In this calculation your account is fully charged for any losses in the stock market but is credited with a maximum positive return of only 7%.

FORBES: Guaranteed to Go Up

Natural gas emits half as much carbon dioxide as coal when used to make electricity, though the calculation fails to take into account the release of methane from natural-gas wells and pipelines, which also contributes to climate change.

WSJ: Rise in U.S. Gas Production Fuels Unexpected Plunge in Emissions

Changing the figure to take account of the cycle would make more sense, but it would introduce a complex and disputable new calculation, with countries arguing endlessly over precisely where they were in the cycle.

ECONOMIST: Europe's fiscal follies