It’s the year 2,500. Civilization has long recovered from the chaos years following the Trump regime.
Alas, even that catastrophe only transiently slowed CO2 accumulation. It continued for decades near the 2020 height of approximately 4*10^13 kg (40 gT) of CO2 a year.
Happily in 2,500 post-AI solar powered nanotech can extract atmospheric carbon and produce diamond sheets and glues. It’s possible to build a diamond wall to hold back the Atlantic and restore the lost cities of Manhattan and Miami.
How much CO2 would that take out of the atmosphere?
Diamond has a density of 3.5 gm/m or 3,500 kg/m3. If the wall is 1000km long by 50m high by 10m wide it will consume about 1.75*10^12 kg of carbon.
So we only have to build 22 such walls to undo a single year of CO2 emissions.
I put the numbers into a Google Sheet. Since the wall only required a few weeks of carbon output I decided to try the diamond base of a floating city modeled as a disc with a radius of 50km and a depth of 200m. That did the job!
A single floating city absorbed 145 years of 2018 carbon production.
It would be easier, of course, to build something attached to a landmass, but where’s the romance in that?