How much CO₂ can we still burn?

It's the goal of the Paris Agreement to limit global warming to 1.5°C above pre-industrial levels. The 2018 IPCC report says that the world has a budget of about 420 GtCO₂, to have a 66% chance to stay within that 1.5°C. By dragging the knob you can see that if we would cut down right now, we would have to get to zero emissions around 2040. (The IPCC itself uses a 50% chance, which means zero emissions at 2050.)

That said, 1.5 degree warming is still pretty bad. It's hard to model the exact consequences on us, but we know some things. By the end of the century:

• coral reefs will have declined by 70-90%
• 14% of the population will be exposed to severe heat once every 5 years
• average droughts will last 2 months
• 271 million people will be exposed to water scarcity.

These two websites have some more information.

After 1.5°C warming, 2.0°C is the next big threshold. We want to stay well under this per the Paris Agreement. The budget is about 1170 GtCO₂ since 2018 (which includes the 1.5°C budget).

It is harder to predict what would happen in this case, since it's further out, but to give you an idea, by the end of the century:

• coral reefs will have declined by 99% (vs 70-90%)
• 37% of the population will be exposed to severe heat once every 5 years (vs 14%)
• average droughts will last 4 months (vs 2 months)
• 388 million people will be exposed to water scarcity (vs 271 million).

These two websites have some more information.

When we get past 2.0°C, things get tricky. Self-reinforcing natural feedback loops get activated, which can accelerate warming automatically, to potentially above 10.0°C. This would cause mass extinction of species, and make much of the planet uninhabitable. See these resources for more information.