Perspectives on Paleoclimate - the growth of a modern science

Dr. Joseph D. Ortiz
Lamont-Doherty Earth Observatory of Columbia University

Saturday Workshops for Educators
January 27, 2001

What processes influence climate on time scales longer than the observational record? On time scales of a few years, climate can be described as the long-term background or "average weather" for a given region. This is easily seen by the predictable change in seasonal temperatures. On much longer time scales however, regional and global climate actually exhibits considerable variability, much of which is driven by external forcing.

The objective of this workshop is to provide teachers with some historical insights into the development of paleoclimate as a modern field of research.  The roots of this modern science can be traced back at least to the 1830's when Louis Agassiz proposed that geographic features of the European landscape were formed by the movement of great ice sheets in the distance past.  Such ideas were radical in their time because they implied the Earth was both old and had experienced dramatic changes in its distant past. These ideas sparked considerable scientific debate in the decades to come. By the mid 1860's James Croll published an astronomical theory that hypothesized the ice ages arose from the precession of the seasons and the eccentricity of the Earth's orbit.  Croll's work was breathtaking for its time because he introduced important concepts such as the climate feedback and posed his questions in terms of testable hypotheses. He was limited however by the lack of computational resources needed to fully describe the variations in Earth's orbit through time, and he lacked a continuous climate record against which he could test his hypothesis.  Some of these limitations were overcome by Milutin Milankovitch in the 1920's who had access to an improved conceptual model of Earth's orbital variability, which now included terms for changes in the tilt of the Earth's axis through time. Despite these advances, Milankovitch like Croll before him, still lacked access to continuous records of climate change.

The evidence needed to test the Croll/Milankovitch hypothesis would not be discovered until the 1950's when methods to recover long, deep sea sediment cores became routine. In the decades that followed, earth scientists developed the tools and methods needed to generate (and date with considerable precision) continuous climate records that relate to changes in terrestrial ice volume. Surprisingly, this climate proxy was developed from subtle changes in the ratio of oxygen isotopes preserved in the shells of microscopic marine organisms! These data finally provided the information needed to conduct a quantitative test of the Milankovitch hypothesis. Much of this research was conducted here at the Lamont-Doherty Earth Observatory.

Considerable progress has been made in the past decades toward unraveling the mechanisms of climate change. It is now generally well accepted that changes in the Earth's eccentricity (~100,000 year cycles in the shape and of the Earth's orbit around the sun and thus the mean Earth-Sun distance), precession (~20,000 year cycles in the timing of the seasons in relations to the Earth's orbital position) and obliquity (~41,000 year cycles in tilt of the Earth's equatorial axis relative to the Earth-Sun plane) play a critical role in the shifts from glacial to interglacial climates on geologic time scales.  Linear scaling of these three terms suggests that on average, obliquity effects should dominate, followed by precession effects, and finally the subtle eccentricity changes.

Despite these vast scientific advances, considerable climate riddles remain. Why does the record of climate over the past million years exhibit a switch in dominance from the 41,000 year obliquity cycle to a dominant 100,000 year cycle?  Recently, climate changes nearly as large as the orbitally paced glacial - interglacial cycles have been discovered on much more rapid timescale. What factors internal or external to the climate system case these changes? These are questions of active research. Results obtained by Leg 162 of the Ocean Drilling Program proved some insights into questions such as these.