The deadlines are driven by the carbon budget. We have about 500 GT of emissions that can be released before we exceed 1.5 - 2 C of temp rise. We are actually seeing what a 1.5 C world looks like due to El Niño (obviously exclusive of the aggregated impacts of further warming (droughts, biodiversity loss, etc as well as tipping points such as the Gulf Stream collapse 4Real mentioned earlier).
This is a function of radiative forcing. Each GHG makes various contributions to radiative forcing. CO2 is the most abundant and most substitutable - things like the PFCs and HFCs, which are nominal but as much as 24,000x more potent at trapping heat, don’t have a really good substitute given their role in refrigeration.
Globally, we release about 50 gigatons annually… so at our current pace, we have 10 years before the budget is consumed.
Not possible to fully decarbonize on that timeline, so there are incremental targets based on the availability of technology. Things like power generation are relatively easy. Decarbonizing cement, marine transport, aviation, agriculture (fertilizers) are much more difficult and will take longer.
You can find the IEA energy models here: