Another issue that comes into play with desalinization is cost. Not just literal cost of equipment and such, but also on-going energy costs (electricity), etc.. Its not insormountable by any means, and probably sounds silly to those us in what are refered to as first-world nations, but there are countries out there that can't even afford mesquitto nets (Which are really just tightly meshed nets, nothing too special) to combat the spread of malaria, so desalinization costs could be a big deal.
I saw an estimate that if we were to provide all the water needs for the US from desalinizated water instead of fresh water, we would increase our power usage by 10% annually. That's 10% of the power consumption of a country with relatively high amounts of power use- it doesn't scale. In other words, a poor country doesn't just need to generate or pay for 10% more electricity, it has to pay whatever the actual cost of 10% of the US' electricity is if it has the same number of people- which could be some gigantic figure like 50% of their GDP.
Unfortunately, the poor get poorer globally with climate change, if it takes its likely path, because countries that already are in trouble economically due to overly plentiful desert areas and lack of abundant resources and clean water (I'm thinking Sub-Saharan Africa here, but its just an example), get more overall desert, with existing desert getting hotter, and less available fresh water. Meanwhile, coastal regions flood and that hits available monetary resources and creates refugees flooding inland. Their infrastrcuture also could repeatedly take a beating with odd climatic events that they are unprepared for.
Ideally, we'd see a massive effort to get these plants built and funded by the richer larger nations for the poorer nations, but our relief efforts for these countries in the past have been underwhelming, and this will be a much higher cost to us at a time when we'll have more of our own problems- some of them the same problems. We might be struggling to get our own domestic desalinsation plants up and running, handle refugees from our own flooding coasts, replace lost infrastructure, figure out what to do about places like Arizona that are already desert but have cities like Phoenix that are huge metropolises and get all of their water from places like the Colorado River, which may be needed for thee exclusive use of closer areas turning to desert that were not previously, as Arizona gets hotter instead of colder. I am not picking on Arizona- I lived out there for 6 months once upon a time and would love to get back out there for a vacation or something someday, I am just saying we're going to be facing a lot of huge regional and national projects that need to be dealt with in this hypothetical future, of which this is just one potential example. Also, the present trend towards increased isolationist movements in the US, UK, France, and beyond doesn't bode well for the prospects of increased foreign aid.
Doing another one these may not be enough
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Not trying to be a downer, because I do think there are some positive things we can do to mitigate the severity of these issues, but they are definitely going to be issues. They already are issues in some places.