06/07/2022
By Brooke Coupal

Understanding how climate changed in the distant past is helping researchers forecast its future.

“People are always asking, ‘Why do you care about the climate from thousands of years ago?’ But really, it helps us contextualize the changes that we’re seeing with the climate right now. We can get a sense of what we can expect to see in the future,” says Environmental Earth and Atmospheric Sciences Asst. Prof. Christopher Skinner. “It’s super important to know the past to project the future.”

Skinner and a team of researchers from across the country investigated how the temperature on Earth changed over the past 10,000 years and discovered that vegetation played a major role.

“About 6,000 years ago, we had a very different distribution of plants on the Earth, and because of that, it was 1.5 degrees Fahrenheit warmer than it is today,” says Skinner, who co-authored a paper published in Science Advances on the team’s findings.

Earth’s orbit shifted during that time period, which increased the amount of energy the planet was getting from the sun. This led to vegetation growth in the Northern Hemisphere, including in the Sahara Desert and the edge of the Arctic. 

“It was wildly different. We have a lot of evidence that it was a lot greener at that time,” Skinner says.

Plants absorb a lot of the sun’s energy due to their dark green color, so when there was an abundance of vegetation, Earth absorbed more solar energy, and that caused the temperature to rise.

Earth’s orbit changes in three distinct ways — through eccentricity (shape of Earth’s orbit around the sun), precession (the direction that Earth’s axis points as it moves around the sun) and obliquity (angle of Earth’s tilt on its axis) — and all three changes need to align to repeat what happened 6,000 years ago. Skinner says the planet is thousands of years away from that occurring again; however, warmer temperatures are projected in the future due to our fossil fuel use, and this will impact the distribution of vegetation on Earth.

“Plants can be very sensitive to the climate, and so as we change the climate with increased greenhouse gases from fossil fuel use, the distribution and characteristics of plants are going to start changing, and that change in plants is going to then have an influence on our climate,” he says. “This is a pretty big deal.”

Skinner adds that it’s important for researchers to take changes in vegetation into account when making predictions about future climates.

“If we’re thinking about climate change and we’re trying to project what’s going to happen, the way we do that is through climate models, and if the models don’t include these changes in vegetation, then our projections are going to be wrong,” he says.

The ENSO Impact

Another factor Skinner says is important to consider when making climate projections is the El Niño-Southern Oscillation (ENSO), which is a recurring pattern of changes in water temperature in the Pacific Ocean. Skinner and a team of researchers looked at historical records and identified ENSO as the main driver of concurrent droughts.

These droughts, which impact multiple regions at the same time, can be detrimental to the Earth’s socioeconomic systems, especially the agricultural sector. The researchers’ findings, which were published in the journal Nature Climate Change, point to the challenges ahead, with concurrent droughts expected to increase throughout the century.

“Drought in one region can be really bad for the people that live there, but hopefully, our global agricultural system is set up such that if there’s a drought in one region that impacts food production, we can make up for that by increasing exports from other regions that grow the same crops,” Skinner says. “But that system sort of fails when multiple regions experience drought at the same time. We don’t necessarily have the support system to handle that.”

Scientists can predict ENSO with some regularity, and that can allow industries to plan accordingly for impending concurrent droughts.

“Generally, several months in advance, you have an idea of whether you’re going to have a particular pattern of sea temperatures in the Pacific Ocean, so if we know that a certain phase of ENSO is coming, we have a better sense of whether or not there’s going to be a concurrent drought in the preceding months,” Skinner says.

Incidents of ENSO are projected to become more likely in the future as the planet gets warmer due to increased fossil fuel use. According to NASA, Earth’s average surface temperature has risen by about 2 degrees Fahrenheit since the late 19th century, an increase caused largely by the rise in carbon dioxide emissions and other human activities.

“If we want to stop these concurrent drought events from happening, it’s pretty clear that the best way to do that is to end our reliance on fossil fuels,” says Skinner. “We need to take action immediately.”