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So planet Earth is like a ball, right? Okay, like, yeah, not exactly, but if you're considering our planet as an approximately-spherical object spinning its way through space, you don't have a lot of reason to think that the top half of that ball would be very different from the bottom half. Except that it is; there are a lot of differences between the Northern and Southern Hemispheres that wouldn't occur to most of us, but would make a ton of difference for people living on both sides.

First up, the Northern Hemisphere is warmer than the Southern hemisphere; temperatures in the north are up to two degrees Celcius warmer than the south. And that's Celcius, so you Fahrenheit people, just multiply everything by two. [(°F-32) x 5/9 = °C] We've known about this since the 1500s, and it's puzzled scientists by centuries.

One hypothesis was that it had to do with the amount of land in the Northern Hemisphere; land warms up faster than water, and if you look at a map, it's pretty obvious that there is more land north of the equator than south of it. But while land does get warmer faster during the day, it also loses heat faster than water at night. So any temperature increase would cancel out on a daily basis. It wouldn't explain why the Northern Hemisphere's average temperature are warmer than the Southern Hemisphere.

A likely culprit is a phenomenon called "meridional heat transport." See, on balance, more solar energy hits the Earth at the equator than at the poles, we know this. But that heat has to disperse somehow, otherwise the equator would be a lot warmer than it is, and every other region would be a lot colder. That heat is carried in the atmosphere and in the ocean, and moved away from the equator by currents and prevailing wind patterns.

Now, the oceans have a phenomenon called "Atlantic meridional overturning circulation." This circulation brings hot, salty water via the Gulf Stream to the bottom of the ocean near Greenland, and it also brings oxygen and nutrients to the deep sea. But it also pulls heat north of the equator, warming up the Northern Hemisphere. When the circulation is strong, the Northern Hemisphere gets warmer, and when it's weak, the Northern Hemisphere gets comparatively colder.

Now, while the climate crisis didn't cause this temperature difference, you might be able to guess that it is making it worse. That's because Arctic sea ice is melting a lot faster than Antarctic land ice, and that creates a feedback effect where the north doesn't reflect as much heat and gets hotter faster. But the temperature isn't the only difference between the two hemispheres. The Sourthern hemisphere is also stormier than the north with about 24% more storm activity, but the reason for that has been more difficult to work out.

In a 2022 paper in the journal PNAS, researchers suggested one cause might be mountains. Because mountains are so high, they can actually disrupt wind patterns, breaking up storms. And the Northern Hemisphere doesn't just have more land area, that land has more mountains. The researchers made a computer model of Earth with the exact same amount of land, but they flattened every mountain, and they saw that the difference in storminess between the two regions decreases about half. The other half of the difference was likely due to ocean circulation, much like what we already described.

Thanks to climate change though, Sourthern Hemisphere storminess is increasing. That's because rising temperatures and melting ice can affect ocean circulation and change weather patterns, and in the Southern Hemisphere, those changes are making it easier for storms to form. And Northern Hemisphere storminess isn't changing, so the Southern Hemisphere is getting stormier faster.

So, the Southern Hemisphere is cooler, but it also has more storms. Which you prefer is a matter of opinion, but there is one more factor where the Sourth might win out. The Southern Hemisphere's air is cleaner - a *lot* cleaner. Research shows that depending on where you look, the Northern Hemisphere's stratosphere has about 4 to 100 times more of certain aerosol pollutants than the Southern Hemisphere. Some of the reasons for that seem like they would be obvious: The Southern Hemisphere has less land and fewer people than the Northern Hemisphere; nearly 90% of the human population lives north of the equator, and more people means more pollution. Things get a bit less clear when you leave the troposphere down here and get up to the stratosphere, but up there, air travel is thought to lead to pollution, and there's still more people travelling in the north.

But this isn't just that the Northern Hemisphere produces more pollution; it's that pollution doesn't cross over into the Southern Hemisphere, it stays in the north, which arguably - fair. Scientist thought that this was because of a boundary called the "Intertropical Convergence Zone" or ITCZ; the ITCZ is a belt of low pressure that circles the Earth roughly at the equator, although it does naturally shift a little to the north or south. It's responsible for a lot of weather patterns, including why the equator is so rainy. Air currents have a hard time crossing this boundary, so researchers reasonably assumed that it's what kept pollution in the Northern Hemisphere.

But the ITCZ *does* wiggle north and south every now and then, meaning that some places on Earth's surface can be either in the northern or southern sides of it. One of the places that can peek above or below the convergence boundary is Darwin on the northern coast of Austrailia. And in a paper published in 2008, researchers showed that the air was still clean even when the ITCZ was south of Darwin. When they flew a plane north looking for some dirty air, they found a separate boundary around 50 kilometers wide with polluted air on the northern side and clean air on the other. They suggested that particular difference was due to the interaction of strong winds and monsoons in the area, but the researchers also say that we need a better explanation for this pollution boundary than *just* the ITCZ.

It turns out that Earth is a lot more than just a sphere in space, and that can make a real difference in our lives now and as the climate crisis intensifies. While none of this stuff is simply down to the Northern Hemisphere having more land, a lot of it *is* related to the positions of the continents in the end. So if you don't like it, just give continental drift 100 million years, and it will be sure to change.

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