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Who are these other researcher? Care to link to their research on this?"
It takes months to write, peer-review, and publish research papers, so given that this Nature Climate Change article is quite recent at the moment there exists absolutely zero peer-reviewed studies or counterstudies following up on it. However here are some researchers who have criticized it in their own informal analysis or press interviews: see links in https://is.gd/ksyKyv — shortened URL for https://thinkprogress-org.cdn.ampproject.org/v/s/thinkprogre...
The researchers in question are:
Dr. Jon Koomey, who was a Lawrence Berkeley National Laboratory (LBNL) scientist for more than two decades.
Professor Eric Masanet, who leads the Energy and Resource Systems Analysis Lab at Northwestern University.
Arman Shehabi, research scientist at Lawrence Berkeley National Lab.
And myself who has criticized it here on HN (my work's been cited in Nature Sustainability and the NYT: http://blog.zorinaq.com/bitcoin-electricity-consumption/ so I guess this makes me a researcher now)
"How do we know that?"
It's common knowledge to anyone in the industry that the Antminer S9 is by far the most popular Bitcoin mining machine. For some official reports on that, see market research reports and IPO filings from multiple mining hardware manufacturers. For example this has been neatly researched and presented by the team lead by Christian Stoll in https://papers.ssrn.com/sol3/papers.cfm?abstract_id=3335781 Read their Supplementary Data, tab "3.4 IPO filing analysis"
"Again, how do we know that?"
As I said elsewhere in this thread, according to https://www.hsdl.org/?abstract&did=801297 CO2 emissions per kWh have already decreased -25% over 11 years. Therefore -80% in 100 years seems easily achievable (the trend only has to continue for 60 years, then there can be 40 years of technological stagnation.)
In fact we don't even need any technological breakthrough to achieve -80%. France already emits 90% less CO2 per kWh than the average country, thanks to nuclear power.