That global warming is currently happening should not be a matter for debate any more. It is happening, and the evidence for that, from many different observable factors, is conclusive. The extent to which human activity is influencing the speed of climate change is a legitimate subject, though. Is there a tipping point? Probably. Have we reached or passed it? We'll have to wait and see. How much of it is 'our fault'? Very difficult to gauge. How much can we do to slow or halt its effects? Personally, I don't know if we as a species are able to curb our excesses to the extent that we can have a dramatic positive effect. In the short period of time we've been 'civilised', we've grown accustomed to having things our own way, and that's a very difficult trend to reverse.
Climate change has happened regularly throughout the planet's long history, and we've only been here for the blink of an eye. A few hundred thousand years of self-awareness, and far, far less with any sense of our position within a wider biological system. But now we're able to have some idea of the timescales involved and look to the past to see what it can tell us about what may lie ahead. Humans have been through an Ice Age before. I'm sure it wasn't very nice, but we made it through without all having to be rescued by mammoths, sloths and sabretooths. Human society as it is now, though, has yet to experience such an all-encompassing meteorological event. We don't know what might happen and we're afraid.
Just to cheer you up, let's look at what happened at the start of the Triassic, 250 million years ago. The mass extinction at the end of the Permian - affectionately known as The Great Dying - had killed off almost everything and left the place looking fairly barren. Volcanoes were getting carried away, spewing lava, ash, smoke and grumpiness all over the planet, and plants - not normally heavily hit by extinction events - began to suffer. Once vegetation began to die off, the carbon dioxide levels rose and this caused temperatures to spiral, making things even worse. A recent study showed that around the equator during this long, hot summer the sea water was around 40°C, ten to fifteen degrees more than now. This the researchers describe as 'lethally hot', so it's probably a bad thing. The Earth in the Early Triassic was nasty. Hot, dry, sandy and empty. It was nice at the poles, apparently, though real estate value there has dipped somewhat since. It took a while - a few million years - for the planet's ecosystems to recover from these unfortunate circumstances. A few million years is a very, very long time to us, so an extinction and resultant greenhouse effect would be a difficult time for humans. If anyone asks if you want a global mass extinction event, say no. It'd be rubbish.