If Earth's axis wasn't slanted, would all earth be tropical? [duplicate]

Question

If Earth's axis wasn't slanted, would all Earth be tropical due to even scattering of sun light beams all over the planet?

Of course, the poles would still be cold, even colder, but earth in general more evenly hotter, rainier and lusher, right?

Answer 1

Probably not.

A perpendicular axis would avoid long periods of darkness at the polar regions, but it would do nothing to alter the oblique angle of sunlight falling there. This oblique angle means sunlight is less intense on the ground near the poles than near the Equator, which then renders the polar regions cold even if they receive sunlight every day. In the world as it actually is, we see this effect during "summer" in both Arctic and Antarctic regions.

Answer 2

Despite the loss of sunlight-driven seasons, a more fundamental change in ocean currents would likely expand sea ice in polar regions. As it is, seasonal thermal variations drive massive global currents that carry enormous amounts of heat around the planet; cold, dense polar waters sink into abyssal regions, distributed by centripetal forces to opposite poles, split, warmed and forced up along the way to lose that heat at the sea surface. That released energy drives much temperate and tropical weather.

Would the loss of seasons mean the loss of these currents and concomitant heat redistribution? Hard to say without a supercomputer and customized climate model. But, consider how melting of the North Polar Ice Cap might lead to the collapse of the Gulf Stream and affect weather in Europe.

Nevertheless, I think it is unlikely that the planet would, on the whole, become more tropical. If anything, ice caps would expand, land surfaces would become drier, and the tropical band currently encircling the globe would narrow considerably. But really, that's just an educated guess.