On the flat, low-lying shores of Lincolnshire, where fields already sit barely above the tideline, the numbers have never looked more unsettling. A study published in Nature Communications calculates that up to 13 million people in the United Kingdom face long-term exposure to coastal flooding — not as a distant abstraction, but as a trajectory already being set by decisions made today in capitals from Washington to Beijing. Sea level around the UK has risen by roughly 20 centimetres over the past century, and scientists say that a further 40 to 60 centimetres by 2100 is now effectively locked in, regardless of what emission cuts are achieved from this point forward. That alone would place approximately half a million additional people at risk of flooded homes, on top of the 2.

5 million who already live in vulnerable coastal zones. Staying within the Paris Agreement's temperature targets, however, could spare around one million people from exposure by 2300 — a figure that underlines how much policy choices made in the next decade still matter across timescales that stretch far beyond any election cycle. What troubles researchers most is not the gradual rise, but the possibility of sudden acceleration. If Antarctic or Greenlandic ice sheets tip into runaway collapse rather than simply melting at the margins, the UK could face up to 15 metres of sea-level rise by 2300 — enough to inundate large parts of London.

Prof Matt Palmer of the University of Bristol, who led the study, describes sea-level rise as the "sleeping giant" of the climate crisis, warning that the oceans and ice systems respond so slowly to warming that their full reaction will unfold across centuries, long after the emissions that triggered them have stopped. Planners who focus only on the next hundred years, the researchers caution, may be systematically underestimating the true scale of what is coming.