From Greenhouse to Icehouse: Understanding Earth's Climate Extremes Through Models and Proxies.

Abstract

On geologic time scales, Earth has fluctuated between greenhouse and icehouse climates. Understanding the mechanisms responsible for these disparate climate states provides valuable insight into long-term climate forecasts. During the Quaternary (2.6-0 Ma), there were a series of large glaciations. The pacing of these glacial cycles is often attributed to orbitally controlled high-latitude summer insolation, because it influences the amount of ice melt. However, this relationship is not well reflected in ice-volume records. For instance, in the early Pleistocene (2.6-0.8 Ma), glacial cycles oscillated mainly with obliquity while summer insolation varied most strongly with precession. Here, Earth system model simulations show that a combination of albedo feedbacks, seasonal offset of precession forcing, and orbital cycle duration differences amplified the ice-volume response to obliquity relative to precession; these results help explain the paradox of the early Pleistocene glacial cycles.

Another enigma of Quaternary is the transition from 41 to 100 kyr glacial cycles with ~50 m greater sea level variability, which arose despite little change in CO2 or orbital forcing. The regolith hypothesis provides a potential explanation for this transition. It posits that glacial cycles gradually eroded pre-existing high-latitude regolith, causing a change in ice sheet response to orbital forcing as the ice bed transitioned from low-friction sediment to high-friction bedrock. Earth system model results provide support for the regolith hypothesis; only with reduced basal sliding does the 100 kyr ice-volume signal of the late Pleistocene (0.8-0 Ma) appear in the simulated ice-volume cycles.

In contrast to the Quaternary, the Cretaceous (145-66 Ma) was a greenhouse climate. Nevertheless, evidence suggests significant climate changes occurred during this period, including a dramatic cooling from the Cenomanian (100-94 Ma) to Maastrichtian (72-66 Ma). Here, two Earth system models and a compilation of proxy records are used to explore the hypotheses that Late Cretaceous (100-66 Ma) cooling was in response to changes in geography or CO2. Results show that a decrease in CO2 is necessary to explain the proxy identified cooling across the Late Cretaceous. However, tectonic evolution caused substantial regional climate changes that must be considered when interpreting proxy records.