PMIP3 conference, Namur, Belgium, May 25-30, 2014
Minutes regarding Last Millennium / Past2k simulations from the PMIP conference Namur May 2014
Last Millennium (PMIP: past1000) simulations have been a very successful contribution to CMIP5. 11 groups provided one or more realizations for the standard past1000 period 850-1849, applying various choices of forcing boundary conditions. For most simulations matching historical simulations are available. Some groups did multiple realizations with different choices for forcing or single-forcing experiments. Ensemble simulations appear to be particular useful since the signal-to-noise ratio is usually small. Last Millennium simulations from PMIP3 as well as pre-PMIP3 exercises were quite prominent in IPCC AR5; not only in chapter 5 (Information from Paleo Archives), but also in chapter 9 (Evaluation of climate models). Emerging achievements obtained in the past1000 context are: Simulations of the late Holocene • serve to put in perspective recent anthropogenic changes with variability due to natural forcing or internal variations • Allow analyzing response to forcing under different boundary conditions • Allow in-depth comparison with the most reliable and diverse collection of paleo-proxies, on global, regional and local scale Past2k and PAGES2K The link between past1000 simulations and the reconstruction community (identified 2012 at the Crewe meeting) has been established successfully. The PAGES/Past2K Madrid workshop was quite instrumental and many analyses projects and a synthesis-paper are underway. Some of the resources obtained within these efforts should be transferred to the Past2K wiki page.
Lessons learned from PMIP3/CMIP5 There is need to make sure that there are continuous simulations from the pre-industrial millennium through the historical period to present. It was difficult to find the “historical” CMIP5 simulations fitting to the past1000 simulations. Regarding the question if one should include the first millennium in future late Holocene transient simulations (0-2010 CE) was discussed controversial. Most of the modellers felt that the first millennium with its uncertain forcing history and probably weak forcing from volcanoes would not add much information. Moreover, the uncertainty in reconstructions is high before 1200 CE. CPU-time might be better spent for ensemble/multiple realizations of the last millennium. Forcing choices should be reviewed for an upcoming past1000 exercise. New reconstructions of volcanoes are underway. There is no convergence/consensus on the amplitude of solar forcing. Although there was some critique on the “freedom of choice” for the forcing boundary conditions (i.e. that there was no recommended “standard” past1000 protocol that every group had to follow), the group felt that flexibility in forcing BCs is OK as long it is ensured that the parameter/BC space was well sampled. In the future this should probably happen in a more organized way. It was also felt that a better documentation of the implementation of forcing BCs is needed as well as sensitivity studies. For volcanic aerosols, there is a VolcMip initiative emerging (D. Zanchettin and others), that would be very helpful.
PMIP experiments in CMIP(6) It is not clear yet which paleo simulations will be recommended as part of CMIP6. The structure and nature of CMIP has changed and there will be many MIPs around a core of experiments, called “DECK”. MIPs have a good chance to be endorsed that demonstrate to • Follow a standard experiment design • Get together a sufficient number of participating groups • Make sure that all models have provided DECK experiments • Ensure data availability Priority will likely be given to projects that tackle the WCRP grand challenges: Clouds, Cryosphere, Extremes, Regional Climate, sea-level rise, water availability
A subset of about 15 p2k people met in Vienna as one of the first activities of the group. The following issues were discussed: WG issues/wishes:
• Coordinate PMIP3/CMIP5 „past1000“ multi-model analyses, promote advanced, mechanism- and feedback oriented studies
• Form a link between PAGES2K and PMIP modelling people, complementing existing modeling expertise in PAGES2K groups, emphasis on themes beyond surface temperature (hydrological cycle, carbon cycle, downscaling, incl. regional modeling)
• Form a discussion forum for new coordinated model experiments (a new initiative on the last 2000 years incl. discussion about requirements in terms of ext. forcing etc.)
• Coordinate „past1000“ activities in preparation of the 2014 PMIP conference in Belgium
As a first major activity, the group will promote a PAGES2K/PMIP-Past2K workshop (see below)
Comparing model simulations with proxy-based climate reconstructions offers the possibility of improving our understanding of the mechanisms contributing to climate variability and their links to external forcing or internal processes. It helps to identify deficiencies in the way climate variability is represented by proxy records or by model simulations, with implications foron future climate change projections. The Past2K group of the Paleoclimate Modelling Intercomparison Project (PMIP3) is promoting model-data comparison activities focusing on the climate of the last millennium. The PAGES 2k Network is fostering similar activities covering the last 2000 years and has promoted the development of regional-scale data syntheses resulting in an ensemble of reconstructions for nearly all the continents (PAGES2k Consortium 2013). This meeting gathered members of both communities with a main focus on comparing the PMIP3 “past1000” simulations with the PAGES2k reconstructions.
Activities started a few months before the actual workshop as most of the 32 workshop participants volunteered to contribute to the analyses of some diagnostics within one of the three working groups (WGs) focused on: 1) PAGES2K regions and PMIP3 simulations; 2) PMIP3 simulations and reconstructed circulation modes; 3) best practices and new approaches in model-data comparison. In addition, participants were identified to prepare reviews. The three-day meeting was structured into review talks (days 1 and 2) and WG presentations (days 2 and 3), allowing time for ample discussion slots. The last day of the workshop focused on discussion and planning future joint activities, including 1) the preparation of manuscripts focused on the comparison of PMIP3 simulations and PAGES2k regional reconstructions, 2) comparison between reconstructed and simulated modes of variability, and 3) on best practices in data-model comparison. Additionally, an outreach event was organized on communicating climate and climate change science.
Several issues from the discussions can be highlighted. For instance, significant correlations between regional temperature reconstructions and climate model simulations (Fig. 1) suggest that, at multidecadal scales and above, regional temperatures respond to external forcing (PAGES2k Consortium 2013; Sundberg et al. 2012; Schurer et al. 2014). The specific fingerprints of volcanic solar and anthropogenic contributions were analyzed from the perspective of various methodologies, with the role of solar forcing being acknowledged as comparatively smaller. The inter-regional correlations were shown to be lower in the reconstructions than within the models’ world, the latter evidencing a more homogeneous spatial temperature response. The implications of these differences for climate reconstructions and also for the assessment of confidence in climate models were discussed.
The reconstructions of modes of atmosphere and ocean variability such as PDO, ENSO, IPO, PNA, NAO, SAM or the gyre system in the North Atlantic were presented. In most cases, they show very limited resemblance to their simulated counterparts. This suggests an overall lack of evidence for a direct external forcing imprint on the variability of many climate modes. Since internal variability seems to be the dominant factor, model simulations are useful for identifying the dynamics explaining some of the reconstructed changes. However, a more fundamental issue, with methodological implications for reconstructions, is the limited agreement found in various reconstructions of particular modes. However, a more fundamental issue is the low agreement in the available reconstructions of some of the modes that has methodological implications for the reconstructions of modes of variability. S Such inconsistencies may reflect large spatial changes in the positions of the centers of action that are not captured with index definitions based on fixed locations or eigenvector approaches. Here, model-guided mode definition and proxy-site selection (e.g. Lehner et al. 2012) appears to emerge as a promising field of research.
Circulo de Bellas Artes, Madrid, Spain, November 4th-6th November 2013