Timescales and timeframes
One of the issues in the debate over CSG and fracking is the timeframe over which the global warming potential of methane (in the form of leakage from both conventional and unconventional natural gas projects) should be assessed. The leading critics of fracking, Robert Howarth and his team at Cornell have used a 20-year time-frame. Since methane has a much shorter residence time in the atmosphere than carbon dioxide, but has a greater warming potential over that time, the use of a 20-year time frame makes methane seem more serious than if a timeframe of 100 years or longer is used.
The original justification put forward by Howarth for the 20 year timeframe was that this was the likely life of a project. This is nonsensical, and (to me at least) undermines Howarth’s credibility. The world is still warming as a result of coal burned in power stations that closed decades ago, and no one suggests that we should not worry about this.
Howarth has now adopted a new justification that, on the surface at least, is more plausible. Most attention in the debate over climate change has been based on the assumption of a gradual increase in mean global temperatures, equilibrating to a new higher level some decades after concentrations of greenhouse gases have stabilized, with effects that will then play out for centuries. Since stabilization is unlikely to be achieved before 2050, that implies that we should be looking at timeframes of 100 years or even longer.
However, there is also a risk that we will pass some tipping point, after which the entire process will be irreversible. We don’t know much about tipping points, but, as Howarth observes, “”the world runs a high risk of catastrophic climate change in the period of 15 to 35 years from now.””
That’s true, but unfortunately for Howarth and for us, it doesn’t help his case.
The one thing we do know about tipping points with reasonable certainty is that they will be reached when the global mean temperature reaches a sufficiently high level and stays there for some time. So, what matters is the peak temperature reached for a given trajectory of emissions.
The unfortunate fact is that, no matter what we do, global temperatures are going to keep rising for several decades to come, simply because of the greenhouse gases we have already emitted. If we reduced all emissions to zero immediately, the peak (apart from the long-run adjustment of oceans) might perhaps be reached in 20 years (that’s what the IPCC uses to estimate transient climate sensitivity, a point used, rather misleadingly by Howarth).
It’s obvious that this won’t happen. Still for someone campaigning, like David Spratt, for an all-out “Climate Code Red” state of emergency to eliminate all fossil fuels as fast as possible, it might make sense to focus on short time frames.
But in the context of practical possibilities, atmospheric concentrations of greenhouse gases are going to keep rising for decades. If there is a tipping point 20 years in the future of our current trajectory, then we are going to reach it, and whether there is more or less methane in the atmosphere at that time won’t make a difference one way or the other.
To put things slightly differently, no remotely likely course of action is going to stop global mean temperatures rising by 2 degrees relative to the pre-industrial level and staying that high for some time to come. If there is a tipping point between 1 and 2 degrees (where we are likely to be in 20 or 30 years time) we, and the global climate, have already exhausted our luck.
On the other hand, if we can stabilize atmospheric concentrations of greenhouse gases below 450 ppm (CO2 equivalents on the standard 100-year timeframe measure) by 2050, and then gradually reduce them, there is a good chance of avoiding a rise of more then 2 degrees by, say 2100. That’s why a 100-year timeframe makes sense.