The Community Earth System Model (CESM) Large Ensemble Project: A Community Resource for Studying Climate Change in the Presence of Internal Climate Variability

Abstract

While internal climate variability is known to affect climate projections, its influence is often underappreciated and confused with model error. Why? In general, modeling centers contribute a small number of realizations to international climate model assessments [e.g., phase 5 of the Coupled Model Intercomparison Project (CMIP5)]. As a result, model error and internal climate variability are difficult, and at times impossible, to disentangle. In response, the Community Earth System Model (CESM) community designed the CESM Large Ensemble (CESM-LE) with the explicit goal of enabling assessment of climate change in the presence of internal climate variability. All CESM-LE simulations use a single CMIP5 model (CESM with the Community Atmosphere Model, version 5). The core simulations replay the twenty to twenty-first century (1920–2100) 30 times under historical and representative concentration pathway 8.5 external forcing with small initial condition differences. Two companion 1000+-yr-long preindustrial control simulations (fully coupled, prognostic atmosphere and land only) allow assessment of internal climate variability in the absence of climate change. Comprehensive outputs, including many daily fields, are available as single-variable time series on the Earth System Grid for anyone to use. Early results demonstrate the substantial influence of internal climate variability on twentieth- to twenty-first-century climate trajectories. Global warming hiatus decades occur, similar to those recently observed. Internal climate variability alone can produce projection spread comparable to that in CMIP5. Scientists and stakeholders can use CESM-LE outputs to help interpret the observational record, to understand projection spread and to plan for a range of possible futures influenced by both internal climate variability and forced climate change.

Spatial Patterns of Sea Level Variability Associated with Natural Internal Climate Modes

“…Efforts are being made in this line. For example, the National Center for Atmospheric Research (NCAR) has completed a 42-member ensemble experiment using the Community Earth System Model (CESM) from 1920 to 2100, with historical all forcing from 1920 to 2005 and RCP8.5 trajectory since 2006 (Kay et al 2015). The 42-member ensemble mean will damp internal climate modes and thus measure the effect of external forcing.…”

Surface and top-of-atmosphere radiative feedback kernels for CESM-CAM5

“…These radiative feedback kernels were calculated with CESM version 1.1.2, the same as that used for the 40-member CESM large ensemble (Kay et al, 2015). The TOA kernels are an update from CAM3 .…”

Impacts of internal climate variability on meteorological drought changes in China

“…Dong and Dai 2015). The ensemble mean (ENS) represents the model-simulated response to the EF, while the spread among ensemble members manifests the ICV effects (Kay et al 2015). This is in contrast to the ensemble simulations from multiple climate models (e.g.…”

“…On regional scales, ICV is a dominant factor for drought and precipitation changes, even though the EF signal is detectable in global-mean drought indices (Dai 2013;Dai and Zhao 2016;Zhao and Dai 2016). The ICV may be as important as anthropogenic climate change at regional scales in particular, at middle and high latitudes and for precipitation ensemble 1 generated initial condition with slight perturbations (Kay et al 2015). Differences among all CESM_LE members only come from the internal variability of the climate system since the influence of the initial condition would be demised only for couple weeks (e.g.…”

“…The Community Earth System Model Large Ensemble (CESM_LE) project is one such project. It provides a set of 40-ensemble simulations for the period 1920-2100 using historical forcing and the RCP8.5 scenario (Kay et al 2015). The CESM_LE products contain long-term and high-resolution meteorological and hydrological variables, which facilitate the investigation of the relative attributions of climate change due to ICV and EF, respectively.…”