But how likely is the "total climate doom" scenario, meaning human extinction caused by climate change? Let's try to calculate the odds of it, using a simplified type of probability analysis.
Factor #1: Fossil Fuel Resource Production
Currently the amount of carbon dioxide in the atmosphere is is about 400 parts per million. How high will this level be at the end of the century? Estimates vary widely. A recent IPCC report includes the following graph, which shows possibilities ranging from about 460 to 1000. Three out of six scenarios shown on the graph predict a lower carbon dioxide level, of only about 460 to 540.
It is very hard to predict how much carbon dioxide will be in the atmosphere at the end of the century, because the matter depends on how high the production rate will be for various types of fossil fuels (the higher that production, the greater the amount of carbon dioxide will end up in the atmosphere). This issue is much more complicated than one might think from statements such as “We have enough coal in the ground to fry the planet.” Such a statement may be as misleading as the statement that there's enough gold in the ocean's seawater to make every person a millionaire (a statement that doesn't mean much because it's too difficult to extract such a resource).
For each fossil fuel resource there is an “availability pyramid” like the pyramid shown below. Most of the resources are at the bottom of the pyramid, and such resources are much harder to extract than the easy-to-extract “low hanging fruit” at the top of the pyramid. At some position in the pyramid it becomes no longer economically viable to extract the resource, but we don't know where that point is. It is therefore very difficult to predict fossil fuel resource production in the remainder of this century. Peak Coal and Peak Oil may put a brake on such production.
There are quite a few experts who predict that the production of coal and oil will plunge later in this century, as we work our way down the coal and oil availability pyramids, and start trying to extract resources that are more and more difficult to extract. For example, David Rutledge of Cal Tech predicts that 90% of the world's coal production will occur by 2070, which means a plunge of coal production later in this century. He suggests that this will mean that carbon dioxide levels will not exceed about 500 parts per million by the end of the century.
For the sake of this simplified probability analysis, let us assume that there is a 50% chance that fossil fuel production will not decline very much because of availability and extraction difficulty reasons (which we can call Condition 1A), and a 50% chance that such production will indeed plunge in this century because of availability and extraction difficulty reasons (which we can call Condition 1B). The first of these two conditions is necessary for the worst-case-scenario of a climate doom bad enough to cause human extinction.
Factor #2: Clean Energy, Conservation and Legislative Measures
Another factor that will determine how much carbon dioxide is in the atmosphere at the end of the century is how much effort humans make to restrict their use of greenhouse gases, and prevent global warming. If we keep on with “business as usual” without doing much to fight global warming, that will make a worst case scenario more likely. But if we conserve and cooperate, build lots of clean energy, eat less meat, and follow international protocols such as successors to the Kyoto Protocol, that will make a worst case scenario less likely.
For the sake of this simplified probability analysis, let us assume that there is a 50% chance that humans will not do very much to restrict global warming through such measures (which we can call Condition 2A), and a 50% chance that humanity will get very serious about reducing global warming through such measures (which we can call Condition 2B). The first of these two conditions is necessary for the worst-case-scenario of a climate doom bad enough to cause human extinction.
If global warming gets much worse during this century, this should increase the chance of people being alarmed enough to start taking the necessary measures to fight global warming through measures such as conservation. So we should not rate the likelihood of Condition 2A as being more than 50%.
Factor #3: Climate Sensitivity
Another factor that will determine how bad global warming will get is what is called climate sensitivity. Climate sensitivity means how much the earth's temperature will go up for after a given rise in carbon dioxide in the atmosphere. Scientists debate how much the temperature will rise if the carbon dioxide in the atmosphere doubles from its current level of 400 parts per million, to become as high as 800 parts per million. A post on RealClimate.org says, “The ‘meta-uncertainty’ across the methods remains stubbornly high, with support for both relatively low numbers around 2ºC and higher ones around 4ºC, so that is likely to remain the consensus range.” The more dire projections of future global warming assume a high climate sensitivity.
For the sake of this simplified probability analysis, let us assume that there is a 50% chance that climate sensitivity is high (which we can call Condition 3A), and a 50% chance that climate sensitivity is relatively low (which we can call Condition 3B). The first of these two conditions is necessary for the worst-case-scenario of a climate doom bad enough to cause human extinction.
