  The Bitcoin network is burning a large amount of energy for mining. In this
paper we estimate the lower bound for the global energy cost for a period of
ten years from 2010, taking into account changing oil costs, improvements in
hashing technologies and hashing activity. Despite a ten-billion-fold increase
in hashing activity and a ten-million-fold increase in total energy
consumption, we find the cost relative to the volume of transactions has not
increased nor decreased since 2010. This is consistent with the perspective
that, in order to keep a the Blockchain system secure from double spending
attacks, the proof or work must cost a sizable fraction of the value that can
be transferred through the network. We estimate that in the Bitcoin network
this fraction is of the order of 1%.
