Climate

As of 2012, the average land and ocean surface temperature on earth has increased by about 0.85°C relative to preindustrial times (before 1880 or so, when human activity had little effect on the climate), and ocean surface pH has dropped by about 0.1 units. It is extremely likely (95% confidence) that more than half of the warming in the last 60 years is due to human activity, and predictions from climate models using observed greenhouse gas emissions and related data over this time period closely match the resulting observed warming [1]. These climate models are the basis of predictions about the future effects of human activity on the climate.

Representative Concentration Pathways

The Intergovernmental Panel on Climate Change (IPCC) [2] is the UN body responsible for assessing and summarizing scientific research related to climate change. In preparation for its fifth assessment report, released in 2014, a set of four Representative Concentration Pathways (RCPs) were developed by the research community to characterize possible trajectories for greenhouse gas concentrations that could happen over the 21st century and beyond [3]. RCPs provide a shared language for researchers using climate models to project future changes to temperature and other environmental factors.

The RCPs are RCP 2.6, RCP 4.5, RCP 6, and RCP 8.5. RCPs are named according to the change in radiative forcing in 2100 relative to preindustrial times, in watts per square meter. As greenhouse gases accumulate in the atmosphere they absorb infrared radiation from the earth's surface and radiate some of that back towards the earth, increasing the total amount of radiation reaching the earth's surface. Gases and aerosols released into the atmosphere as a result of human activity have different effects on this radiation, and the sum total of these effects is the radiative forcing. The gas with the largest effect on radiative forcing is ${\displaystyle {\ce {CO2}}}$, though there are several others.

In the fifth assessment report, radiative forcing as of 2011 is estimated at 2.3 W m-2 [4], which doesn't leave much wiggle room for RCP 2.6. Following the trajectory in RCP 2.6 requires ${\displaystyle {\ce {CO2}}}$ emissions to stabilize more or less immediately and then begin rapidly decreasing, with net emissions becoming negative late in the 21st century as humanity begins removing ${\displaystyle {\ce {CO2}}}$ from the atmosphere [5].

At the other extreme is RCP 8.5, which is a business-as-usual scenario under which emissions continue growing pretty much unabated through the 21st century. RCP 4.5 and RCP 6 are intermediate scenarios. Under RCP 4.5, emissions stabilize and then decline, but not as rapidly as under RCP 2.6. Under RCP 6, emissions continue growing in the 21st century, but not as quickly as under RCP 8.5 [5].

This wiki focuses on the effects of RCP 8.5, representing the worst case scenario for emissions and their effects on corals.

Warming

IPCC reports are mostly based on data generated by the Coupled Model Intercomparison Project (CMIP) [6], which is used to compare climate models and to average the results of many climate models to provide a single ensemble projection of future temperature changes [7]. Under RCP 8.5, global average surface temperature is projected to increase to 2.6°C above preindustrial temperatures in 2045 - 2065, and to 4.3°C in 2081-2100 [4].

Temperature change will not be uniform world-wide, and it's important to see what these models say about specific locations of interest. The below graph uses data from the GFDL model (one of the models used in CMIP) to plot projected changes in sea surface temperature at Moorea during the 21st century under RCP 8.5 [8]. The sixty month running average of temperature erratically increases from a minimum of 27.4°C to a maximum of 29.5°C.

The script which generated this chart is here.

Ocean Carbonate System

Corals create their skeletons by precipitating calcium carbonate, ${\displaystyle {\ce {CaCO3}}}$, from ${\displaystyle {\ce {Ca^{2+}}}}$ and ${\displaystyle {{\ce {CO3^{2-}}}}}$ ions in the water. The more of these ions that are available to the coral, the easier it is for them to precipitate calcium carbonate and grow. The ${\displaystyle {\ce {Ca^{2+}}}}$ ion is a major component of seawater (about 0.04% by weight [9]) and is readily available to corals. The concentration of ${\displaystyle {{\ce {CO3^{2-}}}}}$, which is known as the carbonate ion, mainly depends on the water's acidity. When ${\displaystyle {\ce {CO2}}}$ dissolves in water, some of it forms carbonic acid, ${\displaystyle {\ce {H2CO3}}}$, which disassociates into the bicarbonate ion, ${\displaystyle {\ce {HCO3-}}}$, and the carbonate ion, ${\displaystyle {{\ce {CO3^{2-}}}}}$. This disassociation releases ${\displaystyle {\ce {H+}}}$ ions into the water, acidifying it. This process is described by the following chemical reactions:

