Abstract
Abstract. Studying the climate of the last millennium gives the possibility to deal with a relatively well-documented climate essentially driven by natural forcings. We have performed two simulations with the IPSLCM4 climate model to evaluate the impact of Total Solar Irradiance (TSI), CO2 and orbital forcing on secular temperature variability during the preindustrial part of the last millennium. The Northern Hemisphere (NH) temperature of the simulation reproduces the amplitude of the NH temperature reconstructions over the last millennium. Using a linear statistical decomposition we evaluated that TSI and CO2 have similar contributions to secular temperature variability between 1425 and 1850 AD. They generate a temperature minimum comparable to the Little Ice Age shown by the temperature reconstructions. Solar forcing explains ~80% of the NH temperature variability during the first part of the millennium (1000–1425 AD) including the Medieval Climate Anomaly (MCA). It is responsible for a warm period which occurs two centuries later than in the reconstructions. This mismatch implies that the secular variability during the MCA is not fully explained by the response of the model to the TSI reconstruction. With a signal-noise ratio (SNR) estimate we found that the temperature signal of the forced simulation is significantly different from internal variability over area wider than ~5.106 km2, i.e. approximately the extent of Europe. Orbital forcing plays a significant role in latitudes higher than 65° N in summer and supports the conclusions of a recent study on an Arctic temperature reconstruction over past two millennia. The forced variability represents at least half of the temperature signal on only ~30% of the surface of the globe. This study suggests that regional reconstructions of the temperature between 1000 and 1850 AD are likely to show weak signatures of solar, CO2 and orbital forcings compared to internal variability.
Highlights
The study of the last millennium climate has considerably increased during the last decades because it replaces the evolution of the global temperature for the last fifty years in a multi-centennial context, providing the necessary hindsight for a more comprehensive assessment of the recent climatic change
With a signal-noise ratio (SNR) estimate we found that the temperature signal of the forced simulation is significantly different from internal variability over area wider than ∼5.106 km2, i.e. approximately the extent of Europe
We have seen in the previous section that the Northern Hemisphere temperature in the SGI simulation is driven at 78% by the model response to the forcings, mostly represented by Total Solar Irradiance (TSI) variability
Summary
The study of the last millennium climate has considerably increased during the last decades because it replaces the evolution of the global temperature for the last fifty years in a multi-centennial context, providing the necessary hindsight for a more comprehensive assessment of the recent climatic change. 2008), lake sediments (Hu et al, 2001; McKay et al, 2008), boreholes (Gonzalez-Rouco et al, 2003a), speleothems (Tan et al, 2003; Mangini et al, 2005) and historical documents (Brazdil et al, 2005) This important amount of data has been used to perform empirical reconstructions of Northern Hemisphere (NH) surface temperature over the last millennium, relying on various methods to extract the hemispheric signal (Mann et al, 2008; Ammann and Wahl, 2007; Crowley and Lowery, 2000; Moberg et al, 2005; Hegerl et al, 2007). In the Southern Hemisphere proxy data are still too sparse to produce robust hemispherical temperature reconstructions (Juckes et al, 2007; Mann et al, 2008) This increasingly documented period displays prominent climatic features for the last millennium such as the socalled Medieval Climate Anomaly (MCA, firstly introduced by (Lamb, 1965) as the “Medieval Warm Epoch”) and the Little Ice Age (LIA hereafter; Matthes, 1939).
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