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| How much of the world's fossil fuel can we burn? | |
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| The world is gradually waking up to the true nature of the climate change conundrum, and not a moment too soon. The situation boils down to this: fossil fuel is immensely useful, valuable and politically important, yet if we want to avoid taking unacceptable risks with the planet we need to leave most of that fuel in the ground – either forever or at least until there’s an affordable and scalable way to stop the exhaust gases building up in the atmosphere. | |
| Many of us have been saying this for years (I co-wrote a book about it) but much of the credit for the increased awareness of the need to ‘leave the fuel in the ground’ goes to Bill McKibben, whose brilliant and much-read article in Rolling Stone clarified for many readers the simple and crucial fact that there is far more carbon in existing fossil fuel reserves than we can safely burn. | Many of us have been saying this for years (I co-wrote a book about it) but much of the credit for the increased awareness of the need to ‘leave the fuel in the ground’ goes to Bill McKibben, whose brilliant and much-read article in Rolling Stone clarified for many readers the simple and crucial fact that there is far more carbon in existing fossil fuel reserves than we can safely burn. |
| So far so good, but McKibben’s article has been so influential that the very specific numbers it contains are now often cited as a kind of unchanging gospel truth. Those numbers are as follows. Limiting global warming to the agreed global target of 2C means staying within a ‘carbon budget’ of 565 GT (gigatonnes or billion tonnes). That is a fifth of the 2,795 GT that would be released if all the world’s proven oil, coal and gas reserves were burned. Therefore four-fifths of the fossil fuel must stay in the ground. | So far so good, but McKibben’s article has been so influential that the very specific numbers it contains are now often cited as a kind of unchanging gospel truth. Those numbers are as follows. Limiting global warming to the agreed global target of 2C means staying within a ‘carbon budget’ of 565 GT (gigatonnes or billion tonnes). That is a fifth of the 2,795 GT that would be released if all the world’s proven oil, coal and gas reserves were burned. Therefore four-fifths of the fossil fuel must stay in the ground. |
| But other estimates differ. For example, a recent paper in Nature stated that although we’ll need to leave most of the coal in the ground, we can burn half the gas and two-thirds of the oil – a major difference given oil’s key role in the world economy. So whose figures should we believe? | But other estimates differ. For example, a recent paper in Nature stated that although we’ll need to leave most of the coal in the ground, we can burn half the gas and two-thirds of the oil – a major difference given oil’s key role in the world economy. So whose figures should we believe? |
| The short answer is that all such numbers need to be taken with a pinch of salt. For a start, most of them are out of date. The 565 GT budget in McKibben’s piece, for instance, was first published in a 2011 report by Carbon Tracker, which in turn based it on research by the Potsdam Institute. The figure therefore excludes the last four or five years of emissions, during which time I’d estimate we’ve eaten through about a quarter of the 565 Gt budget.Moreover, all such figures are based on a set of assumptions that are worth being aware of. These relate to four key factors: | The short answer is that all such numbers need to be taken with a pinch of salt. For a start, most of them are out of date. The 565 GT budget in McKibben’s piece, for instance, was first published in a 2011 report by Carbon Tracker, which in turn based it on research by the Potsdam Institute. The figure therefore excludes the last four or five years of emissions, during which time I’d estimate we’ve eaten through about a quarter of the 565 Gt budget.Moreover, all such figures are based on a set of assumptions that are worth being aware of. These relate to four key factors: |
| Evolving climate scienceNo one knows precisely how much warming will be caused by any given build up of CO2 in the air, nor how much of the carbon we emit will stay in the atmosphere, as opposed to getting soaked up by oceans and plants. Instead, the estimated impact of each tonne of CO2 increases or decreases as scientific knowledge improves. A couple of years ago, the last major IPCC report slightly reduced the range of estimates for the climate’s ‘sensitivity’ to CO2, increasing the size of any given carbon budget. The science will continue to evolve. | Evolving climate scienceNo one knows precisely how much warming will be caused by any given build up of CO2 in the air, nor how much of the carbon we emit will stay in the atmosphere, as opposed to getting soaked up by oceans and plants. Instead, the estimated impact of each tonne of CO2 increases or decreases as scientific knowledge improves. A couple of years ago, the last major IPCC report slightly reduced the range of estimates for the climate’s ‘sensitivity’ to CO2, increasing the size of any given carbon budget. The science will continue to evolve. |