Factor #4: Amplifying Feedback Mechanisms
If conditions 1A, 2A, and 3A mentioned above all occur, this will be enough to cause horrifying trouble for the human race, resulting in the ruin of many coastal cities, the loss of countless species, a huge increase in wildfires, and a huge increase in desertification. But by themselves those conditions would not be enough to cause human extinction. The human race could always abandon all areas around the middle of our planet, and move entirely to regions such as Canada and Russia (or, if things got really hot, to the continent of Antarctica after its ice melted). In order for you to have human extinction by global warming, you would also need to have feedback mechanisms that amplify global warming once its gets going very strong.
Pessimists have raised concern that such amplifying feedback mechanisms may exist. One of the most frightening possibilities is the melting of methane hydrates, natural sources of methane that exist in places such as Siberia and Antarctica. Methane is a greenhouse gas 20 times more potent than carbon dioxide. So if man-made global warming starts melting frozen methane hydrates, that might cause the release of more and more methane into the atmosphere. That in turn could cause more and more methane hydrates to melt. It could possibly be a deadly feedback mechanism that could amplify global warming.
Experts disagree over how much we should be worried about the possibility of such feedback mechanisms. Some say it's a huge concern, while other say the risk is small.
For the sake of this simplified probability analysis, let us assume that there is a 50% chance that planetary feedback mechanisms will one day start amplifying global warming (which we can call Condition 4A), and a 50% chance that such feedback mechanisms will not be a very big factor (which we can call Condition 4B). The first of these two conditions is necessary for the worst-case-scenario of a climate apocalypse bad enough to cause human extinction.
Factor #5: Geoengineering
There is one last factor we must consider: the possibility of geoengineering. When people use the term geoengineering when talking about global warming, they are talking about man-made measures designed to directly reduce the effects of global warming. Various techniques have been proposed. One technique would place on the oceans many specially designed ships that would create mist out of sea water, a salty mist that would rise in the atmosphere and increase the amount of sunlight reflected by the atmosphere (thereby lowering temperatures).
Opinions vary over whether geoengineering techniques would work to effectively fight global warming. For the sake of this simplified probability analysis, let us assume that there is a 50% chance that geoengineering techniques would not effectively reduce global warming (which we can call Condition 5A), and a 50% chance that such techniques would be effective (which we can call Condition 5B). The first of these two conditions is necessary for the worst-case-scenario of a climate apocalypse bad enough to cause human extinction. No such climate doom would occur if we had an effective way to prevent it with geoengineering.
So What is the Chance of Total Climate Doom?
So given all these considerations, what is the chance of global warming causing the extinction of the entire human species? In each of the five issues I have discussed, there is a particular condition that must occur in order for total climate doom to happen. Let us assume that the probability of each of these conditions is independent of all of the others. If that is true, then to calculate the combined probability of each condition occurring we multiply together the individual probabilities. This gives us the following formula:
Probability of total climate doom =
probability of Condition 1A (undiminished fossil fuel resource production) multiplied by
probability of Condition 2A (“business as usual” without huge efforts at conservation and clean energy) multiplied by
probability of Condition 3A (high climate sensitivity) multiplied by
probability of Condition 4A (amplifying feedback mechanisms) multiplied by
probability of Condition 5A (no effective geoengineering)
Substituting the probabilities I have listed, this gives us a probability of total climate doom equal to 50% multiplied by 50% multiplied by 50% multiplied by 50% multiplied by 50%. So according to this simplified analysis, the probability of total climate doom (meaning complete human extinction because of global warming) is roughly 1 chance in 32, or about 3 percent.
It would seem, therefore, that the chance of complete human extinction by global warming is quite low, but still large enough to be worth worrying about.
But this does not mean that we can be complacent, or that we should lessen efforts to fight global warming. For one thing, a 3 percent risk must be considered a very substantial risk when one is talking about something such as total human extinction. For another thing, there is a possibility not quite as catastrophic as total climate doom, a possibility that we might call decimation by global warming. As decimation means literally to destroy a tenth, we may define decimation by global warming as the possibility that something like a tenth of the human species will be wiped out by global warming. That possibility is a lot more likely than total climate doom, and might require only three or four of the five conditions necessary for total climate doom. The math would get a little complicated as to how to calculate such a likelihood, but I think the chance is significantly higher than 3 percent. Such an outcome would be far worse than any world war the world has seen, and its avoidance is something all of us have a moral duty to prevent.
Speaking in purely practical terms, I think that to argue for “total climate doom” is perhaps not very helpful in the fight against global warming. If you persuade a man that global warming will cause great trouble for mankind, many deaths, and a risk of extinction, that person may well change his actions to help fight the problem. But if you persuade a man that the whole human race is going to become extinct from global warming, then the person may just tend to think that action is futile, and that it makes no difference how he acts.