The global average atmospheric carbon dioxide in 2018 was 407.4 parts per million (ppm for short), with a range of uncertainty of plus or minus 0.1 ppm. Carbon dioxide levels today are higher than at any point in at least the past 800,000 years.

In fact, the last time the atmospheric CO₂ amounts were this high was more than 3 million years ago, when temperature was 2°–3°C (3.6°–5.4°F) higher than during the pre-industrial era, and sea level was 15–25 meters (50–80 feet) higher than today.

Carbon dioxide concentrations are rising mostly because of the fossil fuels that people are burning for energy. Fossil fuels like coal and oil contain carbon that plants pulled out of the atmosphere through photosynthesis over the span of many millions of years; we are returning that carbon to the atmosphere in just a few hundred years. 

Squeeze or stretch the graph in either direction by holding the Shift key while you click and drag. The bright red line (source data) shows monthly average carbon dioxide at NOAA's Mauna Loa Observatory on Hawai'i in parts per million (ppm): the number of carbon dioxide molecules per million molecules of dry air.  Over the course of the year, values are higher in Northern Hemisphere winter and lower in summer. The dark red line shows the annual trend, calculated as a 12-month rolling average.

According to the State of the Climate in 2018 report from NOAA and the American Meteorological Society, global atmospheric carbon dioxide was 407.4 ± 0.1 ppm in 2018, a new record high. That is an increase of 2.5 ± 0.1 ppm from 2017, similar to the increase of 2.2 ± 0.1 ppm between 2016 and 2017. 

In the 1960s, the global growth rate of atmospheric carbon dioxide was roughly 0.6 ± 0.1 ppm per year. Over the past decade, however, the growth rate has been closer to 2.3 ppm per year. The annual rate of increase in atmospheric carbon dioxide over the past 60 years is about 100 times faster than previous natural increases, such as those that occurred at the end of the last ice age 11,000-17,000 years ago.  

Why carbon dioxide matters

Carbon dioxide is a greenhouse gas: a gas that absorbs heat. Warmed by sunlight, Earth’s land and ocean surfaces continuously radiate thermal infrared energy (heat). Unlike oxygen or nitrogen (which make up most of our atmosphere), greenhouse gases absorb that heat and release it gradually over time, like bricks in a fireplace after the fire goes out. Without this natural greenhouse effect, Earth’s average annual temperature would be below freezing instead of close to 60°F. But increases in greenhouse gases have tipped the Earth's energy budget out of balance, trapping additional heat and raising Earth's average temperature. 

Carbon dioxide is the most important of Earth’s long-lived greenhouse gases. It absorbs less heat per molecule than the greenhouse gases methane or nitrous oxide, but it’s more abundant and it stays in the atmosphere much longer. And while carbon dioxide is less abundant and less powerful than water vapor on a molecule per molecule basis, it absorbs wavelengths of thermal energy that water vapor does not, which means it adds to the greenhouse effect in a unique way. Increases in atmospheric carbon dioxide are responsible for about two-thirds of the total energy imbalance that is causing Earth's temperature to rise.

Another reason carbon dioxide is important in the Earth system is that it dissolves into the ocean like the fizz in a can of soda. It reacts with water molecules, producing carbonic acid and lowering the ocean's pH. Since the start of the Industrial Revolution, the pH of the ocean's surface waters has dropped from 8.21 to 8.10. This drop in pH is called ocean acidification.  

A drop of 0.1 may not seem like a lot, but the pH scale is logarithmic; a 1-unit drop in pH means a tenfold increase in acidity. A change of 0.1 means a roughly 30% increase in acidity. Increasing acidity interferes with the ability of marine life to extract calcium from the water to build their shells and skeletons.

Past and future carbon dioxide

Natural increases in carbon dioxide concentrations have periodically warmed Earth’s temperature during ice age cycles over the past million years or more. The warm episodes (interglacials) began with a small increase in sunlight due to a tiny wobble in Earth’s axis of rotation or in the path of its orbit around the Sun.

That little bit of extra sunlight caused a little bit of warming. As the oceans warmed, they outgassed carbon dioxide—like a can of soda going flat in the heat of a summer day. The extra carbon dioxide in the atmosphere amplified the initial warming.

Based on air bubbles trapped in mile-thick ice cores (and other paleoclimate evidence), we know that during the ice age cycles of the past million years or so, carbon dioxide never exceeded 300 ppm. Before the Industrial Revolution started in the mid-1700s, the global average amount of carbon dioxide was about 280 ppm.

By the time continuous observations began at Mauna Loa Volcanic Observatory in 1958, global atmospheric carbon dioxide was already 315 ppm. On May 9, 2013, the daily average carbon dioxide measured at Mauna Loa surpassed 400 ppm for the first time on record. Less than two years later, in 2015, the global amount went over 400 ppm for the first time. If global energy demand continues to grow and to be met mostly with fossil fuels, atmospheric carbon dioxide will likely exceed 900 ppm by the end of this century.

More on carbon dioxide

NOAA carbon dioxide observations

Carbon cycle factsheet

Carbon dioxide emissions by country over time

Comparing greenhouse gases by their global warming potential

References

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Dlugokencky, E.J., Hall, B.D., Montzka, S.A., Dutton, G., Mühle, J., Elkins, J.W. (2019). Atmospheric composition [in State of the Climate in 2018, Chapter 2: Global Climate]. Bulletin of the American Meteorological Society, 100(9), S48–S50.

Lüthi, D., M. Le Floch, B. Bereiter, T. Blunier, J.-M. Barnola, U. Siegenthaler, D. Raynaud, J. Jouzel, H. Fischer, K. Kawamura, and T.F. Stocker. (2008). High-resolution carbon dioxide concentration record 650,000-800,000 years before present. Nature, Vol. 453, pp. 379-382. doi:10.1038/nature06949.

Woods Hole Oceanographic Institution. (2015). Introduction to ocean acidification. Accessed October 4, 2017.

Lindsey, R. (2009). Climate and Earth’s energy budget. Accessed October 4, 2017.