It isn’t every day that a paper in Geoscientific Model Development makes mainstream news.
The paper, led by Utrecht University Professor Detlef van Vuuren, proposes a revision to the greenhouse gas emissions scenarios that are considered when projecting the magnitude of climate change that we can expect to see in the future.
Let’s take a step back. Emissions scenarios are based on different possibilities of what society’s relationship to energy looks like over the next century. On one extreme, we might rapidly transition to low- and no-emissions energy sources like solar and wind. On the other, we might double down on coal, oil, and natural gas — the dominant sources of greenhouse gases. Climate projections based on greenhouse gas emissions scenarios inform how communities, businesses, and governments understand, prepare for, and respond to the impacts of global warming.
In 2011, a scientific team led by van Vuuren published four scenarios that would be used in the Intergovernmental Panel on Climate Change’s (IPCC) Fifth Assessment Report of 2014. These scenarios, termed “Representative Concentration Pathways” (RCPs), described different possible trajectories of future greenhouse gas emissions through 2100. The authors conceived of a worst-case high-emissions scenario (RCP8.5), a best-case low-emissions scenario (RCP2.6), and two scenarios in the middle.
The question is — as it always has been — in the magnitude of climate risk we are willing to accept.
New research led by van Vuuren, meant to inform the IPCC’s forthcoming Seventh Assessment Report, now suggests both the previous best-case and worst-case scenarios be dropped.
The narrowing of the range of scenarios offers both reassurance and warning: Scientists are increasingly confident the planet is unlikely to warm, by 2100, as much as some of the most severe projections once suggested; at the same time, global climate policy has fallen short of the emissions reductions required to achieve the most optimistic outcome.
However, some public figures who question mainstream climate science have used the retirement of the worst-case scenario to argue that we no longer need to worry about climate risks. Further, they suggest the revised scenarios are evidence climate scientists have misled the public about the risks of climate change, such as sea-level rise, extreme weather, and ecosystem disruption.
But that is not what this paper finds. Different emissions trajectories influence the magnitude of expected climate change, but not the sign. There are no scenarios in the scientific literature in which the fundamental physics of greenhouse gases is altered such that they cease interacting with electromagnetic radiation and stop warming the surface of the planet.
There are also no plausible emissions scenarios in which we don’t experience the impacts of climate change.
The question is — as it always has been — in the magnitude of climate risk we are willing to accept.
The new scenarios suggest an end-century temperature increase of between about 1.5 and 3.5 °C relative to a preindustrial baseline. In even the most optimistic scenarios, we temporarily overshoot 1.5 °C — the level of warming that the Paris Agreement sought to avoid in order to prevent some of the worst impacts of climate change. The IPCC is clear that every increment of warming results in increased climate risk, a point that remains scientifically uncontested.
We should also not lose sight of the new paper’s conclusions: economics and policy have not worked at a pace sufficient to secure our most optimistic climate future.
Narrowing uncertainty is, arguably, the whole goal of science. That we have more confidence in the range of possible climate futures is a welcome scientific development.
Revised scenarios do not mean that climate risk has vanished or that climate action no longer matters. If anything, a clearer understanding of the range of possible futures should help decision-makers of all stripes — from farmers evaluating crop choices to coastal planners preparing for future flood risks in their communities.
To be sure, the retirement of the worst-case emissions scenario is worth celebrating. It is a testament to climate action already underway. The lower cost of renewables and climate policy have worked together to ensure that RCP 8.5 is no longer plausible.
But we should also not lose sight of the new paper’s conclusions: economics and policy have not worked at a pace sufficient to secure our most optimistic climate future.
