You've probably seen a message recently that we've hit a big climate tipping point and can't go back. That’s not what the science says, and that meme sends the wrong message that there is no longer any point to fighting the fossil fuel companies.
It is true that we are above 400 ppm of CO2 and have no plan that will get us under 400. For the foreseeable future (not quite the same as "permanently" as stated in several articles), we will be above 400.
But 400 is not a tipping point. It is a symbolic round number.
What is a tipping point? Think of slowly pushing a bottle over with one finger. At first the bottle wants to settle back upright. At some point, it has tipped too far and falls, spilling your drink. You can't get your drink back. Or think of pushing a cart over a hill. Going up, the cart won't move forward unless you push. After you pass the summit, the cart moves on its own. You have to pull to keep it from running away.
A true climate tipping point comes when we make a change to the earth system that says CO2 levels and temperatures will stay high even as emissions become low for centuries. That comes from things like albedo changes (earth becoming darker and absorbing more heat) resulting from melting ice caps, melting permafrost releasing methane, extinctions irreversibly changing ecosystems, and shutting down ocean currents. Even a temperature increase itself, by changing air moisture levels, can be a tipping point.
We are on the verge of hitting a number of those tipping points. We don't know for sure when we hit any of them. We probably won't know we have hit them until long after it happens. Suggested numbers like getting CO2 levels back to 350 ppm or keeping global temperature increase below 1.5°C are based on maintaining a low probability of hitting a bunch of those tipping points.
The more we do today and the sooner we do it, the better the chance of avoiding tipping points and avoiding catastrophes. That means both reducing emissions to decrease the chance of hitting a real tipping point and learning to live with a new climate to deal with the chance that we hit a tipping point despite reducing emissions.
400 ppm is a wake up call (well, more like the alarm going off for the tenth time after repeatedly hitting “snooze”). As Payal Parakh of 350.org said, “Crossing the 400 ppm threshold is a somber reminder that we haven't taken the action we need.”