The University of Texas at Austin
  • Century-old climate myth still making the rounds

    By Marc Airhart
    Marc Airhart
    Published: Nov. 9, 2010
    Charles JacksonPhoto: Sasha Haagensen

    Myth 2: Increased carbon dioxide (CO2) can’t contribute to global warming: It’s already maxed out as a factor and besides, water vapor is more consequential.

    Some climate skeptics claim that the carbon dioxide (CO2) currently in the atmosphere is already “saturated” in its ability to absorb longwave radiation from Earth and therefore additional CO2 in the air won’t make a difference –won’t, that is, absorb more heat. They also argue that water vapor is a more potent greenhouse gas and therefore increases in CO2 shouldn’t be a concern. These claims have been made in recent years by Hungarian physicist Ferenc Miskoczi and other scientists. They were repeated in the Skeptic Handbook, published in 2009 by science writer Joanne Nova. Yet the seed of the argument actually goes back more than a century.

    In 1900, scientists published results of a laboratory experiment interpreted at the time to signify that all of the long wavelength radiation emitted by Earth is absorbed by the atmosphere already, and that therefore, adding more CO2 couldn’t possibly make a difference.

    Here’s what the scientists did in that early experiment: They sent infrared light through a foot long (30 cm) tube containing a small concentration of CO2 meant to simulate Earth’s atmosphere and then measured how much radiation made it through to the other end. Next, they cut the amount of CO2 by a third and measured how much radiation made it through. As it turned out, the results were nearly the same. Therefore, they reasoned, CO2 is already maxed out in its ability to further warm the planet.

    Steam rising out of a tea pot

    Image: Westy Ford on Flickr/CC

    The flaw, noted climate modeling expert Charles Jackson, was simplifying the atmosphere down into something like a short tube or a thin sheet of glass. In reality, the atmosphere is thick with many layers. As radiation makes its way up through the atmosphere, it gets absorbed and re-emitted many times. And it’s re-emitted in all directions. More CO2 near the surface doesn’t make a big difference, but higher up in the atmosphere, more CO2 means more heat is absorbed and re-emitted (both up and down). The net effect is that it becomes harder for Earth to shed its heat back out to space.

    Then there’s water vapor. It absorbs a wider range of wavelengths of radiation than CO2 and is more abundant overall in the atmosphere. So it seems logical that water vapor would have a larger role in climate change than CO2.

    But, Jackson noted, Air Force experiments in the 1940s showed that in the upper atmosphere — where Earth’s heat is released into space — there is little water vapor and at lower pressures, it is less able to absorb radiation. So CO2 turns out to be more important than water vapor in the region that counts.

    That isn’t to say that water vapor doesn’t matter. All climate models incorporate its effects in their simulations. The difference is that climate scientists consider it a feedback rather than a main driver of climate change. That’s because of observations showing that regardless of changes in global temperatures, global relative humidity stays fairly constant.

    You are invited to post comments and follow-up questions on this site. You can also e-mail climate scientists questions. The scientists cannot respond to all questions individually but will address recurring themes with new entries.

