Humans Have Never Lived in CO2 Concentrations Seen in Earth's Atmosphere Today: 'An Experiment on Ourselves'

As we continue to burn vast amounts of fossil fuels, a group of scientists has said that humans have never lived on Earth with such high concentrations of carbon dioxide in the atmosphere, in what they describe as an experiment "on ourselves."

For a study published in the journal Nature Communications, a team from Texas A&M University and Nanjing University, China, has shown that for the entire Pleistocene era (2.58 million to 11,700 years ago)—during which the majority of evolution in our group of species, Homo, took place—the concentrations of carbon dioxide in the atmosphere averaged about 250 parts per million.

This stands in stark contrast to concentrations of the greenhouse gas today measured in the atmosphere today, which in May this year were measured at highs of nearly 415 parts per million at the National Oceanic and Atmospheric Administration's Mauna Loa Atmospheric Baseline Observatory in Hawaii.

This measurement was the highest seasonal peak in observations taken at the site since 1961, marking seven consecutive years of significant global increases in carbon dioxide (CO2) concentrations. The potent greenhouse gas traps heat, thus causing the planet to warm.

In 1965, by comparison, carbon dioxide concentrations in the atmosphere stood at around 320 parts per million, levels which have not occurred in the past 2.5 million years.

"According to this research, from the first Homo erectus, which is currently dated to 2.1 to 1.8 million years ago, until 1965, we have lived in a low-carbon dioxide environment—concentrations were less than 320 parts per million," Yige Zhang, co-author of the study from A&M, said in a statement. "So this current high-carbon dioxide environment is not only an experiment for the climate and the environment—it's also an experiment for us, for ourselves."

"It's important to study atmospheric CO2 concentrations in the geological past, because we know that there are already climate consequences and are going to be more climate consequences, and one way to learn about those is to look into Earth's history," Zhang said. "Then we can see what kind of CO2 levels did we have, what did the climate look like, and what was the relationship between them."

Given that modern technology for measuring carbon dioxide has only existed for the past few decades, scientists often analyze cores of ice to study concentrations of the gas in the atmosphere further back in the past.

However, these ice cores can only provide records going back 800,000 years, according to Zhang. So to get a picture of the situation before, the team analyzed ancient carbonated soil from the Loess Plateau in central China—where researchers have identified dust that's around 22 million years old.

"Soil, or the parent material for soil, in this case, dust, naturally contains carbonates," Zhang told Newsweek. "During soil formation process, rainwater dissolves carbonates and then re-precipitates them."

This enabled the team to reconstruct carbon dioxide concentrations for the entirety of the Pleistocene era, with the findings agreeing with previous observations of ice cores.

"Our reconstructions show that for the entire Pleistocene period, carbon dioxide averaged around 250 parts per million, which is the same as the last 800,000 years' values," Zhang said.

"There are previous estimates of Pleistocene CO2 using other, mostly marine-based proxies," he told Newsweek. "However, we used a refined terrestrial carbonate proxy to suggest that the Pleistocene CO2 might be lower than previously thought."

The earliest member of the genus Homo is Homo habilis, which first appeared around 2.8 million years ago. Other human species, such as Homo erectus, appeared a few hundred thousand years later, before modern humans (Homo sapiens) finally emerged between around 350,000 and 250,000 years ago in Africa.

According to Zhang, these human species evolved in a low-carbon dioxide environment, so it is unclear how modern humans will adapt to the high concentrations we experience today.

This article was updated to include additional comments from Yige Zhang.

Loess Plateau, China
Analyzing ancient soils from the Loess Plateau, scientists reconstructed the Earth’s carbon dioxide levels. Dr. Yige Zhang