The last millennium is the best-documented period of climate change in a multi-century time frame. Climate has varied considerably during the late Holocene and these changes left their traces in history (Medieval Climate Optimum, Little Ice Age). However, the relative magnitude of natural fluctuations due to internal variability of the Earth’s climate system and to variations in the external forcings (sun, orbital, volcanic) and the present global warming, attributed to anthropogenic greenhouse gases, is still under debate. Since reconstructions of the past climate beyond the instrumental record have to rely on relatively sparse data sources, numerical model simulations are useful tools to investigate the underlying causes of climate change. Model simulated climate data can be used to validate reconstruction methods.
In the third phase of PMIP and the fifth phase of CMIP, simulations of the pre-industrial millennium (“past1000”, 850-1850 CE) have been officialy included and roughly a dozen modelling groups have conducted the experiments under a common experimental design. In contrast to earlier PMIP efforts, the new simulations were carried out with the same model at the same resolution and data are distributed through the CMIP5 data base network (ESG).
The WG aims at coordinating research activities using the past1000 and earlier millennium simulations, providing links to the reconstruction community, in particular to the PAGES2K network and to define new strategies both for upcoming analyses and the next generation of simulations.
Last Millennium simulations proposed for CMIP6
Within PMIP, a considerable number of individual researchers and modelling groups is committed to perform LM simulations. The simulations will base on experience gained in PMIP3/CMIP5. Several studies, partly reflected by entries in the AR5 chapter 5, have highlighted the value of the LM multi-model ensemble. The PMIP3 LM working group (WG Past2K) is closely cooperating with the PAGES initiative PAGES2k promoting regional reconstructions of climate variables and variability modes. Collaborative work has focused on reconstruction-model intercomparison (e.g. Bothe et al., 2013) and assessment of variability modes (e.g. Raible et al-., 2014). Integrated assessment of reconstruction and simulations has led to progress in model evaluation and process understanding (e.g. Lehner et al., 2013; Sicre et al., 2013; Jungclaus et al., 2014).
Simulations of the last millennium (LM) directly address the first CMIP6 key scientific question “How does the Earth System respond to forcing”. Investigating the response to (mainly) natural forcing under climatic background conditions not too different from today is crucial for an improved understanding of climate variability, circulation, and regional connectivity. LM simulations also allow assessing climate variability on decadal and longer scales and provide information on predictability under forced and unforced conditions. These are crucial for near-term predictions and thus address the third CMIP5 scientific question “How can we assess future climate changes given climate variability, predictability and uncertainties in scenarios”. In providing in-depth model evaluation with respect to observations and paleoclimatic reconstructions in particular addressing details of response to forcing, LM simulations serve to “understand origins and consequences of systematic model biases”, thus addressing also the second CMIP6 scientific question.
The science question and/or gap being addressed with this experiment:
The LM experiments will contribute to the WCRP Grand Challenges:
Cryosphere: transient simulations over the last millennium put in perspective the recent changes e.g. in Arctic Sea ice. Assessment of feedbacks under natural forcing (e.g. role of volcanoes/solar forcing for climate shifts including sea ice (Berdahl and Robock, 2013; Lehner et al., 2013, Jungclaus et al., 2014)
Climate Extremes: LM simulations, supported by a growing number of reconstructions for e.g. precipitation or climate variability modes (e.g. Raible et al., 2014), allow assessing extreme events under varying background conditions or under varying regimes during the millennium. Assessment of extremes under natural forcing, e.g. volcanoes.
Regional climate and decadal predictions: Improved assessment of decadal to centennial variability as carrier of near-term prediction potential. Regional assessment of response to natural forcing and interaction with variability modes and teleconnections.
Water variability: Assessment of natural variations in droughts in connection with paleo-reconstructions
Clouds/Circulation: • WCRP Grand challenge Initiative on Leveraging the past record (http://www.wcrp-climate.org/index.php/gc-clouds-circulation-activities/gc4-clouds-initiatives/116-gc-clouds-inititative4).
Possible synergies with other MIPs:
DECK: provide initial conditions for historical+scenario simulation, evaluation of forced vs. internal variability; assessment of forced response.
DECADAL PREDICTION: provide multi-centennial information on low-frequency climate variability as carrier of predictability.
REGIONAL CLIMATE/EXTREMES: provide boundary conditions for regional climate simulations for historical periods. Assessment of extreme events under background conditions different from today.
CARBON CYCLE: A subset of LM simulations will be run with interactive carbon cycle (Jungclaus et al., 2010). Assessment of carbon-cycle evolution and feedbacks between sub-components on multidecadal to centennial time scales. Evaluation of paleo reconstructions of carbon storage.
LAND USE: provide important boundary conditions also in connection with carbon-cycle changes.
CHEMISTRY/AEROSOLS/VOLCANOES: a dedicated VolcMip assesses uncertainties in the climate response to volcanic forcing performing idealized experiments, whereas LM simulations describe the climate response to volcanic forcing in long transient simulations where related uncertainties are due to chosen input data for volcanic forcing: mutual assessment of forced response.
OCEAN/SEA_ICE: Mutual assessment of the role of the ocean in low-frequency variability, e.g. multi-decadal changes in ocean heat content or heat transport. Provide initial conditions for the ocean including long-term forcing history.
Experimental design: See sub-page “boundary conditions and forcing” and join the discussion!
Historical footnote: group initiated at the Crewe 2012 meeting.