NBC News: Trace amounts of CO2 could cause "political instability, severe drought, famine, ecosystem collapse and other changes that make Earth a decidedly inhospitable place to live"

Forget the movies: 9 real ways the world could end - Science

...here are nine apocalyptic visions that scientists foresee.
1. Global warming
The mother of all apocalyptic fears, climate change is the biggest threat facing the planet, many scientists say. Climate change could make extreme weather more severe, increase droughts in some areas, change the distribution of animals and diseases across the globe, and cause low-lying areas of the planet to be submerged in the wake of rising sea levels. The cascade of changes could lead to political instability, severe drought, famine, ecosystem collapse and other changes that make Earth a decidedly inhospitable place to live.
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9. Snowball effect
Though each of these scenarios could happen, most scientists think a snowball effect of multiple events is more likely, Miller said. For instance, global warming could increase the prevalence of pathogens while also causing widespread shifts in climate. Meanwhile, ecosystem collapse could make it slightly harder to produce food, with no bees to pollinate crops or trees to filter agricultural water. So, instead of an epic catastrophe, several relatively small factors would slightly worsen life on Earth until it gradually degraded, Miller said.
In that scenario, the downfall of Earth is not dramatic, "like being attacked by a saber-toothed tiger," Miller told LiveScience. "It's more like being nibbled to death by ducks."