I don’t think Population is the biggest problem, and if we try to cut down on it, it would change society in ways not intended. For example, the economy in all developed nations and those aspiring to become developed, is based on constantly increasing the buying and selling of goods. If the population and/or production is cut significantly, there will no longer be the money to provide for education and welfare, or pensions at the level we have today, let alone increase it. There would have to be a marked change in the way we live and it would require families ( who would be more likely to include unemployed or part timers), to look after their own sick and aged including financially. Unless imposed on the population, it would be the community minded who would have fewer children and vice versa. At the moment in the UK the population is more or less stable but it is changing in make up - googling statistics of population growth shows distinctive trends. I say this without judgement - even without immigration, the population will become significantly more culturally mixed purely due to family size based on current trends. So be it, this is inevitable to some extent whatever happens.
Definitely, giving up meat would make a huge difference, growing our own veg and pulses perhaps with the help of modern technologies such as hydrophonics would make us more self sufficient even in urban areas ( but obviously not completely so). Maybe people who can will go back to keeping a goat which has a lower gas effect!) and some chickens. We need to stop being a throw away society., and revert to some of the ‘old fashioned’ ways of living and passing time without giving up all the benefits of modern society, become more community minded and helping our neighbours because there will not be the money ( already it is running out) for social workers and nurses to step in. In many ways it would resemble our childhoods but with some mod cons. The use of luxury yachts and anything that causes pollution, for leisure alone should be severely taxed and/or restricted (don’t despair if you have loved ones far away, the technology to visit them without harming the atmosphere is bound to come soon).
And by the way, in ten years time we may be in a mini ice age - due to a lack of sun spots.
And for those who blame China for pollution, note that they have assigned 60,000 troops to start planting trees!