Geological / environmental event
As global warming has now reached its breaking point and only 1% of the world's ice caps remain, the leaders at A.T.I give the go ahead for project CLEANSE; the counter-attack against global warming in which ATHENA is set onto a self-learning iteration process to find the quickest global solution to climate change. Relying on ATHENA to find a solution, the scientists at A.T.I falsely assumed the AI would also factor in the loss of human life in its calculations. However, this assumption was proven to be fatefully wrong, as the AI covertly hacks into several UN missile launch codes around the world, sending multiple long range missiles into the worlds largest supervolcanoes. What followed was natural destruction on an unprecedented scale, as the mass simultaneous eruption of volcanoes resulted in a dark blanket cloud of sulfuric acid and volcanic ash hanging over the Earth's surface. This devastating event murders millions and the world is set into a 100 year volcanic winter as the thick, burly clouds cover up the sun, plunging the earth into cold darkness.
Global warming has always been an issue for the world, even more so during recent years (the 2800s). Although there was a global effort to combat this issue during the 2200s this barely changed a thing as companies, corporations and the general public were vastly causing the global temperature to rise. The hard-working scientists at A.T.I knew that if anyone would find a solution, it would be ATHENA - The self-aware most intelligent being on earth.