Climate Change Meeting Report

Characterizing Superwarm Periods in Earth's History

DeepMIP Kickoff Meeting; Boulder, Colorado, 14–15 January 2016

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Periods of climate extremes in Earth’s history provide an exciting natural laboratory in which to explore. As anthropogenic greenhouse gas emissions warm our planet, it becomes important to look to past periods of extreme warmth to challenge our understanding of the relevant mechanisms and to test our climate models.

To this end, 36 scientists from across the modeling-observational spectrum met at the National Center for Atmospheric Research to share their latest exciting research and to plan the Deep-Time Model Intercomparison Project (DeepMIP). This model-data intercomparison focuses on the superwarm early Eocene period (about 50 million years ago), when average yearly temperatures were as much as 10°C warmer than today.

comparison-model-super-warm-eocene-future-warming
Model simulations of (left) the superwarm early Eocene climatic optimum, assuming 6 times the preindustrial carbon dioxide levels, and (right) a possible future, assuming 4 times the preindustrial carbon dioxide levels. The temperature scale represents warming (in °C) relative to preindustrial levels. Simulations were carried out using the Hadley Centre HadCM3L model. Credit: Daniel Lunt

DeepMIP—where “deep” refers generally to climates of the pre-Pliocene, more than 5.3 million years ago—is a working group within the larger Paleoclimate Modelling Intercomparison Project (PMIP), itself an official component of the Coupled Model Intercomparison Project Phase 6 (CMIP6). DeepMIP ultimately aims to evaluate state-of-the-art climate models under warm paleoclimate conditions, informing model developers and the Intergovernmental Panel on Climate Change.

The meeting began with a series of short talks in which all members were able to share their recent research. One of the most exciting revelations was the possibility of resurrecting previously discredited temperature records derived from measuring the oxygen-isotopic composition of calcite in the shells of microscopic marine surface-dwelling organisms, such as foraminifera. Many such records were thought to be rendered useless because of postdepositional changes in the calcite not connected to the original temperature (diagenesis). However, by making detailed measurements of single specimens in which microscopic regions of calcite are better preserved, such records might be remeasured. This measurement could ultimately provide a comprehensive global data set of sea surface temperatures during the superwarm climate of the early Eocene. Such a data set would be an invaluable tool for evaluating climate model simulations of this time period.

Another goal of the meeting was to come to a consensus on a model experimental design, describing in detail the experimental protocols for the intercomparison. This design, which was the main outcome of the meeting, is under review for publication in the journal Geoscientific Model Development.

Such discussions are normally carried out by the modeling community in isolation. However, the expertise of the data community present proved hugely useful, ensuring not only that the core model simulations would be as realistic as possible but also that associated sensitivity studies would fully reflect the uncertainty in our knowledge of the key forcings that made the Eocene so warm. In this regard, working groups were set up to focus on reconstructing atmospheric carbon dioxide levels, assessing uncertainty in paleogeography, and developing the marine and terrestrial data reconstructions.

Finally, the meeting highlighted the fact that our climate models are still not reproducing the level of high-latitude warmth seen unequivocally in the proxy records. This is a decades-old problem in paleoclimate science. The challenge to the data community over the coming years of DeepMIP is to rigorously characterize the uncertainties in the proxy records, using multiple proxies where possible, and to target new records in undersampled regions.

The challenge to the modeling community is whether the recent increases in model resolution and in the scope and complexity of Earth system components and the improved process-based representation of processes such as convection will mean that, finally, our models will be able to recreate the superwarm climate of the Eocene. If they do, the scientific community’s confidence in future climate simulations will also increase.

—Daniel J. Lunt, School of Geographical Sciences, University of Bristol, Bristol, U.K.; email: [email protected]

Citation: Lunt, D. J. (2016), Characterizing superwarm periods in Earth’s history, Eos, 97, doi:10.1029/2016EO056275. Published on 02 August 2016.
© 2016. The authors. CC BY-NC-ND 3.0
  • davidlaing

    There is no hard evidence, experimental or observational, that “anthropogenic greenhouse gas emissions warm our planet,” nor is “consensus” worth anything at all in comparison to hard data. Models are not science. They are suppositions, only, and they depend on data to be worth using. What has been completely neglected is the warming effect of non-explosive volcanism eroding Earth’s ozone layer and letting in more solar UV-B radiation, and the “consensus” simply assumes that it’s all due to CO2. If it’s really erupted chlorine, then the PETM (Paleocen-Eocene Thermal Maximum) becomes perfectly logical. Earth’s tectonic plates have become spread out since then, and their trailing edges, where non-explosive volcanism mainly occurs, have sunk, ineffectively, below sea level. Thus the explosive volcanoes, located over subduction zones on the plates’ leading edges have the advantage, and they have progressively cooled Earth with injections of aerosols. Lesson: simplistic assumptions lead to some very enigmatic results that are hard to explain. Seeing the bigger picture tends to do away with the “clever ideas” and the unnecessary arm-waving in order to try to explain things, and clears things up.