Going up from 280ppm pre-industrial to 400ppm today, with possible large-scale positive feedback loops coming up in the coming decades from changes to Arctic ice cover, permafrost methane emissions.
Climate change will be (future-tensed, because what it is right now is trivial compared to what it will be) a massive disruptor, socially, economically and ecologically and it will affect everyone, with very few, if any winners.
A huge part of whether we manage to limit both CO2 and temperature increase is directly linked to the US position on emissions. While the US public is gradually becoming aware of the risks, this is happening much too slowly.
In any case the current administration has gone of its way to worsen things as much as possible, often for limited actual economic gains except for pushing their ideology:
The US has an outsize effect on climate change both by its direct emissions, which, already high per capita, get amplified by its size and by its technological, economic and "soft power" leadership within the world.
Its cap-and-trade approach to limiting acid rain in the 80-90s was groundbreaking in its outcome and economic efficiency. A similar hard-nosed, number-based, approach based on market mechanisms and pricing emissions could serve as a useful counterpoint to the heavy subsidy model personified by the German Energy Wende, which, both in its cost and relative lack of actual drop in emissions shows its limits.
On the flip side, the "what, me worry?" approach by a sizable proportion of US politicians encourages many countries to do nothing, secure in the knowledge that someone much bigger is making a much worse mess.
Quite simply, while a competent and responsible US leadership is no guarantee that we will get global warming under control, continuation of current US trends in that regard makes it very likely we will not, and that will affect all of us, globally.
Under Xi, that country certainly seems to be trending in the wrong direction, with increasing regional belligerence, internal repression and ongoing widespread intellectual property theft ("acceptable", to an extent, from a developing country, not so from a world power).
Ideally, the world would see a peaceful transition from US hegemony to a benign, non-interventionist, internally liberal China.
If that could not be achieved, the West should consider switching to a containment model, although that would be harder to do than it was when dealing with the economically inept Soviet Union. This could not be done by the US alone, it would have to involve a sizable proportion of the world's countries.
Somewhere in the middle you'd have a modus vivendi where the West would accept China as it is and it would agree to reform its predatory industrial spying and refrain from regional adventurism.
A situation where the US is both stridently, but inconsistently, aggressive towards China, incapable of projecting a more seductive social and economic model and lacks credibility with its allies severely reduces the probability of a peaceful transition or effecting any coordinated approach to dealing with the China challenge.