Arguments
Has the greenhouse effect been falsified?
Posted on 19 May 2010 by sylas
Guest post by Chris Ho-Stuart
Most participants in climate debates can agree that the atmosphere's capacity to interact with thermal radiation helps maintain the Earth's surface temperature at a livable level. The Earth's surface is about 33 degrees Celsius warmer than required to radiate back all the absorbed energy from the Sun. This is possible only because most of this radiation is absorbed in the atmosphere, and what actually escapes out into space is mostly emitted from colder atmosphere.
This absorption is due to trace gases which make up only a very small part of the atmosphere. Such gases are opaque to thermal radiation, and are called "greenhouse gases". The most important greenhouse gases on Earth are water vapor and carbon dioxide, with additional contributions from methane, nitrous oxide, ozone, and others. If the atmosphere was simply a dry mix of its major constituents, Oxygen and Nitrogen, the Earth would freeze over completely.
Observing the greenhouse effect in action
The simplest direct observation of the greenhouse effect at work is atmospheric backradiation. Any substance that absorbs thermal radiation will also emit thermal radiation; this is a consequence of Kirchoff's law. The atmosphere absorbs thermal radiation because of the trace greenhouse gases, and also emits thermal radiation, in all directions. This thermal emission can be measured from the surface and also from space. The surface of the Earth actually receives in total more radiation from the atmosphere than it does from the Sun.
The net flow of radiant heat is still upwards from the surface to the atmosphere, because the upwards thermal emission is greater than the downwards atmospheric backradiation. This is a simple consequence of the second law of thermodynamics. The magnitude of the net flow of heat is the difference between the radiant energy flowing in each direction. Because of the backradiation, the surface temperature and the upwards thermal radiation is much larger than if there was no greenhouse effect.
Atmospheric backradiation has been directly measured for over fifty years. The effects of greenhouse gases stand out clearly in modern measurements, which are able to show a complete spectrum.
Figure 1. Coincident measurements of the infrared emission spectrum of the cloudfree atmosphere at (a) 20km looking downward over the Arctic ice sheet and (b) at the surface looking upwards. (Data courtesy of David Tobin, Space Science and Engineering Center, University of Wisconsin-Madison. Diagram courtesy of Grant Petty, from Petty 2006).
When you look down from aircraft at 20km altitude (Fig 1a), what is "seen" is the thermal radiation from Earth that gets out to that height. Some of that radiation comes from the surface. This is the parts of the spectrum that follow a line corresponding in the diagram to about 268K. Some of that radiation comes from high in the atmosphere, where it is much colder. This is the parts of the spectrum that follow a line of something like 225K. The bites taken out of the spectrum are in those bands where greenhouse gases absorb radiation from the surface, and so the radiation that eventually escapes to space is actually emitted high in the atmosphere.
When you look up from the surface (Fig 1b), what is "seen" is thermal backradiation from the atmosphere. In some frequencies, thermal radiation is blocked very efficiently, and the backradiation shows the temperature of the warm air right near the surface. In the "infrared window" of the atmosphere, the atmosphere is transparent. In these frequencies, no radiation is absorbed, no radiation is emitted, and here is where IR telescopes and microwave sounding satellites can look out to space, and down to the surface, respectively.
The smooth dotted lines in the diagram labeled with temperatures are the curves for a simple blackbody radiating at that temperature. Water vapor has complex absorption spectrum, and it is not well mixed in the atmosphere. The emissions seen below 600 cm-1 are due to water vapor appearing at various altitudes. Carbon dioxide is the major contributor for emission seen between between about 600 and 750 cm-1. The patch of emission just above 1000 cm-1 is due to ozone.
The term "greenhouse"
The term "greenhouse" was coined for this atmospheric effect in the nineteenth century. A glass greenhouse and an atmospheric greenhouse both involve a physical barrier that blocks the flow of heat, leading to a warmer temperature below the barrier. The underlying physics is different, however. A glass greenhouse works primarily by blocking convection, and an atmospheric greenhouse works primarily by blocking thermal radiation, and so the comparison is not exact. This difference is well understood and explained in most introductions to the subject. Where confusion arises, it is usually the glasshouse that is improperly described, rather than the atmospheric greenhouse effect.
The enhanced greenhouse effect
The greenhouse effect itself has always been an important effect on Earth's climate, and it is essential for maintaining a livable environment. Without it, the surface would rapidly freeze.
The existence of a greenhouse effect itself should not be confused with changes to the greenhouse effect. Global warming in the modern era is being driven by increasing concentrations of greenhouse gases in the atmosphere, which leads to an enhanced greenhouse effect. This is covered in more detail as a separate argument: How do we know more CO2 is causing warming?
Many thanks to Chris Ho-Stuart for this guest post. Chris is co-author of the recently published paper Comment on "Falsification of the atmospheric CO2 Greenhouse Effects within the frame of physics" (Halpern et al 2010). which is a peer-reviewed response to the paper by Gerlich and Tscheuschner which claims to falsify the greenhouse effect. Chris also runs Climate Physics Forums which is a very high quality forum featuring substantive and courteous discussions of climate science. There is also a discussion thread on the Halpern et al paper.
