"Let’s go back to 2005 and take a look at how the world’s top energy and climate experts envisioned the range of plausible futures for climate change to 2100, and explore how they might react to our new analysis.
The future is always an unknown place, but if we are going to create desirable futures,
then we need to have some way of reliably projecting where were are headed and how we might alter course if we decide that we are headed in a wrong direction.
In climate policy, expectations for the future have long been characterized as scenarios,
which according to the Intergovernmental Panel on Climate Change allow us to assess “a range of plausible futures,
because human development is determined by a myriad of factors including human decision making.”
There are of course a very wide range of plausible scenarios for the future, defined as “a variety of future states that are considered ‘occurrable’ (could happen).”
The figure below illustrates the future as an expanding cone of possibilities — of which some are possible, plausible and probable, with some futures more preferable than others.
The notion of “business as usual” has long been considered to be where we are currently headed if we don’t change course.
We implement policy to try to shape the cone of future possibilities toward more preferred outcomes.
So we step out of our time machine in 2005.
We find the climate science community, under the auspices of the IPCC, embarking on an effort to produce and assemble a set of plausible scenarios for the long-term climate future
to help guide our thinking and action on climate policy.
Back then, as today, it was generally accepted in policy that futures with less carbon dioxide emissions were preferable to those with more.
Plausible scenarios can help us think through how to achieve that goal.
It took several years, but the climate science community eventually assembled a very large number of scenarios to represent a wide range of what were then considered to be plausible scenarios of the future
— 1,311 scenarios, in fact.
To simplify and streamline research the climate science community initially picked 4 of these scenarios as priorities for research, and later added 7 more.
... Thanks to our new research, in 2022 we know which scenarios remain plausible of the 1,311 IPCC scenarios produced years ago, and the small subset of 11 used most commonly in climate research.
How have we judged which scenarios remain plausible in 2022?
We use two methods:
One is to look at how the world actually evolved from 2005 to 2020.
That provides some real world data against which to evaluate scenarios.
If key elements of an IPCC scenario have already departed from the trajectory of the future envisioned by a scenario, then it is implausible.
A second method is to use the best estimates of energy system experts for the near-term trajectory of key scenario elements.
If the elements of an IPCC scenario have already departed from the trajectory of the future envisioned by a scenario, then it is implausible.
It turns out, using our most restrictive criterion and comparing to real-world data from 2005 to 2020, only 71 (about 6%) of the original 1,311 IPCC scenarios remained plausible in 2022.
Using data and near-term projections 2005 to 2050, only 35 (less than 3%) of IPCC scenarios remained plausible.
You can see the range of plausible scenarios for 2100 (blue) compared to trajectories of all scenarios (grey) in the figure below, along with the 2020 Stated Policies scenario to 2050 of the International Energy Agency (dark blue line).
You will notice that the figure above includes net negative emissions for some scenarios starting later this century.
We don’t know if such scenarios are themselves plausible, because such carbon removal technologies do not yet exist at scale.
So we can also look at the plausible scenarios with the removal of negative emissions and that figure is shown below.
... the mandate of the IPCC is to use only plausible scenarios, so if we can identify those which are implausible, that would make the IPCC work much more relevant to policymakers, which after all is our mission.
Using our more restrictive criterion, we have identified 71 of your 1,311 scenarios that reman plausible based only on what actually happened 2005 to 2020,
and that number is cut in half, to 35, when we also consider the 2020 IEA projections to 2050.
... as the future unfolds, we should fully expect that the subset of available scenarios judged to be plausible shrinks in number over time as reality constrains possibilities as the future becomes the present.”
... the (IPCC) scenario that is identified as “business as usual” and is most commonly used in climate research projects that the world
will build 33,000 new coal power plants by 2100 and global temperatures might increase by 5 degrees Celsius or more.
... all of the plausible scenarios remaining from the ones you are working with today envision less than 3 degrees Celsius total warming by 2100.
In fact, the median projection is for 2100 warming of 2.2 degrees Celsius, which is within spitting distance of the Paris Agreement goal of holding temperatures to a warming of 2.0 degrees Celsius.
... the scenarios ... include some pretty dubious assumptions about future energy use.
... Have a look at this figure, which shows that the most plausible scenario in 2022 is SSP4-3.4, and of the 11 that the climate research community focuses on, the most extreme 3 are so far removed from reality that they should be reserved for exploratory research rather than for projections . . .
In fact, it is 2022 and the IPCC still doesn’t evaluate its scenarios for plausibility. ... "