Millions of trees seem to be dying in the US, both near the coasts and inland near lakes, thanks to rising water levels. The finding may force a re-evaluation of how much carbon is stored in forests, which are important carbon sinks for mitigating the more severe effects of climate change.

Rising sea levels and intense storms linked to climate change have driven widespread tree die-offs along the US Atlantic coast, creating known as ghost forests. North Carolina, for example, is thought to have because of waterlogged soil and higher salt levels resulting from rising seawater.

To see how widely this is happening, at the University of Virginia and his colleagues used aerial imagery from the US . They analysed images captured between 2012 and 2023 to map dead trees within 10 kilometres of the Pacific, Gulf and Atlantic coasts of the US, as well as along the US shorelines of the Great Lakes, down to a resolution of about 1 square metre.

They have identified more than 260 million dead trees and found that more than half of the mortality was concentrated in just 10 per cent of the mapped forests. These hotspots weren鈥檛 only in low-lying coastal forests prone to flooding, but also in high-elevation inland wetlands, far from salty water.

Over the decade they studied, tree mortality tripled in low-lying coastal forests, where water inundation was combined with increased levels of salt.

But mortality rates also doubled around expanding freshwater lakes, including the Great Lakes, and in inland wetlands, which have experienced more precipitation.

鈥淭his is the first map of ghost forests that covers the entire US. It鈥檚 also perhaps the first comprehensive study that takes the concept of sea level rise killing trees and applies it to more inland locations,鈥� says at the Virginia Institute of Marine Science. 鈥淎s a coastal scientist, I immediately think of salt as the culprit. These authors have extended it to water in general.鈥�

at the University of Plymouth, UK, says it is striking how widespread the pattern is. 鈥淧rolonged inundation and salinity are already known to stress and kill trees, but the study suggests that water-related forest mortality may be more widespread than is usually recognised,鈥� he says. 鈥淭hat broadens the issue beyond sea-level rise alone.鈥�

Yeung and his colleagues found that between 2013 and 2020, high levels of precipitation and water runoff drove record-high levels in the Great Lakes, and they suggest that this led to sustained waterlogging, increasing the chances of nearby trees dying.

鈥淭he study provides convincing evidence that changing hydrological conditions are an important contributor to the mortality patterns, but I would be cautious about attributing every tree death directly to inundation or salinity,鈥� says Iwantoro, adding that tree mortality is often influenced by other interacting factors, including storms, drought, pests, local drainage and soil conditions.

He says the work adds to evidence that the water-related effects of climate change could accelerate ecosystem shifts.

As the climate warms, the melting of ice means global mean sea levels are rising at about 4.1 millimetres per year, and saltwater intrusion is expected to of the world鈥檚 coastlines by 2100. What鈥檚 more, climate warming also fuels heavier rain and snow, because for every 1掳C that atmospheric temperature rises, the amount of water vapor in the atmosphere can increase by about 7 per cent.

This means that in coastal and lakeside areas, forests may retreat as conditions become more suitable for wetland vegetation, with knock-on effects on carbon storage, says Iwantoro.

Urban trees in south-east England can鈥檛 cope with the changing climate

In cities such as London, trees are already struggling with hot and dry summers. Botanists warn that we need to start replacing them with drought-adapted species

鈥淭ree mortality and decomposition releases carbon into the atmosphere, and if the forest does not recover, then these areas may not regain their capacity to sequester carbon, and it could represent a more long-term carbon stock loss,鈥� says at the World Resources Institute, a global non-profit.

However, the ecosystem that replaces the forest may also accumulate carbon, says Iwantoro. 鈥淭he important question is how carbon storage changes as the landscape transitions, rather than simply how much carbon is lost when trees die.鈥�

African forests are now emitting more carbon dioxide than they absorb, a fundamental shift that will make it more difficult for the world to cut its net emissions to zero.

Forests and shrubby woodlands on the continent have previously been one of the world鈥檚 biggest carbon sinks, accounting for 20 per cent of all the CO2 taken up by plants. The lion鈥檚 share of this is in the Congo rainforest, the second largest in the world after the Amazon. Sometimes called the 鈥渓ungs of Africa鈥�, it absorbs an estimated 600 million tonnes of CO2 per year. However, that number has been falling as the rainforest is destroyed by logging and mining.

Now, researchers have found that after gaining biomass from 2007 to 2010, African forests lost 106 million tonnes of biomass per year from 2011 to 2017. That is equivalent to roughly 200 million tonnes of CO2 emissions per year. This was driven by deforestation in the Congo rainforest, says at the University of Leicester, UK.