- Most textbooks on climate or atmospheric physics describe the greenhouse effect, and you can easily find these in a university library. Some examples include:
- Elementary Climate Physics, by F. W. Taylor (Oxford Uni Press 2005). Chapter 7 is about the greenhouse effect.
- Physical Principles of Meteorology and Environmental Physics, by D. Blake and R. Robson (World Scientific 2008). Chapter 1, "The Big Picture", can be freely downloaded and includes a good introduction to the greenhouse effect.
- Principles of Planetary Climate, by R. Pierrehumbert (Cambridge Uni Press 2010). This will be available from December 2010, and goes into considerable detail of the greenhouse effect and the associated physics.
- A First Course in Atmospheric Radiation, by G. W. Petty (Sundog publishing, 2006). Covers the underlying physics of the greenhouse effect.
- The Greenhouse Effect, part of a module on "Cycles of the Earth and Atmosphere" provided for teachers by the University Corporation for Atmospheric Research (UCAR).
- What is the greenhouse effect?, part of a FAQ provided by the European Environment Agency.
Actually, I do not want to explain the anomaly in the greenhouse effect, and I say so in the final paragraph of this blog post. This essay aims at a much more basic level: does the greenhouse effect exist at all? Most people do understand that there is a greenhouse effect. However, some people are skeptical even of this. The aim of this blog and this answer is to help people understand, at a very basic level and using direct observational data, simply that there really is an atmospheric greenhouse effect and that it really does give warmer temperatures at the surface than if the atmosphere did not have the greenhouse gases. All the diagrams shown, and your own comments, continue to confirm this basic point, so I presume we don't have a disagreement on this. The distinct question of the impact of changes in atmospheric composition on the magnitude of the greenhouse effect is dealt with as a separate issue in another page, which I have linked previously. However, I will just quickly respond to your comment about the effects of changing concentrations of carbon dioxide, since you may have missed the most important consequence, which is change to the width of the stopband. You say:
The basic theory involved for calculating OLR is covered in some of the more technically detailed textbooks. In particular, Principles of Planetary Climate by Ray Pierrehumbert, due to be published by Cambridge Uni Press in Dec 2010 is excellent and designed to give the student all the tools to do the calculations themselves. This requires a computer to do a large numeric integration through all different frequencies and up a series of graduated steps in altitude of the atmosphere. But in the end you can calculate OLR for a given atmospheric profile. A major primary reference used for the effects of changing CO2 concentration is:
This paper reports the original calculation of the approximately logarithmic effect of carbon dioxide, at about 5.35 W/m2 per natural log. This is the impact on OLR for a given temperature. Of course, the consequence is that temperatures will increase until OLR again matches the solar absorption. What you are likely to find of particular interest is that this calculation reduced earlier calculations of the effect of increasing CO2 by about 15%, because of more thorough consideration of all effects in particular in the stratosphere. The IPCC 2nd AR used about 6.3 W/m2 per natural log CO2. The IPPC 3rd AR and 4th AR used the improved value of 5.35, and this remains the best estimate for the approximately logarithmic relationship. I do not think there is any credible objection to this relation. If the optical depth at a given frequency is very small, or very large, then there is not much consequence for increasing concentrations for that frequency. Given a frequency in the stopband (with a large optical depth) emissions to space all come from high in the atmosphere, and doubling concentrations doesn't make much difference. Similarly, for a very low optical depth (transparency) the changes at that frequency are comparatively slight. The largest impact by far is for those frequencies where optical depth is close to unity. These are the frequencies for which additional concentrations move the effect of the atmosphere most strongly from being transparent to being opaque. Roughly speaking, higher concentrations mean the stopband is a little bit wider, stopping additional frequencies. This is the most important consequence of higher concentrations. The proof of that can be given in various ways. It can be done theoretically, as in calculations explained in Pierrehumber (2010) or reported in Myhre et al (1998). There are also observational confirmations of the enhanced greenhouse effect described in these pages at How do we know more CO2 is causing warming? I don't propose to go into a long further explanation here. There's ample description of the technical details in various references that have been given for people to chase up themselves if they have an interest, and the impact on OLR (the forcing) is taken for granted by major working scientists who happen to be skeptical of AGW, such as Lindzen or Christy. They tend to focus on the more reasonable question of response to forcings, or "climate sensitivity". Changes to atmospheric composition is, of course, a more technically complex question than I cover in this essay. That is deliberate. But I hope this brief account in the comments may go some way to answering your questions. Cheers -- sylas
This is my first post on SkepticalScience. I recognize that I'm over a decade late on this thread, and I don't know whether or not there might be a more appropriate place to post this. Yet, in case it's of interest, I want to share that I recently wrote a new critique of Gerlich and Tscheuschner's 2009 paper purporting to falsify the greenhouse effect. While my essay overlaps some of the insights offered by Halpern et. al. (2010), it offers its own perspective, and might be accessible to readers in a different way.
[BL]
Welcome to Skeptical Science.
None of our threads here are ever closed. As long as your post is on-topic (as yours is) it is appropriate to add a comment. It is always useful to have more information.
Thank you for the link Bob. I felt very validated to see that, as I understood it already at the time, the stupid G&T canard was little more than a play on words leading to a convoluted strawman argument...