As world leaders enter the home stretch of the Paris climate negotiations they should keep in mind a key measure of success in limiting carbon emissions: cheap oil. The lower the global price of oil, the more it stays in the ground – due to the brutal, if counterintuitive, logic of the petroleum marketplace.

Most of the easily extracted oil deposits are long gone. What’s left are high-cost, high-risk long shots such as the Alberta tar sands, deep-water reservoirs off Brazil, and drilling the high Arctic. Companies hoping to profit from the last dregs of the petroleum age need to convince their investors to part with massive amounts of capital in hopes of competitive returns often decades down the road.

Billions have already fled the Alberta oil sands in the last year as the global price of oil collapsed from over $100 per barrel to below $40. Shell has just called a halt to its Carmon Creek project in Northern Alberta, writing off $2bn in booked assets and 418 million barrels of bitumen reserves. A barrel of bitumen will release about 480kg of carbon dioxide from extraction, refining, transport and combustion. This head office write-down means that 200m tonnes of carbon will not be released into the atmosphere.

Two other tar sands projects were also shelved this year with reserves of about 3bn barrels. If these investments stay dead the world will avoid another 1.6tn tonnes of dangerous carbon emissions. Together the cancellation of these three projects alone amount to the equivalent of taking more than 14m cars off the road for the next 25 years.

There a simple correlation between future emissions and the price of oil needed to make that profitable. Such a graph has been compiled by Carbon Tracker, a UK-based non-profit organisation set up to educate institutional investors on the increasing financial risks of the fossil fuel sector.

Its message to investors is simple: the world must limit additional emissions to below 900 gigatons to avoid potentially catastrophic climate consequences – and 40% of this future carbon budget – about 360 gigatons – is projected to come from the oil sector. Anything more than that must stay in the ground – the so-called unburnable carbon.

And what’s the price of oil that could save to world? Anything below $75 a barrel of Brent crude means that companies cannot profitably extract more than 360 gigatons of the world’s remaining reserves – no messy policy solutions required.

Just last year the price of Brent crude was about $110 a barrel, a price that would gainfully produce about 500 gigatons of carbon emissions by 2050. Now it is less than $50, which would only produce 180 gigatons over the same period. If prices stay where they are, the world will avoid some 320bn tonnes of carbon emissions by 2050 in precluded production from uneconomic oil fields.

To put this in perspective, that is 25 times larger than reductions the Kyoto protocol was supposed to achieve if it had worked (it didn’t), and 180 gigatons below the oil emissions limit scientists say we need to avoid a world with more than two degrees of warming. Economic turmoil aside, the global commodities market just served up massive progress on an issue in desperate need of some good news.

Carbon Tracker recently revised its calculations to include the turmoil in the oil market, but the basic correlation is the same: lower fossil fuel prices devastate the economics of future extraction.

Seen through this lens, a key measure of our success in controlling carbon emissions should be keeping commodity prices of fossil fuels low. And while the main driver of the current slump in prices is the current glut of supply, it’s important to realise that almost every policy intervention to avert climate disaster is directly or indirectly aimed at lowering the price or profitability of fossil fuels such as oil and coal.

Efficiency and conservation incentives reduce demand, as do vehicle emission standards and investing in public transit. Carbon pricing means that fossil fuel companies can no longer use the atmosphere as a free dumping ground for CO 2 , so also lowering profitability.

But doesn’t cheap gas mean that people just use more of it? Not really. While there is a weak economic link between declining prices and increasing consumption, key producers like Saudi Arabia are in fact fretting that slowing growth in Asian markets and already peaked demand in developed countries will lead to a long-term decline in the world’s appetite for oil.

I dearly hope that world leaders can somehow negotiate transformative change. But perhaps the best they can do is nudge economic indicators like crude prices in the right direction and get out of the way. The unstoppable forces of the global marketplace will hopefully do the rest.