Past and future carbon dioxide levels

The last time atmospheric carbon dioxide (CO2) was this high humans didn’t exist. 

It is also true that our Planet has been warming and cooling naturally for millions of years and that natural increases in CO2 concentrations have warmed the Earth’s temperature over the past million years or more of ice age cycles. One way this happened was with slight variations in the Earth’s orbit around the sun, for example. These caused small increases in sunlight which led to the oceans getting warmer and to outgas CO2. The extra CO2 in the atmosphere helped amplify the solar-driven warming, resulting in a warm interglacial interlude.

The important thing to remember however is that the current warming is not natural and we need to go back some 3 million years to the Mid-Pliocene to find the last time atmospheric CO2 was so high. Back then global surface temperatures were 2.5 – 4°C warmer than during the pre-industrial era and sea levels were 5 – 25 m higher than in 1900. 

Atmospheric CO2 never exceeded 300 ppm in the past million years or so, according to ice core data. Before the Industrial Revolution, atmospheric CO2 was 280 ppm or less and the Earth’s average temperature was relatively stable, but human activities have increased greenhouse gases, upsetting the balance. By 1958, when continuous CO2 observations began, atmospheric CO2 had already reached 315 ppm. More worryingly still, the increase in atmospheric CO2 in the last 60 years is 100 times faster than previous natural increases. 

In 2023, the world saw a new record high of 419.3 ppm for the average atmospheric CO2 level, according to NOAA’s Global Monitoring Laboratory. 

If we continue to meet our increasing global energy demands by burning fossil fuels and woody biomass, human emissions of CO2 could increase from 40 billion currently to 75 billion tonnes per year by the end of the century. Atmospheric CO2 could then top 800 ppm, conditions not seen on Earth for close to 50 million years.