"Café des Sciences" on March 25th - Julien Emile-Geay
Café des Sciences on March 25th 2010
Speaker : Julien Emile-Geay, Ph.D.
Title : Of Climate and Men
Subject : Climate dynamics
The speaker for the third edition of the "Café des Sciences" in Los Angeles is Julien Emile-Geay, Assistant Professor at USC Department of Earth Sciences. Julien Emile-Geay graduated from the "Ecole Normale Supérieure de Paris" with a major in Earth Sciences and received a Master of Sciences degree in Ocean and Atmosphere Dynamics from the "Université Paris VI" and "Ecole Normale Supérieure de Paris". He then obtained his Ph.D. from the Columbia University in New York after studying ENSO dynamics and the Earth’s climate. He joined USC after a post-doctoral fellowship at Georgia Tech, on the tropical Pacific’s role in low-frequency climate change of the last millennium.
In the first part of his seminar, Julien Emile-Geay presented the present understanding of Earth’s climatic system and in particular the greenhouse effect. He explained this phenomenon by reviewing the interaction between solar rays and two major atmospheric compounds : water and carbon dioxide (CO2) in gaz forms. Contrary to what is commonly thought, 60% of the greenhouse effect is due to water, thanks to its high concentration in the atmosphere. However, even if CO2 has a minor role in the climate systems, it is one of the main protagonists in climate change. This stems from its long life span of the molecule in the atmosphere (400 years as compared to 10 days for a water molecule) and its lower concentration in the air, making it more sensitive to variations provoked by human emissions. In addition, a long-lasting correlation exists between Earth’s temperature and the concentration of CO2 in the atmosphere. However, this system involves a great number of parameters - most of them varying with each other - so that the carbon cycle and the climate variations form a positive feedback loop. In conclusion, though the greenhouse effect is physically well-know, there are lots of other non-linear effects that are the cause of most of the uncertainties in climate prediction.
Julien Emile-Geay then illustrated the influence of climate on human development. The geological era called Holocene - spanning the last 10,000 years - is characterized by a remarkable temperature stability at the scale of the planet. This stability was probably one of the important factors in the development of sedentary civilizations, as human tribes were no longer compelled to nomadism by climate variations. Yet, human civilizations remain very sensitive to climate variations, however small in amplitude and localized they may appear at the geological scale. Two examples illustrate this : the Akkadian Empire - an offspring of Sumer - and the Anasazi Indians who lived in the Southern parts of the United States. In spite of their knowledge of advanced agricultural techniques and good adaptation to their natural environments, these two civilizations waned after a several years of drought induced by minute climate changes. Interdependency between regions in the vast Akkadian Empire and a burst in Anasazi demograpy a few years before the droughts are two samples of the sensitivities to climate change displayed by human advanced civilizations.
To conclude, Julien Emile-Geay presented the previsions in terms of climate change such as changes in pluviometry at the planet scale or ice melting at the poles and in major mountains as well as their expected consequences on human activity. The bulk of these previsions points to increasing issues in water supply for our civilization. Hence the questions : is our modern civilization really less sensitive to climate changes than the Akkadians and the Anasazi and will we be able to learn from the past mistakes? These questions constituted the core of the following discussions.