This is wrong on multiple levels.

One thing I want to point out: Climate change has negative consequences, but this is not a thing that is either going to happen or not in a black-or-white manner. In contrary, there are many degrees of possible severity as the Earth's climate system is going to progressively break down with every increase of the amount of CO2 which is released into the atmosphere. These negative effects are not linear, but they are of a complex, interconnected, highly dynamical system of processes which are linked to each other. As a consequence, additional amounts of CO2 do NOT have a diminuishing effect, but an ever increasing effect.
This is a very clear effect of climate science.

What we see is already a disintegration of the large ice shields in Antarctica, an acceleration of extreme weather events, droughts and so on. Each additional amount of CO2 is going to make these consequences ever more severe. There will no be saturation of effects. That means that even if the general course of events is not what we want, the influence we can take is ever increasing. So, we are probably still in the situation that we can, for example, avoid a break-down of the North Atlantic circulation which would have a devastating impact on Northern Europe and the East coast of the USA.

Let's put that into a picture. A catastrophic, highly undesirable situation which will cause harm to a lot of people. Let's imagine you are sitting in a heavy truck where the steering control has broken down and which is movin with a high speed and with little control towards a group of 50 school children. All you can do is to brake, or not do it.

So what is ethical to do? If you brake, you cannot prevent that some of the children are harmed - you are already too close, the truck is going too fast. Collision of a child with a hard, fast-moving object causes injuries and will be often deadly. But, the effect of speed is that the higher the speed, the higher is the harmful effect, in a non-linear way. That means, that it is not the right thing to do to do nothing and let the disaster happen. It is not even relevant who is to blame here, the only question is what you can do?

The right course of action is to brake as much as possible. Every reduction of speed means to disproportionately reduce the harm which the situation is causing. Maybe you cannot prevent it entirely, but the truth is that you have room to act, and every action does have disproportionate effects.

There is one more, not-that-obvious parallel to climate change: A lot of the destruction will be caused by the speed of the changes. It is a big differences whether the Earths climate changes slowly over one million years, or within just a century. The latter will cause far more extinctions of species, and also will be much more of a problem to human societies.

And, by the way, if I were some industrial lobby group which wants to inhibit that people act on climate change, the message "it is to late to do something about" is exactly what I would spread. It is the new climate denial - denial that our actions do have a lot of effect. In my opinion, also a pathetic way of living. Remember we are leaves of the great tree of life. As Helen Fox-Keller says, real life was always a risk. Don't be a shame for our ancestors. The first sulphur bacterium on Earth which crawled out of its comfy underwater hot pool and participated in the evolution of Life was way more courageous than that.