FROM THE FIELD
You Asked

You Asked: If CO2 Is Only 0.04% of the Atmosphere, How Does it Drive Global Warming?

by |July 30, 2019

Got a burning question about climate change? Feeling curious about conservation? “You Asked” is a series where Earth Institute experts tackle reader questions on science and sustainability. To submit a question, drop a comment below, message us on Instagram, or email us here.

A reader named Paul submitted this question on one of our previous posts:

Why does CO2 drive global warming when there is only 0.04% of it in the atmosphere? And why isn’t water vapor the major driving factor?

Yochanan Kushnir

Yochanan Kushnir is a research professor at Lamont-Doherty Earth Observatory, in the Division of Oceans and Climate Physics

Answer provided by Yochanan Kushnir

Earth absorbs energy from sunlight, but as the surface warms, it also emits energy in the form of infrared radiation (which we know of as heat) out into space. Water vapor and CO2, however, act like a cap, making it more difficult for Earth to get rid of this energy. Without gases like these to absorb the energy, our planet’s average surface temperature would have been near zero degrees Fahrenheit.

About 99 percent of the atmosphere is made of oxygen and nitrogen, which cannot absorb the infrared radiation the Earth emits. Of the remaining 1 percent, the main molecules that can absorb infrared radiation are CO2 and water vapor, because their atoms are able to vibrate in just the right way to absorb the energy that the Earth gives off. After these gases absorb the energy, they emit half of it back to Earth and half of it into space, trapping some of the heat within the atmosphere. This trapping of heat is what we call the greenhouse effect. Because of the greenhouse effect created by these trace gases, the average temperature of the Earth is around 15˚C, or 59˚F, which allows for life to exist.

CO2 makes up only about 0.04% of the atmosphere, and water vapor can vary from 0 to 4%. But while water vapor is the dominant greenhouse gas in our atmosphere, it has “windows” that allow some of the infrared energy to escape without being absorbed. In addition, water vapor is concentrated lower in the atmosphere, whereas CO2 mixes well all the way to about 50 kilometers up. The higher the greenhouse gas, the more effective it is at trapping heat from the Earth’s surface.

The burning of fossil fuels affects the concentration of CO2 in the atmosphere. Before the industrial revolution, the amount of CO2 in the atmosphere was about 288 ppm. We have now reached about 414 ppm, so we are on the way to doubling the amount of CO2 in the atmosphere by the end of this century. Scientists say that if CO2 doubles, it could raise the average global temperature of the Earth between two and five degrees Celsius. We are already increasing the amount of energy that bounces back to the Earth. Because of the greenhouse effect, this is causing global warming with its many destructive impacts.

Both water vapor and CO2 are responsible for global warming, and once we increase the CO2 in the atmosphere, the oceans warm up, which inevitably triggers an increase in water vapor. But while we have no way to control water vapor, we can control CO2. And because we are increasing the amount of CO2 in the atmosphere by continuing to burn fossil fuels, even in relatively small amounts compared to the entire mass of the atmosphere, we are disturbing the entire heat balance of the planet.

Get our newsletter

I'd like to get more stories like this.
Email address
Secure and Spam free...

13
Leave a Reply

avatar
8 Comment threads
5 Thread replies
2 Followers
 
Most reacted comment
Hottest comment thread
12 Comment authors
Lonnie kingHelmut WoberMattRolf EMary M. Douthwaite Recent comment authors
  Subscribe  
newest oldest most voted
Notify of
JLocke
Guest

If CO2 increases follow temperature increases how can it be the cause? It makes sense that it could exacerbate the rise but how can it be the cause of the initial rise?

Miki
Guest
Miki

It’s the initial cause because it’s emitted in the first place
And that emission rises the amount of greenhouse gas and therefore the amount of heat trapped

Mary M. Douthwaite
Guest
Mary M. Douthwaite

JLocke asked above, “If CO2 increases follow temperature increases how can it be the cause?” Miki answered, “It’s the initial cause because it’s emitted in the first place.” ? The response not appear to answer the question. How can a rise of CO2 be emitted in the first place if it follows the rise in temperature?

Matt
Guest
Matt

CO2 can be both a primary and secondary cause of warming. CO2 can be the primary cause of rising temperatures, as it has been in the last couple centuries. CO2 can also be release from other sources (e.g. permafrost) in response to rising temperatures. And so, 1) in previous warming periods that were I initially started by non-CO2 causes, CO2 rises predictably follow temperature rises (and then further drive more increase). 2) In other cases (including the current warming period) it looks like CO2 is the primary cause of warming, but similar to the above this warming will cause even… Read more »

Abhaya Thiele
Guest
Abhaya Thiele

Thank you for this. To all those doing climate research or reporting . . . As many Americans think in terms of Fahrenheit, might you always accompany a Celsius temperature with its Fahrenheit equivalent?

Uno Hansson
Guest
Uno Hansson
Jonathan
Guest
Jonathan

Wow, that was a weak answer, with a lot of “scientists say” type references. Not convinced at all. I was hoping for so much better.

Neobiognosis
Guest
Neobiognosis

Can you articulate why you remain unconvinced and clarify the reasons you made this comment.

No CAGW
Guest
No CAGW

Water vapor has a window? It’s 5x the ghg and 10x more than co2. Co2 has “windows” too, it only absorbs in three small bands leaving 92% transparent to IR.

So, when the IR is “back radiating” down (not heat btw) it is more likely to be intercepted by N2, o2 which is 99% of the atmosphere, and repeat the process back to space. This happens at the speed of light. Chances of the same IR making it back to the surface from the TOA is unlikely.

Neobiognosis
Guest
Neobiognosis

It is not unlikely, it is certain that a proportion of it reaches the earth’s surface, and the earth’s heat energy balance is then lost. The energy reflected back to the earth’s surface is 2.9 Wm-2. You may examine this, and decide it is a small increment on the radiation already reaching earth, you may not understand the significance of the unit, so I’ll add some clarification. The unit represents the amount of energy reaching every square meter of the surface of earth every second. A simple extrapolation takes you to the amount of energy we are incementing our budget… Read more »

Rolf E
Guest
Rolf E

What are the formulas to calculate the temperature change from H2O- and the CO2- concentration in the athmosphere given a certain start temperature and a certain time with constant solar radiation.

Helmut Wober
Guest
Helmut Wober

If CO2 is such a greenhouse gas, especially 50 km up, how come that the surface of Mars with an atmosphere that consist of 95% CO2 has an average surface temperature of – 40 degrees Celsius? The 288 K, risen to 288.8K in the last 100 years on the surface of the earth are primarily due to adiabatic compression of the atmosphere, not green house effects. Yes , rises in CO2 follow global warming with a time lag of 600 – 800 years, due to degassing of the Oceans, which cover about 71% of the surface of the earth. Yes,… Read more »

Lonnie king
Guest
Lonnie king

So if co2 is so high up in the atmosphere how do plants in the ocean and on land absorb it? Do certain densities make it come down ? Is the earth supposed to have ice on it? Have there been periods on this planet where it had no ice? If most of earths heat is released through the hole in the atmosphere in the Antarctic does the size of that hole change? Is earth’s orbit around the sun the exact same every year or is it moving closer and farther over the years? How much have the ocean levels… Read more »