Environmental Science and Engineering Seminar

Models disagree about the average state of the tropical Pacific when subjected to enhanced greenhouse gas forcing and this uncertainty emphasizes the importance of reconstructing past variability in tropical Pacific climate, through episodes of known radiative forcing. Measurements of the modern and ice age tropical Pacific allow us to reconstruct the zonal gradient in nutrients over the past 4 million years—a gradient principally responding to upper ocean dynamics. Variability of eastern equatorial Pacific upwelling strength/thermocline depth (as inferred from the zonal nutrient gradient) over the Pleistocene was highly correlated with local boreal fall seasonal insolation and therefore planetary precession. By contrast, upwelling strength/thermocline depth at this time was insensitive to the 100,000-year cycle that dominated other aspects of the Pleistocene ice ages, including greenhouse gas concentrations. Additional measurements from the Pliocene suggest that, despite greenhouse gas concentrations as high as the present and a suggested Pliocene "permanent El Niño," Pliocene nutrient dynamics resemble the late Pleistocene. These results suggest that this aspect of the tropical Pacific climate-system (eastern upwelling/thermocline depth) is insensitive to changes in greenhouse gas radiative forcing.