One of the most persistently misunderstood concepts in public discourse is “global warming”. That misunderstanding is part of the reason the fashionable term now is “climate change”, which could of course mean almost anything. But I have not seen a good explanation of why “global warming” is not a good term.
I will provide part of an answer here.
Most people intuitively think that if the Earth warms, the temperature will rise. That intuition is wrong, because Earth is an aqueous world, and water behaves in a peculiar way. Unlike many substances, which go from solid to liquid to vapor as the temperature rises, water has a temperature at which solid, liquid, and vapor can all exist at the same time, and introducing heat into the system does not raise temperature on the average, but causes a shift from some of those phases into the others, while the temperature doesn't change much. The following diagram is intended to illustrate that behavior.
The important thing about the Earth is that the average temperature is fairly close to the triple point. Adding heat can cause ice to melt into water, or either of those phases to evaporate into vapor. It is only when almost all the water is vapor that temperature will begin to rise. Of course, water vapor is also a greenhouse gas, so that can cause an even greater rise.
Nothing in the geological record indicates that such a runaway rise has ever occurred. Water has always acted to buffer temperature changes within a fairly narrow range. We can speculate that enough heat, together with the sublimation of deep-sea methyl clathrates, together with releases of methane and CO2 from melting tundra, might combine to produce runaway temperature rise, but that is only speculation, without geological evidence. One would expect that if it had a significant chance of occurring, it would have done so at least once in Earth's past.
That of course does not mean it may not occur in Earth's future. In the far distant future we can expect it to occur.
So what we can expect from global warming is the melting of ice, the rising of sea levels, and more clouds with more rainfall. That is also likely to result in more stormy weather, with more flooding and wind damage.
Now because Earth is not spherically symmetric, and there are irregularities on the surface, we can expect average surface temperatures to rise seasonally over land, causing seasonal changes to shift toward the poles and occur earlier as the process unfolds. The great deserts that encircle Earth are likely to shift toward the poles, causing the desiccation of now green areas and increases in widespread forest and grass fires. Once fertile land may become infertile, and infertile land to become fertile again after a span of thousands of years. North Africa and the Levant were once green, about 6000 years ago, before the desert band moved northward.
So could sea levels rise as much as 200 feet? Yes. In the past they have been that high. That could inundate low lying coastal areas like Bangladesh, Florida, and the Netherlands, and perhaps even the northern part of the Central Valley of California. It may not be a good time to invest much in coastal resort property.
So is this global warming/climate change caused by human activity? Much of it is, although coal fired power plants and automobiles may not be the ,main sources of CO2. The isotopic abundances of the carbon in atmospheric CO2 indicate much of the source is fossil fuels. More of it may come from things like the tilling of topsoil, causing it to oxidize, from runoff of topsoil into the oceans, the burning of grassland and forest, and other such activities over which public policy doesn't have much control. Even if the developed countries were to stop producing CO2 altogether and go entirely to wind, solar, and other “green” sources, it would make little difference. China alone would overwhelm the rest. The notion that we can influence other countries by “setting a good example” is ludicrous. They would just laugh at us.
No, the only thing we can do is to develop alternative energy sources. There are two likely sources: thorium nuclear plants, and solar power collected in space, probably on the Moon, and beamed to earth.