Fingerprinting climate change

I'll take up one of the questions from the question place, how do we 'fingerprint' current climate change as being from CO2, rather than from any of the many other things that affect climate?  Same questioner asked several other interesting questions, and they'll be topics for later posts.  One thing I've mentioned is that there are indeed many things which affect climate.  A few to start with are: CO2 (or, more generally, the non-water vapor greenhouse gases*), solar input, volcanic aerosols, and orbital variations.

I don't really like 'fingerprint' as the description, myself.  I think of the process more as the 'duck test' -- if it looks like a duck, walks like a duck, and quacks like a duck, it's probably a duck.  We start by finding out what would be true if a given candidate were driving climate change, the more implications the better, and then go look at what is going on in the atmosphere.  So I'll approach it that way.

Orbital cycles, we know very well.  They're slow -- taking 20,000 to 100,000 years to cycle.  The magnitude can be large -- about 5 C for global mean temperatures between the coldest parts of an ice age and the warmest parts of the period between ice ages.  But this gives us a rate of change for temperature of 0.5 degree per 1,000 years (5 degrees from the trough to the peak in 10,000 years -- the fastest orbital cycle). 

Volcanoes affect climate by dumping aerosols into the stratosphere.  Or, rather, if they affect climate, this is how they do it.  It takes a pretty major eruption to get junk in to the upper atmosphere.  Mt. St. Helens had no real effect on climate, for instance.  It takes something like Mount Pinatubo (1991) or El Chichon (1982) to have a significant effect.  The aerosols do two things to climate.  First, they warm the stratosphere.  The aerosols absorb solar energy, warming themselves and their surroundings.  Second, they cool the surface -- the energy they absorb in the upper atmosphere is not available to keep us warm at the surface.  But there's a different characteristic to these aerosols -- they fall out of the atmosphere quickly.  After 2-3 years, you can't see their effect on temperatures any more.  During the 2-3 years, you can see something like a 0.5 C cooling (Pinatubo's peak).  If you started your observation near the peak of a volcano's effect, you could see a very large warming -- 0.5 C in 2 years.  But it won't continue.

Solar output also varies.  For data, see, for instance, the NGDC archive or PMOD's site.  The latter also presents graphics that tries to remove the sensor to sensor offsets, so that you can get a proper overall picture.  One thing we'd expect is, when the solar output (total solar irradiance at the sites) is higher, the earth should be warmer.  And when it's lower, the earth should be colder.  We can't expect instantaneous response to the variations, for the same reasons that noon is not the hottest time of day, or that December 21st is not the coldest (northern hemisphere) or warmest (southern) day of the year.  The climate system takes time to respond.  But the cycle should be present.

When the sun is brighter, we expect both the stratosphere and the surface to be warmer.  Both see more energy coming from the sun.  Since the sun varies on about an 11 year cycle, we also expect solar-induced climate change to show an 11 year cycle.  Further, if you look at the solar output over the period of satellite record, we expect to see a slight cooling, if anything.  Solar output has a declining trend over this period.  We can also use the http://moregrumbinescience.blogspot.com/2008/09/summary1-of-simplest-climate-model.html">simplest climate model to estimate just how much temperature should change over the 11 year cycle, or due to the slight declining trend.  For the 1 Watt per square meter variation during a solar cycle (but remember to divide by 4 because the earth is not a flat disk always pointed squarely at the sun), we expect a temperature change of 0.05 C.  The much smaller declining trend in solar output would contribute, then, some much smaller than 0.05 C trend over the satellite period.

If carbon dioxide were the source of a climate change, we expect some rather different things than for those other mechanisms.  As with the solar changes, we expect the surface to warm.  Unlike with the sun, however, we expect the stratosphere to cool.  We can repeat the response calculation for the greenhouse gases.  They've contributed an increase of about 2.25 Watts per square meter (which is for all square meters of the earth's surface).  So with this being the only consideration, we'd expect about 0.45 C warming since pre-industrial times. 

All these descriptions, I hope it is obvious, are simplified.  You'd have a lot more work to do to publish in the scientific literature on climate change attribution.  On the other hand, while simplified, they all point in the correct directions.  (Namely, you'll see these directions in that same literature).

So now let's look at some climate observations, and see which mechanisms look most like that.

1) Over the past century, global mean temperatures have warmed by about 0.75 C (1.3 F).
-- that rate is 7.5 C per thousand years
2) The surface has warmed but the stratosphere has cooled
-- for the latter, see, for example, Thorne et al., 2005 Revisiting radiosonde upper air temperatures from 1958 to 2002, J. Geophysical Research, 110, D18105, doi:10.1029/2004JD005753.

How does this match against the mechanisms?

Orbital cycles: The observed magnitude could be explained, but it is 15 times too fast.  For the same reason as with solar input, this also fails to explain the second observation -- stratospheric cooling.

Volcanoes: The magnitude is ok, but the time scale is far too long.  Recovering from a major volcanic eruption would show a cooling of the stratosphere (as the aerosols fell out) and warming of the surface.  And it could provide a 0.75 C warming, but it would take only a few years, not a century. 

Solar output: Warmer sun would warm the stratosphere, which is the opposite of what is seen.  The trend in solar output is even or down, but the observed temperatures are substantial warming (satellite estimates for the lower atmosphere being about 0.5 C over the 30 years of satellite temperature observations).

Greenhouse gases: Cool the stratosphere, which is observed.  Warm the surface, which is observed.  On their own, should warm the earth by about 0.45 C, and a greater warming than that is observed.  This, too, is expected as water vapor is expect to act as a feedback.


In other words, the greenhouse gas candidate looks pretty much like a duck, it walks like a duck, it quacks like a duck, and it's about the same size as a duck.  In contrast, none of the other candidates look even vaguely ducklike.  You'll certainly want to continue research -- find more characteristics of greenhouse gas climate change, make more precise predictions for them, and get enough (and good enough) observations to match against your prediction.


* I know that some will complain about my separating out water vapor.  The thing is, water vapor is a responder to the other things that affect climate, not something that drives the changes in the first place.  Water cycles very rapidly through the atmosphere -- days to weeks.  If there's a bit too much at the moment, water vapor turns in to clouds, and then in to rain or snow -- taking it back out of the atmosphere.  If there is too little, you get more evaporation and rain and snow, until there's enough.  It is these other factors (dry greenhouse gases, sun, orbit, aerosols, ...) which set what is 'enough'.