鈥淚f we are losing the tropical forests as one of the means of mitigating climate change, then we basically have to reduce our emissions of greenhouse gasses from fossil fuel burning even faster to get to near-zero emissions,鈥� he says.

Balzter and his colleagues estimated the amount of biomass with satellite measurements of the colour and moisture content of the forest canopy, as well as its height at certain points. They compared this with measurements taken on the ground, although these are sparse in Africa.

But at University College London says satellite data can鈥檛 detect the type of trees in a forest and isn鈥檛 reliable for estimating the carbon absorbed by high-biomass intact forests or emitted in forests degraded by selective logging. A dense hardwood like mahogany might hold more carbon than a light balsa wood of the same size, for instance.

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鈥淒eforestation in Democratic Republic of Congo鈥� is higher than it was in the 2000s. And we all know that,鈥� he says. 鈥淏ut whether that is enough to tip the whole carbon balance of the entire continent is unknown.鈥�

The study also didn鈥檛 include the wet peatlands found underneath much of the Congo rainforest, which absorb a small amount of CO2 each year and hold about 30 billion tonnes of ancient carbon.

The Amazon rainforest, which was also once a major carbon sink, emitted more CO2 than it absorbed in several recent years. But whereas deforestation in the Amazon has fallen under a government crackdown, it has been growing in the Congo.

In the Democratic Republic of the Congo, impoverished farmers often destroy rainforest for slash-and-burn agriculture. Companies, many of them foreign-owned, illegally log colourful hardwoods like African teak and coralwood.

At the COP30 climate summit in the Amazon this month, Brazil announced the Tropical Forests Forever Facility, a fund that will pay its investment returns to tropical countries at a rate of $4 per hectare of forest left standing. But so far countries have donated only $6.6 billion to the fund, far short of the $25 billion goal.

Balzter says this mechanism could be more effective than carbon credits, which reward 鈥渁voided鈥� emissions and have in many cases been found to be worthless.

鈥淚t鈥檚 really important to make this Tropical Forest Forever Facility work, and make it work quite quickly, to try and reverse this trend of the African tree biomass actually releasing carbon into the atmosphere,鈥� he says.

Atmospheric carbon dioxide levels jumped by a record amount in 2024 to push concentrations to their highest point since measurements began, the UN鈥檚 World Meteorological Organization (WMO) has reported.

Between 2023 and 2024, the global average concentration of CO2 surged by 3.5 parts per million (ppm) to reach 423.9 ppm, the WMO has said. This is the largest increase since modern measurements started in 1957 and is well in excess of the 2022 to 2023 increase of 2.3 ppm.

It marks the latest in a trend of accelerating annual increases, with growth rates tripling since the 1960s. The last time Earth experienced a comparable concentration of CO2 was 3 million to 5 million years ago.

Excess CO2 in the atmosphere will have a warming effect on the planet for centuries to come, the WMO warns. 鈥淭he heat trapped by CO2 and other greenhouse gases is turbocharging our climate and leading to more extreme weather,鈥� at the WMO said in a statement.

Ongoing emissions from fossil fuels, alongside a surge in emissions from wildfires and a slump in the carbon uptake by the world鈥檚 lands and oceans, were the key drivers of last year鈥檚 record surge, according to the WMO.

Researchers expected a slump in the uptake of carbon by oceans, forests and other ecosystems in 2024 due to the recent El Ni帽o weather pattern, which pushed up global temperatures and dulled carbon absorption by driving heat, fires and drought in key regions like the Amazon. The amount of tropical forest lost in 2024 was double that of 2023, scientists noted earlier this year. 鈥淚t is normal for some tropical lands to be drier and store less carbon during warm El Ni帽o years such as 2024,鈥� says at the University of Reading, UK.

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But there is concern that this dip in carbon uptake by the planet 鈥� particularly by the land 鈥� is part of a longer-term trend that could mean climate change is weakening the planet鈥檚 ability to soak up excess carbon.

鈥淭here has been some suggestions that the land sink was particularly low in 2023 and 2024, even for El Ni帽o years, and that there has been a worrying reduction over time, particularly in the northern hemisphere outside the tropics,鈥� says r, a climate scientist at the Breakthrough Institute. 鈥淚n short, there are worrying signs that the land sink in particular is declining, but it鈥檚 too early to know with confidence without another few years of data.鈥�

In the meantime, it is more urgent than ever for humanity to curb ongoing fossil fuel emissions, says at the University of Leeds, UK. 鈥淭he biggest reason for the ongoing increase [in CO2 concentrations] is fossil fuel emissions being at a persistent all-time high and not yet coming down.鈥