Friday, March 22, 2013

The Egg Was First, No, the Chicken, No Never Mind

An evergreen in Eli's business is which came first exiting the ice age, the temperature or the greenhouse gas concentrations.  Since the forcing is orbital changes, it is reasonable to expect at least some initial lag in the greenhouse gas and that includes water vapor, but the length of this lag has been an issue ever since Monnin, et al's analysis of the EPICA Dome C core inferred a lag of 800 + 200 years.  This has become an evergreen amongst the less serious, and indeed, there has even been considerable discussion by reasonable folk. Jeff Severinghaus provided an answer

So one should not claim that greenhouse gases are the major cause of the ice ages. No credible scientist has argued that position (even though Al Gore implied as much in his movie). The fundamental driver has long been thought, and continues to be thought, to be the distribution of sunshine over the Earth’s surface as it is modified by orbital variations. This hypothesis was proposed by James Croll in the 19th century, mathematically refined by Milankovitch in the 1940s, and continues to pass numerous critical tests even today.
The greenhouse gases are best regarded as a biogeochemical feedback, initiated by the orbital variations, but then feeding back to amplify the warming once it is already underway. By the way, the lag of CO2 of about 1000 years corresponds rather closely to the expected time it takes to flush excess respiration-derived CO2 out of the deep ocean via natural ocean currents. So the lag is quite close to what would be expected, if CO2 were acting as a feedback.
As pointed out by EJ Brook, the experimental  issue is that for this core it is untrivial to correlate the age of the ice along the core with the age of the gas in it. There has been significant work on this recently.  Pedro, Rasmussen, and van Ommen narrowed the lag to less than 400 years by looking at different Antarctic cores ( these cores had their own problems).  This month, Parrenin and colleagues (here as a pdf) reanalyzed the EPICA Dome C core by using 15 N enrichment to determine the depth at which air is locked in providing better relative dating. (TI is Termination I coming out of the last ice age, aCO2 is CO2 atmospheric concentration, AT is temperature anomaly)
We infer the aCO2-AT phasing at the four break points using a Monte-Carlo algorithm (supplementary materials): the onset of TI (Transition I) (10 ± 160 years, 1σ, aCO2 leads), the onset of the Bølling oscillation (–260 ± 130 years, AT leads), the onset of the Younger Dryas (60 ± 120 years, aCO2 leads), and the onset of the Holocene (–500 ± 90 years, AT leads). The uncertainty takes into account the uncertainty in the determination of the break points and the uncertainty in the determination of Δdepth. The only significant aCO2-AT lags are observed at the onsets of the Bølling oscillation and the Holocene. It should be noted that during these two events, the associated sharp increases in aCO2 were probably larger and more abrupt than the signals recorded in the ice core, due to the diffusion in the gas recording process (17). This atmosphere–ice core difference biases our break point determination toward younger ages. If we use these fast increases to determine the break points in aCO2, we find a lag of –10 ± 130 years (1σ) for the Bølling onset and –130 ± 90 years (1σ) for the Holocene onset; that is, no significant phasing. If, instead of using aCO2 we use the radiative forcing of aCO2 (18) [rCO2 = 5.35 W/m2 ln(CO2/280 parts per million by volume)], the inferred phasing is not significantly changed


Of course, bunnies also need to think about how this ties in with the picture painted by Shakun, et al previously described at RR

4 comments:

Steve Bloom said...

To clarify for the bunnies, the Severinghaus statement is only true for more or less static boundary conditions, i.e. in the geologic short term. Those conditions can and do change over time; e.g. plate tectonic processes eventually will raise CO2 high enough so that orbital cycles can no longer trigger ice ages. (This BTW will start to occur in a few tens of millions of years when the continents reach maximum dispersion and begin the long trip back toward unity.) Or too-clever-for-their-own-good plains apes might think of a way to add enough CO2 to warm things up in a big hurry.

Anonymous said...

It is an error to consider ice ages as a consequence of global average temperature.

The orbital variation which causes them doesn't, after all, change the global average top of the atmosphere insolation. It does change theregional ( Arctic and high northern latitudes ) and seasonal ( summer ) insolation.

Neo Nate

old_salt said...

And, one final point, temperature changes in the northern polar region will affect the south and deep ocean circulation. So, comparing CO2 to southern polar temperatures may not be the appropriate metric. Most everybody except those that poorly understand think that CO2 change is a feedback to solar orbital forcing.

William Connolley said...

> This hypothesis was proposed by James Croll in the 19th century, mathematically refined by Milankovitch in the 1940s,

That's a grotesque over-simplification. M was badly wrong; indeed the entire theory is an excellent exemplar of something that was continually got wrong (copyright Stoat, 2005: http://mustelid.blogspot.co.uk/2005_09_01_archive.html).

I put together some notes on this on wiki:

http://en.wikipedia.org/w/index.php?title=Talk:Milankovitch_cycles&oldid=534157937#Needs_more_work