Greenhouse to icehouse: Understanding the role of CO2 and non-CO2 forcings in warm climate intervals
Abstract
The Earth system has evolved significantly over the past 65 million years. A relatively ice free world dominated the Eocene ∼45 million years ago (Ma), until the late Oligocene (∼34 Ma) when the Antarctic Ice Sheet (AIS) developed in relatively short time period. Throughout the Oligocene and Miocene (23 to 5.3 Ma) temperatures gradually decreased as atmospheric CO2 continued to fall, vegetation biomes shifted, ocean circulation moved into its modern positions, and ocean gateways opened and closed. This transition from the warm and humid Eocene climate to the icehouse world we currently live has largely been attributed to a gradual decline in atmospheric CO 2. Acknowledging the fact that CO2 was the dominant driver in the gradual cooling over the last 65 million years, here we explore the less constrained feedbacks and forcings within the Earth system. These non-CO 2 forcings are important and could prove pivotal as we continue to constrain future climate prediction. Here we explore the climatic impact and forcing of the AIS, the oceanic response to AIS forcing, the temperature and precipitation patterns induced by changes in the El Niño southern Oscillation, and the impacts of El Niño and AIS forcing in the mid-Miocene Climatic Optimum (MMCO). Specifically, we find that the distribution of sea surface temperature (SSTs) in the eastern equatorial pacific has a teleconnected fingerprint throughout the world and more El Niño like conditions is a possible explanation of the wetter conditions in the mid-latitudes during the Pliocene and Miocene. The effective forcing and temperature impact of the Antarctic Ice Sheet depends on the mean climate state as modern climate responds differently to removing the AIS than at the Eocene-Oligocene transition and during the MMCO. The differing temperature and climate sensitivity response is largely controlled by low cloud and sea-ice feedbacks during these time periods and the efficacy of AIS forcing in the Eocene is not necessarily close to one and is likely to be model and state dependent. We also find that adding the AIS into the unglaciated Eocene world cools the deep ocean comparable to previous modelling studies that opened southern ocean gateways. The modelled δ18O anomaly induced by glaciation is comparable to the change detected in the proxy records across the transition suggesting that the AIS can induce changes in ocean circulation and thermal structure, thus reversing the hypothesis that gateways caused a reorganization of ocean circulation and glaciation across the EOT. Finally, Simulating the MMCO at 400 ppm CO2 using a recently released state of the art modelling framework produces a model data mismatch in global MAT and at high latitudes. The discrepancy is comparable to that introduced by a full doubling of CO2. It is noteworthy that including two of the most discussed Earth system feedbacks (El Niño and reduced ice volume) had small impacts on improving the model predictions even when we included uncertainty from orbital forcing. In summary, the Earth system is complex and explaining the warmth in past greenhouse climates requires many changes to boundary conditions, the right climate modelling framework, and better understanding of the non-CO 2 climate forcings.
Degree
Ph.D.
Advisors
Huber, Purdue University.
Subject Area
Climate Change|Paleoclimate Science|Atmospheric sciences
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