Warming by CO2 is basic science. CO2 reflects back infra red radiation emitted by the earth. So the more CO2, the warmer the Earth. The details are tricky to work out, but the basics are simple. CO2 does warm the Earth. The problem is that weather is complex. They think that for a few years the oceans have been warming, acting as a heat sink. And warming of tundra will release methane, and even stronger greenhouse gas. But more CO2 will encourage plants to grow more, which will absorb CO2.
The CO2 we are releasing was captured by forests in the past when dinosaurs rules. It was then laid down in the ground. We are releasing millions of years of CO2 in a few decades.
If you want to see how the greenhouse effect works, walk into a greenhouse.
Oddly enough recent reports suggest CO2 release is reducing, due to efforts to increase use of solar, wind, nuclear etc. I use CFL bulbs, very good they are too, and they save me money. My eco loo is pants, there's not enough water in the cistern, because they want to reduce water usage. So you flush it twice. My VW Up is very nice, 65 mpg on a good commute. What's wrong with more efficient products?
Why not create flood barriers much higher than the present need? Money.
Probably because the size of flood barrier to protect the Somerset Levels, for example, would be ridiculous. Or maybe we don't give a damn about some yokels. They built a nice big barrier in the Thames to protect London cos that's important innit bro?