Along stretches of the American east coast, whole stands of trees have gone the colour of old bone. Bare branches, no leaves, roots sitting in brackish water, dead for years and in no particular hurry to fall down.

Nobody had ever counted them.

A machine has now done it. Henry Chi Hang Yeung, a doctoral researcher at the University of Virginia, built the tool with his supervisor, Xi Yang: a deep learning model trained to spot dead trees in aerial photographs of the coastline. The tally came back at more than 10 million, scattered from Maine down to South Carolina, and the results were published in Nature Sustainability in December.

How to count ten million dead trees

Yeung did the tedious part by hand. As documented by Science, he spent hundreds of hours labelling more than 50,000 of them by hand, teaching the model what a corpse looks like from above: a pale grey crown and the scraggly shadow that bare branches throw across the ground.

Resolution was the whole game here. Earlier surveys leaned on Landsat satellite data at roughly 30 metres per pixel, which turns a single dead pine into a smudge of nothing much. This map uses aerial photography at better than one metre. When the team checked their results against existing coarse-resolution maps of coastal forest loss, the two barely overlapped: most of what they had found had never registered before.

Yeung is upfront that 10 million is probably an undercount. His method misses trees that have already toppled, trees hidden beneath a living canopy, and anything with a skinny crown. Some of the dead were also killed by insects or drought rather than seawater.

Salt gets there before the flood does

Trees drink through their roots by osmosis, which only works while the water outside the root is less salty than the sap inside it. Push salt into the soil and that gradient collapses. A tree ends up standing in water it can no longer use, and dies of thirst with its feet wet.

Yang told VPM that the killing comes from three directions at once: salt creeping through soil and groundwater below, flooding at the surface, and sea spray driven inland by storms.

Underground appears to be the busiest of the three. More than 6 million of the dead trees stand in forests below five metres of elevation, and the paper attributes that concentration mainly to salinisation rather than flooding on its own. Storm surges and droughts push the salt line further inland from there, finishing what the tide started.

The roads that are holding back the sea

One result reads almost as slapstick. Where roads and levees happen to sit between the water and the woodland, the team calculated that forest loss dropped by 40 and 79 per cent respectively, which means rural America has built itself a coastal defence network entirely by accident, out of embankments meant for pickup trucks.

Yeung and his co-authors are careful about what that buys. Barriers hold the line temporarily while the water keeps rising underneath them.

What moves in afterwards

Dead forest does not stay bare for long. Salt marsh takes over, salt-tolerant grasses and shrubs colonising ground the trees have vacated, which sounds like a fair swap until somebody checks the carbon ledger.

Researchers at North Carolina State University tracked the changeover on a single stretch of the North Carolina coast and found that 15 per cent of unmanaged public land in the study area, some 167 square kilometres, flipped from coastal forest to ghost forest between 2001 and 2014. In a paper published in Environmental Research Letters, they put the aboveground carbon loss at about 130,000 tonnes across those thirteen years. Lead author Lindsey Smart pointed out that people tend to file sea level rise under long-term threats, while the landscape is quietly reorganising itself over a decade or so.

Marshes earn their keep. They buffer storms, feed fisheries and lock carbon into their sediments. They just hold far less of it above ground than the trees they replaced, so the swap runs at a loss for a long while.

The lag nobody has priced in

Forests are slower to die than the sea is to rise, which turns out to matter enormously. Yinan Chen and Matt Kirwan (the latter at the Virginia Institute of Marine Science) examined mid-Atlantic forest retreat against local sea level records and reported in Global Change Biology that land conversion is running decades behind the water.

Their conclusion cuts two ways. Coastal ecosystems have more staying power than the direst models assume, and the full consequences of sea level rise already locked in have yet to arrive.

Kirwan, who was not involved in the mapping work, told Science that the die-off is not a future event. Ten million trunks bear him out.

The interactive version of the map released alongside the study is public, and it zooms. Anyone can find their own patch of coastline, scroll in, and count the marks where woodland used to be.

Whether that reads as a forecast or a receipt depends mostly on how far above sea level you happen to live.