1. Earth’s temperature would rise drastically.
Water absorbs a lot of heat without showing a significant rise in temperature. Because of this ability, the presence of a large amount of water in the oceans helps in keeping the Earth cool. Also, evaporation of water from the oceans and rivers helps in regulating the Earth’s temperature. If water and land switched places, the Earth’s temperature would rise very quickly and it would become hotter. Large parts of the land masses would become arid and dry.
2. The amount of oxygen in the atmosphere would decrease.
Marine plants supply about 70% of the oxygen present in our atmosphere. Without large bodies of water, many aquatic plants would quickly vanish, which in turn would lead to less oxygen in the atmosphere. Bodies of water also act as a sink and absorb the carbon dioxide that we release into the atmosphere. So, if there was a smaller percentage of water, on one side oxygen would decrease and on the other side, carbon dioxide would increase in the atmosphere. And, because carbon dioxide is a greenhouse gas, this would also result in drastic global warming.
3. Earth’s fauna would change.
The swapping of land and water would have many effects on Earth’s lifeforms. The temperature would rise drastically, the amount of oxygen in the atmosphere would decrease, and the amount of carbon dioxide would increase. All of this would make living on the planet hard. And for animals and plants to be able to thrive, they would have to modify themselves to be able to survive and prosper. Due to less availability of plants, the extinction rate would accelerate with herbivores facing the brunt more than carnivores. The weather conditions would also make cold-blooded animals more successful than us warm-blooded ones.
4. Land transport would become easier.
With large swaths of land connected without any disruption caused by bodies of water, land transport would become easier. On the flipside, marine transport would be hit hard. Countries that depend on the export of seafood would be the worst sufferers.
5. The human civilization and economy would get altered beyond recognition.
Water is a source of life. With water being scarce, countries would strive to control as much water as they could. This quest could escalate into a full-blown war. Economies that are reliant on marine life for survival would get affected, and economies that were not dependent before on marine life might find it to be the only source of survival.
Bonus: Everest will no longer be the highest mountain
While we all know Mount Everest (8,840 m / 29,002 ft) to be the highest peak, in fact, it the highest peak above sea level. Mauna Kea standing at 10,000 m (33,000 ft) is the tallest mountain if we consider all mountains—submerged and above water.
So, if water and land were to switch places Mt. Everest would go underwater and the now submerged mountain Mauna Kea would then be present, making it the tallest peak above sea level.