ITCZ always migrates away from cooler regions and towards warmer regions. The most relevant studies pointed to the Atlantic multidecadal oscillation that caused cooling waters in the north Atlantic (as illustrated here in blue) and warming waters in the south Atlantic.
Climate scientists from NOAA also tested for effects from greenhouse gases but reported that the IPCC’s climate models failed to simulate those contrasting ocean temperatures or the ITCZ 's southern shift suggesting the droughts were "likely of natural origin"
The major drought events also coincided with small reductions of solar radiation associated with sunspot minimums
The Sahel's major Little Ice Age droughts of the 1600s and 1700s coincided with the Maunder sunspot minimum
The 1830s drought with the Dalton Minimum.
The 1910s drought again with low sunspots. Then as sunspots increased the Sahel received more rain culminating with a decade of steady rains in the 1950s.
But sunspots declined again resulting in the droughts of the 60s and 70s Then wetter weather returned in the 90s as sunspots increased
But as 21st century sunspots have approached the same low numbers as the 1910s, the Sahel has recently experienced 3 droughts between 2002 and 2012
The ITCZ and solar cycles are global phenomena, so as expected, the ITCZ shifts also affected people of the Americas. The Mayan population centers occupied Mexico’s Yucatan peninsula. There, situated at the northern limits of the ITCZ, summers brought abundant rains.
But during winter the ITCZ moved far to the south bringing winter drought. So, the Mayans adapted to winter dryness and increasing ITCZ variability by building extensive reservoirs and irrigations canals
But as the ITCZ continued to move southward, the Mayans began abandoning their cities around 200AD. And Mayan society finally collapsed by 800AD
As the southward migrating ITCZ approached the Little Ice Age between 1500 and 1800 AD, the Yucatan experienced increasing dryness and weather variability.
Droughts that devastated the people of the Sahel during the little ice age also happened all around the sub-tropical latitudes. In the 1400s the Mayan culture, and Aztecs of central Mexico suffered massive drought-induced famines . The Little Ice Age reduced the North American monsoons, and droughts devastated New Mexico's Pueblo culture. The drought of 1638 prompted a revolt by Pueblo people against the Spanish. . Little Ice Age droughts brought by the contraction of the Asian monsoons caused Cambodia's Khmer empire to abandon its capital of Angkor in 1431And as the Little Ice Age droughts dislocated more and more societies, china's Ming dynasty expanded and fortified the great wall to prevent a growing number of invaders. But the droughts finally triggered the downfall of the Ming dynasty in 166 The changes in the ITCZ have increased the El Nino cycles which also alter the locations of dry and wet pressure systems which exacerbates droughts and floods around the world So up next: part 4 of how pressure systems control the climate: how El Nino and ocean oscillations influence droughts and floods
Until then embrace renowned scientist Thomas Huxley’s advice:
“Skepticism is the highest of duties; blind faith the one unpardonable sin"