Can megafauna shift the carbon and surface radiation budgets of the Arctic?

One of the key challenges and concerns when considering 21st century climate change is the identification and avoidance of positive feedbacks (which may lead to "tipping points") in the biosphere carbon cycle, where parts of the biosphere respond to climate change by becoming major emitters of greenhouse gases to the atmosphere. High latitude tundras are particular regions of concern, as they hold substantial reserves of permafrost carbon -especially the Yedoma soils of northeast Siberia and north-western North America- and are also substantial sources of atmospheric methane. Although these regions are now dominated by wet shrub- and moss-dominated tundra and forest-tundra vegetation, there is evidence that throughout Pleistocene glacials and interglacials the region was dominated by highly productive grasslands ("the mammoth steppe"), the most extensive land biome on Earth, which supported high animal biomass despite the cold temperatures. SA Zimov (1995, 2012) proposed that the mammoth steppe was created and maintained by the abundance of large herbivores (e.g. bison, horses, rhinoceros, mammoths), and that it was the extinction of these megafauna, most likely caused by the spread of human hunting populations into the Arctic in the Late Pleistocene and early Holocene, that led to the collapse of the mammoth steppe and its replacement by the current low productivity wet tundra vegetation. Moreover, he proposed that the introduction of a guild of megafauna herbivores with diverse feeding strategies such as horses and bison into the Arctic could lead to the rebirth of this lost cold high-latitude ecosystem. This would stabilise soil carbon reserves and act as mechanism to diffuse the threat of a carbon cycle positive feedback in the permafrost regions. In 1996, SA Zimov established the "Pleistocene Park" in northeast Siberia to demonstrate the feasibility of megafaunal introduction in the Arctic and its potential to shift ecosystem states from tundra to grassland. While the experiment has succeeded in initiating a vegetation shift from wet tundra and forest-tundra to open, grass-dominated landscapes, to date no detailed and systematic monitoring has been implemented to test the core components of SA Zimov ecosystem-climate hypothesis. These outline how such an ecosystem shift would affect land surface radiation and water budgets, soil and surface temperature and moisture, and net carbon balance. Here, we propose to work closely with Sergey and Nikita Zimov to conduct the first detailed evaluation of the above hypotheses, using state-of-the art techniques to assess the carbon, water and radiation budgets of the land surface with and without megafaunal rewilding. We will measure the net flux of carbon and water from the ecosystem to the atmosphere using flux measurement towers and soil CO2 efflux measurements, coupled with detailed measurements of soil and atmospheric conditions and energy balance, and scaled using drone-based maps. Detailed observation of ecosystem and microclimate processes in the field will provide parametrisation of key aspects of the system in two Earth System Models (BNU-ESM & CAS-ESM), allowing exploration of the potential impacts of different possible scenarios of high-latitude biome shift on planetary climate and biogeochemical processes. This work would provide unique mechanistic insights into the present, past and potential future ecosystem and climate dynamics of large parts of the Arctic