Hello; Afraid I am not knowledgeable of the source material of these particular "tipping points". I have known of the concept of tipping points for several decades. While my recall is a bit foggy of the post graduate Limnology class i took back in the late 1970's, I do think oxygen levels in bodies of water were well covered. The class focused more on ponds, lakes and streams as we were in Richmond KY.
Those who fish lakes likely know of the thermocline layers. Dive deep into a lake in late summer and you can get into the colder water below a thermocline layer.
In the fall the upper layer of water stays warm after the air temps drop.
A fisherman also likely knows to fish a bit deeper during late summer & early fall days. The big fish do tend to stay deeper, below the warm upper water layer and in the cooler water.
I like to top water fish with a fly rod and have learned it is best before June in my area. During the rollover when the water layers start to even out in temperature and the deeper water upwells mixing with nutrients and oxygen. Better chance of catching bigger fish when the water layers overturn.
One of the tipping points mentioned is well known. That being the excess fertilizer runnoff which tends to create "dead zones" in the Gulf of America (known as the Gulf of Mexico for most of my life) at the mouth of the Mississippi River. Can happen in ponds with extra fertilizer from sources. The fertilizer contributes to algae blooms which can deplete oxygen levels. Can be runoff from grazing pastures or agriculture.
I enjoy the videos made on ice covered lakes or ponds. Someone pokes a hole in the ice and uses a flame to light up the gas which escapes. They then try to imply it is Global warming or the newer and more generic "climate change". The gas is indeed methane but the phenomena is not new nor recent. The methane forms in the mud layers on the bottom of the pond/lake from anerobic bacteria. The gas is released all year around. Can be called swamp gas.
Afraid I am not up to date on the oxygen depletion of ocean waters. I cannot say if the situation is real or not. I tend to be a bit skeptical when such is used to push what is clearly an agenda. Off the top of my head, I would want to know more of the mechanisms. Could it be that the extra carbon in the water will become a resource for the phytoplankton in the parts of the ocean where cool waters are supposed to be warming up some?
But back to the thread topic a bit. My quest for ZPG back 50 + years was to a large degree based on known impacts of human populations of the time. Could be seen that a number of people flushing toilets had an impact on an area. Figure to double or triple the number of toilets and the impact doubled or tripled.
I lived near Wilmington NC for a couple of years. Back in 1998-1999 the city was beginning to have problems with the ability of the sandy soil to support septic tank fields.
I rented a room in Boiling Springs Lake, NC. There was a housing boom at the time with many new homes built while I was there. I rode my bike and saw how the septic fields were set up. They did not go deep like in TN or KY. They trucked in a different sort of sand to cover the filed pipes. One different sort of sand over the common sand of the area.
Seemed strange to me at first, but I was there during some torrential rains. The water could flood roads and ditches to the point of my being happy to have a 4x4 pickup. Then in an hour or so the water could be gone into the sandy soils. The man I rented from is still a friend. We exchange letters with Christmas cards. He tells me there are no empty lots around his home anymore. I wonder how the septic fields cope?
