The recent publication of an unprecedented critique against the so-called “WWS” 100% renewable energy (RE) scenario has re-ignited the debate about the feasibility of renewable only energy scenarios in the United States and abroad. This is a long-overdue debate the world sorely needs, and everyone who has the slightest interest in climate change mitigation should pay careful attention. At stake is nothing less than whether or not our climate policy measures are based on sound science or pie-in-the-sky optimism.

As many of the critics of 100% RE plans – myself included – have repeatedly pointed out, the problem here is not that 100% RE plans are being developed. We definitively need research that tries to solve the issues related to large-scale deployment of renewable energy sources, and it is a very good thing that such plans are made. Even if the plans themselves never come to fruition, their existence serves to increase the ambition level of other plans and policy proposals; and if it turns out that we can power the planet with nothing else but renewable energy yet limit the environmental and social damages to an acceptable level, I believe we should do so.

But the burden of proof lies with those who assert that we definitely do not need certain solutions, usually nuclear energy and carbon capture and storage. At this moment, no country on Earth has managed to decarbonize its economy even close to the extent required by climate science. Despite encouraging progress of renewable energy sources, the “new” renewables that would have to shoulder most of the burden in renewable-only decarbonization plans are still a minor fraction of the world’s total energy supply. And the fact is that no combination of renewable energy sources and ambitious climate policies has ever been able to reduce emissions even to the levels that the rapid roll-out of nuclear energy was repeatedly able to do – entirely by accident.

History of per capita CO2 emissions in four countries, and their best 10-year CO2 emission reduction achievements. Data from CDIAC Carbon Project, figure from our book Climate Gamble.

In short, it is far too early to state with any confidence that the “best” climate strategy is to use only renewable energy sources. We really don’t know yet whether it is even possible to make the transition to 100% RE sources in time to stave off dangerous climate change, and whether, if it is possible, we can amass the political capital required to do so. These uncertainties can only be resolved through experience and simply cannot be eliminated by energy systems modeling, no matter how detailed and scrupulous – and certainly not by modeling that contains as many dubious assumptions as most 100% RE plans currently do. And even if the current critique results to better models in the future, which I hope it will, the uncertainties remain uncomfortably large. Modeling energy system futures is very different from, say, climate modeling: while the climate can be modeled based on invariant laws of physics, energy system models are essentially predictions (or educated guesses) about how human beings make decisions, how technology develops, and how the economy functions.

One reason for doubt: the required sustained new energy build rates and efficiency increase rates in 100% RE scenarios are far in excess of both historical records and those demanded in technology neutral mitigation scenarios. Source: Loftus et al. (2015), graphic from our book Climate Gamble.

We all know, or at least should know, that reliable predictions of future technologies or future economy are very hard, if not fundamentally impossible. As a rule, we should treat all such predictions with considerable skepticism. After all, we have plentiful examples of failed predictions in the energy sector as well: as a historian of technology, I cannot but be amazed by how the 100% RE promoters of today re-enact almost word-to-word the confident assertions and other rhetorical strategies (like framing the debate to be between the forces of progress and the luddites) originally used by nuclear energy promoters in the 1960s. Back then, serious scientists told us that by the year 2000, all the legacy energy companies would be bankrupt and even oil wouldn’t be profitable to pump from the ground, thanks to relentless and inevitable progress of cheap atomic energy. There were only some minor technical details to be worked out, and some irritating but uninformed critics to silence. Just as RE advocates of today, these technocrats were also almost totally ignorant of the political and social aspects of their proposed solutions.

A vision of a “City of the Future” from the 1960s. All the energy used in the city comes from a single nuclear power station at the center, and the daily life is re-arranged accordingly. Can you spot the conceptual similarities to current ambitious energy plans? Picture from Finnish Seura magazine, with thanks to Esko Pettay.

In another striking resemblance to renewable plans of today, nuclear proponents could not think that people might have any reasons to resist the nuclear buildup – after all, it was not only the embodiment of “progress” but also the cleaner and safer energy source that promised considerable economic benefits. (A concrete example of how local opposition is derailing ambitious RE plans comes from Germany, where grid expansion plans have been delayed by a decade by strong and almost entirely unforeseen local opposition. The expansion underway is very modest compared to what 100% RE plans absolutely require.) Another striking similarity is how the methods for dealing with the opposition are again from the “ridicule it” playbook; at one extreme, a Harvard professor whose expertise in energy systems came entirely from the now-criticized WWS study called those who support not only renewable energy but nuclear as well “climate denialists!”

