The paper represents a sharp departure from a long-standing paradigm in paleoclimatology, which holds that the tropical Pacific ocean-atmosphere circulation during the peak of the last ice age was comparable to a persistent or prevailing La Niña, a circulation mode marked by enhanced equatorial upwelling, east-west temperature contrast, and zonal trade winds. We proposed on the basis of new paleoceanographic data that the more proper analogy is instead with El Niño, i.e. marked by weaker upwelling, weaker temperature gradients, and weaker winds near the equator. This new hypothesis challenges the old premises in the field of tropical paleoclimatology, and provides a novel framework for reinterpreting parallel lines of evidence, data, and mechanisms relating to Earth's climate history of the last 30,000 years and beyond. Many authors are finding this a compelling and intriguing new concept and are eager to examine their own work in its light.

  Does it describe a new discovery or a new methodology that's useful to others?

The rational for distinguishing between ancient El Nino or La Nina conditions relied on the pattern of ocean temperature in space: El Nino produces uniform temperatures near the equator, whereas La Nina produces sharp gradients, or fronts.

The paper describes a previously unrecognized and counter-intuitive pattern of climate change over ice age cycles. It presents a new paleoclimate record, unique because of its strategic location near a key oceanographic feature of the tropical Pacific Ocean known as the equatorial cold tongue, which is formed by divergent upwelling at the equator.

Sea surface temperature in the cold tongue is very sensitive to the prevailing wind regime and is a robust index for the strength of upwelling which in turn is related to the basin-wide circulation mode (e.g. El Niño vs. La Niña), and has global climatic repercussions. Aside from the inherent value of the record itself, the main conclusion relied on a previously underutilized (though well-known) concept, namely that the oceanographic front between the equatorial cold tongue and the Intertropical Convergence Zone (ITCZ) north of the equator strengthens and weakens very predictably with ENSO (El Niño/Southern-Oscillation). By tracking the change in this front in the past it is possible to deduce ENSO-like oceanographic variability, as well as shifts in the latitude of the ITCZ, both highly useful descriptors of past tropical climate states. For the case of the last glacial maximum the data indicated a shift toward an El Niño-like circulation mode and a southward shift of the ITCZ relative to the Holocene (the present interglacial period).

  Could you summarize the significance of your paper in layman's terms?

The last glacial maximum, i.e. the peak of the last ice age approximately 20,000 years ago, was a time when a profoundly different, much colder climate prevailed around the globe. While ample evidence from temperate and polar regions bears testament to much colder, drier, and stormier conditions, the situation in the tropical regions, including the oceans, atmosphere, and continents, is far less clear. Of central interest is the issue of ice age conditions in the tropical Pacific Ocean, because today that region dominates global year-to-year climate variability through the quasi-periodic El Niño/Southern Oscillation (ENSO). A plausible but speculative extension of ENSO theory holds that long-term changes in the character of ENSO, e.g. the frequency, intensity, and duration of El Niño and La Niña episodes, may occur in tandem with, or even be the cause of, long-period climate cycles such as the ice ages. In extreme cases the tropical Pacific may even become locked in a perennial El Niño- or La Niña-like circulation with hard-to-predict consequences for global climate. So far the prevailing opinion has been that La Niña conditions, manifested today by the occasional appearance of unusually cold waters in the eastern equatorial Pacific, would be the more appropriate analog for the long-term ice age circulation. But our work suggested the opposite, i.e. El Niño-like conditions seem to have prevailed. The results were based on the concentration of magnesium in skeletal remains of foraminifera (a type of plankton organisms) preserved in ocean sediments, which is related to the water temperature at the time these organisms lived. The rationale for distinguishing between ancient El Niño or La Niña conditions relied on the pattern of ocean temperature in space: El Niño produces uniform temperatures near the equator, whereas La Niña produces sharp gradients, or fronts. The relative absence of fronts can be a tell-tale sign of El Niño, and this is what our data indicated. This unexpected result has forced a rethinking of basic tropical-extratropical climate interactions over ice age cycles, and has implications for the evolution of the modern climate and its future course.

This work was carried out for my doctoral thesis at Columbia University with my advisor, Professor Jean Lynch-Stieglitz and our co-workers, Tom Marchitto (University of Colorado) and Julian Sachs (Massachusetts Institute of Technology). The initial idea was to produce a body of paleoceanographic data from the eastern Pacific to test recent theories about the role of ENSO variability on glacial-interglacial cycles. The project began as a rather broad data collection enterprise and for some time it proved challenging to make sense of the data we were generating because they ran contrary to mainstream views. It took careful rethinking and reevaluation of long-standing assumptions to reach a consistent interpretation and formulate a plausible alternative hypothesis.