I can't find a link to the it now but in the late 90s/early 00s there was a great research paper that found that we have lost a significant fraction of our phytoplankton biomass, estimated at about 10-20% of the population between 1970 and 1995 due to pollution in the oceans. Our most important oxygen producers are also the most vulnerable to pollution and ocean acidification.

The sky isn't "falling" but combined with all of the other trends we've been seeing, we have no idea just how bad it can get it or if there is a cliff where our economic/political systems begin to rapidly collapse under the pressure of environmental effects. From what I can tell from the last few years of research, our best climate models have been woefully underestimating negative second order effects and since our ecosystems and economies are interdependent there are few to no positives.