Why are you assuming that the "total biological and ecosystem diversity" will diminish and what metric are you using for it?
My metrics for "total biological and ecosystem diversity" are the total numbers of ecosystems and species, and the degrees to which they're represented. As for why I'm "assuming" they will diminish, this the conclusion, not the presumption, of most of the research in that area.
In broad terms, we should expect this kind of thing to happen because so many species are adapted very strongly to very specific niches. When circumstances change rapidly, those species are unable to cope, and die out, but there are no species which are strongly adapted to the new circumstances to adequately replace them. Because ecology is much more complex than a simple intersection of weather conditions, the result is that rather than ecosystems and species being shuffled around, a lot just ends up being lost entirely. But of course, there's a whole lot of research indicating specific ways in which this is already happening and is likely to happen in the future, outside the general principles that predict it.
Wouldn't the reverse also happen -- some "impoverished" systems will get replaced by highly productive ones?
Yes, and over timescales of millions of years, with temperature increases, these effects would likely dominate. But over timescales humans are more practically concerned with, the dominance will tend to be in the other direction.
As for whether a low biological productivity ecosystem being bad in itself, I'll simply say this. Some such impacts are likely to have significant direct influence on our economy. Some are not, and there are those who care deeply about them regardless, who are willing to go to great expense to prevent them, and those who are not, and regard such expense as worthless. But in my experience, I have not known anyone to regard those impacts which are unlikely to directly influence human economy as positive and worth paying for.
Are you claiming that the climate change (as envisaged, say, by the latest IPCC projections) is going to be a mass extinction event?
Yes. By itself, anthropogenic climate change is unlikely to result in a mass extinction on the level of one of the Big Five, but it would still almost certainly be visible in the fossil record as a mass extinction event (I emphasize again that the degree of impact here is due to the rapidity, not the degree, of climate change.) Along with other forms of human influence though, a mass extinction event on the level of the Big Five is distinctly within the realm of possibility. Our current estimated rate of species extinction is around ten thousand times the usual background rate-and accelerating.
As for why I'm "assuming" they will diminish, this the conclusion, not the presumption, of most of the research in that area.
What you call "conclusion" is probably a forecast, since we're talking about the future, right?
...In broad terms, we should expect this kind of thing to happen because so many species are adapted very strongly to very specific niches. When circumstances change rapidly, those species are unable to cope, and die out, but there are no species which are strongly adapted to the new circumstances to adequately replace
Note: Please see this post of mine for more on the project, my sources, and potential sources for bias.
I have written a couple of blog posts on my understanding of climate forecasting, climate change, and the Anthropogenic Global Warming (AGW) hypothesis (here and here). I also laid down the sources I was using to inform myself here.
I think one question that a number of readers may have had is: given my lack of knowledge (and unwillingness to undertake extensive study) of the subject, why am I investigating it at all, rather than relying on the expert consensus, as documented by the IPCC that, even if we're not sure is correct, is still the best bet humanity has for getting things right? I intend to elaborate on the reasons for taking a closer look at the matter, while still refraining from making the study of atmospheric science a full-time goal, in a future post.
Right now, I'm curious to hear how you formed your views on climate change. In particular, I'm interested in answers to questions such as these (not necessarily answers to all of them, or even to only these questions).