One reason why large-scale RE plans may be uniquely vulnerable to local opposition: harvesting a dilute energy resource puts many more people within sight of the energy infrastructure. The graphic shows the land use for ca. 27 terawatt hours of annual electricity generation, in scale and based on actual projects. Graphic from our book Climate Gamble.

Being usually rather ignorant of the history of their own discipline, the promoters of 100% RE scenarios nevertheless very rarely if ever even admit that there may be some uncertainties or unknowables in their models. After a decade of following the energy discussions, I’m hard pressed to remember a single instance of a 100% RE promoter publicly discussing the caveats of their plans, or the assumptions required to make 100% RE scenarios work. On some rare occasions, the promoters have admitted (usually as a response to criticism) that the plans, while possible in a sense that they do not break any known physical laws, may not be very realistic (see one such case here). More often, though, the promoters seem to be entirely unburdened by doubts and confidently assert that the 100% RE scenarios “prove” that 100% RE future is “possible”, and as a consequence, we should only use renewable energy sources in the climate fight. (The phenomenon where the most confident tend to get the most airtime has been discussed in e.g. Dan Gardner’s book about predictions, “Future Babble.” Gardner also notes that as a rule, the confidence someone has about his – almost always his – prediction unfortunately tends to correlate negatively with the accuracy of the said prediction.) This transition from “possible” to the “only possible” is often subtle, and the difference seems to be lost to many listeners, politicians in particular.

And here lays the greatest drawback of 100% RE scenarios: taking them too seriously. For a politician, the allure of 100% RE can be overwhelming. Almost every single one of the prominent 100% RE scenarios promises that we can lick the climate change easily with nothing else but clever new technologies – and even profit at the same time. Such a plan is bound to attract political attention: promises of free lunches usually do. By following the 100% RE promoters, the politicians are also saved from having to do unpleasant decisions and released from the burden of having to champion deeply unpopular solutions, such as nuclear energy or carbon rationing. Most pleasingly for the politicians, the risks of promoting 100% RE strategies are small: the politicians can always say that they only followed scientific advice, and the long timelines all but ensure that if it becomes undeniable that the plan did not pan out, the politicians of today are likely to be safely retired already. (Furthermore, many politicians sincerely believe that we should use only renewable energy sources, and it is easy to listen to studies that confirm one’s pre-existing beliefs.)

And if all else fails, scapegoats can be found: witness how many nuclear advocates are still claiming that the ambitious nuclear plans of the 1960s were perfectly fine, if only the sinister cabal of pesky environmentalists and fossil fuel interests hadn’t colluded in stopping nuclear expansion. I have no doubt that I for one will be indicted in the future as one of the reasons why 100% RE plans didn’t come to fruition, as one of those “negative thinkers” who now “sow doubts” whether the plans are really feasible. After all, finding people to blame will always be easier than admitting that one’s favored political plan had some inherent technical or political limitations.

What all this translates to is that the aggressive, almost certainly overconfident promotion of 100% RE scenarios and the conflation of “possible” with “the only possible” threatens to shut down support for what may very well be necessary components of effective climate policy. Many towns, countries and political parties are now making commitments to back only renewable energy sources; while quitting fossil fuels is commendable, there is a real risk that such plans exclude e.g. nuclear power and lead to premature closure of what still remains the second most important low carbon energy source in the world. In the U.S. for example, right now the greatest climate risk is in the premature shutdowns of nuclear plants. Committing to renewables only also undermines desperately needed support for research into new energy technologies and climate mitigation tools (e.g. carbon capture and solar radiation management), leaving us to only hope that the Ultimate Victory of renewable energy really is looming just around the corner this time.

Some renewable energy predictions and actual share of RE in the United States. But this time they must be correct, right?

Given the history of failed energy predictions, I would not be holding my breath. But if the techno-optimists are wrong this time as well, the problem is that we really don’t have the time to try again. We have one shot and one shot only at decarbonization, and being overly confident we can do it easily with only some of the possible tools is just as dangerous as pooh-pooing the dangers of climate change.