BitcoinEmissions – A project to calculate CO2 emissions of mining bitcoin
So, to offset a Bitcoin, plant 1.6 trees and keep them alive for a century [1].[1] "On average, one broad leaf tree will absorb in the region of 1 tonne of carbon dioxide during its full life-time (approximately 100 years)." http://www.carbonfootprint.com/plantingtrees.html