Catastrophic climate change can still be averted even without increasing one of the biggest sources of carbon-free electricity.

That was the message sent to International Atomic Energy Agency (IAEA) officials on Monday, who convened their first-ever conference exploring how nuclear power could help mitigate climate change. Reactors currently supply about a third of the world’s low-carbon electricity, but that share could tumble as economies turn to cheaper technologies.

“You have to address the high capital costs and long time for construction, risks of accidents and proliferation, long-term storage of nuclear waste and social opposition,” Patricia Espinosa, a United Nations envoy on climate, said in a message read to the 550 delegates.

Atomic power is still seen by some environmental and business luminaries including Bill Gates and Stewart Brand as an essential ingredient in keeping global warming below 1.5 °Celsius. But rising costs, lengthening construction times and continued safety concerns have merged to raise barriers against new nuclear projects.

“There is considerable potential as well as uncertainty for nuclear power,” said Hoesung Lee, the chair of the Intergovernmental Panel on Climate Change (IPCC). “The challenge for nuclear power is to be a cost-effective alternative to other non-fossil technologies and to deploy nuclear power much faster than in the past.”

IPCC scientists studied 89 different energy scenarios that could keep global temperatures under thresholds that would trigger more superstorms, droughts and famine. While the role of nuclear modestly expanded until mid century under most of those scenarios, its share of power dramatically declined in others.

“One reason for this large variation is that the future development of nuclear can be constrained by societal preferences,” according to Hoesung, who was met with other delegates by scores of protesters outside IAEA headquarters in Vienna on Monday.

One of the biggest costs of nuclear when it comes to mitigating climate change is the amount of time it takes to build new reactors, according to the World Nuclear Industry Status Report published last month. Wind and solar generation backed up by batteries can be deployed quickly whereas new nuclear reactors can take a decade or more to build.

“Any solution that saves less greenhouse gas emission per dollar, or does so slower, than it could have will stabilise the Earth’s climate less and later than it should have,” the report read. “Costly and slow options avoid less carbon per dollar and per year than cheaper and faster options could have, and thus make climate change worse than it should have been.”

Nuclear power currently displaces one or two gigatons of carbon dioxide equivalent a year that would be generated from natural gas or coal, according to Liu Zhenmin, a top United Nations official dealing with economic affairs. The trouble is that about two-thirds of those units are operating in industrial economies and are approaching the end of their lives, he said.

“The cost competitiveness of nuclear power will remain an issue as renewable power has become increasingly more cost competitive,” Liu said. “Few private investors are willing to go it alone given the large costs. Government and public acceptance will be critical for new nuclear.”