Consider the following points:
1) Climate lag. There is a 40 year delay between emissions and effects on the climate so the effects we are now seeing are from the emissions from the late 70s early 80s. We have emitted more GHG in the last 40 years than between 1850 and 1980. Even if a miracle happened and we reached zero emissions today we have a huge climatic bill ahead of us in the next 40 years.
https://skepticalscience.com/Climate-Change-The-40-Year-Dela...
2) Self sustaining climatic systems commonly called feedbacks. These systems once triggered will keep affecting the climate even if we reach zero emissions. Eg: melting of Arctic ice, permafrost methane, etc.
https://www.wri.org/blog/2018/08/why-positive-climate-feedba...
3) There are currently 405ppm of CO2 in the atmosphere which will remain there for centuries and will keep having an effect on the climate unless we remove it.
From the last IPCC report:
> There is sufficient uptake capacity in the ocean to incorporate 70 to 80% of foreseeable anthropogenic CO2 emissions to the atmosphere, this process takes centuries due to the rate of ocean mixing. As a result, even several centuries after emissions occurred, about a quarter of the increase in concentration caused by these emissions is still present in the atmosphere.
4) It is usually stated that we are at about 1ºC of warming right now, but until recently the cooling effects of aerosols in the atmosphere were not well understood. A recent paper from 2019 showed that it's likely there is a lot more cooling going on and we could already be at about 2ºC of warming. This would mean we are in a much more difficult position than commonly believed.
https://science.sciencemag.org/content/363/6427/eaav0566
https://www.sciencedaily.com/releases/2019/01/190122104611.h...