Is Water Vapor Worse Than CO₂?

Written By Teacher: Elizabeth Ward

My name is Elizabeth Ward. I am a former Early Childhood, Elementary, and English as a Foreign Language educator. I have taught third grade Science and Social Studies as well as Kindergarten in both urban and rural Oklahoma public schools. I taught online EFL to students of all ages in China for four years. I also have experience in curriculum development and content design for teachers in the physical and digital classroom. As a former teacher I have a passion for supporting teachers and making their jobs easier. I currently live in the greater Houston area with my husband and four dogs. 

Water vapor is the most abundant greenhouse gas, but carbon dioxide plays a larger role in long-term climate change because it stays in the atmosphere much longer and drives temperature increases. Understanding the differences between these gases helps students grasp why CO₂ is the primary focus of climate policy and mitigation efforts. This topic encourages critical thinking about atmospheric science and the factors influencing global warming. Resources such as climate summaries, climate models, and scientific explanations can help bring this concept to life in the classroom.

Written By: MIT Environmental Solutions Initiative

The MIT Climate Change Engagement Program, a part of MIT Climate HQ, provides the public with nonpartisan, easy-to-understand, and scientifically-grounded information on climate change and its solutions.

With all the attention given to humans’ climate-warming carbon dioxide (CO2) emissions, you might be surprised to learn that CO2 is not the most important greenhouse gas affecting the Earth’s temperature. That distinction belongs to water.

We can thank water vapor for about half of the “greenhouse effect” keeping heat from the sun inside our atmosphere. “It’s the most important greenhouse gas in our climate system, because of its relatively high concentrations,” says Kerry Emanuel, professor emeritus of atmospheric science at MIT. “It can vary from almost nothing to as much as 3% of a volume of air.”

Compare that to CO2, which today makes up about 420 parts per million of our atmosphere—0.04%—and you can see immediately why water vapor is such a linchpin of our climate system.

So why do we never hear climate scientists raising the alarm about our “water emissions”? It’s not because humans don’t put water into the atmosphere. Even the exhaust coming from a coal power plant—the classic example of a climate-warming greenhouse gas emission—contains almost as much water vapor as CO2. It’s why that exhaust forms a visible cloud.