The general principle seems to be that any deviation at all from the present situation is necessarily detrimental – or, in other words, that we now live in the best of all possible worlds. The major conclusions that are highlighted in the report are mostly meaningless, weasel-worded platitudes, such as "A global assessment of data since 1970 has shown it is likely that anthropogenic warming has had a discernible influence on many physical and biological systems." The apparent implication is that any "discernible" effect of human activities is a major global disaster that we must resist at any cost.
It is simply not true that all changes due to human activities are deleterious.
Some actions (for example, deforestation that creates farmland or prairie) may exterminate some species, but it also creates new habitat for other species. It is not obvious a priori that it is better to preserve a threatened species than to extend the range of some other species, or to encourage evolution of new species.
Severe climate changes can certainly cause difficulties for many species, including us, but they are a major evolutionary force. Eventual extinction has been the common fate of all species: 97% of those that once existed have disappeared. The history of life on Earth has been one of repeated catastrophes – but without them, primitive pond scum would be the only life there is. In particular, it was probably the selective stresses due to the brutal ice ages of the Pleistocene that led to the evolution of Homo sapiens. If our ancestors had not been exposed to severe glaciation, interrupted every thousand centuries by brief warm interglacials, we would still be primitive homini, mere prey animals for the big cats that roamed the Rift Valley in East Africa.