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" Life on earth is based on a salt-water metabolism: we all carry around salt-water inside us to bathe our cells which evolved to expect that environment. So, with only fresh water, evolution would have had to happen in a completely different way: basically you're looking at rewinding the clock 4 billion years and seeing what happens without sodium and or chloride. My suspicion is that the various chemical processes which are running all the time in our cells could be done differently, with different elements, and you would get life which was broadly similar to the life we know, until you got down to the level of biochemistry. However, maybe the dependence on the alternative chemicals would push life in a completely different direction.
Basically you're looking at a total reboot of evolution, from the bottom up, and it's quite tricky to speculate what you're going to end up with after that.
I forgot to think about weather and geology. I'm not particularly knowledgable about meteorology, geography and geology, but my
guess would be thus:
The current state of the earth's physical geography is due to three main processes: rock formation, plate tectonics and weather. The rocks are formed, smashed together/pulled apart on the big scale by tectonics to make mountain ranges and valleys, and then weather erodes the rocks. In addition to this, tectonics and perhaps other geophysical processes move landmasses under and above sea level over huge time scales, so you have land that used to be on the sea bed now forming the earth's walkable surface, etc.
Since water evaporating from the sea doesn't take the salt with it anyway, the weather wouldn't be very different. In other words, i don't think that the oceans' salt content plays much part in the Earth's weather, because i don't think there's much salt in the atmosphere. I doubt that the salt content of the rocks plays much part in tectonics either. So, the only remaining aspect of the earth's surface which could be affected by the removal of sodium and/or chlorine would be the actual rock formation: perhaps we would have different kinds of rocks - in particular, we might have a smaller range of crystalline rocks in existence. But, i suspect we would still see the same broad processes, wherein tectonics and volcanoes make mountains, and the weather smoothes them out again, etc, and you have things like topsoil and silt and other "stuff" which change our planet's land surface from a bare rocky landscape into a softer "earthy" terrain which can support plants etc."
"If all the salt dissolved in the ocean disappeared instantly, then the oceans would convert into a giant freshwater body, which would have the ability to feed the humanity freshwater for a really long time.
There would be no need for desalination plants, since only basic filtration processes would suffice to feed the humans potable water. Countries would stop worrying for water problems, because now a virtually unlimited amount of water would be available for all domestic and industrial purposes.
However, most of the saltwater species that can’t thrive in freshwater will die. Therefore, we would lose a number of discovered and undiscovered flora and fauna species when the oceans go fresh.
Can Saltwater Fish Live in Fresh Water?
However, many euryhaline species that can live in both freshwater and saltwater would be just fine, and we would still have Salmons to eat, but not Tuna.
This would have a geographical effect as well. Since there are a large number of ocean currents, the directions of which depend on various factors that
include water salinity, we would have their routes slightly affected. This would not be much, but we might experience subtle differences. We won’t have to worry much about that.
There is a catch, however. The rivers continuously bring silt and minerals with them, which is one of the primary reasons of ocean salinity. They would continue to do so. The salinity of the oceans would start increasing immediately after this reset. We won’t notice it for thousands of years. However, after a sufficient time lapse, for example, like 500,000 years, we would begin to notice, and notice well, the oceans reverting to their initial states. In a million year or so, the oceans would pretty much have become saline enough that they could not be drunk from. Hurricanes, anticyclones and ocean currents would help in homogenizing the salt content throughout the oceans. We would need the desalination plants again.
Of the 1.386 billion cubic kilometers of water on earth, about 97.5% is seawater, of which 3.5% is salt. Thus, we have 1 cubic kilometer as 0.24 cubic miles. One cubic mile of seawater has about 120 million tonnes of salt. Thus, we have about 135,135,000 cubic kilometers of seawater, which means that we have the mass of salt in the oceans as
135,135,000×0.24×120,000,000,000 kilograms, which is 3.89E18 kilograms. This much mass would be removed from earth. It is not much, given the mass of the earth, but it would still have some effect. For example, earth will rotate faster and we would have smaller days, just like it used to be in the age of dinosaurs. And it would eventually get slower with time again, to return to the current day length, just like it has been doing for billions of years."
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Phase 1: The Extinction
Most life in the sea would die out almost immediately. Saltwater fish, invertebrates, and most importantly plants and microbes are finely-tuned for a particular level of salinity and their cells would burst under the osmotic pressure.
Sea mammals and reptiles can tolerate the change better, but since these animals tend to be in the middle or top of the marine food chain they would quickly starve. Some of them, like sea lions which are capable of leaving the water might be able to survive for a short time by turning to eating what they find outside the water, but it is unlikely that these specialized fish-eaters could survive for long enough to evolve back into a fully-terrestrial lifestyle. The fact that marine mammals are pretty much all obligate carnivores doesn't help their case either.
The Deadly Shores
Some carnivorous whales and a few salt-tolerant shark species might migrate to the estuaries and beaches, where large prey still lives. They will be hungry and competition will be fierce, with the larger individuals inevitably turning to cannibalism out of desperation. Orcas and bull sharks will plague the shorelines of the world - stay out of the water for a few months if you want to live. Their population will drop off sharply, but a few lucky ones might survive.
Short of Breath
Don't think that living on land means you'll have it easy. Between 50 and 80 percent of the world's oxygen comes not from trees, but from marine algae - the same marine algae that is now dead. Atmospheric oxygen will take a nose-dive. All animal life will suffer as a result, and many will go extinct, starting with warm-blooded, high-energy animals like birds and mammals. A CO2 buildup will accelerate global warming to absurd degrees. If we're lucky, land plants will take advantage of the extra heat and CO2 to multiply rapidly and pick up the slack before we run out of air. Hopefully the next phase will begin before we all suffocate.
Phase 2: The Green Sea
After marine life has all but disappeared, river life would slowly begin to evolve in order to fill the abandoned ecological niches. This would naturally begin with photosynthetic microbes seeking out the extra space in the empty seas. After a relatively short time (perhaps no more than a few months or years, thanks to the extreme ability of microbes to evolve quickly) the seas would once again bloom with algae evolving from salt-tolerant estuary strains. Animal life can once again breathe easy. Perhaps too easy...
The Neo-Permian
Remember how the vast majority of oxygen is made by photosynthetic algae? In our world, that algae is regularly grazed by all kinds of herbivorous animals and microbes. The carbon-overdosed world will suddenly experience a violent shift in the other direction as the new algal blooms pump the atmosphere full of oxygen. Animal life will make a comeback, but the oxygenated atmosphere might present a prime opportunity for massive firestorms to erupt. Also insects might once again gain the ability to grow to sizes not seen in millions of years. Along with the earlier mass extinction of birds and mammals, the world's ecosystem may once again resemble the Permian era.
Phase 3: The Return
Over time, river-dwelling animals would return to the oceans. Salt-tolerant estuary-dwellers and fish capable of altering their own salinity such as salmon would be first, with the handful of surviving species diversifying to fill the empty ecosystem.
What is particularly interesting about this new situation is that river-dwelling reptiles and mammals would adapt more quickly to retaking the seas than most fish would, since they are already salt tolerant (assuming they survived the low oxygen levels, that is). Manatees and turtles might be among the pioneer species, leaving the safety of the estuaries to graze the green pastures of the relatively predator-free seas, and crocodiles would likely follow them.
From here, it is virtually impossible to predict how evolution will proceed. What is certain is that the world will be greatly altered. This would likely be the biggest extinction event the world has ever known, but life would survive.
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