What would the equator be like if Modern-Day Earth were as hot as the Cambrian?

Question

This is a map of Earth's climate today:

This is how the Koppen classification colors the particular climates, so I don't know why the lower latitudes were dominated by several shades of the color blue. What does matter, though, is that the equator is dominated primarily by tropical forests. The recipe is perfect for such habitats--endless warmth, endless wetness and no seasonal variations in sunshine.

But suppose, for a moment, that in an alternate universe, the only difference to 21st century Earth is that atmospheric CO2 is at 4500 parts per million, as much as the Cambrian. How will this affect the climate of the equator? Will it still be jungle? Or will deserts dominate it instead?

Oh, and before you ask as to why there are two Koppen maps of Earth, it's because it's the only one I could find that is accurate and neither too blurry nor "too big".

Answer 1

Equatorial regions will not be affected much as regards climate

The question:

atmospheric CO2 is at 4500 parts per million, as much as the Cambrian. How will this affect the climate of the equator? Will it still be jungle? Or will deserts dominate it instead?

How hot was the Cambrian?

https://www.britannica.com/science/Cambrian-Period

Average global temperatures during much of the Neoproterozoic Era (1 billion to 541 million years ago) were cooler (around 12 °C [54 °F]) than the average global temperatures (around 14 °C [57 °F]) of the present day, whereas the global temperature of Cambrian times averaged 22 °C (72 °F).

Can we get informed by climate change models?

Climate change models use several scenarios of "representative concentration pathways" or RCPs to model future global warming scenarios.

Four pathways were developed based on their end-of-century radiative forcing: RCP2.6 (indicating a 2.6 watts per metre squared – W/m2 – forcing increase relative to pre-industrial conditions), RCP4.5, RCP6.0, and RCP8.5.

RCP 8.5 is the most extreme of these.

https://www.theatlantic.com/science/archive/2019/01/rcp-85-the-climate-change-disaster-scenario/579700/

“God help us if 8.5 turns out to be the right scenario,” Jackson told me. Under RCP 8.5, the world’s average temperature would rise by 4.9 degrees Celsius, or nearly 9 degrees Fahrenheit. “That’s an inconceivable increase for global temperatures—especially when we think about them being global average temperatures,” he said. “Temperatures will be even higher in the northern latitudes, and higher over land than over the ocean.”

RCP 8.5 takes averages up 4.9C - so that would mean 14C now + 4.9 = 18.9 C. That is short of 22C. But a look at proposed Koeppen maps for the various RCP scenarios will let us extrapolate changes to further warming to 22C.

This image is from Global hydrological droughts in the 21st century under a changing hydrological regime January 2015Earth System Dynamics 6(1) DOI:10.5194/esd-6-1-2015.

Global warming makes the most difference (as regards Koeppen classification) for northerly regions. Deserts make some gains but with these climate models, equatorial regions are affected the least. The text states the same thing.

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I here assert that (within reason) further temperature increases will push Koeppen classification of various regions along in a similar direction. Of course there would have to be tipping points - average global temperature of 60C would make things very different.