There is a technical distinction between a scenario and a prediction, though the line is fine and often disputable. Many scenarios are merely intolerably loose, and therefore useless, predictions. The difference is that a prediction lays out its conditions for all to see, while a scenario keeps at least one condition hidden. The conditions are both on the “left hand side”, i.e. what conditions must hold for the prediction to be in force, and on the “right hand side”, i.e. what is the measurable, observable event that is being predicted.
An example of a scenario is the so-called Doomsday Clock touted by the Bulletin of the Atomic Scientists, a group composed of folks from the left wing of science. On 10 January 2012, the BAS announced
It is five minutes to midnight. Two years ago, it appeared that world leaders might address the truly global threats that we face. In many cases, that trend has not continued or been reversed. For that reason, the Bulletin of the Atomic Scientists is moving the clock hand one minute closer to midnight, back to its time in 2007.
The BAS said, “Two years ago, it appeared that world leaders might address the truly global threats that we face. In many cases, that trend has not continued or been reversed.” By “threats” they mean climate change, energy use, terrorism, and other matters. Nuclear weapons being loosed is on the list, but seemingly as an afterthought.
The BAS clock is a scenario for two reasons. (1) We do not know what will happen if the clock does strike midnight: the clock is supposed to represent “doomsday”, but that term is meaningless (in secular terms). There is also no date by which we can verify the prediction. (2) We do not know each condition that went into the decision to shift the clock. We know some of the conditions, because the BAS announced some (“harmonized domestic regulation [of CO2]”), but we do not know them all (the BAS publicly stated their conditions were only “a minimum” set).
What we have at the BAS is a secular clerisy saying, “Do something: we won’t tell you what, but it had better be something. If you do this something, we’ll turn back our frightening clock. If the clock strikes midnight, something bad will happen: so bad we cannot tell you what it is.” I mention in passing (and with no value judgment) that humanity has suffered through both (repeated) climate change and the use (and stockpiling) of nuclear weapons, yet it has not approached annihilation.
The BAS clock is thus political theater, capable of convincing only those unwilling to put in any thought about what the clock means, and it is quite useless in making any real-world decision.
The Intergovernmental Panel on Climate Change issues what it openly calls scenarios on the path of “emissions” and their effect on global temperature (all data and quotes taken from this public website). Are these genuine scenarios?
Without burdening us with details, it is possible to operationally define what “global average temperature” (GAT) is, even though the term has little physical meaning (no thing experiences a GAT; plants, animals, rocks, experience temperatures; nothing experiences a global average temperature). This definition of GAT is statistical.
A forecast of the GAT is a scenario unless the operation definition of what the GAT is is known (and thus the scenario becomes a prediction). That is, the list of (ever in flux) surface stations, satellites, buoys, etc. must be known; plus all the statistical manipulation that is used to massage the data from these sources must be explicated for a prediction to hold. The GAT is a prediction, but the loose statements which litter the media about other weather effects are generally scenarios.
Other associated scenarios are the bad things that await us if the GAT increases, which is to say, if the “climate” changes. Greenpeace is a serial offender and issues steady streams of scenarios. For example, they would like to “promote sustainable agriculture” which they claim is endangered because of climate change. This is a scenario because by taking the word at its plain English definition, agriculture is already “sustainable” because the fruits of the land are, in fact, sustaining billions. Incidentally, Greenpeace knows this and so is forced to begin their scenario with the words, “Over the past 50 years, we have nearly tripled agricultural outputs.” (they also say, “lack of food is not the cause of hunger” which is again at odds with agriculture not being “sustainable”).
“Sustainable”, then, is always a scenario word.
The IPCC’s energy-use scenarios are, however, predictions. They are predictions which seek to evade the force of predictions by calling them scenarios. In their words:
Scenarios are alternative images of how the future might unfold and are an appropriate tool with which to analyse how driving forces may influence future emission outcomes and to assess the associated uncertainties. They assist in climate change analysis, including climate modeling and the assessment of impacts, adaptation, and mitigation. The possibility that any single emissions path will occur as described in scenarios is highly uncertain.
The IPCC admits that “Any scenario necessarily includes subjective elements and is open to various interpretations”, which is of course part of the definition of a scenario. But their scenarios are predictions. The IPCC directly states that if energy use is this-and-such, then that-much warming will occur; and that if energy use is instead that-or-so, then this-not-that much warming will occur. It is true that the IPCC does not state in exact terms what are likelihood of each condition is, but this does not matter because if one of these conditions obtains, the specific prediction of warming is then in force.
The IPCC’s ploy is obvious: if these predictions are a bust then the theory/model which gave rise to the predictions will not have to be abandoned or radically modified. That is, if the prediction fails, the IPCC hopes to say that it was only a scenario and was on no use whatsoever in making any decisions. But this is false: unlike the BAS, the IPCC clear states that the action it requires is reducing carbon-based energy use. They intend fully that these so-called scenarios be treated exactly as predictions.
Who was it that said something about a beautiful theory being murdered by ugly facts? It is this wholesale slaughter that the IPCC would escape by claiming, after the fact, that what actually happened was just not part of any “scenario,” therefore its theories must live. Theories live or die by their ability to make skillful predictions or not. And this is so if the predictions are mislabeled scenarios.
We’ll look at some detailed prediction/scenarios at another date.