The darkest climate change scenario can officially be written off, thanks to falling renewable energy prices and emerging climate policies. At the same time, even the most rosy prospects are now outdated: global warming will not be limited to one and a half degrees Celsius. This is evident from last week published publication of the CMIP, an international working group for scientific climate models.
The CMIP models have been the main basis for the authoritative IPCC reports for years and feature in countless scientific publications and policy documents worldwide. By disappearing the most unlikely scenarios, it is now becoming clearer what future awaits in a warming climate.
The study published last month contains seven new future scenarios, ranging from “high” for the future with the largest fossil emissions to “very low” for the scenario in which emissions decrease the fastest. They replace the five CMIP scenarios that have been central to climate science until now. All new scenarios fall within the extreme limits identified by the previous scenarios. The publication now calls the former highest emissions scenario, which has existed in several variants since 2010, “unlikely.”
A draft version of the study was made public more than a year ago, and the final version was published a month ago, without much publicity. “I think it is more policy relevant than I realized,” says lead author Detlef van Vuuren, professor at Utrecht University and program leader at the The Netherlands Environmental Assessment Agency, when asked why the publication of the study was not marked with a press conference or press release. Van Vuuren was also involved in the development of the earlier models through the CMIP.
Received as a victory
Last week, the long-awaited publication was noticed by critical followers of climate science, such as American environmental professor Roger Pielke who wrote a post about it on his blog. He mainly paid attention to the deletion of the most pessimistic scenario, not to the optimistic visions of the future that were lost. The research has since been hailed as a victory in climate-skeptical circles on social media.
The most pessimistic scenario that is now disappearing was intended from the start as a possible, but very extreme vision of the future. However, that disaster scenario gradually took on a life of its own in the media and politics. Under the heading ‘business as usual’, it played an increasingly central role in climate reporting. It was not intended for that, Van Vuuren said two years ago in an interview with NRC: “We included the scenario to show the bandwidth of possible future emissions.”
It had been clear for some time that the most pessimistic scenario would disappear from the official estimates, just like the most optimistic scenario in which warming would remain below one and a half degrees. Van Vuuren called both scenarios outdated in that interview.
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The court in The Hague nevertheless referred to the pessimistic scenario when it ruled early this year in a case brought by Greenpeace against the Dutch state. That case revolved around the Netherlands’ obligation to protect residents of Bonaire, part of the Caribbean Netherlands, against the consequences of climate change. In the worst case, the sea level around the special municipality would have risen by 85 centimeters in the year 2100, according to calculations of the scenario.
Van Vuuren thinks it is no problem that the judge takes an extreme climate scenario into account. “For example, if you look at how high a dike should be, you don’t want that dike to collapse anyway. Then you have to take into account a small chance of a high risk.” The most disastrous outcome is unlikely, but the associated temperatures and sea level rise are still not impossible, Van Vuuren emphasizes. “If the judge looks at the scenarios that way, then that’s fine.”
Another factor is that the scenarios that Van Vuuren develops say something about greenhouse gas emissions, not about global warming or the resulting sea level rise. These are other models that convert expected emissions into likely consequences. If the climate turns out to be more sensitive to greenhouse gas emissions, the earth could still warm faster. Van Vuuren refers to so-called feedback effects, such as forest fires that are caused by climate change and further strengthen the greenhouse effect. “There is still a chance that that temperature will indeed be four degrees above pre-industrial levels.”
