The weather system of a rotating planet that faces a star with its equator (such as the Earth) naturally organizes its precipitation into cells. These cells naturally transport energy and water vapor towards the poles from the equator. The **Hadley Cell**, which advects energy from the equatorial surface upwards and transports it along the upper troposphere to where it meets the **Ferrel Cell**, begins at the equator and typically ends at 30 degrees latitude. Its interaction with the Ferrel Cell causes a downdraft of cool and dry air around 30 degrees latitude and causes a high pressure zone called the **subtropical ridge**. Because air flows from high to low pressures, the high pressure from the subtropical ridge pushes away encroaching air from its surroundings and the result can lead to insufficient water vapor to condense into clouds and form precipitation. 

This mechanism is responsible for many of the world's deserts and is a natural result of the Earth's equitorial heating and rotational inertia. Various computational models have also confirmed that this effect is present on other worlds that rotate and face a star like Earth. There will always be a precipitation deficit around 30 degrees latitude so it is impossible to convert the entire Earth into farmable landscape. On the other hand, however, the Hadley cell also creates a natural precipitation rich area around the equator called the **Intertropical Convergence Zone** and there is a smaller excess anomaly around 60 degrees latitude. These areas are favorable to farmland and as a result can host a similar amount of vegitation in a smaller area compared to if precipitation was evenly spread out across Earth.

While we can not convert the entire Earth into farmland, the Sahara desert has turned green in the past. During the **African Humid Period** approximately 6,000 years ago, a combination of increased solar flux, global warming, and increased axial tilt, caused a natural low pressure over the Sahara that drew water vapor in from a northerly displaced Intertropical Convergence Zone. As a result, the Sahara was a grassland with some ancient civilizations using the numerous rivers as irrigation for farmland. So while the Sahara can be naturally turned into farmland, it is impossible to turn the whole world into self-irrigating farmland. Artificial farming in deserts is plausible but it would require solving difficult engineering challenges that have never been attempted at such a scale before.
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