Holocene temperature response to external forcing: assessing the linear response and its spatial and temporal dependence
Previous studies show that the evolution of global mean temperature forced by the total forcing is almost the same as the sum of individual orbital, ice sheet, greenhouse gas and meltwater single forcing runs in the last 12 000 years in three independent climate models: Community Climate System Model 3 (CCSM3), Fast Met Office/UK Universities Simulator (FAMOUS) and Loch-Vecode-Ecbilt-Clio-Agism Model (LOVECLIM). This validity of the linear response is useful because it simplifies the interpretation of the climate evolution. However, it has remained unclear if this linear response is valid on other spatial and temporal scales and, if valid, in what regions. Here, using a set of TraCE-21ka (Simulation of the Transient Climate of the Last 21,000 Years) climate simulations, the spatial and temporal dependence of the linear response of the surface temperature evolution in the Holocene is assessed approximately using the correlation coefficient and a linear error index. The results show that the response of global mean temperature is almost linear on orbital, millennial and centennial scales in the Holocene but not on a decadal scale. The linear response differs significantly between the Northern Hemisphere (NH) and Southern Hemisphere (SH). In the NH, the response is almost linear on a millennial scale, while in the SH the response is almost linear on an orbital scale. Furthermore, at regional scales, the linear responses differ substantially between the orbital, millennial, centennial and decadal timescales. On an orbital scale, the linear response is dominant for most regions, even in a small area of a midsize country like Germany. On a millennial scale, the response is still approximately linear in the NH over many regions. Relatively, the linear response is degenerated somewhat over most regions in the SH. On the centennial and decadal timescales, the response is no longer linear in almost all the regions. The regions where the response is linear on the millennial scale are mostly consistent with those on the orbital scale, notably western Eurasian, North Africa, subtropical North Pacific, the tropical Atlantic and the Indian Ocean, likely causing a large signal-to-noise ratio over these regions. This finding will be helpful for improving our understanding of the regional climate response to various climate forcing factors in the Holocene, especially on orbital and millennial scales.