Ultimately, those fossil fuel increases outpaced sharp improvements in solar and wind generation, both of which climbed by double digits in 2018. Even nuclear generation grew at modest levels, rising 3.3%, mainly due to new turbines in China and four reactors that went back online in Japan, according to the IEA.

But figures deeper in the report highlight a systemic issue that’s making it harder to drive down emissions in a consistent way.

From 2000 to 2018, while the portion of global electricity generation from solar and wind grew by 7%, nuclear declined by the same percentage. Meanwhile, coal only dipped by 1% over that time, while natural gas, which emits just more than half as much carbon dioxide, climbed from 18% to 23%.

In other words, renewables mainly picked up market share forfeited by another source of carbon-free power, rather than seizing it from fossil fuels. Once you add that to the increasing use of natural gas and coal use to fuel economic growth, it’s no surprise that the world still isn’t making a real dent in energy emissions, decades after the threat of climate change became clear.

“If you’re replacing one zero carbon source with another, you’re not really changing the carbon intensity of electricity,” says Nikos Tsafos, at the Center for Strategic and International Studies in Washington, DC, who highlighted this issue on Twitter. So while “certain data points say we’re making tremendous progress … if you flip the perspective a bit, you find, ‘Oh, we’re not really moving the dial here.’”

To be sure, the rapid increase in renewables, as well as gains in energy efficiency, and the shift to less climate polluting natural gas, have all at least helped to slow the increase in global emissions – and bring it down in some nations, including the US in recent years (though notably, not in 2018).

But achieving bigger and consistent declines will likely require a lot more renewables, a lot more nuclear, and other big changes to our energy systems and practices.

Many nuclear power stations around the world, however, are due for retirement or are already being decommissioned. Meanwhile, little new capacity is coming online thanks to tougher regulations and safety concerns intensified by Japan’s Fukushima disaster in 2011, as well as steep operating and development costs (see: “Meltdown of Toshiba’s nuclear business dooms new construction in the US”). In particular, nuclear has struggled to compete against the dirt cheap costs of natural gas, which accounted for almost 45% of the increase in energy demand last year, according to the IEA report.

But despite the current distaste for nuclear, it has a critical advantage over other clean sources. It generates electricity that doesn’t fluctuate depending on the time or day or weather conditions, so it can help to balance out intermittent wind and solar generation, without requiring vast amounts of expensive storage or transmission upgrades to the grid. A next generation of plants that are cheaper, safer, and easier to build might also help assuage a skeptical public’s fears.

Most models from the UN’s climate research body call for a substantial uptick in nuclear power. Under the IEA’s Sustainable Development Scenario, which lays out a path to achieve a stable climate and universal energy access, the world would need to add 17 gigawatts of nuclear capacity every year, nearly doubling our current fleet by 2040.

As things stand, the world’s retiring plants will remove around 200 gigawatts by 2040. That will make it nearly impossible to reach those targets unless companies and policymakers decide to extend the life of those facilities, or get busy building many more.

Update: This story was updated to clarify the causes of nuclear's shrinking share of electricity generation, and underscore the role of renewables in slowing emissions gains and future decarbonization.