If all, or most, fresh water was frozen due to temperature drop over the space of a few generations, what would be the effects on living organisms such as humans and animals?
Take in to consideration that there are already places where most fresh water is frozen and people live there.
When I say adapt, I don't mean by evolution, I meant as in coping with the dwindling of outdoor live stock and crops. Having to create warmer homes or living quarters, etc.
When the point is reached where all fresh water is frozen it means that global temperatures will have dropped enough that we have much bigger problems than finding drinking water, such as being extinct.
Have you considered the implications of what you're asking?
All fresh water being frozen means that every stream and lake - even the ones at the equator - are now frozen solid. Even the massive Amazon River.
That in turn implies that temperatures have dropped well below zero at the point on the Earth which is supposed to be the warmest. It's not a massive leap to assume that the rest of the planet is probably covered in massive glaciers at that point.
Most surface life would be long extinct if the situation is that dire, including, most likely, humanity. Even any bunkers we might build for ourselves would be under so much ice that we'd never be able to leave them again.
There would be no question of "warmer clothing", or better insulated living spaces. The Earth would be experiencing an ice age the likes of which it never has before.
Only a cataclysmic event, such as being knocked out of our regular orbit, or having a massive volcanic eruption cloud the sky (worldwide) with ash for generations could cause that sort of weather.
Several things have to happen to cause all natural, accessible, fresh water to freeze.
With temperatures that cold, life will go extinct, or nearly so. No crop plants will survive; only a few ultra-hardy lichen will manage to live. A few tiny organisms will survive, but otherwise everything will die, if not from the cold then from starvation. Finally, without plants, animals, fish, birds, or even insects to eat, humans will go extinct as well.
The extinction process will roughly follow the temperature decline; if the temperature falls over the course of a thousand years, animals and people will slowly move towards the equator, altering migration habits enough to survive, at least for a while. If it takes place over a century, most creatures will not be able to adapt quickly enough to escape the cold. If it takes place in less time than that, even humans won't have time to prepare. It will be a mass extinction.
If there is some time to prepare, humans will have to build huge underground structures, stocked with a diverse animal and plant population. These habitats will have to be enormous to hold even a fraction of the food, water, and other supplies the population of the world will need. And even with a well-stocked bunker, impervious to outside temperatures, life will only be about surviving. Unless the Earth re-heats quickly, humanity will still be doomed... and even then, life will be permanently altered.
Water freezes at 32 degrees Fahrenheit (or 28-ish with seawater), or 0/-2 degrees Celsius. So if the temperature has dropped that much, then you can expect that the organisms on Earth will die out because "a few generations" is not enough to adapt biologically.
Mammals won't suddenly be growing extra long coats of hollow hair. Plants won't have enough time to need less resources. You'll see a lot of organisms go extinct.
Basically, for the water to freeze, the environment needs to be at least that cold, and if it is, it's bad news for everyone. All the chaotic implications it has on the weather would also have to be accounted for. It would be hard to migrate or take steps to protect oneself, even for humans.
You can expect a lot of side-effects of the cold as well. Aside from hypothermia, you'll get respiratory failure, muscle spasms, decreased immunity to diseases, etc. It's just bad news.
Even if the animals can adapt by finding warmer places - perhaps using insulation of a sort on land - they still have to worry about their food sources. Humans will suffer a lot even with their intelligence because the grain and wheat will go bye bye.
Oceanic creatures will fare better with thermal vents. The ocean would retain a lot of sufficient heat for a very long time, as well, so the ecosystems wouldn't suffer as drastically and quickly.
Once everything is frozen, then you have mass extinction. I could imagine surviving humans with a lot of preparation and technology (which would make for a great second question for Worldbuilding) but aside from that, only the extremophiles will survive... errant bacteria clinging to life in the deepest, darkest nooks and crannies of the Earth.
...over the space of a few generations what would be the effects would it take on living organisms such as humans and animals?
They'd all die. A few humans survive by reverting to Arctic living techniques, clinging to the equatorial coastlines and fishing from a ravaged and depleted ocean. Others might undergo a crash development program to survive permanently in underground bunkers.
All land dwelling life, including plants, not already adapted to permafrost would go extinct. Only sea life and lithotrophs (organisms which eat minerals) would remain. Even land animals which feed on sea life and are adapted to the cold such as polar bears and penguins would die from the extreme temperature drop within their territory and the extreme disruption to the ocean ecosystem.
To freeze all the fresh water on the planet you need to freeze the water at the equator, a place which runs an average global temperature of 30C and up. Global warming is already causing problems at a 1C change and will mess things up badly at 5C. Dropping the worldwide average temperature by 30-40C everywhere over a few decades would be so disruptive it would likely kill even land organisms already adapted to freezing conditions.
Even the Arctic thaws a little each year; not any more. The mean temperature in the Arctic goes from -10C to -40C. Land-animals shift away from their native areas too slowly to reach "warmer" climates 5000 miles away even over decades. Seasonal migratory species no longer have anywhere to migrate to.
With land-based plant life wiped out the whole land-based food chain collapses. Now it's a purely sea-based food chain. But dropping global temperatures will disrupt ocean ecosystems and currents as well leaving far less food available.
Arctic human natives would not be able to survive the extreme drop in temperature, disruption of their fisheries, and loss of land animals to hunt. They would have to migrate move south or die. Perhaps a few pockets of humanity would survive by adapting Arctic native living techniques near the equator if they could find people with the experience to teach them. Clinging to the coastline and fishing for whatever is left in a cold and disrupted ocean, it would not be pleasant.
There will, of course, be some people who will try to live underground. As we have yet to successfully figure out artificial permaculture, they would likely run out of stockpiles of food, energy and spare parts in a few years and die. Perhaps, given the temperature drop is over a few generations, a crash program could be initiated to learn how to permanently live underground, but given our track record of climate change denial I have my doubts.
This happened in the Earth's past, in a more extreme form. It's referred to as Snowball Earth. All fresh water and most of, probably all of, the surface of the Earth was frozen.
Global temperature fell so low that the equator was as cold as modern-day Antarctica. This low temperature was maintained by the high albedo of the ice sheets, which reflected most incoming solar energy into space. A lack of heat-retaining clouds, caused by water vapor freezing out of the atmosphere, amplified this effect.
Surprisingly, this didn't cause a mass extinction event. However, at 650 to 750 million years ago life was fairly simple and was largely confined to the sea. This was covered in an episode of Catastrophe.
All the bigger animals will be extinct far before all the water become ice. The humans probably will try to build some artificial habitat which probably can make them (and some animals) survive at least in the short term.
This habitat probably will be underwater to exploit the fact that the water under the ice have a costant temperature above zero, so the survivors will end to live in a big underwater dome (how under and how big are an interesting questions and they depend on the reason that caused the temperature drop), in a perpetual artifical winter and low light, with all the activities indoor.