• ${\displaystyle {\ce {CO2 + H2O <=> H+ + HCO3-}}}$
• ${\displaystyle {\ce {HCO3- <=> H+ + CO3^{2-}}}}$

These reactions do not run to completion but instead form an equilibrium. A sample of seawater will have a fixed amount of dissolved inorganic carbon, or DIC, the total amount of ${\displaystyle {\ce {CO2}}}$, ${\displaystyle {\ce {HCO3-}}}$, and ${\displaystyle {{\ce {CO3^{2-}}}}}$. After the reactions reach equilibrium, the distribution of the carbon compounds depends on the concentration of ${\displaystyle {\ce {H+}}}$, which also determines the water's acidity. The chart below shows this distribution as a function of the water's pH. This illustrates that the concentration of ${\displaystyle {{\ce {CO3^{2-}}}}}$ is dramatically affected by changes to pH, especially when the pH is close to that of seawater (around 8.1). At a neutral pH of 7, the amount of ${\displaystyle {{\ce {CO3^{2-}}}}}$ will only be 8.6% of that at a pH of 8.1, while at a more basic pH of 9 there will be 4.1 times as much ${\displaystyle {{\ce {CO3^{2-}}}}}$ as at pH 8.1.

The script which generated this chart is here.

Ocean Acidification

Atmospheric ${\displaystyle {\ce {CO2}}}$ is in equilibrium with ${\displaystyle {\ce {CO2}}}$ dissolved in the ocean. As atmospheric ${\displaystyle {\ce {CO2}}}$ increases due to human activity, it slowly dissolves in the ocean to restore equilibrium conditions.

As the dissolved ${\displaystyle {\ce {CO2}}}$ concentration increases, the carbonate system equilibrium is affected. Per the chart above, at a pH of 8.1 almost all of the newly dissolved ${\displaystyle {\ce {CO2}}}$ will disassociate to ${\displaystyle {\ce {HCO3-}}}$ and ${\displaystyle {{\ce {CO3^{2-}}}}}$ in order to reach equilibrium. If the water's pH were to stay unchanged, the concentration of ${\displaystyle {{\ce {CO3^{2-}}}}}$ would increase as a result. However, while disassociating ${\displaystyle {\ce {H+}}}$ ions will be generated. This acidifies the water, lowering the pH and decreasing the fraction of ${\displaystyle {{\ce {CO3^{2-}}}}}$ in solution. The net effect is a decrease in the amount of ${\displaystyle {{\ce {CO3^{2-}}}}}$ ions available for corals.

As for projections of warming, climate models are used to project future changes to ocean pH. Also as for warming, pH changes will not be uniform world-wide. The graph below uses data from the GFDL model to plot projected changes in surface pH at Moorea during the 21st century under RCP 8.5 [8]. The sixty month running average of pH steadily decreases from 8.08 to 7.76. The pH scale is logarithmic, and this change roughly doubles (2.09x) the number of ${\displaystyle {\ce {H+}}}$ ions in the water.

The script which generated this chart is here.

The following chart shows GFDL model projections [8] for ${\displaystyle {{\ce {CO3^{2-}}}}}$ concentrations around Moorea during the 21st century. The concentration changes due to changes in pH, DIC, and (to a lesser degree) temperature and salinity. The sixty month running average of ${\displaystyle {{\ce {CO3^{2-}}}}}$ at the end of the century decreases by 40% compared to that at the start of the century, a very large reduction in the amount of these ions that are available for use by corals.

The script which generated this chart is here.

Aragonite Saturation State

The aragonite saturation state ${\displaystyle \Omega _{a}}$ is widely used to measure the effect of ${\displaystyle {{\ce {CO3^{2-}}}}}$ concentrations on corals. Aragonite is the crystal form of ${\displaystyle {\ce {CaCO3}}}$ which corals precipitate, and has different physical properties from other forms of ${\displaystyle {\ce {CaCO3}}}$. In solution, it reacts according to ${\displaystyle \Omega _{a}}$:

• When ${\displaystyle \Omega _{a}=1}$ the solution is saturated and aragonite is in equilibrium with ${\displaystyle {\ce {Ca^{2+}}}}$ and ${\displaystyle {{\ce {CO3^{2-}}}}}$.
• When ${\displaystyle \Omega _{a}<1}$ the solution is undersaturated and aragonite dissolves into ${\displaystyle {\ce {Ca^{2+}}}}$ and ${\displaystyle {{\ce {CO3^{2-}}}}}$ ions to restore equilibrium.
• When ${\displaystyle \Omega _{a}>1}$ the solution is supersaturated and aragonite will slowly precipitate from ${\displaystyle {\ce {Ca^{2+}}}}$ and ${\displaystyle {{\ce {CO3^{2-}}}}}$ to restore equilibrium. Corals can precipitate the aragonite themselves, and the higher the saturation state the easier it is for corals to precipitate.

