By Roger Andrews
One of the many potential threats posed by rising CO2 and climate change is “ocean acidification”, a term I put in quotes because with a pH around 8.1 the ocean is still a very long way from becoming acidic. I’ve chosen it as the topic for this post for three reasons: first because the basics can be handled briefly, second because some comments on recent threads have expressed concern about it, and third because the way the data are usually presented gives an exaggerated idea of the rate at which the oceans are being “acidified”. Here’s an example:
We see CO2 in the atmosphere rising at a rapid clip, we see CO2 in seawater rising with it and we see ocean pH decreasing at what appears to be about the same rate.
But what we’re actually looking at is a decrease in pH of about 0.034 units on a scale that goes from zero to fourteen, i.e. a change of a little over 0.2 percent. We see the decrease in its true perspective when we re-plot the data (from the Hawaii Ocean Time Series project ALOHA site) using an expanded Y scale that begins at pH=7 (neutral):
A downward trend is still visible, but it’s very, very small (note that only actual pH measurements, not estimates, are plotted). If it continues at the same rate ocean pH will still be close to 8 in 2100 and the oceans won’t become truly acidic (pH=6.9) until about the year 2800. And even then the level of acidity will be minimal (natural rainwater has a pH of 5.6).
But the trend probably won’t continue at the same rate. The ocean absorbs CO2 from the atmosphere, so the rate of ocean pH decrease will be largely (although not entirely) dependent on future atmospheric CO2 concentrations. Before we can make estimates of future ocean pH values we therefore need some future CO2 scenarios, and the IPCC provides us with four in the AR5:
RCP2.6: Atmospheric CO2 increases to 440 ppm by 2100
RCP4.5: Atmospheric CO2 increases to 580 ppm by 2100
RCP6.0: Atmospheric CO2 increases to 650 ppm by 2100
RCP8.5: Atmospheric CO2 increases to 930 ppm by 2100
We also need to establish a numerical relationship between atmospheric CO2 and ocean pH, and a trend line drawn through an XY plot that compares the ALOHA pH data with Mauna Loa CO2 shows pH decreasing by 0.094 units for every 100 ppm increase in atmospheric CO2:
Applying this rate of increase to the IPCC CO2 concentrations gives predicted 2100 ocean pHs of 8.0 for RCP2.6, 7.9 for RCP4.5, 7.8 for RCP6.0 and 7.6 for RCP8.5. The scenario that gives the lowest pH is the RCP8.5 “worst case”, and here it is plotted up:
There are, however, reasons to believe that this plot is worse than a “worst” case. As Dave Rutledge pointed out in his recent post the RCP8.5 scenario burns twice as much coal by 2100 as is known to exist, and we also aren’t taking into account the fact that the more CO2 the ocean contains the less it absorbs. The Keeling Institute puts it thus:
“ …. although the oceans presently take up about one-fourth of the excess CO2 human activities put into the air, that fraction was significantly larger at the beginning of the Industrial Revolution. That’s for a number of reasons, starting with the simple one that as one dissolves CO2 into a given volume of seawater, there is a growing resistance to adding still more CO2.”
The likelihood is therefore that both CO2 concentrations and the amount of CO2 absorbed by the oceans will have been overestimated, in which case the projected 2100 ocean pH of ~7.6 for the RCP8.5 case will be too low. But let’s assume anyway that ocean pHs in 2100 will be in the 7.6 range. What impact will this have on marine life? No one knows for sure, but according to Hofmann et al.there are parts of the ocean which routinely reach pH values this low, or lower:
Here, we present a compilation of continuous, high-resolution time series of upper ocean pH, collected using autonomous sensors, over a variety of ecosystems ranging from polar to tropical, open-ocean to coastal, kelp forest to coral reef. These observations reveal a continuum of month-long pH variability with standard deviations from 0.004 to 0.277 and ranges spanning 0.024 to 1.430 pH units (see note). The nature of the observed variability was also highly site-dependent, with characteristic diel, semi-diurnal, and stochastic patterns of varying amplitudes. These biome-specific pH signatures disclose current levels of exposure to both high and low dissolved CO2, often demonstrating that resident organisms are already experiencing pH regimes that are not predicted until 2100.
(Note: The highest pH measured was 8.356 and the lowest 6.699 for measurements taken over 30-day periods. The annual pH range could be significantly larger.)
So is “ocean acidification” a threat? The basic data say it probably isn’t, but there are a number of additional complexities that for reasons of space have not been considered, including:
1. Mixing of surface water with deep water
2. The removal of CO2 by marine organisms
3. The CO2/HCO3/CaCO3 equilibria
4. CO2 dissolution rate as a function of CO2 saturation and temperature.
If any of these are potential game-changers, feel free to say so.