Alph is correct. Humans will not all die. Some of the poorer nations will suffer as they warm and things such as heatwaves increase. While climate change is worldwide the effects of that climate change are regional and each region will respond differently. In the more northern areas of the world there may be a boon in agriculture. In coastal regions there may be increased rainfall and flooding. In drought ridden areas there will be increased drought occurrences. Some species will go extinct and have been facing hardships. That is a big deal.

Are you asking for the worst *possible* outcome, or the worst *likely* outcome?

The worst possible outcome would be that some kind of runaway greenhouse effect is triggered, the oceans boil away, and we all die. But that is vastly, vastly, vastly, vastly unlikely. We'd probably, at a minimum, need to convert pretty nearly *all* of the Earth's carbon into CO2 for that--including carbonate rocks and the like. Just burning fossil fuels would be unlikely to get us there.

I think the worst remotely likely outcome is, essentially, the collapse of civilization. Hundreds of millions (possibly even 1 or 2 billion) dead, wars over resources, chaos, destruction, loss of infrastructure, et cetera. And even that is, imho, pretty unlikely, unless we do absolutely nothing to slow AGW until we've changed the climate so far that the Earth can, effectively, no longer support anything like the current world population.

There are several gradiations below that, down to the (imho equally unlikely) "essentially no change at all". Which is most likely depends entirely on how soon we start taking AGW seriously and start taking action.

We are not all going to die.

The worst case scenario is where all of the ice melts and sea level rises by 70 metres. Most of the inhabited world is more than 70 metres above sea level. But the flooding would cause more damage than all of the hurricanes, earthquakes, tornadoes and wars in history.

The worst case scenario is that an asteroid large enough to sterilize the world would hit, It's a bit pointless considering the worst case.

Read the IPCC reports directly, not what you 'hear' or get from blogs.

There is no way 7billion people will die or that earth turns into venus.

NOTHING. 1 foot increase in the sea level, and species adapt just as the fish have in the southern hemispher.

Earth will turn to Venus

Some have said Earth will turn to Venus

What's the worst thing that could happen from global climate change? I've heard answers ranging from "WE'RE ALL GOING TO DIE!" to "No big deal, sea levels will rise and some species will go extinct". I don't expect the answers from here to be that much better, but it's worth a shot. So what is the worst case scenario from global climate change and how likely is that to happen?