>It's cheaper still to build barriers
for some definition of cheaper. For full cost you need to factor in political costs of implementing such a a collective action as well as opportunity costs. And once built it needs to be maintained. It requires continuous involvement into political fight to insure that the maintenance is happening to avoid Katrina type situations. The implicit threat of non-maintenance becomes a tool of political blackmail and pressure. It makes for high-risk business environment which is just bad for business.
>it seems incredibly stupid to be burning subsidized fossil fuels.
well, removing the subsidies and decreasing the fossil fuel burn is an immediately available very low cost collective action to slow climate change and delay water rise... We can see how it's been going so far.
>A proper carbon taxing scheme with tariffs on countries that don't play along would make so much sense.
Russia for example has all the sanctions and tariffs (and when there is no tariff imposed from outside, Russia usually takes care about that by having self-inflicted inside tariff there :) so it is hard to make much more, and Russia is anyway going full-steam into that warmer future (the global warming was taught in middle school even back in 198x in USSR) pumping oil and gas like crazy. Many Russian territories are becoming much more conductive for living and doing agriculture there. Short of WWIII there is not much can be done here (and Russia is actively building bases in the North and Far East as well as new strategic weapons system - the point is that they don't bet much on global international cooperation :). I'm pointing at Russia because i know it better than other countries, and of course it may be that Russia is large outlier, yet i somehow think that a number of other countries are also not betting much on successful global international cooperation in particular when it comes to global warming.