A research team from the University of Southampton has estimated the amount of carbon dioxide present during the warmest part of the Pliocene epoch about 3.3 million years ago. At the time, the seas were much higher and the polar caps were much smaller, with the planet being 3°C (5.4°F) warmer than it is today. The researchers believe that by 2025, there will be a higher amount of CO2 in the atmosphere than any time during the last 3.3 million years. The findings are published in Scientific Reports.
The team used microscopic fossilized zooplankton from deep ocean sediments from the Caribbean Sea to determine atmospheric CO2. The composition of their shell depends on the pH of seawater, which in turn is influenced by atmospheric CO2. The work focused on a period of 200,000 years between 3.35 and 3.15 million years ago, with a focus on 3.3 million years ago at the beginning of the mid-Piacenzian Warm Period, when the climate became slowly warmer until the mean global temperature was much higher than today. In doing so, the researchers were able to produce a picture of how carbon dioxide levels have changed every 3,000 to 6,000 years.
"Knowledge of CO2 during the geological past is of great interest because it tells us how the climate system, ice sheets and sea-level previously responded to the elevated CO2 levels. We studied this particular interval in unprecedented detail because it provides great contextual information for our current climate state," Dr Elwyn de la Vega, who led the study, said in a statement.
Dr de la Vega and colleagues established that during the warmest part of the period, carbon dioxide levels were between 380 and 420 parts per million (ppm) CO2 in the atmosphere. In May 2020, it crossed the 417 ppm threshold for the first time since records began.
The value changes constantly due to several factors, but the increasing trend is well established. Over the last decade, the level has increased by about 2.4 ppm per year. Even assuming a conservative value for the current level, it is clear that in five years’ time it will exceed a level unseen since the Pliocene.
"Having surpassed Pliocene levels of CO2 by 2025, future levels of CO2 are not likely to have been experienced on Earth at any time for the last 15 million years, since the Middle Miocene Climatic Optimum, a time of even greater warmth than the Pliocene," Concluded Dr de la Vega.
It is not just the amount of carbon dioxide in the atmosphere that concerns experts, it is also how quickly it has been released and the combined effects with other greenhouse gases such as methane.