The MWP has been vigorously argued to be a regional northern hemisphere phenomenon only, but this new study finds it in South America. In this new paper they write:
“The most striking features in the reconstruction are the warm temperatures from ∼1050 to ∼1300 AD compared to the preceding and following centuries, the persistent cooler temperatures from ∼1400 to ∼1800 AD, and the subsequent rise to warmer temperatures which eventually seem to exceed in the last decades of the 20th century the range of past variation.
While the onset of the warm period around 1050 AD is almost consistent within the dating error (±60 years at 1000 AD) with the perception of a Northern Hemisphere (NH) MWP from 950 to 1100 AD, it is remarkable to note that it seems to have lasted about 200 years longer in the tropics.”
When I first saw this paper (PDF here h/t to Dr. Leif Svalgaard) I was intrigued by the idea, especially since it demonstrated the MWP very well. The authors also say that “The last decades of the past millennium are characterized again by warm temperatures that seem to be unprecedented in the context of the last ∼1600 years.”
But then a question arose in my mind; how well have they separated all of the Ammonium from worldwide Ammonium fertilizer use (which started in the late 1800’s) from the proxy samples? They do touch on the subject, but I’m not convinced that they have separated the impact. They cite Mann and Jones in the references, and use principal component analysis methods (PCA) so that makes me wonder even more. Perhaps Steve McIntyre or Jeff Id can have a look to see how the PCA was carried out, since I’m no expert in it.
The authors do mention dust accumulation related to tracking South American weather patterns, saying they appear unchanged, but I’m not sure dust is a good indicator of lack of mixing and transport via the global water cycle of Ammonium (NH3 and NH4) which is highly soluble in water.
Chemical fertilizers were introduced in the mid 1800’s; Ammonium Sulfate and Ammonium Nitrate were commonly used. Ammonium Sulfate was originally a by-product from coal gas manufacturing. The quality wasn’t great as a fertilizer but it was used some in the 1800’s.
Ammonium Nitrate was used mostly after World War I as it was the principal ingredient used in explosives. WWI demand increased Ammonium Nitrate production and following the war, was used in agriculture in significant volume. World War II caused even more of an increase in production, and again after WWII production increased globally.
UPDATE: reader “Jaymam” points out this graph, production of Ammonia
When rainwater and Ammonium Nitrate NH4NO3 mix, you get ammonium ions (NH4+) and nitrate ions (NO3–) in solution. Some nitrogen gets used by plants, the remainder is either retained in the soil or transported in runoff. As we all know, rainwater runoff of fertilizer is a big problem, and it is clear that ammonium ions get into the water cycle. This is confirmed by Tiwari et al who did a study of ions in rainwater and found “High concentrations of ammonium in rainwater due to agricultural activity…”
So with increasing use of Ammonium fertilizers in the 20th century, how can we be sure it hasn’t contaminated the Ammonium ice proxy record during the same century?
Still, I find the study interesting for it’s uniqueness.
Ammonium concentration in ice cores: A new proxy for regional temperature reconstruction?
T. Kellerhals,1,2,3,4 S. Brütsch,2 M. Sigl,1,2 S. Knüsel,1,2 H. W. Gäggeler,1,2
and M. Schwikowski2,3 Received 15 June 2009; revised 22 December 2009; accepted 8 March 2010; published 31 August 2010.
 We present a reconstruction of tropical South American temperature anomalies over
the last ∼1600 years. The reconstruction is based on a highly resolved and carefully dated
ammonium record from an ice core that was drilled in 1999 on Nevado Illimani in the
eastern Bolivian Andes. Concerning the relevant processes governing the observed
correlation between ammonium concentrations and temperature anomalies, we discuss
anthropogenic emissions, biomass burning, and precipitation changes but clearly favor a
temperature‐dependent source strength of the vegetation in the Amazon Basin. That given, the reconstruction reveals that Medieval Warm Period– and Little Ice Age–type episodes are distinguishable in tropical South America, a region for which until now only very limited temperature proxy data have been available. For the time period from about 1050 to 1300 AD, our reconstruction shows relatively warm conditions that are followed by cooler conditions from the 15th to the 18th century, when temperatures dropped by up to 0.6°C below the 1961–1990 average. The last decades of the past millennium are
characterized again by warm temperatures that seem to be unprecedented in the context of the last ∼1600 years.
Citation: Kellerhals, T., S. Brütsch, M. Sigl, S. Knüsel, H. W. Gäggeler, and M. Schwikowski (2010), Ammonium concentration in ice cores: A new proxy for regional temperature reconstruction?, J. Geophys. Res., 115, D16123,