The concentration of greenhouse gas will soon be higher than it has ever been in the past millions of years. But what does that mean for the future?
The way to the secrets of the earth’s history often leads to the sea floor. There are fossils from bygone times in the rock layers – a treasure for geologists like Gavin Foster from the University of Southampton. He and his team examined fossilized zooplankton from the Caribbean sea floor, about the size of a pinhead, in order to draw conclusions about the CO₂ content in the atmosphere millions of years ago – and to compare it with today’s values.
Because the acidification of the oceans is closely related to the CO₂ in the air. Because carbon dioxide reacts with water to form carbonic acid, the oceans absorb a large part of the greenhouse gas from the atmosphere. Foster and his colleagues used this relationship between the pH value and CO₂ in the atmosphere in their studies of fossilized plankton. The boron content in its shells told them how high the pH was in the water when the tiny organisms drifted through the oceans. Based on the acidity, the scientists were then able to infer the CO₂ content in the air at that time.
The results sound worrying: By 2025 there will probably be as much CO₂ in the atmosphere as there has not been for around 3.3 million years , the scientists report in the journal Scientific Reports . In that period, the Pliocene, it was significantly warmer than today, an average of two to four degrees. Trees grew in Antarctica, gazelles jumped across Europe, and the sea level was 15 to 25 meters higher.
“There’s no blueprint for what’s happening. We’re playing with fire.”
When the CO₂ content soon leaves the maximum values from the Pliocene behind it, it will move towards a new phase in geological history: the Miocene, which dates back to 23 million years. But its CO₂ maxima could also be cracked over the coming decades if humanity continues to emit as much carbon dioxide as it does at present.
Climate change deniers like to use such historical CO₂ highs as an argument that climate change is not a real threat. The earth and its inhabitants would have survived the CO₂ concentration at that time, they say. But what do such comparisons say? And if there was already as much CO₂ in the atmosphere as there is today, why did the CO₂ content decrease again?
Mojib Latif from Geomar in Kiel is one of the most renowned German climate researchers. The CO₂ record reports worry him. “However, historical maximum CO₂ values can only be compared with the current situation to a limited extent,” says the chairman of the German Climate Consortium. What Latif means: Before humans started building machines and cars, the CO₂ content in the atmosphere changed much more slowly than since the beginning of industrialization. Fluctuations stretched over tens to hundreds of thousands of years. Changes in the earth’s orbit and axis alternately resulted in more and less solar radiation.
As a result, temperatures rose and fell, as did the CO₂ concentration in the atmosphere. In the current climate change, however, these factors – scientists speak of the Milanković cycles – play no role. The period in which we humans drove up the CO₂ values is far too short. “There is no blueprint for what is happening,” says Latif. That is exactly what makes climate change so dangerous. “We’re playing with fire.”
Nevertheless, a lot can be learned from the past warm and cold periods, says Latif. Because even if the causes were different back then – the consequences are, at least in principle, the same today. However, it also applies here that everything is happening much faster at the moment, emphasizes Latif.
“Knowledge of CO₂ levels in the past shows us how the climate system, the ice caps at the poles and the sea level have reacted to it,” says Elwyn de la Vega, one of the researchers on Foster’s team. After the last glacial period, for example, the sea level rose over a very long period of time, but by no means continuously at a snail’s pace, but occasionally jerkily in short and violent spurts. “Using simulations, we are trying to find out exactly how that happened back then – and whether there could be similar attacks in the coming years as a result of climate change,” says Latif.
Gazelles are unlikely to be hopping across Europe anytime soon
As an ingredient for climate simulations, historical CO₂ values are quite useful – but completely unsuitable for general comparisons, because back then it is about many millennia and now about a few decades. In the past, living beings had enough time to adapt to increased temperatures and higher sea levels: over the course of several generations, animals moved, and evolution brought about new species.
The current, rapid climate change, however, leaves nature and humans hardly time to react – gazelles are unlikely to be hopping through Europe anytime soon. “Our civilization today only developed significantly after the last great warm periods,” says Georg Feulner from the Potsdam Institute for Climate Impact Research. “In earlier warm periods, for example, there were no cities near the coast that could have been endangered by rising sea levels.”
According to estimates by the United Nations, around ten percent of humanity lived in coastal regions in 2017 that are less than ten meters above sea level – if the water levels were to rise to the Pliocene level again, these areas would be significantly below sea level. According to Gavin Foster and his colleagues, the fact that coastal cities have not yet been flooded is solely due to the fact that it will take a while for the earth to fully react to the higher CO₂ content in the atmosphere. However, the consequences of man-made climate change are already evident today. Meteorologists recently reported that the past twelve months were among the warmest on record , with an average of almost 1.3 degrees Celsius warmer than before industrialization.