We are standing on the precipice of environmental change on a scale so massive that it will likely destabilize our civilization and lead to millions, if not billions, of deaths.
Climate change will flood densely populated coastal areas, damage agriculture, destroy ecosystems, force mass human migration orders of magnitude larger than ever before, and likely have many effects we haven't yet imagined.
Ocean acidification will dissolve calcareous plankton and collapse most marine food chains, ending half of the photosynthesis that takes place on planet earth and starving massive numbers of people who depend on the sea for food.
The collapse of the oceanic food web and loss of most of the life in the ocean will also shut off one of the main mechanisms for carbon sequestration in the earth system--the poop and dead bodies of organisms that float down to the ocean floor and get buried (each one a small puff of carbon that is no longer floating around the atmosphere).
Nitrogen and phosphorous pollution will simultaneously cause massive algal blooms that will generate unlivable anoxic zones throughout vast swaths of the ocean.
At the current rate of deforestation, there will no longer be tropical rainforests by 2100, and the rainforests that survive today (more than 60% have been destroyed) are so badly degraded that they emit more CO2 than all of the traffic on all of the roads in the entire United States.
Currently, extinctions are estimated to be happening at a rate that is at least 100-1000x the baseline frequency of extinction, meaning we have initiated the 6th mass extinction in the history of planet earth.
Over the past 30 years, flying insect populations in wildlife reserves in Germany (and plausibly most of the industrialized world) have dropped by 3/4--insects have only been affected by one other mass extinction in the earth's history, and it was the most severe.
We are likely killing the insects by a combination of wholesale habitat destruction and over-application of extremely potent pesticides that then spread throughout the environment (some studies have recently discovered higher pesticide concentrations in the grasses and forests surrounding farms than on the crops themselves).
Without flying insects, much of the plant life on earth can't reproduce.
Our economic, political, and social structures are only able to function by exploiting the exact processes that are generating the unimaginable destruction that accelerates all around us every day. Fixing any one of these potentially-civilization-ending problems would require species-wide cooperation the likes of which we can scarcely conceptualize; fixing all of them is an idea that is almost ludicrous. I feel so helpless. In the face of problems like this, individual action means nothing. The earth is a massive, seething bundle of chaotically interacting processes that can settle into qualitatively different stable states when perturbed from a local minimum. Looking back into the paleoclimate record, we can see examples of abrupt, large-scale changes being driven by relatively small forcings. Please love your family and throw yourself with all your might into whatever you care about. Things are very likely to spiral out of control during our lifetimes.