    • Quote 2
      Charles Jackson said on Nov. 14, 2010 at 7:59 p.m.
      In response to DirkH's comment about CO2 causing a cooling high in the atmosphere: That is correct, the effect of CO2 induces cooling high in the atmosphere (in the upper stratosphere above 15 km). As you stated, it is expected and we see this cooling in observations. It is one of the "fingerprints" of global warming (see Myth 3). However it is the mid to upper troposphere (~5 to 10 km) that is most effectively radiating Earth's heat energy to space. This is the emission level where most of the incoming solar energy is balanced by outgoing heat energy. Adding CO2 in the atmosphere raises the elevation of the emission level. When this happens, the bulk of the troposphere will also heat up. The direct effect of CO2 is not to change the planetary energy balance, rather it redistributes where heat energy leaves the atmosphere.
    • Quote 2
      Charles Jackson said on Nov. 14, 2010 at 4:45 p.m.
      To the best of what we can measure, CO2 and global temperatures vary together. There are two issues one needs to take into account when comparing CO2 and temperature proxies from Antarctic ice cores. First is that the CO2 that is trapped in bubbles is always younger than the surrounding ice (from which one generates a record of temperature). That is because it can take up to a thousand years for the pore spaces to close up and seal in the record of atmospheric CO2. This gas-age/ice-age difference would make it seem that temperature changes would lead CO2 changes. The other issue is that the ice core is a point measurement of climate and would be affected by other factors such as changes in Earth's orbital geometry, ocean circulation, etc. The question of the relative phasing of CO2 and temperature is a good one as there remains many questions about the mechanisms that regulate long term changes in atmospheric CO2. There have been many studies that have considered this very question. However when you consider all the factors, we can not say that the warming that brought us out of the last ice age led CO2 increases.
    • Quote 2
      BestTimesNow said on Nov. 11, 2010 at 10:19 p.m.
      Water vapor is the most important greenhouse gas and has been responsible for most of the climate change for millions of years. IMHO Let’s look at progression of examples, starting with the warmest Earth, so you can see the effect on the planet. To keep things simple, we’ll set the composition of dry atmosphere, to the same as the present time, in all examples, so that we can see the effects of the water vapor in the air. Earth 1 – Water World We’ll start out with a hypothetical “Water World” type Earth, without any land. This will be the overall warmest Earth, with no ice caps at the polls. “Why would it be warmer than the present day Earth? Water just rains out.” Climate models are not required, just basic understanding and logic to find the answer. What’s different in “Water World”? All elevations are at sea level in “Water World”, so the height of atmosphere (greenhouse effect) is at a maximum level all over the world. The very humid, middle latitudes will be the warmest, with no relief, with the humidity. If it rains, the moisture in the air will be replaced very quickly. This part is similar to the very humid, present day, tropical ocean areas, but now covers 100% of the middle latitudes in “Water World”. Also, because of the insulating effect of the very moist atmosphere and unrestricted ocean currents, the middle latitudes (45N to 45S) will have a very constant, but warm, temperature with only a few degrees temperature swing between the daytime highs and the morning low temperature. The warm water from the middle latitudes will also mix with the polar oceans due to the unrestricted ocean currents, this will cause these regions to be much warmer than our present time polar regions, and there will not be ice caps at the poles. The air in the Polar Regions will also hold more moisture than our present time polar regions and will add to the greenhouse effect. Earth 2 - Cretaceous period type land mass Some low elevation continents are formed and cover 20% of the surface of the Earth. The land mass will disrupt the ocean currents and will allow the Polar Regions to cool. The middle latitudes will still be very humid (similar to Florida) because the most of the land mass will be at low elevations. Some of the larger continents will allow areas of lower humidity, which will allow some cooling at night. This planet Earth 2 will be cooler than Earth 1, but still very warm due to the greenhouse gas, water vapor. Earth 3 - Paleogene (Eocene) period type land mass The continents now cover about 25% of the Earth and mountain ranges are forming. Although ocean currents are a little more restricted than Earth 2, it’s the larger continental areas, with mountain ranges that will allow Earth 3, to be cooler than Earth 2. As the prevailing winds pass over the mountains, the moisture in the air precipitates out, allowing for large areas of lower humidity, over some of the continents. The large areas of low humidity allow for even more night time cooling. Planet Earth 3 will be cooler than Earth 2, but still very warm due to the greenhouse gas, water vapor, but the water vapor is loosing its grip on keeping the Earth’s temperature elevated. Earth 4 – Pre Industrial Revolution The continents cover about 30% of Earth 4 and very high mountain ranges have formed. Ocean currents are very restricted, with the connection of North and