The forest-tundra-short wave feedback is the dominant short wave (SW) vegetation feedback at mid-to-high northern latitudes and is an important feedback in Earth’s climate system, 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. It is hypothesized that the feedback has generally strengthened over the last four million years. The feedback mechanism is hypothesized to be 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 generally south across the interiors of the large continental land masses. To test the hypothesis of the weakened/strengthened feedback, an Earth System Model of Intermediate Complexity is used that consists of a newly-developed simple dynamic terrestrial vegetation model coupled to a general circulation atmospheric model and a slab ocean. The response to the same orbital forcing ("cold orbit", favorable to Northern Hemispheric glacial inception) is analyzed for two eras: the PRISM mid-Pliocene Warm Period and the pre-industrial Holocene.
The change in top-of-atmosphere short wave net radiation (TOASW net) that is attributable to including interactivity of vegetation in the systemic response to orbital forcing is decomposed into the product of three terms: the short wave vegetation feedback, an effective orbital forcing term, and the amplification of this effective orbital forcing by the climate system when vegetation cover is held fixed. Further analysis is carried out to determine why these terms differ between each era (mid-Pliocene and pre-industrial).
The results show that the change in TOASWnet that is attributable to including interactivity of vegetation in the systemic response to orbital forcing is about four times as strong in the pre-industrial as in the mid-Pliocene. The mid-to-high latitude SW vegetation feedback is about twice as strong for the pre-industrial as for the mid-Pliocene. This SW vegetation feedback is stronger in the pre-industrial mostly because its climate system is more sensitive in boreal spring to climate-induced changes in vegetation for various reasons, many of which boil down to geography. Surface albedo change is the principle mechanism by which the forest-tundra-short wave feedback operates, but it is discovered that there is also a component of this feedback that operates through the interactions between atmospheric reflectivity and vegetation. The results suggest that the forest-tundra-short wave feedback in glacial inception strengthens as the baseline climate cools from early Pliocene levels, with important implications for the start of major Northern Hemispheric glaciation and the increasing amplitude of glacial-interglacial oscillations over the last few million years.
|Advisor:||Elison Timm, Oliver|
|Commitee:||Keesee, Robert G., Lapenas, Andrei, Rose, Brian E. J., Swann, Abigail L. S.|
|School:||State University of New York at Albany|
|Department:||Atmospheric and Environmental Sciences|
|School Location:||United States -- New York|
|Source:||DAI-B 79/01(E), Dissertation Abstracts International|
|Subjects:||Physical geography, Paleoecology, Paleoclimate Science|
|Keywords:||Boreal forest, Feedback, Glacial inception, Pliocene, Taiga, Tundra|
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