This post tries to help visualizing, or understanding better, the greenhouse effect.
By the way, if you are new to this subject and think CO2 is an insignificant trace gas, then at least take a look at Part One.
I tried to think of a good analogy, something to bring it to life. But this is why the effect of these invisible trace gases is so difficult to visualize and so counter-intuitive.
The most challenging part is that energy flowing in – shortwave radiation from the sun – passes through these “greenhouse” gases like they don’t exist (although strictly speaking there is a small effect from CO2 in absorption of solar radiation). That’s because solar radiation is almost all in the 0.1-4μm band (see The Sun and Max Planck Agree – Part Two).
But energy flowing out from the earth’s surface is absorbed and re-radiated by these gases because the earth’s radiation is in the >4μm band. Again, you can see these effects more clearly if you take another look at part one.
If we try and find an analogy in everyday life nothing really fits this strange arrangement.
Upwards Longwave Radiation
So let’s try and look at it again and see if starts to make sense. Here is the earth’s longwave energy budget – considering first the energy radiated up:
Of course, the earth’s radiation from the surface depends on the actual temperature. This is the average upwards flux. And it also depends slightly on the factor called “emissivity” but that doesn’t have a big effect.
The value at the top of atmosphere (TOA) is what we measure by satellite – again that is the average for a clear sky. Cloudy skies produce a different (lower) number.
These values alone should be enough to tell us that something significant is happening to the longwave radiation. Where is it going? It is being absorbed and re-radiated. Some upwards – so it continues on its journey to the top of the atmosphere and out into space – and some back downwards to the earth’s surface. This downwards component adds to the shortwave radiation from the sun and helps to increase the surface temperature.
As a result the longwave radiation upwards from the earth’s surface is higher than the upwards value at the top of the atmosphere.
Here’s the measured values by satellite averaged over the whole of June 2009.
Of course, the hotter parts of the globe radiate out more longwave energy.
Downwards Longwave Radiation
But what does it look like at the earth’s surface to an observer looking up – ie the downwards longwave radiation? If there was no greenhouse effect we should, of course, see zero longwave radiation.
Here are some recent measurements:
Note that the wavelengths have been added under “Wavenumber” (that convention of spectrum people) and so the graph runs from longer to shorter wavelength.
This is for a winter atmosphere in Canada.
Now what the scientists did was to run a detailed simulation of the expected downwards longwave radiation using the temperature, relative humidity and pressure profiles from radiosondes, as well as a detailed model of the absorption spectra of the various greenhouse gases:
What is interesting is seeing the actual values of longwave radiation at the earth’s surface and the comparison 1-d simulations for that particular profile. (See Part Five for a little more about 1-d simulations of the “radiative transfer equations”). The data and the mathematical model matches very well.
Is that surprising?
It shouldn’t be if you have worked your way through all the posts in this series. Calculating the radiative forcing from CO2 or any other gas is mathematically demanding but well-understood science. (That is a whole different challenge compared with modeling the whole climate 1 year or 10 years from now).
They did the same for a summer profile and reported in that case on the water vapor component:
As an interesting aside, it’s a lot harder to get the data for the downwards flux at the earth’s surface than it is for upwards flux at the top of atmosphere (OLR). Why?
Because a few satellites racing around can measure most of the radiation coming out from the earth. But to get the same coverage of the downwards radiation at the earth’s surface you would need thousands or millions of expensive measuring stations..
Measurements of longwave radiation at the earth’s surface help to visualize the “greenhouse” effect. For people doubting its existence this measured radiation might also help to convince them that it is a real effect!
If there was no “greenhouse” effect, there would be no longwave radiation downwards at the earth’s surface.
Calculations of the longwave radiation due to each gas match quite closely with the measured values. This won’t be surprising to people who have followed through this series. The physics of absorption and re-emission is a subject which has been extremely thoroughly studied for many decades, in fact back into the 19th century.
How climate responds to the “extra radiation” (radiative forcing is the standard term) from increases in some “greenhouse” gases is whole different story.
More in this series -
Part Seven – The Boring Numbers – the values of “radiative forcing” from CO2 for current levels and doubling of CO2.
Part Eight – Saturation – explaining “saturation” in more detail
CO2 Can’t have that Effect Because.. – common “problems” or responses to the theory and evidence presented
Measurements of the Radiative Surface Forcing of Climate, W.J.F. Evans & E. Puckrin, American Meteorological Society, 18th Conference on Climate Variability and Change (2006)