Understanding global warming: our atmosphere is like a bathtub about to overflow

Finally, I have found a cogent and easy to understand explanation of the process of global warming. It is in the article “The Carbon Bathtub” in the December 2009 issue of National Geographic. Explaining the issue of global warming in a manner that inspires people to take action has been difficult for environmentalists as well as politicians. Sadly, the most memorable visual about global warming for many is the hungry polar bear swimming fruitlessly for polar ice in the movie an Inconvenient Truth.

To date it has been hard to get more support of global warming mitigation and this may be because the process is hard to understand and seems removed from the average person’s life. According to John Sterman, a professor at the MIT Sloan School of Management, this is because of a cognitive limitation of most humans. Dr. Sterman found out that even his smart MIT graduate students couldn’t get a grasp of how exactly CO2 is building up in the atmosphere using the standard climate change jargon. This was until he explained the process using the metaphor of a bathtub. Like a bathtub with water pouring in from a tap and the drain open, when more water pours in that can drain out, the level of the water rises and will eventually overflow. Dr. Sterman explains that this is similar to how the level of CO2 is rising in our atmosphere. More CO2 is flowing in than can drain out. See a graphic of the bathtub-like process of global warming on the National Geographic web site.

Last year 9.1 metric tons of C02 was released into the atmosphere but only 5 billion metric tons was “drained” by being absorbed by plants, soils and oceans. “At the current emissions rate, CO2 is released into the atmosphere nearly twice as fast as it is removed–so the bathtub will continue to fill,” the National Geographic article stated. The leftover 45% of “un-drained” CO2 that remains in the atmosphere is causing global warming. The excess carbon dioxide absorbs more heat radiation coming from the Earth’s surface and re-radiates it downward, warming up the atmosphere.

Where does most of the human-created CO2 come from? “Four-fifths [of the C02 emissions released by human activity] is from burning fossil fuels. Nearly all the rest is from deforestation and other changes in land use,” according to the National Geographic article.

Even if we stop increasing the amount of C02 emissions there will still be global warming for a while according to climatologist David Archer, author of The Long Thaw. It will take hundreds of years for the planet to absorb the CO2 created by industrialization. In 2008 the average amount of CO2 in the atmosphere was 385 parts per million (ppm). The pre-industrial level of carbon dioxide in the atmosphere was 271 parts per million. The amount of CO2 hasn’t been this high for “at least 800,000 years, say the oldest air bubbles found in Antarctic ice cores,” the National Geographic article reports. The highest ice core reading of CO2 in the atmosphere was 299 parts per million, dating 333,000 years ago.

To stop the level of CO2 at 450 ppm, still too high according to many scientists, would require the world cutting emissions by 80% by 2050. To do this we will have to make a massive shift in our global carbon-based industrial economy to cleaner sources of energy such as wind, solar, or aquatic energy. This will require a global understanding of the climate change process and a political will to enact expensive changes. The industrial revolution created global warming. There will have to be a sustainable living revolution to un-create it. Hopefully, the Copenhagen COP15 Climate Change Talks will be a productive step forward towards stopping global warming before it’s too late. The consequences of global warming include rising sea levels, more droughts, more flooding, less ice and snow and more extreme weather events such as Hurricane Katrina that wiped out coastal communities of the entire Gulf Coast in the United States.


Ministry of Climate and Energy of Denmark
2009, “What consequences can we expect, and what can we do?” COP15, retrieved on November 16, 2009, from: http://en.cop15.dk/climate+facts/what+consequences+can+we+expect

Sterman, John and David Archer
2009 “The Carbon Bathtub,” National Geographic, December 2009, P.26-29.

Sterman, John and David Archer
2009 “The Carbon Bathtub,” National Geographic, retrieved on November 15, 2009, from http://ngm.nationalgeographic.com/big-idea/05/carbon-bath