The tilt of the earth and habitable conditions

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Dear Tom,
If the axis of the Earth didn’t tilt and the sun were always directly above the equator, would our planet still be habitable for human life?
— John Rohr, Darien
Dear John,
It’s an interesting speculation, and my belief is that it would be. Let’s assume the distribution of land and oceans would be the same as exists today. The primary change in the global climate would be the lack of seasons, everywhere. The gradation of temperature from warm equatorial to cold polar would be the same, or perhaps a bit more extreme: equatorial regions a little warmer and polar regions a little colder, but not significantly so. Equatorial and midlatitude regions would still be fit for human habitation, but the poles, too cold now, would be unfit, then as now. Low- and high-pressure systems moving across the landscape would be the same.