ND: Gotcha, so that’s the GHG abatement half of the story. Would restoring this ecosystem also sequester additional carbon from the atmosphere on top of protecting the carbon that’s already there?
GL: On average, Mammoth Steppe soils of the past seem to have accumulated carbon at around 0.1 tonnes of carbon per hectare per year, slightly slower than in the northern peatlands today. And like peatlands, some of the soils once covered by the mammoth steppe now hold thousands of tonnes of carbon per hectare in preserved peat.
The current tundra landscape in Siberia is dominated by unproductive mosses, which have almost no roots, and sparse shrubs. By transiting tundra back to the deep-rooted and productive grasses of the Mammoth Steppe, you’d likely see a surge in net carbon sequestration in the first few decades as the grasses pumped extra carbon below the soil surface, where temperatures and thus rates of decay are lower. Unsurprisingly, there are few data specific to boreal grasslands yet, but in temperate soils the return of grasses to bare or cultivated soil can drive sequestration of 0.3-0.6 tonnes of carbon per hectare per year.
That still may seem small compared to the emissions from melting permafrost. Yet over the vast area once covered by grasslands, this could amount to sequestration of tens to hundreds of millions of tonnes per year. But to answer this question fully, we’ll need the data on soil carbon accumulation from Pleistocene Park itself.
ND: If we replace northern forests with grasslands, isn't there a huge biomass carbon deficit that will exacerbate climate change for decades before soils have had a chance to accumulate significant amounts of carbon?
GL: It’s not clear how much the expanding Mammoth Steppe would really replace much true boreal forest, as opposed to tundra and shrubland. At least until humans are able to reintroduce mammoths to the ecosystem!
But if it does, there would likely be some emissions associated with the loss of the forest. Then again, 80% of carbon in the boreal forest is typically below the surface in soils. And tree species adapted to permafrost soils can be damaged or even killed if the permafrost melts, when subsidence or erosion leads to collapse of their foundations.
So, the net climate impact would still depend on the alternative fate of that piece of forest.
ND: What about the balance between soil carbon sequestration and the methane emissions from massive reintroduction of grazers?
GL: Sergey and Nikita have estimated the numbers of animals per square kilometre at the peak of the Mammoth Steppe from the bones they have uncovered in the permafrost: five bison, seven and a half horses, fifteen reindeer and one woolly mammoth!
Based on estimates of typical methane emissions from these species, we can estimate that animals at the ecosystem’s peak were producing 0.3-0.5 tCO2e/ha/year in methane (based on a 100-year global warming potential).
That’s the same order of magnitude as potential carbon sequestration, and a lot lower than the emissions we’d expect if the permafrost started melting, which may be tens of tonnes of CO2-equivalent per year. But it is enough to become a significant new source of methane if the Mammoth Steppe is restored at scale, and needs to be accounted for.
ND: How much do you think it would cost to realize the Zimov’s vision, in terms of dollars per hectare or per tonne of carbon avoided?