Some tropical land regions may warm more dramatically than previously predicted, as climate change progresses, according to a new CU Boulder study that looks millions of years into Earth's past.
Using lake sediments from the Colombian Andes, researchers revealed that when the planet warmed millions of years ago under carbon dioxide levels similar to today's, tropical land heated up nearly twice as much as the ocean.
The study was published February 2 in the Proceedings of the National Academy of Sciences.
"The tropics are home to about 40% of the world's population, yet we've had very little direct evidence of how tropical land temperatures respond to climate change," said lead author Lina Pérez-Angel, who conducted the study as a doctoral student at CU Boulder's Institute of Arctic and Alpine Research (INSTAAR) and the Department of Geological Sciences. "If we want to study climate change to help people, we need to pay more attention to the regional changes so those living there know what to expect."
Climate archive from sediments
About 2.5 to 5 million years ago, giant sloths still roamed Earth. The planet was on average 2.5 to 4 °C (4.5 to 7.2°F) warmer than today, and Greenland was largely ice-free.
This period, known as Pliocene, was the last time Earth had carbon dioxide levels similar to what they are today. As such, it is one of the best analogs for what would happen if Earth's temperatures continued to rise.
Sediment cores are one of the main tools scientists use to reconstruct Earth's past climate. As sediments slowly accumulate layer by layer, they trap chemical signals, fossils and minerals that reflect temperature, rainfall and atmospheric conditions at the time they were deposited. By drilling and extracting a column of these sediments, scientists can retrace past climate.
Most of what scientists know about Earth's ancient temperatures comes from ocean cores. This is because sediments on the seafloor build up slowly and remain largely undisturbed, whereas on land, rapid landscape changes from erosion, landslides, shifting rivers and mountain building often scramble older sediments, making continuous records hard to come by.
In 1988, a team of Dutch and Colombian scientists retrieved an impressive 580-meter (1,902 feet) long sediment core from the Bogotá basin in Colombia. Pérez-Angel grew up in the region, located at nearly 2,550 meters above sea level in the Andes. The lush high-plain basin is home to Colombia's capital, Bogotá, South America's second most populous city with about 11 million people.
Formed millions of years ago, the basin has preserved sediment continuously and largely undisturbed since the late Pliocene.
For the study, Pérez-Angel, senior author Julio Sepúlveda, associate professor in the Department of Geological Science, and their team analyzed a type of fat in bacteria preserved in the core. This enabled them to reconstruct a temperature record of the region from the Pliocene to the Pleistocene, or Ice Age.
They found that compared to the Holocene, which is the current epoch, this land region of the tropical Andes was about 3.7 °C (6.6°F) warmer than today, whereas the tropical sea surface was only 1.9 °C (3.4°F) warmer. This means that land temperatures in the tropics changed about 1.6 to nearly 2 times more than the tropical ocean.
Feedback loop
Pérez-Angel, now a senior research associate at Brown University, said that the Pacific Ocean had a nearly permanent El Niño condition during the late Pliocene, which in turn heated up the tropical Andes even more.
Modern El Niño events have already caused significant warming and drought in the northern Andes. The team warned the area could experience additional warming with El Niño potentially happening more frequently due to climate change.
"If you compare the temperature records for the past couple of decades with what climate models predicted a few decades ago, you see that all the real-world data is at the uppermost end of those predictions," said Sepúlveda, who is also a fellow at INSTAAR. "This is partly because there are so many feedback mechanisms in nature, and crossing certain thresholds could trigger a series of cascading events that amplify changes."
Overlooked land
The tropics don't get as much attention as other regions in climate science, Pérez-Angel said, partly because most of the leading institutions studying climate change are located in middle and high latitude areas, like North America and Europe. The tropics are also not warming as fast as colder regions like Greenland or Antarctica.
But in a region where temperatures are already very high, any increase could push it beyond the threshold of what people and wildlife can tolerate.
"When we model climate change, we tend to focus on how temperatures are going to change globally. But people experience climate change at the regional level," Pérez-Angel said. With only two high-income countries across the entire tropics, many communities have limited resources to adapt to climate change.
"Understanding what the future might look like for people, ecosystems and the land they depend on is very important for building resilience at a regional level," she said.
