Although some researchers have raised concerns about possible negative effects of rising CO2 on ocean surface pH, there are several lines of evidence demonstrating marine ecosystems are far more sensitive to fluxes of carbon dioxide from ocean depths and the biosphere’s response than from invasions of atmospheric CO2. There is also ample evidence that lower pH does not inhibit photosynthesis or lower ocean productivity (Mackey 2015). On the contrary, rising CO2 makes photosynthesis less costly.
Furthermore in contrast to researchers arguing rising atmospheric CO2 will inhibit calcification, increased photosynthesis not only increases calcification, paradoxically the process of calcification produces CO2 and drops pH to levels lower than predicted by climate change models. A combination of warmer tropical waters and coral reef biology results in out-gassing of CO2 from the ocean to the atmosphere, making coral reefs relatively insensitive to the effects of atmospheric CO2 on ocean pH.
Sixty million years ago proxy evidence indicates ocean surface pH hovered around 7.4. If surface pH was in equilibrium with the atmosphere, then CO2 concentrations would have hovered around 2000 ppm, but there is no consensus that CO2 reached those levels. However as will be discussed, there are biological processes that do lower surface pH to that extent, despite much lower atmospheric CO2 concentrations.
Over the next 40 million years corresponding with the rearrangement of the continents and ocean currents, the formation of the Antarctic Circumpolar Current and initiation of Antarctic glaciers, the evolutionary expansion of diatoms and their increasing abundance (diatoms are the most efficient algae for exporting carbon to ocean depths), ocean carbonate chemistry was greatly altered. As a result ocean surface pH gradually rose above pH 8. Then for our last 20 million years, ocean surface pH has fluctuated within this new equilibrium between 8.4 and 8.1, as seen in Figure 1 below (Pearson and Palmer 2000). For the past 400,000 years, pH rose to about 8.35 during the depths of each ice age. Then during each warm interglacial period, when both land and marine productivity increased, pH fell to ~8.1 (Honisch 2005).Although it is commonly assumed atmospheric CO2 and ocean surface pH are in equilibrium, studies examining various time frames from daily and seasonal pH fluctuations (Kline 2015) to the millennial scale transitions from the last ice age to our warm interglacial (Martinez-Boti 2015), demonstrate surface ocean pH has rarely been in chemical equilibrium with atmospheric CO2. Because oceans contain over 50 times as much CO2 as the atmosphere, surface pH is more sensitive to changes in the rates of upwelling of low-pH, carbon-rich deep waters. It is the response of photosynthesizing organisms and the food webs they support that largely determines how much carbon is sequestered in the surface layers and then sent to deeper waters (the biological pump).
As discussed in an earlier essay, the “biological pump” modulates how much CO2 is sequestered and how much CO2 will out-gas to the atmosphere. It has been estimated that without the biological pump, pre-industrial atmospheric CO2 would have out gassed and raised atmospheric CO2 to 500 ppm, instead of the observed 280 ppm. Ironically the processes that build coral reefs also increase surface CO2 concentrations and lower regional pH to levels lower than expected by equilibrium with atmospheric CO2. This biological control of the earth’s chemistry is the essence of Gaia theory.
Gaia theory stimulated great scientific interest (as well as controversy) since the 1970s, and stimulated more extensive investigations into how the biosphere affects the earth’s chemistry. Gaia’s founding scientist James Lovelock formulated Gaia theory while working for NASA seeking chemical signatures of life on other planets. For example, due to living organisms our atmosphere maintains about a 21% concentration of oxygen. Without photosynthesizing organisms, the atmosphere would contain extremely low amounts of oxygen. Thus Lovelock argued, “if life has merely a passive role in cycling the gases of the air, then the concentrations will be set by equilibrium chemistry; in fact they most certainly are not.” Shedding the mystical connotations that many “New Age” adherents attributed to Gaia, several universities have now created departments studying Gaia’s effects, but under less anthropomorphic titles such as Earth System Sciences or Biogeochemistry.