P2C2: Toward Improved Projections of the Climate Response to Anthropogenic Forcing: Combining Paleoclimate Proxy and Instrumental Observations with an Earth System Model

PROJECT SUMMARY

We propose to combine paleoclimate and instrumental observations with a new Earth system Model of Intermediate Complexity (EMIC) to improve constraints on key climate parameters including those governing dynamical and potentially abrupt responses to forcing. We will focus on dynamical mechanisms associated with the El Nino/Southern Oscillation (ENSO) and the Atlantic Meridional Overturning Circulation (AMOC). We will adopt an existing EMIC (“LOVECLIM”) as a starting point. Using ‘anomaly coupling’ in the tropical Pacific, a new version of LOVECLIM is capable of simulating ENSO quite realistically. ENSO teleconnection patterns will be corrected statistically to provide more reliable assessments of extratropical expressions of ENSO such as changes in western United States precipitation/drought. We will assemble observational constraints from the paleoclimate and instrumental record spanning a period of roughly the last millennium. Specifically, we will use paleoclimate proxy information about:

  1. Surface temperature changes on hemispheric scales,
  2. Measures of ENSO-related changes in tropical Pacific climate (e.g. measures of mean state and variance) from fossil corals,
  3. Reconstructions of precipitation and drought from ENSO-sensitive regions such as the western U.S and equatorial East Africa, and (with appropriate caveats)
  4. Proxy evidence of changes in the AMOC in past centuries.

We will additionally use information from the instrumental record of: (i) surface air temperature changes, (ii) changes in ENSO, and (iii) changes in oceanic heat content, and (iv) oceanic density structure.

We hypothesize that the additional information available in the paleoclimate record, which, though noisier than modern observations, provides a considerable extension in the length of available observations, will reduce the uncertainty in probabilistic future projections of ENSO and the AMOC. The use of multiple paleoclimate and instrumental constraints would represent a significant advancement relative to past studies that used only a single constraint, such as the estimated Northern Hemisphere average temperature. We will assimilate observational constraints into the EMIC using a Bayesian framework and an ensemble of runs to obtain simultaneous model parameter estimates of vertical ocean diffusivity, climate sensitivity, tropical air-sea coupling strength, and atmospheric horizontal mixing. These improved model parameter estimates will then be used to derive an updated and, we anticipate, improved set of probabilistic projections of a suite of societally relevant climate variables. These projections will include not just the mean expected warming, but also the equally important potential changes in ENSO, the AMOC, and their associated impacts on extratropical precipitation and drought patterns, in response to future anthropogenic climate forcing.

Intellectual Merit: The project introduces a statistically rigorous framework for fusing information from the paleoclimate and instrumental record with the mechanistic understanding afforded by an Earth System Model. We expect the project to provide key new insights into dynamical responses associated with ENSO and the AMOC, the coupling between them, and their potential for giving rise to abrupt climate change in response to anthropogenic forcing. A potentially transformative element of the proposed research is an explicit framework for incorporating diverse paleoclimate information from past centuries into probabilistic projections of future climate change.

Broader Impacts: The project will provide (i) an improved representation of ENSO in an Earth system model of intermediate complexity, (ii) refined estimates of key climate parameters, and (iii) improved projections of potential changes in AMOC and ENSO (and associated drought impacts) in response to anthropogenic forcing. The project will provide a general methodological framework that can be exploited by the larger climate research community as improved proxy climate data become available, and/or as computational resources become available for employing more elaborate climate models in the required large ensemble runs. The project promises to further the professional development of several early career researchers through their inclusion in a novel, interdisciplinary Earth System project. Methods and results from the project will be integrated into undergraduate and graduate curricula.