State dependency of the forest-tundra-short wave feedback: comparing the mid-Pliocene and pre-industrial eras

The forest-tundra-short wave net radiation feedback is an important climatic feedback, especially due to its potential role in modulating glacial cycles. Little research has been done on how the strength of this feedback might vary with the background climate state. We propose that the feedback has generally strengthened over the last four million years and that it is weaker under warm Northern Hemispheric conditions when tundra is primarily confined to the high Arctic than under cooler conditions in which the forest-tundra boundary lies south across the interiors of the continental land masses. An intermediate-complexity climate-vegetation model was used to analyze the strength of the feedback for two eras, the mid-Pliocene Warm Period (MP) and the pre-industrial Holocene (PI). We applied a “cold” orbital forcing, favorable to Northern Hemispheric glacial inception. The results show that, in the mid-to-high northern latitudes, orbitally-induced vegetation changes alter top-of-atmosphere short wave net radiation about four times as strongly, and the mid-to-high latitude short wave (SW) vegetation feedback is about twice as strong in PI as compared to MP, primarily due to a more southward position of forest and tundra. This SW feedback operates mostly via surface albedo change. Our modeling results support the hypothesized overall increase in the strength of the forest-tundra-short wave feedback as the climate cools from the mid-Pliocene to late Pleistocene, further suggesting an active role for vegetation dynamics in the onset of major Northern Hemispheric glaciation and the increasing amplitude of glacial-interglacial oscillations over the last few million years.