Atmospheric concentrations of CO2 have never been higher in human history. Devastating climate anomalies are going to become more commonplace in years to come, with extreme heatwaves already bringing scorching temperatures in countries like India this year, putting the health and lives of many thousands of people at risk.

This summer’s global heatwaves are just the latest reminder that the need for large-scale decarbonization is more pressing than ever. Renewables are thankfully gaining ground across much of the planet, but despite being the fastest-growing energy sector, a new International Energy Agency report has determined renewables will account for only 30% of global energy capacity by 2023. In coming years, “renewables are forecast to meet more than 70% of global electricity generation growth, led by solar PV and followed by wind, hydropower, and bioenergy,” the IEA explains. “Hydropower remains the largest renewable source, meeting 16% of global electricity demand by 2023, followed by wind (6%), solar PV (4%), and bioenergy (3%).”

Nuclear power, the world’s second-largest low-carbon energy source (behind hydropower), accounts for 10% of global energy generation. The 452 nuclear reactors currently in operation worldwide provided 2,700 TWh of electricity last year. “In advanced economies, nuclear has long been the largest source of low-carbon electricity, providing 18% of supply in 2018,” the IEA notes in a new report, called Nuclear Power in a Clean Energy System, the first such report by the agency on nuclear energy in two decades.

Yet despite its pivotal role in countries such as the United States and Japan, the overall share of nuclear is on the decline outside of a handful of countries, most notably China and Russia. Those two countries account for most of the 11.2 GW of new global nuclear capacity last year. The energy agency warns this projected decline in nuclear power should be highly concerning to those worried about the progress of CO2 emissions reductions efforts. Ageing plants are beginning to close down “partly because of policies to phase them out but also as a result of economic and regulatory factors,” the IEA observes.

“Without policy changes, advanced economies could lose 25% of their nuclear capacity by 2025, and as much as two-thirds of it by 2040,” it adds. “The lack of further lifetime extensions of existing nuclear plants and new projects could result in an additional 4 billion tonnes of CO2 emissions.”

This sharp uptick in emissions boils down to one simple, disturbing fact: coal and other fossil fuels, rather than renewable energies, will in many cases end up taking the place of decommissioned nuclear plants. “Without an important contribution from nuclear power, the global energy transition will be that much harder,” notes Dr. Fatih Birol, the IEA’s executive director. Birol, a Turkish economist and energy expert, calls the ongoing phasing-out of nuclear “the most urgent policy challenge today” in climate change mitigation efforts.

“Alongside renewables, energy efficiency and other innovative technologies, nuclear can make a significant contribution to achieving sustainable energy goals and enhancing energy security,” Birol elucidates. “But unless the barriers it faces are overcome, its role will soon be on a steep decline worldwide, particularly in the United States, Europe and Japan.”

Diminishing profit margins in highly competitive energy markets, including the U.S., have placed several nuclear plants at risk of shutting down far earlier than originally planned, even though some experts argue nuclear energy is more economically competitive than the conventional wisdom lets on. In a Financial Times piece published earlier this year, Jonathan Ford demonstrated that the supposed price differences between nuclear and offshore wind capacity in the United Kingdom are far smaller than they appear at first glance, with wind power’s hidden costs including the need for a substantial of backup power generation capacity; 1 GW of wind power generation, Ford explains, would require 2 GW of wind capacity and almost 1 GW of (gas-fired) backup power.

Dr. Kirill Komarov, a deputy director at the Russian nuclear operator Rosatom, has pointed out that nuclear builds achieve economies of scale when a design is employed across multiple projects. Komarov, like the IEA, also argues that the real competitor for nuclear capacity isn’t renewables – it’s coal.

That helps explain why the IEA sees setbacks to nuclear power as troubling for effective climate change mitigation. Renewables would have to be deployed at unprecedented levels to keep up with this shortfall. “In the past 20 years, wind and solar PV capacity has increased by about 580 gigawatts in advanced economies,” the agency explains. “But over the next 20 years, nearly five times that amount would need to be added. Such a drastic increase in renewable power generation would create serious challenges in integrating the new sources into the broader energy system.”

Without that clean baseload, energy transitions in advanced economies would also come with a hefty price tag: $1.6 trillion in additional investment over two decades. That would mean higher electricity bills for most consumers, since large-scale transitions to renewables often lead to significant hikes in electricity prices.

That, Birol says, is why nuclear should play a key role in future energy plans: “Electricity market desig[s] must value the environmental and energy security attributes of nuclear power and other clean energy sources [in tandem].” Otherwise, the fossil fuels responsible for creating this moment of crisis could dig us even further into it.