Bold claim: the Amazon rainforest is on a path toward a hypertropical climate that could redefine whether trees can survive this era. New research suggests this regime—unknown on Earth for at least 10 million years—could bring hotter, drier spells that intensify and extend droughts, potentially triggering widespread tree die-offs. By 2100, hot droughts could push the Amazon to endure around 150 days of extreme heat and dryness, even intruding into the wet season, according to a study published in Nature.
Historical context shows hypertropical climates last appeared roughly between 40 and 10 million years ago, during the Eocene and Miocene. Back then, global temperatures averaged around 28°C (82°F)—about 14°C warmer than today—while forests near the equator tended to have fewer mangroves and evergreen trees. Today, the Amazon already faces hot drought conditions for portions of the year, but climate change is stretching its dry season (typically July through September) and increasing the share of days that are hotter than average.
To explore this, researchers led by Chambers analyzed three decades of data from a forest patch north of Manaus, Brazil, including temperature, humidity, soil moisture, and sunlight. They also used sensors tracking water and sap flow inside tree trunks to understand how trees cope with drought. During droughts, trees struggle to access water and limit CO2 uptake, as evaporation drives soil moisture down. Trees respond by closing leaf pores (stomata) to conserve water, but this also throttles CO2 intake, constraining growth and repair.
When droughts become extreme, trees can die from CO2 starvation. Additionally, once soil moisture falls below about one-third saturation (roughly 33%), trees develop embolisms—gas bubbles in the xylem that disrupt water transport. If enough embolisms form, a tree can die. The researchers found a remarkably consistent soil-moisture threshold triggering embolisms across two El Niño years (2015 and 2023) and aligned with another Amazon study site, a finding that surprised the team.
Current Amazon tree mortality sits just above 1% annually; under future scenarios, it could rise to about 1.55% by 2100. While that increase may seem small, it translates into a substantial cumulative effect when spread across the entire rainforest. Fast-growing species appear more vulnerable to hot droughts because their growth relies on abundant water and CO2. This could tilt the forest composition toward slower-growing trees like yellow ipê (Handroanthus chrysanthus) and Shihuahuaco (Dipteryx micrantha), assuming they can tolerate increasing water stress and faster temperature shifts.
The study also raises the possibility that other tropical rainforests—such as those in western Africa and Southeast Asia—could be moving toward this hypertropical regime, with profound consequences for the global carbon cycle. Rainforests currently store vast amounts of CO2, acting as crucial carbon sinks; a shift toward hypertropical conditions could alter how much CO2 is absorbed from the atmosphere.
These projections assume CO2 emissions remain high. In other words, if humanity reduces greenhouse-gas output, the timing and scale of a hypertropical shift could slow or even be averted. As Chambers notes, continuing to emit greenhouse gases without constraint makes this climate transition more likely—and sooner.
In brief, scientists warn that the Amazon and potentially other tropical forests may be approaching a tipping point where hotter, drier conditions alter tree growth, composition, and carbon storage. The question now is whether actions to curb emissions can prevent or delay this hypertropical shift, or whether rapid systemic changes in forest dynamics will proceed regardless. Do you think aggressive emission reductions can meaningfully delay such a regime shift, or will ecological changes outpace policy efforts?
