» Home > In the News

Expansion and Contraction of the Intertropical Convergence Zone

4 July 2024
Ancient history, Climate change, Environmentalism, Geology

The expansion and contraction of the Intertropical Convergence Zone [ITCZ for short] is a well known feature of past climate. It is known to have impacted ancient societies for example, and HH Lamb describes it in his books. For example, Mayan civilisation has had its ups and downs, times of plenty puctuated by drought and famine. The Galapagos Islands on the other side of South America have also had well known ups and downs where drought is associated with a disappearance of the ITCZ. It did not of course disappear – but moved elsewhere. This is where the expansion and contraction comes in. HH Lamb discusses why and how this occurred. He begins by dividing each hemisphere into zones. The tropical, subtropical, desert, temperate and polar. The contraction of the ITCZ is concomitant with the movement of the monsoon track. As far as the historical past is concerned. This comes about as cooler weather occurs in the northern part of the temperate zone. It goes hand in glove with an expansion of the polar zone – and a movement southwards of the jet stream. As a result of that the bulk of Russia is affected by such episodic changes. The size of the Caspian Sea expands and contracts – depending on how much rainfall feeds the rivers of the steppe. A southward moving jet stream provides the water that expands the Caspian. Or that is how some see the situation. Others would disagree. What seems to happen is that a cooler global climate expands the polar zone at the expense of the Temperate zone which itself impacts on the subtropical and desert  zones, by sqeezing them southwards. This is why the monsoon track moves south and droughts become commonplace in the Near and Middle East, on a historical basis. As with the Maya in the New World so too with the civilisations that rise and fall in the Old World. These things are all connected. Yet, you can read article after article that blame the movement of the monsoon track on small orbital changes – a variation on the theme of Milankovitch.

At https://tallbloke.wordpress.com/2024/06/27/new-research-looks-at-orbital-factors-in-expansion-and-contraction-of-the-intertropical-convergence-zone/ … which comes with graphs and maps etc. It provides a good overview of the ITCZ and its boundaries. One can see quite clearly that the Galapagos Islands and the Maya homeland are close to the boundaries of the zone. Hence, a contraction of that zone would lead to drought and crop failure – or reduced levels of harvest. This may be one reason the Maya also gardened the forest outside their living area, as the wild harvest would be something to fall back on in times of scarcity. Just as a movement southwards of the monsoon track benefited some people, and their habitat, but was a disaster in other regions left outside the rainy season. So too with the ITCZ. In a new study orbital changes are blamed once again – but in this instance, bound up with the idea of potential climate change. We are told orbital factors may play a role with the trade winds in the northern and southern hemispheres by affecting the boundaries of the ITCZ zone. These boundaries result in heavy cloud formation and heavy rainfall. Hence, not such a modest target for the climate change lobby. In this instance they seem to be saying that in a warmer world the boundaries may move northwards. Would they? I suppose that is a logical train of thought if cool weather causes the ITCZ to contract. However, has that happened in the past and how far north were those boundaries?

See also https://phys.org/news/2024-06-climate-shift-tropical-northward.html … where the speculation is that carbon emissions will force tropical rains to shift northwards in  coming decades. No sign of it yet though.

On a different subject, at https://phys.org/news/2024-06-newly-hydrothermal-vents-depths-meters.html … newly discovered hydrothermal vents have been found 3000m below the ocean surface, off Svalbard, in the North Atlantic. Hydrothermal vents are seeps on the sea floor from which hot liquids escape. They were discovered by a remote controlled submersible on a 500 km long ridge. It is thought the ridge is part of a plate boundary – but one  that is spreading rather than diving under or over each other.

Skip to content