Depends what you mean.
Any species going extinct is by definition irreversible.
Anything less than that is probably reversible if enough effort is devoted to it.
Most people only consider the species that are big enough for them to see.
Most species are actually too small to be seen with the unaided eye (by a factor of a thousand or more).
Some changes to climatic systems can reach tipping points where positive feedbacks cut in, and it takes a great deal of energy to swap them back to previous patterns. If one does not have control of such high energy systems, then those changes become irreversible in a practical sense (whatever theoretical considerations might apply).
Under various sets of agent based modelling systems it seems that we have a couple of major systemic inflection points approaching, one in about 5 years and another in about 12 years. At both of those it is entirely possible for the system that is humanity to go in directions that result in the death of most individual human beings.
I am cautiously optimistic that we can avoid those outcomes, and create outcomes that are of benefit to all people and the environment; and those outcomes are not possible in any form of “business as usual” approach. All forms of “business as usual” all levels, seem clearly to lead to failure.
So we all have difficult things to think about and difficult choices to make.
The only class of actions that seem to me to deliver any sort of long term security are those that involve global cooperation, and demonstrate universally that the highest values are individual life and individual liberty. And that demands of every individual that we accept that there are real and necessary constraints on liberty, and that they include demands for actions that are socially and ecologically responsible.
There is plenty of room for diversity in what the details of social and ecological responsibility look like, but no room to deny the requirement for both.
So we live in “interesting times”.
I actually believe that we have a better than even chance of surviving, but I normally like the odds of survival to be very much greater than the toss of a coin (billions to one greater).
Is there cause for optimism? Yes – the odds are better than a coin toss (just).
Is there cause for concern? Yes – our world is currently organised on fundamentally competitive principles, and those competitive systems must fail if they are pushed any further (no mathematical escape from that).
Are there alternatives? Yes – there are. There are alternatives that are both cooperative, and respect individual life and individual liberty; and they have the demand that markets and money take a secondary role in social organisation. That transition will be difficult for some. One possibly useful transition strategy is a universal basic income, and that can only work if a great deal of very high technology is devoted to meeting the reasonable needs of every individual on the planet in ways that are environmentally sustainable – and there is no possible purely market based sets of incentives to achieve that (there will always exist market incentives to undermine such a systemic outcome).
It does seem to me to be possible to create a world where every individual experiences the sorts of choices and security that I have – secure housing, plenty of fresh clean water, plenty of food, good healthcare, good communications, a reasonable selection of toys and things to do, reasonable freedom to travel across the planet (say one year travelling for every 4 years I stay at home). Delivering that sort of outcome to every other person on the planet is technically possible, and it comes with some demands, like keeping families down to one child per family (on average), being responsible for what we do in the natural environment, not littering, not dumping stuff in the ocean, making some reasonable effort to clean up the messes we as a species have made getting to this point. Relatively simple stuff like that.
So security for all is doable, it is possible and it is by no means certain.
It is only possible in a cooperative context.
It is only possible if there is acceptance of diversity, and every one of us has a point beyond which such acceptance is difficult. Liberals find it hard to accept conservatives, conservatives find it hard to accept liberals. There are many other sets of dimensions present in human diversity. And every one of us has our liberal and our conservative aspects, at different strengths in different contexts.
It is really hard, when someone else’s diversity challenges something we value. That can take real work, real negotiation, a real willingness to listen and to try out new ideas, to get past.
So not much that is simple on this path, and it seems to be the path that reality has to offer. We seem to have run out of alternatives that actually involve survival.