| Acceptable riskGiven the inherent uncertainties, the best that scientists can do is tell us the likelihood that any given carbon budget will cause any given level of warming. Picking a budget therefore involves choosing how much risk we’re prepared to take of overheating the planet. | Acceptable riskGiven the inherent uncertainties, the best that scientists can do is tell us the likelihood that any given carbon budget will cause any given level of warming. Picking a budget therefore involves choosing how much risk we’re prepared to take of overheating the planet. |
| This is unavoidably a value judgement as it entails weighing up the cost and inconvenience of reducing fossil fuel use against the risks of exceeding 2C (which itself splits scientific opinion: some experts think exceeding even 1.5C could be disastrous, while others are less nervous about overshooting 2C a little). The Carbon Tracker/Bill McKibben numbers are based on accepting a 20% chance of exceeding 2C. By contrast, the Nature paper is based on 50/50 odds, while the most recent IPCC report shows a 33% chance of failure. Changing the acceptable odds can make a big difference to the budget. | This is unavoidably a value judgement as it entails weighing up the cost and inconvenience of reducing fossil fuel use against the risks of exceeding 2C (which itself splits scientific opinion: some experts think exceeding even 1.5C could be disastrous, while others are less nervous about overshooting 2C a little). The Carbon Tracker/Bill McKibben numbers are based on accepting a 20% chance of exceeding 2C. By contrast, the Nature paper is based on 50/50 odds, while the most recent IPCC report shows a 33% chance of failure. Changing the acceptable odds can make a big difference to the budget. |
| Fuel reserves and emissions dataThe world’s fossil fuel reserves are just a tiny slice of the total fuel resource that we know exists. To count as a reserve, the fuel has to have a good chance of being profitably extracted. Reserves in turn break down into proven reserves (usually defined as those with a 90% chance of eventually being taken out of the ground) and much bigger probable reserves. Fossil fuel being a finite resource, you might assume that proven reserves would shrink each year as we all fill up our cars and heat our homes. In reality, though, in recent years these reserves have stayed flat or even increased slightly as ever more unconventional sources such as tar sands and shale gas get upgraded to proven status thanks to new technologies, such as fracking, and (until recently) high fuel prices. But as carbon emissions continue, our remaining budget does get smaller every year – and therefore the proportion of fuel we can burn. | Fuel reserves and emissions dataThe world’s fossil fuel reserves are just a tiny slice of the total fuel resource that we know exists. To count as a reserve, the fuel has to have a good chance of being profitably extracted. Reserves in turn break down into proven reserves (usually defined as those with a 90% chance of eventually being taken out of the ground) and much bigger probable reserves. Fossil fuel being a finite resource, you might assume that proven reserves would shrink each year as we all fill up our cars and heat our homes. In reality, though, in recent years these reserves have stayed flat or even increased slightly as ever more unconventional sources such as tar sands and shale gas get upgraded to proven status thanks to new technologies, such as fracking, and (until recently) high fuel prices. But as carbon emissions continue, our remaining budget does get smaller every year – and therefore the proportion of fuel we can burn. |
| Beyond fossil fuelsAlthough CO2 is the main driver of global warming, there are plenty of others, such as soot, methane, nitrous oxide and even aeroplane vapour trails. Tackling these other warming agents quickly would increase the CO2 budget for any given odds of 2C, whereas allowing them to increase would have the opposite effect. As the last Intergovernmental Panel on Climate Change (IPCC) report shows, success or failure on these other fronts could make a big difference to our carbon budget. And that’s not all: although fossil fuel combustion is the main source of CO2, we also release the same gas by clearing forests and producing cement. The more of our carbon budget we eat through in these other ways, the less will be left for burning fossil fuels. | Beyond fossil fuelsAlthough CO2 is the main driver of global warming, there are plenty of others, such as soot, methane, nitrous oxide and even aeroplane vapour trails. Tackling these other warming agents quickly would increase the CO2 budget for any given odds of 2C, whereas allowing them to increase would have the opposite effect. As the last Intergovernmental Panel on Climate Change (IPCC) report shows, success or failure on these other fronts could make a big difference to our carbon budget. And that’s not all: although fossil fuel combustion is the main source of CO2, we also release the same gas by clearing forests and producing cement. The more of our carbon budget we eat through in these other ways, the less will be left for burning fossil fuels. |
