Scenarios are not predictions, just ways of describing possible futures; useful in order to be prepared for unexpected events. The only rule in scenario building is that the assumptions should not be too improbable; such as involving time machines. And, yet, it seems that in some cases involving climate projections, time machines are a built-in assumption.
The COP21 conference in Paris has brought again climate to the attention of the public and, from now on, there starts the real challenge: what can we really expect for the future of the earth's climate? As always, predictions are difficult, especially when there are many variables involved. Nevertheless, climate change is the result of physical factors that we can understand and we know that the build-up of greenhouse gases in the atmosphere - if it continues - is going to lead us to a very unpleasant future.
If we look at the long-term future, the whole question rotates on whether we manage to stay below an increase in temperature that is believed to be "safe" (it might be 2 degrees C, but we don't know for sure), or we pass the limit and we find ourselves above the "climate tipping point" after which the system starts moving by itself toward more and more warming with all the associated disasters.
So, I thought I might engage in a little exercise of qualitative "scenario building" with a special focus on climate. Here are some scenarios; listed in no particular order. Some you could see as horrible, some as unlikely, others as overoptimistic. But they are just that: scenarios. The COP21 was a step in the right direction. Avoiding the worst outcomes will not be easy, but it is up to us.
1. Business as usual. In this scenario, things remain mostly as they are today; just gradually worsening. There are no major wars, no abrupt economic collapses, no sudden climate disasters. But temperatures keep increasing while the world's economic system is battered by one crisis after the other. So, the economy gradually loses the resources necessary to keep alive the structures that study and understand global problems: universities and research centers. As a consequence, global problems slip away from the collective consciousness. People get killed by heat waves, starved by droughts, and swept away by monster hurricanes, and still no one is able to connect all that to climate change, while the burning of fossil fuels, although reduced because of depletion, continues. In the long run, that would lead to the end of civilization by a whisper, rather than by a bang.
2. The climate panic. This is the symmetric and opposite scenario to the above. As the climate crisis gets worse, we may arrive at a "perception tipping point;" maybe generated by some spectacular event (e.g. a monster ice calving from Antarctica or Greenland) or, simply, by the accumulation of evidence. A wave of climate panic would lead to a scramble to "do something" and things might worsen rather than improve if some extreme forms of geoengineering were attempted. However, it might also lead to positive results. For instance, a push for reforestation and for renewable energy would effectively mitigate climate change. It is not obvious that our civilization needs a burst of panic to be saved, but that might give us an extra chance.
3. The Seneca collapse. Before being hit by some climate disaster, the world's economy could experience a "Seneca collapse" as the result of resource depletion. such a situation, people would have no time to worry about anything but their immediate survival and that would lead to climate change being completely forgotten. On the other hand, the economic collapse would cause a reduction in emissions probably well beyond even the wildest dreams of environmentalists and might be sufficient to avoid to go above the "climate tipping point". Then, of course, such a collapse would be a disaster in all other terms; causing the probable disintegration of the whole world's economy. Nevertheless, it might still give us a "window of opportunity" to restart from scratch with a new renewable energy infrastructure and then rebuild a new and better society.
4. The warring states. The present situation has been likened to the beginning of the first world war and there are serious risks that the ongoing conflicts will escalate into a major worldwide confrontation. In such case, all the worries about climate change would be immediately forgotten. A major war would likely boost the efforts to extract as much fossil fuels as possible, including, probably, the oil shales that pure market forces seem to be unable to extract (it may be that the current drive for war arises in part from this kind of considerations). That would lead to emissions spiking up, at least for the duration of the war. On the other hand, it is likely that any major war would rapidly peter out because of the lack of energy and resources to carry it on. So, the carbon spike won't last long. Still, it could do a lot of damage, making things even more difficult.
5. The nuclear holocaust. A variant of the war scenario, it assumes that one or more contenders would decide to play the nuclear card. That could take the shape of tactical or strategic nuclear bombing or also that of attacking the adversary's nuclear plants utilizing conventional weapons. In all cases, we would see a rapid drop of the carbon emissions as large industrialized areas would be destroyed or just rendered uninhabitable. A massive nuclear exchange would also generate so much dust in the upper atmosphere that the result could be described as a "nuclear winter" causing an extreme cooling that might do even more damage than warming. However, that would do nothing to change the long-run effect of the greenhouse gases already emitted in the atmosphere. The dust would eventually settle down and the warming restart with a vengeance.
6. Depopulation. Most current projections assume that the human population will keep smoothly increasing throughout the 21st century, plateauing at around 9-10 billion individuals, or perhaps more than that. However, the historical record shows that human populations rarely follow this kind of trajectory, more often tending to collapse after having peaked. A good case in point is that of Ireland, between 1845 and 1850, when population crashed to about half of the size it had at the peak. The world's population might collapse in the same way as the result of wars, epidemics, pollution, of someone playing games with biological weapons and it might not be impossible to lose several billion people in a few decades, or even faster. The result would be a strong reduction of the greenhouse gas emissions, albeit obtained at a price that nobody would want to pay. However, people would continue burning fossil fuels and the cumulative amount of the greenhouse gases in the atmosphere would continue increasing. So, it is not obvious that even this extreme scenario could lead to avoiding the climate tipping point.
7. The renewable revolution. Renewable energy is the wild card of the situation. It is already efficient enough that it can outcompete fossil fuels and it could grow fast enough to replace them before it is too late. Assume that people understand both the advantages of renewable energy and the desperate need we have to stop burning fossil fuels, then we could arrive at a "bottom-up" revolution in which we don't need government-enforced emission trading or a carbon tax. A situation in which even climate science deniers wouldn't be so silly to pay more for dirty fossil energy when they can have cheaper and clean energy. In the end, the battle for climate would be won when a consortium of renewable companies buys Exxon and closes it down. Problem solved and it is the beginning of a new era.
We could combine some of these scenarios together, or think of different ones. The only rule is that they shouldn't be too improbable. For instance, we shouldn't include scenarios dealing with an alien invasion of the planet or with the COP97 being held in Siorapaluk, in Northern Greenland, in 2074 finally arriving at a binding treaty on the phasing out of fossil fuels. Apart from this, the future always surprises us. Just don't forget that the future cannot be predicted, but that you can be prepared for it.