Paleoclimates offer a unique perspective to understand the Earth's climate sensitivity and its stability. Paleoclimate archives tell us that Earth has experienced a wide range of climates over various time scales, and that transitions in Earth's climate can take place rapidly. We know that many of these past climates were determined by changes in external forcing factors. To the extent that climate models can reasonably simulate past warm and cold climates, we gain confidence that the models can be used to study Earth's future climate.
CMIP5, PMIP3, and IPCC
The Coupled Modeling Intercomparison Project (CMIP5) and the Paleoclimate Modeling Intercomparison Project (PMIP3) recognize the importance of using observations of past climate change to validate models that will be assessed in the next IPCC. For the first time, international modeling groups are requested to perform simulations for three past time periods with the same model and same resolution as their 20th century and future projection simulations. In 2011, we completed the CMIP5 Tier 1 Last Glacial Maximum (21,000 years ago) and Mid-Holocene (6000 years ago) simulations and Tier 2 Last Millennium simulation with the CCSM4 1° CN model configuration of CESM. Forcings and boundary conditions follow the protocols developed by the PMIP3 for CMIP5.These simulations are being archived on the CMIP5 databases to allow the community to compare past and future climate variability and change, including detection and attribution. These experiments for long-term climate understanding are designed to make better predictions toward future climates with contributions to the USGCRP and IPCC assessment products.
The Last Millennium
The CCSM4 Last Millennium (LM) simulation includes the reconstructed forcings for volcanic eruptions, solar variability and orbital changes in solar insolation, Greenhouse gases, and land use. It reproduces many of the large-scale climate patterns suggested by historical and proxy-data records, including a Northern Hemisphere (NH) cooling from the Medieval Climate Anomaly (MCA, 950-1250 CE) to the Little Ice Age (LIA, 1400-1700 CE) and a pronounced increase in surface temperatures from 1850-2005 CE (Fig. 1). The CCSM4 LM simulation tends to have a greater response to volcanoes than the proxy-records, and a greater warming in the latter half of the 20th century than in the observations. NH temperatures after 1950 are greater in both the LM simulation and most proxy reconstructions than at any other time in the last millennium. Geographically, LM simulated surface temperatures are warmer at the MCA than LIA, with significant polar amplification, and the LM simulation shows a decrease in winter-time precipitation in the southwestern North America during the MCA, in general agreement with paleoclimate precipitation reconstructions.
Climate Sensitivity for Different Climate Forcings
The CCSM4 Last Glacial Maximum (LGM) simulation includes the reconstructed forcings for lower atmospheric concentrations of the Greenhouse gases, orbital changes in solar insolation, ice sheets, and changed paleogeography associated with the lower sea level. The estimated equilibrium global mean annual surface temperature cooling is ~5.5°C in the full LGM simulation and ~2.6°C in the LGMCO2 simulation, suggesting that just less than half of the global LGM cooling is due to lowered CO2 alone. The equilibrium warming estimated for the 4xCO2 simulation is ~6.2°C. The full LGM and 4xCO2 simulations predict an Effective Climate Sensitivity (ECS) comparable to the Equilibrium Climate Sensitivity found with the CCSM4 slab ocean model of 3.2°C. The LGMCO2 simulation shows a greater ECS of ~4.2°. The CCSM4 shows a greater sensitivity at high northern latitudes to a lowering of CO2 only, with a decrease in the strength of the Atlantic Meridional Overturning Circulation (AMOC), significant expansion of sea ice, and greater polar amplification in the LGMCO2 simulation.
2012 and Beyond
CMIP5 and PMIP3 are long-term protocols designed to address outstanding scientific questions for the IPCC AR5 and beyond. The NESL/CGD Paleoclimate group, in collaboration with university and international partners, will continue to analyze the CMIP5 simulations of past climate to understand the mechanisms and feedbacks that provide the responses seen in proxy reconstructions and to assess how past climate sensitivities can be used to constrain future climate sensitivities.
We will also provide additional past climate simulation, including:
- Completion of simulations with CCSM4 for the Last Interglacial (~125 thousand years ago) and Mid-Pliocene Warm Period (~3 million years ago) to allow a model-data assessment of polar amplification seen in proxy reconstructions. These simulations are part of the PMIP3 protocol and will be assessed in the IPCC AR5.
- Simulations with CESM to understand and quantify the carbon cycle – climate feedbacks for the Last Millennium and Last Glacial Maximum.
- Simulations with the Community Ice Sheet Model (CISM) coupled to CESM for the Last Interglacial to assess the contributions of the Greenland and Antarctic ice sheets to the estimated 6-9 meter higher sea level for this past time period with warmer poles.