The saturation state depends on the water's ${\displaystyle {{\ce {CO3^{2-}}}}}$ concentration, temperature, and salinity. Variations in the latter two during the 21st century will only have a small effect on the saturation state (~1%), though, so saturation state is essentially in direct proportion to the ${\displaystyle {{\ce {CO3^{2-}}}}}$ concentration. Around Moorea the aragonite saturation state is projected to decrease from 4.1 to 2.5 by the end of the century, the same 40% decrease as was seen above for ${\displaystyle {{\ce {CO3^{2-}}}}}$ concentrations [10].

References

1. IPCC, 2014: Climate Change 2014: Synthesis Report. Contribution of Working Groups I, II and III to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change [Core Writing Team, R.K. Pachauri and L.A. Meyer (eds.)]. IPCC, Geneva, Switzerland, 151 pp.
2. https://www.ipcc.ch/
3. Richard Moss, Mustafa Babiker, Sander Brinkman, Eduardo Calvo, Tim Carter, Jae Edmonds, Ismail Elgizouli, Seita Emori, Lin Erda, Kathy Hibbard, Roger Jones, Mikiko Kainuma, Jessica Kelleher, Jean Francois Lamarque, Martin Manning, Ben Matthews, Jerry Meehl, Leo Meyer, John Mitchell, Nebojsa Nakicenovic, Brian O’Neill, Ramon Pichs, Keywan Riahi, Steven Rose, Paul Runci, Ron Stouffer, Detlef van Vuuren, John Weyant, Tom Wilbanks, Jean Pascal van Ypersele, and Monika Zurek, Towards New Scenarios for Analysis of Emissions, Climate Change, Impacts, and Response Strategies, Intergovernmental Panel on Climate Change, Geneva
4. IPCC, 2013: Summary for Policymakers. In: Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change
5. J. T. Houghton, Global Warming: The Complete Briefing, Cambridge University Press (2015), page 134
6. https://cmip.llnl.gov/
7. Stocker, T.F., D. Qin, G.-K. Plattner, L.V. Alexander, S.K. Allen, N.L. Bindoff, F.-M. Bréon, J.A. Church, U. Cubasch, S. Emori, P. Forster, P. Friedlingstein, N. Gillett, J.M. Gregory, D.L. Hartmann, E. Jansen, B. Kirtman, R. Knutti, K. Krishna Kumar, P. Lemke, J. Marotzke, V. Masson-Delmotte, G.A. Meehl, I.I. Mokhov, S. Piao, V. Ramaswamy, D. Randall, M. Rhein, M. Rojas, C. Sabine, D. Shindell, L.D. Talley, D.G. Vaughan and S.-P. Xie, 2013: Technical Summary Supplementary Material. In: Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change [Stocker, T.F., D. Qin, G.-K. Plattner, M. Tignor, S.K. Allen, J. Boschung, A. Nauels, Y. Xia, V. Bex and P.M. Midgley (eds.)]. Available from www.climatechange2013.org and www.ipcc.ch.
8. Dunne, John; John, Jasmin; Shevliakova, Elena; Stouffer, Ronald; Griffies, Stephen; Malyshev, Sergey; Milly, P.; Sentman, Lori; Adcroft, Alistair; Cooke, William; Dunne, Krista; Hallberg, Robert; Harrison, Matthew; Krasting, John; Levy, Hiram; Phillips, Peter; Samuels, Bonita; Spelman, Michael; Winton, Michael; Wittenberg, Andrew; Zadeh, Niki. NOAA GFDL GFDL-ESM2M, rcp85 experiment output for CMIP5 AR5, served by ESGF, 2014
9. F. Millero, Chemical Oceanography, (CRC Press, 2013), 67
10. https://github.com/bhackett1024/coral/blob/master/src/wiki/saturation.js