That's a big if in the first sentence, as well - we're currently on track for about 3°C.Melting of the Antarctic ice sheet will cause sea level rises of about two and a half metres around the world, even if the goals of the Paris agreement are met, research has shown.
The melting is likely to take place over a long period, beyond the end of this century, but is almost certain to be irreversible, because of the way in which the ice cap is likely to melt, the new model reveals.
Even if temperatures were to fall again after rising by 2C (3.6F), the temperature limit set out in the Paris agreement, the ice would not regrow to its initial state, because of self-reinforcing mechanisms that destabilise the ice, according to the paper published in the journal Nature.
Quite a lot of the world's biggest and most 'important' cities are under 2.5 m above sea level. London, New York, Hong Kong, Shanghai, Rio de Janeiro, not to mention entire countries like the Maldives.
Obviously in addition to the increase in mean sea level, with extra water in the system high tides tend to get even higher, increasing the risk from extreme events. The UK is currently spending billions upgrading coastal flood defences to cope with 0.5 m of sea level rise, and even then are having to make a "managed retreat" from large areas of low-lying coastal land (this is code for allowing it to flood sacrificially, protecting adjacent areas with more economic value).The Antarctic ice sheet has existed in roughly its current form for about 34m years, but its future form will be decided in our lifetimes, according to Levermann. “We will be renowned in future as the people who flooded New York City,” he told the Guardian.
Temperatures of more than 20C were recorded for the first time in the Antarctic earlier this year.
So we're not engineering our way out of it.
It would be entirely doable to limit carbon emissions and thus climate change to lower levels, and thus reduce the amount of locked-in melting. But we'd have to accept expense and inconvenience immediately, rather than drawn out over several centuries.
Presumably that's why most of the people striking for climate action are kids, and most of the people tutting about it are older folks who'll be safely dead so would rather not sacrifice anything for a collective effort.