Ocean Acidification pCO2 Calculator
Find the equilibrium pH, carbonate ion and DIC of surface seawater for a given atmospheric CO2, and see how far pH falls from the preindustrial level.
🐚 What is Ocean Acidification and How Does Atmospheric pCO2 Set Surface pH?
Ocean acidification is the long-term fall in seawater pH caused by the ocean absorbing carbon dioxide from the atmosphere. When CO2 dissolves it forms carbonic acid, which releases a hydrogen ion and leaves bicarbonate. Those extra hydrogen ions then attack carbonate ions and convert them to more bicarbonate. The net result is a lower pH, a lower carbonate ion concentration and a higher total dissolved inorganic carbon (DIC). Surface seawater is still slightly basic, so it is becoming less alkaline rather than turning acidic in the literal sense, but the chemistry is moving in the acid direction.
This calculator finds the equilibrium surface ocean carbonate system for a chosen atmospheric CO2 mole fraction. The air and the water are assumed to be in balance, so the dissolved CO2 follows Henry's law with the Weiss (1974) solubility. Total alkalinity is held fixed, because dissolving CO2 does not change it, and the hydrogen ion concentration is then found by solving the alkalinity balance of bicarbonate, carbonate and borate. From that one number the page reports pH, bicarbonate, carbonate, DIC, and how each shifts relative to a comparison CO2 such as the preindustrial 280 ppm.
Students and teachers use this kind of calculation to see why a 40 percent rise in CO2 produces roughly a 30 percent rise in hydrogen ion, to explain why shell-forming organisms are stressed as carbonate ion falls, and to compare warm tropical water with cold polar water. Researchers use the same equations inside CO2SYS and seacarb to turn measured quantities into pH and saturation states. Policy readers can use it to connect an emissions scenario, expressed as a CO2 level such as 560 or 800 ppm, to the chemistry the surface ocean would eventually reach.
Two points are easy to get wrong. First, the observed pH drop of about 0.1 units is smaller than the fixed-alkalinity equilibrium drop for the same CO2 change, because the real ocean has not caught up with the atmosphere and because alkalinity and temperature differ between places. Second, pH change and acidity change are not the same size: pH is a logarithm, so a 0.1 unit fall is about a 26 percent rise in hydrogen ion, and a 0.3 unit fall roughly doubles it. This tool shows both side by side. It ignores pressure, so it applies to the surface, and it does not model the slow exchange with deeper water.
It is a companion to, not a replacement for, the ocean carbon uptake calculator. That page asks how much carbon the mixed layer absorbs for a given rise in seawater pCO2 using a Revelle factor. This page asks what the pH and carbonate chemistry look like once the water is in equilibrium with a given atmospheric CO2.
📐 Formula
📖 How to Use This Calculator
Steps
💡 Example Calculations
Example 1 — Today vs Preindustrial in Warm Surface Water
400 ppm vs 280 ppm, TA = 2300 µmol/kg, 25 °C, S = 35
Example 2 — Cold Polar Water at 420 ppm
420 ppm vs 280 ppm, TA = 2300 µmol/kg, 2 °C, S = 35
Example 3 — From Today to 560 ppm
560 ppm vs 420 ppm, TA = 2300 µmol/kg, 25 °C, S = 35
Example 4 — A High-Emission 800 ppm Future at 15 °C
800 ppm vs 420 ppm, TA = 2300 µmol/kg, 15 °C, S = 35
❓ Frequently Asked Questions
🔗 Related Calculators
How does atmospheric CO2 change ocean pH?
CO2 dissolves in seawater in proportion to its partial pressure, then reacts with carbonate to form bicarbonate and releases hydrogen ions. At fixed alkalinity, raising CO2 from 280 to 400 ppm at 25 C and salinity 35 lowers pH from 8.167 to 8.043, a drop of 0.124 units.
What pH does 400 ppm CO2 give in seawater?
At 400 ppm, 25 C, salinity 35 and total alkalinity 2300 umol/kg the equilibrium pH is 8.043 on the total scale, with DIC near 2008 umol/kg and carbonate ion near 213 umol/kg. Colder water gives a slightly lower pH at the same CO2.
How much has ocean pH dropped since preindustrial times?
Observations show a fall of about 0.1 units in the global surface ocean, from roughly 8.2 to 8.1, which is about a 26 to 30 percent rise in hydrogen ion concentration. This calculator gives a larger equilibrium drop because the real ocean has not yet equilibrated.
Why does a small pH change mean a large change in acidity?
pH is the negative base-10 logarithm of hydrogen ion concentration, so each 1.0 pH unit is a tenfold change. A 0.1 unit fall multiplies [H+] by 10^0.1, or 1.26, a 26 percent increase. A 0.3 unit fall roughly doubles [H+].
What is the difference between pCO2 and fCO2?
pCO2 is the partial pressure of CO2 in the gas phase and fCO2 is the fugacity, which corrects for non-ideal gas behaviour. At the ocean surface they differ by only about 0.3 percent, so this calculator treats them as equal and sets pCO2 equal to the mole fraction times one atmosphere.
Does the calculator include water vapour pressure?
No. A full treatment multiplies the mole fraction by total pressure minus water vapour pressure, which lowers pCO2 by about 3 percent at 25 C. The calculator ignores that correction, so its pCO2 equals xCO2 times 1 atm, and the effect on pH is about 0.01 units.
What is carbonate ion and why does it matter?
Carbonate ion, CO3 2-, is the building block of calcium carbonate shells and skeletons. It falls as CO2 rises because the extra hydrogen ions convert carbonate to bicarbonate. At 25 C and 400 ppm it is about 213 umol/kg, 19 percent below the preindustrial value.
Why is total alkalinity held constant?
Dissolving CO2 does not change total alkalinity, because it adds dissolved carbon without adding or removing net charge. Alkalinity changes only through processes such as calcification, dissolution, and freshwater input, so it is the right fixed quantity for an atmospheric CO2 experiment.
Which equilibrium constants does this calculator use?
CO2 solubility K0 from Weiss (1974), carbonic acid K1 and K2 from Lueker et al. (2000) on the total pH scale, and boric acid KB from Dickson (1990). These are the choices recommended by the Guide to Best Practices for Ocean CO2 Measurements (Dickson et al. 2007).
Is the alkalinity calculation complete?
It includes bicarbonate, carbonate and borate, which make up nearly all of open-ocean alkalinity. It omits hydroxide, phosphate, silicate and minor species, which together shift pH by a few thousandths of a unit in typical surface water.
How is this different from the ocean carbon uptake calculator?
The ocean carbon uptake calculator uses a Revelle factor to estimate how much carbon a mixed layer absorbs. This page solves the full carbonate equilibrium to give pH, carbonate ion and DIC for a chosen atmospheric CO2, alkalinity, temperature and salinity.