Attribution: Blowing in the Wind

Attribution (5)

This is the fifth in a series on the attribution of climate change; that is, how do we determine to what extent the observed warming is caused by humans? The earlier entries are cataloged at the end. (This one should, perhaps be the first!)

First a return to basics: I received a good letter from a reader about the difficulty of determining trends and attribution from, primarily, the last 150 years of observations. The challenge seems even more daunting with the observational evidence from the distant past of a cycle between ice ages and temperate periods. One of the reasons that we can predict with confidence that the globe will warm is the large, observed increase in carbon dioxide. Carbon dioxide warms the planet; this has been known since about 1800. The warming comes from carbon dioxide holding infrared radiation, heat, near the surface of the Earth. The quantitative physical description of this process is simple and well known.

Carbon dioxide, therefore, is different than in the past. It is much larger. It can “force” the temperature to be warmer. (Old, yet relevant, blog) It is the fact that we have this carbon dioxide forcing that we can both make confident predictions and look for signals of attribution. I have been trying to think of a good metaphor to describe forcing, that also maintains some relevance to climate change. (Help?) Here is one that I pose. Imagine that you have a small bell hanging on a string from a beam on your porch. If there is wind, then it will blow the bell and it will ring. If there is more wind, then the bell will ring with different characteristics, perhaps louder, more frequently, more erratically. You could designate the wind as “forcing” the bell by blowing it around. Call this the “natural forcing.” If you were compelled to science you could keep a record of wind speed and direction (perhaps other variables) and a record of the characteristics of the bell ringing.

Now imagine that you keep a small mallet on the porch, and that you hit the bell. This is “anthropogenic forcing.” (Let’s see: human-caused, manmade, womanmade --- isn’t it interesting that manmade is a “word,” and “womanmade” is not?) Hitting the bell, a new type of forcing, will have a distinctly different sound. There will be a sharp sound, followed by a ringing of the bell’s body, and then, in the end, because the hit will cause the bell the swing on the string, it will sound much like it was blowing in the wind. There are a set of characteristics of the ringing from hitting the bell that are distinctly different than the ring from blowing in the wind.

It is the difference in the characteristics of the bell blowing in the wind (“natural”) and the bell hit by the mallet (“anthropogenic”) that allow the definition of a “fingerprint.” This fingerprint can be used to determine whether the bell has been hit – or not?

When we look at atmospheric observations and measure that it is warming up, we are faced with a far more complex problem than a bell dangling from the porch in the wind. Still, though, the basic ideas are the same. We have a known anthropogenic forcing agent, the carbon dioxide (plus others!), and we have a set of fingerprints. Examples of the fingerprints include greater surface heating at the North Pole than at middle latitudes and at the South Pole. The complexity and importance of the climate problem requires that we identify a thorough set of fingerprints. These include the spatial and temporal structure of changes at the Earth’s surface, changes in the vertical temperature structure of the atmosphere, changes in the vertical temperature structure of the ocean, changes in ecosystems, and the list goes on.

One path to attribution of warming to human activity is to identify enough characteristics of the fingerprint to make a convincing determination. Imagine that you generate a long list of attributes of the fingerprint of climate change and some you find in the observations and some you do not find. Because you find one does not prove human-caused climate change. Because you do NOT find one does NOT disprove human caused climate change. One is faced with the analysis of a complex, varying system and the determination of uncertainties.

The figure below from Ben Santer at Lawrence Livermore National Laboratory is a summary picture of the variables in which human-caused signals of climate change have been identified. This is from a lecture in my class in 2008, and the entire lecture is here.




Figure 1. Taken from class lecture by Ben Santer. This figure is a summary of geophysical parameters in which fingerprints of human-caused climate change have been found.


WU blogs on Attribution of Climate Change to Human Activities:

WU Blog on Models and Attribution

Attribution (1)

Attribution (2)

Attribution (3)

Attribution (4)