TITLE:

THE MATHEMATICS OF PALEOCLIMATE RECONSTRUCTIONS

DATE:

Friday, Sep 28th, 2012

TIME:

3:30 PM

LOCATION:

GMCS 214

SPEAKER:

Julien Emile-Geay.
Department of Earth Sciences and Center for Applied Mathematical Sciences.
University of Souther California.

ABSTRACT:

In 1998, a seminal study by Mann, Bradley and Hughes took advantage of climate signals embedded in an array of high-resolution paleoclimate proxy data to conclude that “Northern Hemisphere mean annual temperatures for three of the past eight years are warmer than any other year since (at least) AD 1400.” The so-called “hockey stick” reconstruction showed relatively stable temperatures for most of the millennium, until the start of the Industrial Revolution, when reconstructed temperatures began a steep rise to a level not seen in the last millennium.
Since 2001, when the third assessment report by the Intergovernmental Panel on Climate Change featured the √¢¬Ä¬úhockey stick√¢¬Ä¬ù prominently, this graph has become the emblem of the debate on anthropogenic global warming. No other picture conveys how anomalous recent climate change is in the context of natural variations in temperature over the past millennium. Defended as definitive proof of global warming by many climate scientists and sympathetic members of the public, hailed as a “misguided and illegitimate investigation” by some politicians, it remains one of the most hotly debated climate studies ever published. After many challenges on the blogosphere and a questionable a congressional inquiry conducted under the aegis of Edward Wegman (then president of the American Statistical Association), most statisticians are now convinced that the “hockey stick” is a fluke due to the overfitting of noisy data.

In this talk, I will describe a new climate field reconstruction method using Markov Random Fields. The new method’s performance is assessed in a realistic geophysical context and shown to outperform competing methods in the most relevant cases. Applications to global and regional climate reconstructions over the past millennium will be presented, together with objective measures of uncertainty. Consequences for the “hockey stick” debate will be discussed.

HOST:

Dr. Sam Shen.

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