A WIDELY read cover story on the impact of global warming in this week’s New York magazine starts ominously: “It is, I promise, worse than you think.” It goes on to predict temperatures in New York hotter than present-day Bahrain, unprecedented droughts wherever today’s food is produced, the release of diseases like bubonic plague hitherto trapped under Siberian ice, and permanent economic collapse. In the face of such apocalyptic predictions, can the world take solace from those who argue that it can move, relatively quickly and painlessly, to 100% renewable energy?

At first glance, the answer to that question looks depressingly obvious. Despite falling costs, wind and solar still produce only 5.5% of the world’s electricity. Hydropower is a much more significant source of renewable energy, but its costs are rising, and investment is falling. Looking more broadly at energy demand, including that for domestic heating, transport and industry, the share of wind and solar is a minuscule 1.6% (see chart). It seems impossible to eliminate fossil fuels from the energy mix in the foreseeable future.

But all energy transitions, such as that from coal to hydrocarbons in the 20th century, take many decades. It is the rate of change that guides where investments flow. That makes greens more optimistic. During the past decade, solar photovoltaics (PV) and wind energy have been on a roll as sources of electricity. Although investment dipped slightly last year, the International Energy Agency, a global forecaster, said on July 11th that for the first time the amount of renewable capacity commissioned in 2016 almost matched that for other sources of power generation, such as coal and natural gas. In some countries the two technologies—particularly solar PV in sunny places—are now cheaper than coal and gas. It is no longer uncommon for countries like Denmark and Scotland to have periods when the equivalent of all their power comes from wind. Ambitions are rising. The Senate in California, a state that is close to hitting its goal of generating one-third of its power from renewables by 2020, has proposed raising the target to 60% by 2030; Germany’s goal is to become 80% renewable by 2050. But whether it is possible to produce all of a country’s electricity with just wind, water and hydro is a subject of bitter debate. In 2015 Mark Jacobson of Stanford University and others argued that electricity, transport, heating/cooling, and industry in America could be fully powered in 2050-55 by wind, water and solar, without the variability of the weather affecting users. Forswearing the use of natural gas, biofuels, nuclear power and stationary batteries, they said weather modelling, hydrogen storage and flexible demand could ensure stable supply at relatively low cost. But in June this year Christopher Clack, founder of Vibrant Clean Energy, a firm, issued a stinging critique with fellow researchers in the Proceedings of the National Academy of Sciences, the journal in which Mr Jacobson et al had published their findings. They argued that a narrow focus on wind, water and solar would make tackling climate change more difficult and expensive than it needed to be, not least because it ignored existing zero-carbon technologies such as nuclear power and bioenergy. They claimed the models wrongly assumed that hydroelectricity output could continue for hours on end at many times the capacity available today, and pointed to the implausibility of replacing the current aviation system with yet-to-be-developed hydrogen-powered planes. In their view, decarbonising 80% of the electricity grid is possible at reasonable cost, provided America improves its high-voltage transmission grid. Beyond that is anyone’s guess.

Others take a wider view. Amory Lovins of the Colorado-based Rocky Mountain Institute, a think-tank, shrugs off the 100% renewables dispute as a sideshow. He takes comfort from the fact that it is increasingly common for renewables sustainably to produce half a location’s electricity supply. He believes that the share can be scaled up with ease, possibly to 80%. But in order to cut emissions drastically, he puts most emphasis on a tripling of energy efficiency, by designing better buildings and factories and using lighter materials, as well as by keeping some natural gas in the mix. He also sees clean-energy batteries in electric vehicles displacing oil demand, as petroleum did whale oil in the 19th century.

Some sceptics raise concerns about the economic ramifications if renewables’ penetration rises substantially. In an article this month, Michael Kelly of Cambridge University focused on the energy return on investment (EROI) of solar PV and wind turbines, meaning the ratio between the amount of energy they produce to the amount of energy invested to make them. He claimed that their EROI was substantially lower than those of fossil fuels; using renewables to generate half of the world’s electricity would leave less energy free to power other types of economic activity.

Critics note that his analysis is based on studies of PV returns in Spain from more than half a decade ago. Since then solar and wind costs (a proxy for EROI) have plunged, raising their returns. What is more, other studies suggest returns from fossil-fuel-derived energy have fallen, and will decline further as they incur increased costs associated with pollution and climate change. A high share of renewables may be less efficient at powering economic growth than fossil fuels were in their 20th century heyday. But if the climate doomsayers are to be proved wrong, a clean-energy system must be part of the solution.