| With all these factors at play, there is plenty of scope for conflicting, confusing and out of date estimates. If I had to run the numbers myself (as I did for the Guardian’s new interactive fossil fuel counters) I would start with the IPCC stat signed off last year by almost every government in the world: a carbon budget of 1,000 GT of CO2 from 2011 to give a 66% chance of staying under 2C. Subtract the roughly 150 GT we’ve already burned since then and that leaves about 850 GT for all sources of CO2. | With all these factors at play, there is plenty of scope for conflicting, confusing and out of date estimates. If I had to run the numbers myself (as I did for the Guardian’s new interactive fossil fuel counters) I would start with the IPCC stat signed off last year by almost every government in the world: a carbon budget of 1,000 GT of CO2 from 2011 to give a 66% chance of staying under 2C. Subtract the roughly 150 GT we’ve already burned since then and that leaves about 850 GT for all sources of CO2. |
| If we rapidly stopped deforestation and pushed down hard on the other drivers of global warming, we might be able to stretch our fossil fuel budget to 1,000 GT – which would let us burn around a third of proven reserves. Let deforestation and other warming agents run amok, however, while also aiming for better odds of staying below 2C, and we might have as little as 300 GT left for fossil fuels – which would be closer to a 10th of proven reserves. | If we rapidly stopped deforestation and pushed down hard on the other drivers of global warming, we might be able to stretch our fossil fuel budget to 1,000 GT – which would let us burn around a third of proven reserves. Let deforestation and other warming agents run amok, however, while also aiming for better odds of staying below 2C, and we might have as little as 300 GT left for fossil fuels – which would be closer to a 10th of proven reserves. |
| In other words, while the familiar Bill McKibben/Carbon Tracker numbers are within the sensible range, nothing is written in stone. Everything from our view of risk to our efforts to reduce nitrous oxide emissions from cars and methane emissions from cows will determine how much of the world’s fossil fuel we need to leave in the ground. And that’s not to mention any disruptive carbon capture technologies that might come along to help us burn more of the fuel without cooking the climate. | In other words, while the familiar Bill McKibben/Carbon Tracker numbers are within the sensible range, nothing is written in stone. Everything from our view of risk to our efforts to reduce nitrous oxide emissions from cars and methane emissions from cows will determine how much of the world’s fossil fuel we need to leave in the ground. And that’s not to mention any disruptive carbon capture technologies that might come along to help us burn more of the fuel without cooking the climate. |
| For now, however, all of this detail remains academic. Political leaders negotiating at the UN have failed to even discuss a total global carbon budget, while fossil fuels companies (both state owned and private) continue to pump huge sums of money into finding and developing yet more reserves. | For now, however, all of this detail remains academic. Political leaders negotiating at the UN have failed to even discuss a total global carbon budget, while fossil fuels companies (both state owned and private) continue to pump huge sums of money into finding and developing yet more reserves. |
| So while it is good to understand what factors will determine our carbon budget, it is much more important to call on politicians and investors alike to get a grip on this issue and face up to the simple and incontestable reality: there’s far more fossil fuel than we can burn, and the more of it that we take out of the ground, the greater the risk of an irreversible climate catastrophe. | So while it is good to understand what factors will determine our carbon budget, it is much more important to call on politicians and investors alike to get a grip on this issue and face up to the simple and incontestable reality: there’s far more fossil fuel than we can burn, and the more of it that we take out of the ground, the greater the risk of an irreversible climate catastrophe. |