LongRunMIP: Motivation and Design for a Large Collection of Millennial-Length AOGCM Simulations

Abstract

We present a model intercomparison project, LongRunMIP, the first collection of millennial-length (1,000+ years) simulations of complex coupled climate models with a representation of ocean, atmosphere, sea ice, and land surface, and their interactions. Standard model simulations are generally only a few hundred years long. However, modeling the long-term equilibration in response to radiative forcing perturbation is important for understanding many climate phenomena, such as the evolution of ocean circulation, time- and temperature-dependent feedbacks, and the differentiation of forced signal and internal variability. The aim of LongRunMIP is to facilitate research into these questions by serving as an archive for simulations that capture as much of this equilibration as possible. The only requirement to participate in LongRunMIP is to contribute a simulation with elevated, constant CO2 forcing that lasts at least 1,000 years. LongRunMIP is an MIP of opportunity in that the simulations were mostly performed prior to the conception of the archive without an agreed-upon set of experiments. For most models, the archive contains a preindustrial control simulation and simulations with an idealized (typically abrupt) CO2 forcing. We collect 2D surface and top-of-atmosphere fields and 3D ocean temperature and salinity fields. Here, we document the collection of simulations and discuss initial results, including the evolution of surface and deep ocean temperature and cloud radiative effects. As of October 2019, the collection includes 50 simulations of 15 models by 10 modeling centers. The data of LongRunMIP are publicly available. We encourage submissions of more simulations in the future.

Escalating global exposure to compound heat-humidity extremes with warming

“…In this context, it should be recognized that our analysis-which is based on transient climates following RCP8.5 and RCP4.5 trajectories over the 21st century-likely understates the long-term benefits of temperature stabilization. Compared to a stabilized global climate, the transient response of surface temperature to radiative forcing shows a spatial pattern with elevated warming over land and the Northern Hemisphere, whereas in a stabilized climate a 'recalcitrant' pattern emerges with more hemispheric symmetry and a more even land/ocean warming pattern [37,49,50]. It can thus be expected that, compared to a climate that has stabilized for many centuries, a transient warming state with the same GSAT imposes higher heat stress on the global population, which virtually all dwells on land.…”

Spatial Radiative Feedbacks from Internal Variability Using Multiple Regression

“…The terms l 1,1 and l 2,2 are local radiative feedbacks, while l 1,2 and l 2,1 are nonlocal radiative feedbacks (where our sign convention ensures that a negative l implies a negative, stabilizing feedback). Nonlocal radiative feedbacks (Rugenstein et al 2016;Zhou et al 2017;Po-Chedley et al 2018;Dong et al 2019) are changes in a region's top-of-atmosphere flux that occur due to changes in surface temperature elsewhere, independent of local surface temperature changes. For example, in Fig.…”

“…To test the methods discussed above on atmosphereocean general circulation models (AOGCMs), we use simulations from LongRunMIP, an archive of fully coupled millennial-length simulations of complex climate models (Rugenstein et al 2019). We chose the six models with millennial-length control and abrupt43 simulations for which we have monthly output.…”

“…These observational methods often assume that the climate feedback is constant, but many studies have shown that it typically changes with time in simulations (e.g., Murphy 1995;Watterson 2000;Senior and Mitchell 2000;Armour et al 2013;Jonko et al 2013;Andrews et al 2015). While the temperature dependence of feedbacks can cause this to occur under sufficient (and likely strong) warming (Meraner et al 2013;Bloch-Johnson et al 2015), the change occurs even after relatively small amounts of warming (e.g., Armour et al 2013;Andrews et al 2015;Rugenstein et al 2016). Since warming in different regions sets off radiative feedbacks of different strengths, the inconstancy of the climate feedback is likely caused by the change in the spatial pattern of warming with time (Winton et al 2010;Armour et al 2013).…”

Constraints on long term warming in a climate mitigation scenario

“…Though TCRE is relatively robust in transient scenarios in which emissions remain mostly positive (Zickfeld et al, 2012;Krasting et al, 2014;Herrington and Zickfeld, 2014), its robustness in complex models under large negative emissions is relatively unexplored (Boucher et al, 2012;Vichi et al, 2013;Cao and Caldeira, 2010). Although an experimental design to test the long term robustness of TCRE under zero or negative emissions (Jones et al, 2019) or the dynamics of equilibrium response to forcing (Rugenstein et al, 2019) have been proposed and would be highly valuable, Earth System Models have not generally performed this type of experiment to date. Though such experiments have been performed in simple climate (Ricke and Caldeira, 2014;Millar et al, 2017c) and some intermediate complexity models (Zickfeld and Herrington, 2015) where cumulative emissions-temperature proportionality has been observed, it rests to thoroughly test whether these findings arise due to oversimplified model structure or prior assumptions on model parameters.…”