In its attempt to maintain its social licence to keep looking for more oil and gas, the prospecting industry has used two arguments — that our economy relies on oil so we must keep drilling, and that gas can replace coal with benefits to the climate. Both are wrong.

First, that oil is useful and the economy depends on it. That argument doesn't take us very far, given that the economy depends even more on a stable climate for almost everything we do.

Does the industry really believe intense storms, sea level rise of a metre or two, heat waves, new pests and diseases in a warmer climate, shortages of fresh water as glaciers retreat, and millions of refugees from the Pacific and South Asia don't have an economic impact? There are alternatives to oil, but no alternatives to a liveable climate.

As long as fossil fuels are as cheap as they are now, and as long as they are subsidised by governments around the world, including our own, alternatives such as rail for freight, waste wood for boilers, renewable electricity for light vehicles and biodiesel for trucks, will stagnate.

The second argument says gas has "half the emissions of coal". This is deliberately misleading, it counts only carbon dioxide emissions. Yes, they are only a little more than half the carbon dioxide from coal — but carbon dioxide is only part of the story.

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Natural gas is mainly methane, a hugely more powerful warming gas than carbon dioxide. When it is burned it converts to carbon dioxide, as coal and oil do, but unburned methane leaks out from all stages of the gas supply chain — from wells, pipelines, tanks, processing plants, combustion plant, abandoned wells, as well as blowouts and deliberate venting.

No one measures or counts or reports most of these "fugitive" emissions, so they are treated as though they don't exist; but they have a powerful climate changing effect.

Over 100 years a tonne of methane has an equal warming impact to 21 tonnes of carbon dioxide. But the IPCC now calculates (2013) that over its first 20 years, before it decays, it is equal to 86 tonnes. The next 20 years is the timeframe we have to deal with now, when warming risks pushing climate beyond the point of no return.

If we haven't got emissions under control by then, it is probably game over for the climate. This makes fugitive methane emissions from gas hugely more important than we have previously thought, and adds at least 50 per cent to the impact of its carbon dioxide.

It is very hard to estimate just how much methane is escaping because it varies a lot across the system. It isn't priced, so there is no economic incentive to control it. The oil companies would prefer not to know as it demolishes their argument that shifting to natural gas from other fuels would be cleaner.

They are also unwilling to put money into fixing a large number of small leaks, and monitoring authorities – central and regional government in our country – are reluctant to spend money to find them.

We do know that fracking increases the rate of escape, some analysts say, by several times.

The oil and gas industry loves to claim that gas is a "bridge" to a renewable future. In fact it is a bridge to nowhere. Taking all greenhouse gases into account, it is not true that gas is better for the climate than coal.

The industry exists to invest in new infrastructure: wells, pipelines, power stations, processing plants, with an expected life of 40 years. The Government, in accordance with the Paris Accord on climate change, has a target of reducing our net emissions to zero by 2050. Is the industry planning for a whole string of stranded assets, and have they told their shareholders this, or are they planning to ignore our national target?

Coal, oil and gas companies have already discovered, and hold on their balance sheets, five times more fossil fuel than would take the world past the 2C of warming the world has agreed must not be exceeded. What is the point in looking for more?

• Jeanette Fitzsimons, a former co-leader of the Green Party, has been researching and campaigning on climate change for 30 years.