How can such a small amount of carbon dioxide in the atmosphere—only around 420 parts per million—cause so much warming?

The small fraction of CO₂ in the atmosphere can be misleading: consider instead the mind-boggling amount of CO₂ we’ve added.
 

June 12, 2023

In June 2023, the National Oceanic and Atmospheric Administration announced that measurements of carbon dioxide (CO₂) at their Mauna Loa Atmospheric Baseline Observatory had reached 424 parts per million (ppm).¹
 
This means that of every one million molecules in the atmosphere, 424 are CO₂. It can be hard to imagine how a chemical compound that makes up such a small fraction of the atmosphere—less than 0.05%—can be responsible for so much global warming. Yet focusing on the fraction of CO₂ in the atmosphere can blind us to just how big a change this represents.
 
First, we are more perceptive to the effects of small fractions than you might think. Consider your daily cup of coffee. Its power to raise your alertness, energy and heart rate comes from caffeine, which, by coincidence, is present at around 400 ppm. As NASA, who made the analogy, put it: “Small amounts of powerful substances have big effects.”
 
There are many aspects of CO₂ that make it a “powerful substance” that’s particularly good at trapping heat, as we’ve explored at Ask MIT Climate before.
 
Second, according to Jesse Kroll, professor of civil and environmental engineering and chemical engineering at MIT, more important than the current fraction of CO₂ in the atmosphere is the change that has taken place since the Industrial Revolution.
 
“In 1750, that number was actually 280,” Kroll explains. “So we’ve increased the number by 50%, and this increase is the cause of climate change.”
 
Going back to the coffee analogy, imagine you’re used to drinking a low caffeine variety such as Aramosa, which has about 50% less caffeine than Arabica coffee. Then one day you switch to Arabica. All that extra caffeine might give you a headache, muscle tremors, make it difficult to sleep, and make your heart beat faster.
 
This is similar to what’s happened in our atmosphere. For thousands of years before the Industrial Revolution, the amount of CO₂ in the atmosphere hovered around 280 ppm, as the natural world absorbed about as much CO₂ as it emitted. That 280 ppm of CO₂ kept a steady level of heat inside our atmosphere, helping to set the temperature of the pre-industrial world.
 
This balance is now thrown off when billions of tons of extra CO₂ are entering the atmosphere each year, allowing CO₂ to accumulate and keep in more heat. This extra CO₂ has meant the global average surface temperature has risen by 1° C (2° F), and is still rising.² To play on the NASA quote: “50% more of a powerful substance can have even bigger effects.”
 
Lastly, it’s important to understand that when it comes to something as large as our atmosphere, a small fraction like 424 ppm actually represents a truly massive number of CO₂ molecules. 
 
Consider a one-liter bottle full of air. At “standard temperature and pressure” (0° C at sea level), that single liter of air contains 2.7 x 10²² molecules: 27,000 billion billion molecules.
 
What’s 424 ppm of this bottle of air? It’s 11.4 billion billion molecules of CO₂, of which almost 4 billion billion were added by human activities. And any infrared radiation trying to get through our bottle—which is how the Earth gives off heat into space—will have to run the gauntlet of all those CO₂ molecules, each one of which can absorb infrared light and keep it from escaping to space.
 
And that’s the number of CO₂ molecules in just one liter of air. (Depending on temperature and pressure—in the real atmosphere, our bottle would hold fewer molecules as you go higher up.)

So don’t get distracted by how small a percentage 424 ppm is: this is still a mind-boggling amount of CO₂ we’ve added to the atmosphere, and a monumental change from the 280 ppm humanity has experienced for most of our history. “These molecules are strong infrared absorbers,” says Kroll, “and their concentration is going up.”