I WANT you to imagine a giant bubble the size of Wembley Stadium. That’s roughly the volume of methane gas that “accidentally” spewed into the atmosphere when a 25-year-old methane natural gas wellhead at Aliso Canyon in California ruptured last October. Unfortunately, it is the amount of methane – a much more potent greenhouse gas than carbon dioxide – that escaped from the well in a single day. Unfortunately, the well went on venting methane for another 112 days, pouring more than 100,000 tons of methane into the atmosphere in what is the worst gas leak in US history.

The Aliso leak made very little impression in the UK media when it occurred (surprise, surprise). But to put this colossal environmental disaster into context, we are talking about an event on the scale of the BP Gulf oil spillage back in 2010.

The leakage was equivalent to annual emissions from half a million cars. At peak, gas belching from the leak was running at double the methane emissions from the entire Los Angeles city basin. Imagine a built-up area filling the entirety of central Scotland from Paisley to Musselburgh and you’ll grasp the scale.

The leakage also raised serious public health issues. More than 2,500 households in the immediate area have been evacuated. Most of the medical ailments identified so far are linked to an additive known as mercaptan. This is added to natural gas so leaks can be identified by a foul smell.

Unfortunately, the gas company was so slow to admit anything was wrong that the additive caused coughs, burning in the chest, headaches, and nausea. Result: schools in the area have been closed.

Why is the Aliso event significant? Firstly, because it wasn’t an isolated such incident. Secondly, because any move to creating a giant gas industry in the UK based on fracking will run the risk of excess (and usually uncounted) methane adding to the greenhouse effect. Be warned.

Every gas drilling and storage project is different and US environmental regulation is a good deal weaker than in the UK. All that I acknowledge up front. It is also the case that the Aliso leak came from a field drilled long before recent fracking operations.

However, the basic lesson we need to draw from the experience of American domestic on-shore gas production is how the environmental risks multiply inordinately when you move to operating on an industrial, nationwide scale.

Any single fracking project can be regulated and kept moderately safe. That is not the point. Frack domestically on a scale to (say) replace the UK’s imported fossil fuels and you end up statistically with a string of Aliso Canyon disasters on our doorstep. Because we are dealing with methane rather than simply carbon dioxide, you actually exacerbate greenhouse gas emissions.

Back to Aliso Canyon. The source of the leak is a broken metal pipe a mere 500 feet underground. But this pipe connects to a giant methane reservoir that is 8,000 feet deeper. This is an old field that was first drilled in 1953. It used to have a safety valve – a so-called blow-out preventer – that would have stopped atmospheric leaks by sealing the field at source (ie, 8,000 feet down at the reservoir). Amazingly, this safety valve was removed in 1979 as it was old. It was never replaced because of cost.

Here is my point. We can expect the real gas infrastructure failures to occur not early on, when there is plenty investment cash and the regulators are on the ball because of public concern. The big danger comes years later when gas fields are old, companies are too cash-strapped to do preventative maintenance, and the regulators believe that because nothing has gone wrong for 25 years it never will. Oops, we have a catastrophic methane leak.

In the US, methane emissions are now routinely tolerated as the “cost of doing business” on an industrial scale. Despite the fact that methane is the most powerful greenhouse gas, though it degrades faster in the upper atmosphere than carbon dioxide, there is little regulation. For instance, only eight out of 8,000 US natural gas producers voluntarily disclose their methane emissions.

Last month, the US Environmental Protection Agency released fresh estimates of America’s greenhouse gas emissions inventory. According to the agency, methane emissions are 27 per cent higher than originally thought. And that does not count one-off “super-emitters” like the Aliso event.

The independent Environmental Defense Fund claims that US methane emissions could be anywhere from 25 to 75 per cent higher than the EPA’s new estimate. And a study by scientists at Stanford University in California suggests that methane emissions could be 50 per cent higher than standard projections. True, carbon dioxide is still the biggest driver of climate change. But because methane emissions are on the rise and exercise such a powerful effect in the short-term on the greenhouse effect, we need to be on our guard.

What about the UK? Around a third of former onshore oil and gas wells are leaking methane gas, according to recent research led by scientists at Durham University. And that is with far tighter regulation than in the US.

Admittedly, the leaks are on a tiny scale and dwarf methane emitted in agricultural use. However, it this not an “either or” situation: we get both farm methane plus the well leaks. Also, the number of old on-shore wells is relatively small. Developing fracked gas in the UK on an industrial scale will multiply those leaks dramatically.

The Conservative government at Westminster is still hell-bent on developing fracked gas on an industrial scale. This comes second only to nuclear power in Tory energy policy. My worry is that when the inevitable delays occur in building new atomic power stations, the Tories will turn to fracking as an alternative. It will be sold (speciously) as a “clean gas” bridge to a renewable electricity future and as a means of energy security. Do not be fooled.

Any move to fracking on an industrial scale will reverse the long-term trend to reduce the UK gas burn, which is down circa nine per cent since 2000 because of “cleaner” industrial technology and more available electricity from renewables. The capital required to develop a UK fracking and gas storage infrastructure would necessitate a massive increase in the gas burn to justify the expenditure, with a commensurate increase in greenhouse gas emissions. Worse, methane “lost” into the atmosphere in the fracking process would add further to emissions. Inevitably – someday – we will have our own Aliso Canyon disaster.

Fortunately, the SNP government has imposed a moratorium on both unconventional gas extraction and underground coal gasification. Some are worried that a mere moratorium, to collect information on the impact of fracking, is too weak a response and that an all-out ban is required. But the Scottish Government cannot “ban” planning applications without due cause, or it risks judicial appeal and the Supreme Court declaring open season for the frackers.

Besides, a party led by a First Minister who joined CND before she joined the SNP is not going to allow Scotland to be the site of the next Aliso Canyon disaster.