South America and Europe, Asia and Africa forming large land areas. The vast land areas of lower humidity allow the middle latitudes to cool. Night time cooling is becoming greater over larger areas of Earth 4. (Just think back to Earth 1) Some of the higher mountain ranges remain snow capped all year long. Earth 4 is much cooler than Earth 3. Earth 5 – The present day Earth Due to the extreme population growth, humans are having an effect on the climate. What was desert and semi-arid land has been converted into farms. The effect of irrigating millions of acres is similar to increasing the amount of the Earth that is covered with water. Most of the irrigation water is evaporated on a daily basis, which increases the greenhouse gas, water vapor and contributes to global warming. The amount of water we are adding to the atmosphere is substantial. We are adding about 160 billion gallons per day to our atmosphere, in the US. This increase is about 5.7 % more than what would have evaporated from surface water bodies, land surface, and vegetation transpiration.
    • Quote 2
      Amoeba said on Nov. 11, 2010 at 3:41 p.m.
      Phin Sprague said on Nov. 10 at 11:15 a.m. "The increase in base Co2 lags temperature change (Cite Vostok Ice Core data) and there for can not be the cause of global warming. Show the world I am wrong and I will thank you." The fact that warming causes CO2 to be out-gassed from carbon sinks does not disprove that CO2 causes warming. Your logic is flawed, it is like saying that chickens do not lay eggs, because they have been seen to emerge from them. Objective scepticism is being persuaded only by evidence. Pseudo-scepticism aka skepticism is dismissing all science as fraudulent and accepting any pseudo-scientific baloney that supports one's preconceived ideas / political beliefs / paymasters.
    • Quote 2
      padmashree said on Nov. 11, 2010 at 12:32 a.m.
      how does river sand extraction impacts the environment like river bank erosion or sea erosion? do river sand extraction contribute to sea level rise?
    • Quote 2
      DirkH said on Nov. 10, 2010 at 3:42 p.m.
      "More CO2 near the surface doesn’t make a big difference, but higher up in the atmosphere, more CO2 means more heat is absorbed and re-emitted (both up and down). " More CO2 in the upper atmosphere actually increases cooling. As you correctly wrote, it is a good absorber and re-emitter in its LWIR absorption band. Now, it will re-emit in all directions with the same probability. High up in the atmosphere, there are more directions pointing into space than there are pointing towards the ground. So, while a denser CO2 fog will make it necessary for LWIR radiation to undergo more absorption-emission cycles than a less dense fog, its density doesn't change the overall outcome. Shine a lamp into a fog; a denser fog doesn't reflect more light to you than a less dense one - they appear equally bright. Only the radius of visibility is higher in a less dense fog.
    • Quote 2
      Marc Airhart said on Nov. 10, 2010 at 1:41 p.m.
      Phin, Thanks for your response. In our eight installment, we'll be addressing the myth that CO2 can't cause global warming today because there were past periods of warming in which temperature rose before CO2. It's a good one, so stick around for that early next week. Regarding the term skeptic, I certainly agree that good scientists are skeptical of new hypotheses. But specifically with the hypothesis that rising CO2 can't raise Earth's temperature, the overwhelming scientific evidence has disproved it. The science doesn't support a skeptical view on that.
    • Quote 2
      Phin Sprague said on Nov. 10, 2010 at 11:15 a.m.
      1. Don't mix up correlation with causation. Don't mix arguments. The increase in base Co2 lags temperature change (Cite Vostok Ice Core data) and there for can not be the cause of global warming. Show the world I am wrong and I will thank you. If anthropogenic contribution of CO2 is contributing to warmer temperatures and CO2 lags cooling it's contribution to maintaining the warmer global temperature AFTER the warming cycle ends is of interest and should be appreciated. Please note the the similarity of the previous four cycles in the past 450,000 years. 2. "Skeptic" is the proper role of scientific method and is the very foundation of paradigm shifts in understanding. This very important analytical process has been turned into a pejorative by those who wish to avoid providing substantive data for peer review, and obscenely manipulate the information provided to the public for political purposes. The present use of the word "Skeptic" has become an adhomenem attack and therefore is an admission of the inability to address the data and fails as a logical argument.
    • Quote 2
      co2hound said on Nov. 10, 2010 at 7:56 a.m.
      Nice article! This is exactly the kind of info we need to get to the general public. Short, to the point, backed up with historical references and presented in language and concepts that are easily understood. You get an A for your paper!
    • Quote 2
      Greg said on Nov. 9, 2010 at 9:28 p.m.
      How come the BBC showed on national TV with Climate expert a bottle of CO2 and how it "works" with 100% CO2 in it? So it's ok for the climate believers to use bottles full of CO2 for experiements only when it produces the answers you want?
    • Quote 2
      More Nonsense From Austin | Real Science said on Nov. 9, 2010 at 7:07 p.m.
      [...] [...]
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