Summary: Progress in the climate policy debate comes too slowly. The use and misuse of RCP8.5 shows why. At this pace, the climate will give final answers before we get a consensus. We can’t afford this.
In 2015 I gave one of the early critiques of the RCP8.5 scenario. Is our certain fate a coal-burning climate apocalypse? No! And then Manufacturing climate nightmares: misusing science to create horrific predictions (it was the first Dr. Curry saw about the issue). I – and the many who followed – said two easily proven things.
- The RCP8.5 scenario was a good worst-case scenario, showing what might happen if many things go wrong. It is either unlikely or impossible.
- The RCP8.5 has been described as the “business as usual” (BAU) scenario and so become the central scenario for both researchers and policy-makers. It is not BAU, and should not be the main case for either group.
In most other fields, there would have been debate and then RCP8.5 would have been used only in an appropriate way – as a worst-case scenario. But this is climate science, and five years later the debate continues to chase its tail. But this might be changing.
For an introduction to the RCPs, see “Understanding The Great Climate Science Scenario Debate” by Roger Pielke Jr. (Professor, U CO-Boulder) at Forbes. That these kinds of articles appear the major journals show that climate scientists might be seeing the obvious: “Emissions – the ‘business as usual’ story is misleading” by Zeke Hausfather and Glen P. Peters in Nature – “Stop using the worst-case scenario for climate warming as the most likely outcome; more-realistic baselines make for better policy.” I recommend reading it in full, especially their conclusions.
“A sizeable portion of the literature on climate impacts refers to RCP8.5 as business as usual, implying that it is probable in the absence of stringent climate mitigation. The media then often amplifies this message, sometimes without communicating the nuances. This results in further confusion regarding probable emissions outcomes, because many climate researchers are not familiar with the details of these scenarios in the energy-modelling literature. …
“Happily – and that’s a word we climatologists rarely get to use – the world imagined in RCP8.5 is one that, in our view, becomes increasingly implausible with every passing year. …
“We must all – from physical scientists and climate-impact modellers to communicators and policymakers – stop presenting the worst-case scenario as the most likely one. Overstating the likelihood of extreme climate impacts can make mitigation seem harder than it actually is. This could lead to defeatism, because the problem is perceived as being out of control and unsolvable. Pressingly, it might result in poor planning, whereas a more realistic range of baseline scenarios will strengthen the assessment of climate risk.”
The fun for activists is over when even the BBC runs the headline “Climate change: Worst emissions scenario ‘exceedingly unlikely’.” It took a decade to make this simple point.
The skeptics contribute to the confusion
Many skeptics – both scientists and laypeople – say that RCP8.5 is “bad science” or “impossible.” Both are absurd. The first is easy to dismiss. The RCP’s are well-constructed and part of a decade-long program.
The second objection is more complex. If it is impossible, then RCP8.5 is not a useful worst-case scenario for researchers and policy-making. It is not impossible. There are many ways to get to a given concentration pathway. It can be driven by emissions of greenhouse gases from burning fossil fuels or melting permafrost. It can be driven by the strength of carbon cycle feedbacks or a dozen other climate dynamics.
Each of those, in turn, can occur for several reasons. Technological progress could slow. Population growth might be faster than predicted, perhaps because fertility in Africa slowed less than expected. Petroleum might be less than estimated; coal deposits might be less than estimated (they probably are). Either of these would force tapping lower-quality deposits – which would release more CO2.
The worst-case scenario reminds us that bad things happen. Sometimes our luck goes bad and many bad things happen. These rare events make history. Let’s not become history like that.
Many things went wrong to make the Titanic famous
At 11:40 pm on 14 April 1912 the RMS Titanic was on its last voyage. The captain disregarded warnings of icebergs, and ordered steaming at high speed (22 knots) under a dark sky (no moon, no Venus) with no wind (so no waves breaking on the ice).
The lookouts peer ahead, but without binoculars. Second officer David Blair had the key to the locker holding the binoculars. He was transferred off the ship before it left on its maiden voyage from Southampton and accidentally took the key with him.
The lookouts sounded three bells for an object dead ahead. First Officer William Murdoch ordered the rudder “hard astarboard” and the engines “full astern” – intending to steer around the iceberg. That was not the “book” response, and did not work well. Reversing the engines reduced the flow over the rudder and its effectiveness. Even so, the Titanic almost made it. The hull gently brushed against the ice. Water entered through 230′ rips where plates buckled and seams opened.
The Californian was close and could have rescued its passengers. Through incredible negligence, it did not do so (its captain was broken for negligence).
The rest is history. All of these things were necessary for the disaster. What were the odds of all these things happening on one voyage?
Conclusions
Well-designed worst case scenarios are unlikely or impossible. And sometimes they happen. We need to prepare for those. But it is imprudent to bankrupt ourselves to prevent them or ignore other high-priority needs.
How should we use all these scenarios? There is a large body of knowledge and experience in the field of risk management. Of course they have little role in climate science and climate policy, a madness that we can fix.