Various accounting systems for carbon sinks have entertained periods other than 100 Years before this. “Carbon accounting methodologies have been devised especially for sinks projects, taking into account the technical differences in relation to other types of emission reduction projects,” according to a 2002 Winrock report for the US EPA.. “The treatment of permanence, therefore, influences and is influenced by the choice of carbon accounting methodologies, the timeframes chosen for carbon accounting, and the approach chosen for dealing with liabilities (i.e., the need to return or replace carbon credits if carbon is released to the atmosphere.”)
There are IPCC precedents for accounting periods of 20, 30 and 60 years. The Milan conference of the UN Framework Convention on Climate Change established two types of emission offsets under the Clean Development Mechanism (CDM), valid for afforestation and reforestation activities. ‘In order to account for the non-permanent nature of carbon storage in forests, these credits expire after a predefined periods, after which the buyer needs to replace them.
The Verified Carbon Standard (VCS), the most widely used carbon accounting standard among projects issuing credits in the voluntary market, allows for a period of 25 years. Redd Forests, the Australian based carbon project developer, has achieved validation of its Tasmanian Improved Forestry Management projects that avoids the emissions of greenhouse gases resulting from the logging, chipping and pulping of the timber into short-lived paper products. Instead the forests will be protected and managed by their owners for 25 years.