Global warming is certainly a threat to civilisation as we know it, but hardly the worst threat there is.

Humans still possess nuclear arms in such a large numbers that there can be a nuclear holocaust many times over. The immediate threat of a nuclear war ended with the fall of the Soviet Union (thank God), but we cannot rule the possibility out. As long as humans know how to make them, nuclear arms will be a fact. Some people have even suggested that there will be conflicts because of global warming so bad that some nations will resot to nuclear war.

At any time, a devastating plague can reappear, just like the Black Death in the 1300's. More and more bacteria evolve multi resistance against antibiotics, what if the medical industry researching for new antibiotics turn out to fail? Sure, mankind survived without antibiotics for millenia, but an airborne disease can spread very fast in this day and age.

There is also a risk of Earth being hit by a large meteorite. It has happened before and may happen again. Astronomers try to chart every object in the Solar System, but are far from finished. Consider if the Tunguska meteorite would have hit a big city instead of the unpopulated reaches of Siberia in 1908? That was not a big meteorite.

There may be other threats as well. It just seems fashionable to speak of global warming right now.