Climate Change: Atmospheric Carbon Dioxide

October 17, 2017

The global average atmospheric carbon dioxide in 2016 was 402.9 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.

Atmospheric carbon dioxide concentrations in parts per million (ppm) for the past 800,000 years, based on EPICA (ice core) data. The peaks and valleys in carbon dioxide levels track the coming and going of ice ages (low carbon dioxide) and warmer interglacials (higher levels). Throughout these cycles, atmospheric carbon dioxide  was never higher than 300 ppm; in 2016, it reached 402.9 ppm (black dot). NOAA, based on EPICA Dome C data (Lüthi, D., et al., 2008) provided by NOAA NCEI Paleoclimatology Program.

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 2016 report from NOAA and the American Meteorological Society, the amount by which atmospheric carbon dioxide increases each year has roughly quadrupled since the 1960s:

The global growth rate of atmospheric CO2 has risen from 0.6 ± 0.1 ppm per year in the early 1960s to an average of 2.3 ± 0.6 ppm per year during the past ten years. However, the increase in global annual mean CO2 from 2015 to 2016 was 3.5 ± 0.1 ppm (0.88% ± 0.03%), the largest annual increase observed in the 58-year atmospheric measurement record.

The annual rate of increase in atmospheric carbon dioxide over the past 60 years are 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, 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.

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.

time series graph showing contribution of different greenhouse gases to Earth's heating imbalance

(left vertical axis) The heating imbalance in watts per square meter relative to the year 1750 caused by all major human-produced greenhouse gases: carbon dioxide, methane, nitrous oxide, chlorofluorocarbons 11 and 12, and a group of 15 other minor contributors. Today's atmosphere absorbs about 3 extra watts of incoming solar energy over each square meter of Earth's surface. According to NOAA's Annual Greenhouse Gas Index (right axis) the combined heating influence of all major greenhouse gases has increased by 40% relative to 1990. NOAA graph, based on data from NOAA ESRL. 

Carbon dioxide also 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.

(left) A healthy ocean snail has a transparent shell with smoothly contoured ridges. (right) A shell exposed to more acidic, corrosive waters is cloudy, ragged, and pockmarked with ‘kinks’ and weak spots. Photos courtesy Nina Bednarsek, NOAA PMEL. 

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. 

carbon dioxide graph

Monthly global carbon dioxide in parts per million from 1980 through 2016.  A long-term increase is superimposed on the natural, seasonal ups and downs. NOAA graph, based on data from NOAA ESRL. Photo of Cholla power plant in Arizona, by John Fowler, used under a CC license. 

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


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