From all indications there will be no functionally effective, multinational agreement to radically and precipitously ratchet down carbon emissions any time soon. Anything less than that just won’t do.
So we will be at the mercy of how rapidly the free market and government controlled economies can produce, export and scale up clean, renewables to global usage, Anything less than something like 80% of global emission at the 1990 level will not suffice. The reason for that is CO2 concentration/rate of accumulation is already well past the point where at least 2C of warming will occur relative to pre-industrial times when CO2 hadn’t exceeded 280ppm. It is now over 390ppm and rising at a rate of 2ppm per annum.
It’s been at least 3 million years since CO2 levels exceeded 400ppm and temperature was as warm as we stand to soon experience. Atmospheric CO2 of over 400ppm coincided with global temps 4-6C warmer than today going back 20 million years ago. Since the mid Cretaceous Period about 100 million years ago when CO2 hovered around 600ppm and global temps were as much as 10C warmer than today, CO2 has experienced a general decline in the atmosphere to reside generally between 180ppm and 280ppm over the past 3 million years of northern hemisphere periodic glaciation.
Natural geologic process regulate the amount of CO2 resident in the atmosphere on multi-million year time frames. Once in the atmosphere CO2 level acts to regulate surface temperature on Earth much the same way as a thermostat does. How it finds itself into the atmosphere in the first place is dependent on the ambient temperate and how this affects the chemistry of rock weathering and CO2 concentration within the oceans. Warmer seas hold less CO2 in solution, so the warmer it becomes due to an enhanced rate of rock weathering, the less capable the oceans are of scrubbing that CO2 out of the atmosphere. The added CO2 then acts to enhance the greenhouse effect. Atmospheric CO2 acts therefore as both a feedback to warming and also a cause of warming. This is a classic positive feeback loop.
Now mankind comes along and becomes a source for introducing CO2 to the atmosphere at a much greater rate than what occurs by the process of natural rock weathering. Natural CO2 sinks were in balance with natural sources of CO2 given overall conditions. Humans are now responsible for about 3% of the total CO2 engaged in the natural carbon cycle as the Earth system literally inhales and exhales CO2. That 3% is the marginal difference which has disrupted the natural balance. The Earth, given current geological conditions, is capable of inhaling only so much, which used to be close to equilibrium with how much it was exhaling. That extra 3% contributed by man’s activities is building in the Earth’s oceans and atmosphere and it will remain there for 100′s to 1,000′s of years, only slowly falling to natural background levels as natural process eventually catch up with the imbalance. The higher that CO2 concentration becomes in parts per million, the longer will be required to reestablish balance over coming decades and centuries.
That basically in a nutshell is what has and is going on according to scientific investigation.