"Existential risk to humans" tends to be at the outer range of my own fairly pessimistic set of scenarios, but several global warming situations could run that far out.

Yonatan Zunger posted an especially pessimisstic bit about global warming in April of 2013. In particular he noted:

_The last big spike like this was the Paleocene-Eocene Thermal Maximum, about 55 million years ago. Average temperatures rose by 6C over a period of 20,000 years -- which is enough to look like a giant, sharp spike on the history-of-the-entire-planet graph...._

_When the biota of a planet get rewritten, the creatures that require the most delicate maintenance die first. This tends to mean really big creatures, that rely on large supplies of their foods; apex predators, which rely on the entire food chain beneath them; and "canary" species like many frogs, which are very sensitive and tend to be the first to die when something is going wrong. Historically, the cutoff for "large creatures" (that tend to not survive extinction events) seems to be in the ballpark of 20 pounds; things bigger than that just require the ecosystem to be too healthy._

IPCC's estimates call for a global rise in temperatures of anywhere from 2-6°C, (3.6 - 10.8 °F). First thing to realize is that this is an average rise. Which means that in some areas (mostly over oceans, with higher albedo and greater thermal mass) it will be lower, and in others (mostly inland regions) it's likely to be much higher. Overland temperatures much over 49°C (about 120°F) are problematic as they tend to rule out much plant life. Above about 65°C (about 150°F), many forms of animal life cannot keep themselves cool, even in the shade. The result would be potentially large areas of land in which life would literally cook to death.

Even if we don't go that far, there are a number of other challenges humans face, all of which tie back to population, resource consumption, and overflowing pollution sinks.

If you think of systems layered on top of one another, you've got the global financial system, global, regional, national, and local economies, governmental systems, social systems, infrastructure, and more, layered on top of ecological, biological, meteorological, oceanographic, and other systems. Disruptions of lower systems will propogate through higher ones.

Disrupt enough human systems and things start to fall apart. The claim by numerous authorities in the Collapse space is that the collapse of Western Civilization isn't something that's going to happen, it's something that's happening, and likely will be for some time. There are definitely global trends which have been pointing downward for some time, many since the 1970s, some from before that.

Both the Arab Spring and the Ebola outbreak are examples people point to. Paul Mason's Why It's Kicking Off Everywhere gives a capsule summary of the dynamic of the wave of global revolutions which started in 2009. The original essay coves the basics:
http://www.bbc.co.uk/blogs/legacy/newsnight/paulmason/2011/0...

The Ebola outbreak is being exacerbated by numerous factors -- perversely, both lack of technology and its presence are playing a role. Many people, living close to their limits, with few resources, in crowded conditions, but with access to transportation (both ground and air) have created conditions for the disease and facilitated its spread. Lack of literacy, ineffective communications, and other factors are also at play. Airplanes could transfer the disease around the planet in under a day. Nightmare scenarios have it breaking out, not in New York or London, but in Kolkata, Manilla, Cairo, or Sao Paulo.

The CDC predicts as many as 500,000 cases (of a disease with 70-90% mortality), I've seen projections far above that, though from less credible sources. Much of this depends on how rapidly exponential growth takes off, and when, if ever, inhibitory effects start slowing the spread.

Where things get interesting though, for ... interesting values of interesting, is when you consider the systemic effects of disruption.

In her letter to President Obama, Liberia's President Sirleaf gives one small example of this:

With blanket travel bans, border closures and interactions on vessels berthing at our ports, this has become more than a humanitarian emergency. In a country that has barely emerged from a 30-year period of civil and political unrest, with the presence of a large youthful(mainly unemployed)population, some of whom were child soldiers-this health emergency threatens civil order. What is even more heartbreaking is that we are unable to reopen our basic and secondary health facilities because terrified health workers, who have watched colleagues die, are afraid to return to work.

That is: disease has disrupted transport, which means commerce, which means civil unrest.

At a global scale, breakdowns in one portion of a globalized system (finance, trade, energy, raw materials, gas, water, food) could lead to a domino effect in others. David Korowicz's "Financial System Supply-Chain Cross-Contagion: a study in global systemic collapse" takes a look at this: http://www.feasta.org/wp-content/uploads/2012/06/Trade-Off1....

While it might not reduce the total population to 5 million or fewer, it could well disrupt systems to a level that modern industrialized economies simply could not function. Which might be a pretty big deal in some areas.