In an article published Thursday -- Castro's first since November 2011 -- the 85-year-old retired leader also took aim at the United States and at gas shale "fracking," a new source of fossil fuels condemned by environmentalists.
He did not, however, address rumors of his death, which were denied by an official Cuban blogger after they surfaced on Twitter earlier this week.
"Many dangers threaten us, but two of them, nuclear war and climate change, are decisive and are drifting further away from a solution," he wrote in an article entitled "The March Towards the Abyss."
Since handing over power to his 80-year-old brother Raul in 2006 because of health concerns, the leader of the 1959 revolution has rarely appeared in public but has penned regular editorials on world events.
In his latest article, Fidel blamed the "inexorable" decline in part on "demagogic talk, statements and speeches from the tyranny imposed on the world by the United States and its powerful and unconditional allies."
He also wrote about the risk of nuclear war, particularly in the Middle East, where he accused Israel -- the region's sole if undeclared nuclear-armed state -- of having "hundreds" of nuclear weapons.
On the subject of climate change, Castro expressed fears about hydraulic fracturing -- or "fracking" -- a process by which water, sand and chemicals are blasted into the ground to release oil and gas trapped in solid rock.
The process has been hailed by oil companies and other backers as a key new source of fossil fuels, but critics have expressed fears about its impact on the environment, health and safety.
Rumors of Castro's death have periodically surfaced since his retirement, most recently on the micro-blogging website Twitter earlier this week.
But official Cuban blogger Yohandry Bloquera, who is usually well-informed and reliable regarding events in Cuba, denied the reports, saying Castro was reading them himself and "laughing" at them.
"The CIA tried to kill him more than 700 times and Twitter is going down that road," Bloquera wrote on Tuesday.