Coral reefs may be losing the race against rising seas

We tend to think of the end of the last ice age as a slow, gentle thaw. Glaciers eased back over many centuries, and the world warmed by degrees too small to feel in a single lifetime.

A broken stalagmite from a tourist cave in Brazil has now put real numbers on that thaw. Sealed in its layers is a precise record of how fast this corner of the world warmed – and how that pace compares with the heat we are driving today.

The cave is called Rei do Mato, tucked into the hills of Minas Gerais in southeastern Brazil. Tourists walk a path through its main hall, past stone columns built by centuries of dripping water.

In 2009, a research team found one of those columns snapped off, and lying in a pile of rubble near the walkway. Angela Ampuero, a paleoclimate researcher at the University of São Paulo (USP), led the effort to read it.

Its layers spanned a remarkable stretch – from about 22,500 to 9,300 years ago. That window runs from the depths of the last ice age into the warm, steady world we still live in.

Most ancient temperature records lean on indirect clues, and in the tropics, the error bars often top 3°F (1.7°C). Ampuero’s team wanted something better: something more direct – the water itself.

As a stalagmite grows, it traps microscopic droplets of its own drip water. These sealed pockets, called fluid inclusions, hold real water from the day each layer formed, and its density depends on the cave’s temperature then.

Reading it required a clever trick. A laser pulse sparked a tiny vapor bubble inside a single droplet, and the team warmed the sample until the bubble vanished – pinning the ancient temperature to within about half a degree.

Across the full record, the cave warmed by about 10 degrees Fahrenheit. It rose from roughly 58 degrees in the coldest depths of the ice age to about 68 degrees by the early Holocene, the mild world that followed.

Much of that lined up with expectations. The warming closely tracked rising carbon dioxide and mirrored the swings recorded far away in Antarctica – a link other tropical records had hinted at.

The warming came in two steps, not one. A brief cooling of about 1ºF (0.6°C) split the warming in half. The same dip shows up in glacier and lake records high in the Andes.

The most dramatic moment came near the end. Around 13,000 years ago, the cave heated by more than 2°F (1.1°C) in barely two centuries. It was the quickest burst in the entire record.

What makes it strange is the timing. The jump landed at the start of the Younger Dryas, a cold snap dragging the Northern Hemisphere back toward the ice. As the north froze, this patch of South America was heating fast.

The likely culprit was the Atlantic ocean. When the Atlantic’s main heat-carrying current – a system known as AMOC – weakens, heat stops flowing north and pools in the South Atlantic instead. Researchers had long suspected this kind of seesaw. Rarely had they caught it so sharply on land.

Warmer and wetter usually travel together in the tropics, so the team checked whether rainfall was steering the temperature. The same stalagmite tracks how wet each era was, recorded in the chemistry of its layers.

But the two signals refused to line up. Warming kept rising long after the big rain events had faded. The jump at the Younger Dryas – during a wet spell, not a dry one.

That split changes how these records get read, with heat and rainfall on separate tracks. A study of a cave in Borneo, using the same method, found the very same surprise.

To untangle the forces, the team ran their record against a climate model, testing one influence at a time. The fit was close enough to trust what the model showed.

Carbon dioxide did the heavy lifting over the long term. Rising CO2 set the slow, steady upward march of temperature across thousands of years – the same engine warming the planet now.

The ocean current handled the shorter, sharper swings, above all that abrupt jump at the Younger Dryas.

Two forces worked on different clocks – slow greenhouse warming underneath, with faster lurches each time the Atlantic faltered.

The fastest warming this cave ever recorded came at the Younger Dryas. Even that abrupt spike unfolded close to three times slower than the warming this region has experienced since 1980. A few human decades outpaced nature at its most violent.

The warming is not finished. Because the climate takes time to catch up with the carbon dioxide already in the air, the region is locked into more warming ahead, even before anyone adds more.

That hits home here. The cave sits in the Cerrado, a tropical savanna already strained by heat, where recent research found the current drought is the worst in over 700 years. For a region with almost no precise climate history, the stalagmite now marks how unusual today’s pace really is.

Like what you read? Subscribe to our newsletter for engaging articles, exclusive content, and the latest updates.

Check us out on EarthSnap , a free app brought to you by Eric Ralls and Earth.com.