Fifteen years to build a different way of fuelling society, or 20 years of watching spiralling methane emissions? It seems a no-brainer.
If you support fracking, you should be pleased with the latest report from the Inter-governmental Panel on Climate Change (the IPCC).
It's given the green light to the use of shale gas as a short-term way to slow climate change.
The report is the third and final part of the latest IPCC assessment on climate change (known as AR5).
While it puts considerable emphasis on the need for more renewable energy - including solar, wind and hydropower - it says emissions of greenhouse gases can be cut in the medium term by replacing coal with less-polluting gas, though the gas will itself ultimately have to be phased out.
Shale gas as a 'bridging fuel'
On shale gas, obtained by the controversial fracking process, Ottmar Edenhofer - co-chair of the working group that produced the report - said it was quite clear that the fuel "can be very consistent with low carbon development and decarbonisation".
Among the objections to fracking is the fact the process releases quantities of methane, a greenhouse gas often reckoned to be at least 20 times more powerful than carbon dioxide at warming the atmosphere.
That is the comparison we have often used in the Network's reporting. It's right, so far as it goes. But by some calculations it doesn't go nearly far enough.
Or is it an own goal
Recently an observant reader pointed out that methane is 20 times more potent than CO2 when its impact is measured over a century - but that in the short term it is a far greater problem. Over the space of two decades it is estimated to be at least 84 times more damaging than carbon dioxide.
Robert Howarth is professor of ecology and environmental biology at Cornell University. He and his colleague Drew Shindell of the US National Oceanic and Atmospheric Administration have made a worrying prediction.
Unless emissions of methane (and black carbon) are reduced immediately, they say, the Earth will warm by 1.5°C by 2030 and by 2.0°C by between 2045 and 2050, whether or not carbon dioxide emissions are reduced.
Professor Howarth puts the global warming potential of methane higher still. He has written: "At the time scale of 20 years following emission, methane's global warming potential is more than 100-fold greater than for carbon dioxide." (Shindell et al. 2009)
Some critics will conclude that the IPCC's search for a bridging strategy to move us rapidly to a world of clean energy has scored an own goal by failing to rule out a fuel which entails a large and avoidable increase in greenhouse emissions.
The cost of the infrastructure needed to exploit shale gas on a large scale may also work to prolong its use.
Ironically, the clean energy world the IPCC seeks need be no more than 15 years away, according to one US expert. Mark Z Jacobson is professor of civil and environmental engineering at Stanford University, California, and director of its atmosphere and energy program.
He believes that wind, water and solar power can be scaled up cost-effectively to meet the world's energy demands, ending dependence on both fossil fuels and nuclear power.
Professor Jacobson described in Energy Policy in 2010 how he and a colleague had analysed "the feasibility of providing worldwide energy for all purposes (electric power, transportation, heating/cooling, etc.) from wind, water, and sunlight", known as WWS.
He continued: "We suggest producing all new energy with WWS by 2030 and replacing the pre-existing energy by 2050. Barriers to the plan are primarily social and political, not technological or economic. The energy cost in a WWS world should be similar to that today."
It sounds like a less risky path to a world of clean energy than the IPCC is urging. Fifteen years to build a different way of fuelling society, or 20 years of watching spiralling methane emissions? It seems a no-brainer.
Alex Kirby writes for the Climate News Network.