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Carbon Dioxide Levels

Carbon dioxide is a primary driver of climate change because of its heat-trapping properties. CO2 levels have risen by over 100 PPM than the hottest period over the last 800,000 years. Scientists use ice cores and basic physics to measure what CO2 levels were and how they affect the climate.

Key Takeaways


Carbon dioxide levels have reached an 800,000+ year high

Over many ice age and warmer periods, global carbon dioxide levels have not reached where they are today in at least the last 800,000 years.


Humans are the cause

The record levels of carbon dioxide are not natural. Thousands of independent studies have confirmed that humans release carbon dioxide when we burn fossil fuels, which is the cause for the high levels of CO2.

What We Know

Carbon dioxide levels have reached record highs in the past fifty years and the cause is human release of greenhouse gas emissions. Scientists have linked carbon dioxide to combusting (burning) fossil fuels for two hundred years. It is clear to scientists that the increase in carbon dioxide is directly caused by human activities.

Carbon dioxide is a greenhouse gas, meaning that it traps heat in the atmosphere. Although this “greenhouse effect” is a natural part of the earth, too much carbon dioxide is leading to warmer temperatures. Scientists have been measuring carbon dioxide directly since 1958. The interactive graph below, with data from NOAA, shows carbon dioxide levels from June 1958 to June 2021.

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As you can see, since 1958, there has been an increase of over 100 PPM (the unit used to measure CO2, meaning how many molecules of carbon dioxide per million molecules) to almost 417 PPM. To put this number in perspective, over the past 800,000 years, CO2 levels have never gone above 300. This period includes both glacial periods (ice ages) and interglacial periods (warm periods). That means that our climate is warmer and has more greenhouse gases than these warm periods.

NOAA graph showing that through multiple ice age cycles, global carbon dioxide levels have never reached the point they are at today.
Through multiple ice age cycles, global carbon dioxide levels have never reached the point they are at today.

How We Know

Scientists use basic physics and chemistry to determine that fossil fuels have caused the increase in carbon dioxide emissions. Fossil fuels are carbon-based and when they burn, the carbon combines with the oxygen in the air to form carbon dioxide. Scientists have also determined that fossil fuels are the leading cause of greenhouse gas emissions through complicated means. The IPCC (the leading international body on climate change) and other scientific groups use climate modeling to analyze what drives climate change. In 2013, the IPCC concluded that humans are the “dominant cause of the observed warming.” In other words, humans are the leading cause of increases in carbon dioxide emissions.

To measure carbon dioxide levels before 1958, scientists analyze ice cores. Ice cores are pieces of ancient glaciers that have trapped remnants of history in layers. These ice cores tell scientists how the earth's climate has changed over hundreds of thousands of years — long before humans were on the planet. Scientists use these ice cores to measure ancient, carbon dioxide levels, temperatures, and more. These measurements form the basis of modern climate analysis.


IPCC. “Chapter 1: Framing, Context, and Methods.” In Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change, edited by V. Masson- Delmotte, P. Zhai, A. Pirani, S. L. Connors, C. Péan, S. Berger, and N. Caud, Vol. 1. Cambridge University Press, 2021.

Rebecca Lindsey. “Climate Change: Atmospheric Carbon Dioxide.” NOAA Accessed August 22, 2021.

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