By 2025, atmospheric CO2 levels – a key contributor to global warming and climate change – are expected to be higher than when the planet was 3C warmer 3.3 millions of years ago. During a period known as the Pliocene epoch, Earth was hotter than today with smaller ice caps and higher sea levels. At its warmest, atmospheric CO2 levels during the Pliocene were between 380 and 420 parts per million.
Today’s CO2 levels are at a comparable 414ppm and levels are rising.
Dr Thomas Chalk, who co-authored the study, said: “Focusing on a past interval when the incoming insolation from the Sun was the same as today gives us a way to study how Earth responds to CO2 forcing.
“A striking result we’ve found is that the warmest part of the Pliocene had between 380 and 420 parts per million CO2 in the atmosphere.
“This is similar to today’s value of around 415 parts per million, showing that we are already at levels that in the past were associated with temperature and sea-level significantly higher than today.
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“Currently, our CO2 levels are rising at about 2.5 ppm per year, meaning that by 2025 we will have exceeded anything seen in the last 3.3 million years.”
The Southampton researchers analysed tiny fossils from the Caribbean Sea to reconstruct Earth’s ancient atmosphere.
Dr Elwyn de la Vega, who led the study, said: “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”.
By 2025 we will have exceeded anything seen in the last 3.3 million years
The CO2 levels were determined with the use of boron – an element present in the shells of zooplankton.
The zooplankton, known as foraminifera or forams, only measure half-a-millimetre in size but have accumulated in vast quantities across the seabed.
The composition of the boron in their shells depends on the acidity or pH levels of the water around them.
And the acidity of the water is largely dependant on atmospheric CO2.
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Professor Gavin Foster, who took part in the study, said: “The reason we don’t see Pliocene-like temperatures and sea-levels yet today is because it takes a while for Earth’s climate to fully equilibrate (catch up) to higher CO2 levels and, because of human emissions, CO2 levels are still climbing.
“Our results give us an idea of what is likely in store once the system reached equilibrium.”
According to Dr de la Vega, once the planet surpasses the Pliocene CO2 levels, future levels will likely be higher than anything experienced in the last 15 million years.
Such levels were not seen since the Middle Miocene Climatic Optimum when the planet was even warmer than the Pliocene.
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Climate change: Latest CO2 measurements are at 414 parts per million
According to the US space agency NASA, CO2 measurements taken in May 2020 show atmospheric CO2 is 414ppm.
Although natural events such as volcanic eruptions can raise CO2 levels, most of the greenhouse gas has been attributed to emissions from human activities.
In particular, the burning of fossil fuels, as well as deforestation, are big contributor.
Carbon dioxide is a greenhouse gas, meaning it traps heat from the Sun from escaping into space after it reaches our planet.
NASA said: “Life on Earth depends on energy coming from the Sun.
“About half the light reaching Earth’s atmosphere passes through the air and clouds to the surface, where it is absorbed and then radiated upward in the form of infrared heat.
“About 90 percent of this heat is then absorbed by the greenhouse gases and radiated back toward the surface.”
Scientists have been tracking the effects of global warming and climate change since the Industrial Revolution.