Coral reefs may start dissolving when atmospheric CO₂ doubles

Abstract

[1] Calcification rates in stony corals are expected to decline significantly in the near future due to ocean acidification. In this study we provide a global estimate of the decline in calcification of coral reefs as a result of increase in sea surface temperature and partial pressure of CO 2 . This estimate, unlike previously reported estimates, is based on an empirical rate law developed from field observations for gross community calcification as a function of aragonite degree of saturation (W arag ), sea surface temperature and live coral cover. Calcification rates were calculated for more than 9,000 reef locations using model values of W arag and sea surface temperature at different levels of atmospheric CO 2 . The maps we produced show that by the time atmospheric partial pressure of CO 2 will reach 560 ppm all coral reefs will cease to grow and start to dissolve. [2] The sharp anthropogenic increase in atmospheric CO 2 has changed the carbonate chemistry of the surface oceans [Sabine et al., 2004]. Model estimates suggest that the average surface ocean pH has decreased by 0.1 units relative to its pre-industrial revolution (PIR) value and it is expected to decrease by an additional 0.4 units by the close of the 21st century [Caldeira and Wickett, 2003]. Such a drop in pH at constant total alkalinity will decrease the aragonite degree of saturation (W arag ), which may lower the biogenic precipitation rate of aragonite (the carbonate mineral deposited by reef building corals) in the ocean [Kleypas et al., 1999;Orr et al., 2005] Where, the rate constant (k T ) and the order of the reaction (n T ) change as a function of temperature according to the empirical relations reported by Burton and Walter [1987]. The rate constant (k T ) has units of mmol Á m À2 Á hr À1, which are converted to units of mmol Á m À2 Á day À1 by multiplying the value of k T by 24 (hours per day) and dividing by 1000 (mmol per mmol). Application of this rate law for coral reefs includes the horizontal area fraction of the reef actively depositing CaCO 3 , i.e., percentage of live coral cover in decimal fraction equivalent (A c = 0 -1) multiplied by a specific area constant (k 0 r ). This constant (k 0 r ) is the ratio between the total area of the reef actively precipitating CaCO 3 and its planar area (i.e., m 2 /m 2 ) for a reef with 100% live coral cover (A c = 1). An additional term in the modified equation for coral reefs is the rate of community CaCO 3 dissolution (D). The modified rate law is therefore:Where the gross calcification is defined as:The linear fit between the measured and calculated net calcification for the Eilat reef yielded a slope of A c Á k . The D estimated from the correlation value is similar to the average dissolution rate measured in situ on the Eilat reef (13 ± 7 mmol Á m À2 Á day À1

Multiple driving factors explain spatial and temporal variability in coral calcification rates on the Bermuda platform

“…Therefore, we need to move beyond simple correlations in order to evaluate what is controlling the calcification rates measured in this study. Silverman et al (2009) and Anthony et al (2011) took the approach of defining functions that attempted to describe the combined effects of temperature and X arag on coral calcification. These functions did not include the effect of light, which certainly is important in explaining seasonal differences nor did they take into account that coral growth often has an asymmetric response to temperature (Houck et al 1975;Reynaud et al 1999).…”

Pacific-wide contrast highlights resistance of reef calcifiers to ocean acidification

“…Most empirical studies of the effects of OA on coral reefs have reported a positive correlation between calcification rates and aragonite saturation state (V arag ) [5], supporting pessimistic projections forecasting the disappearance of most coral reefs before the end of the current century [6][7][8]. However, recent studies now indicate more nuanced responses to OA for select reef calcifiers [9], with a compilation of laboratory studies of corals suggesting that coral calcification will decline approximately 10-20% (rather than ceasing) for a doubling of present-day partial pressure of CO 2 (pCO 2 ) [10].…”

Genetic diversity and connectivity in a brooding reef coral at the limit of its distribution

“…Dramatic deterioration of reef health has occurred over the past few decades and is predicted to continue due to rapidly increasing sea surface temperatures (SSTs) and ocean acidification, resulting in increased bleaching frequency and severity, and reduced calcification and reef accretion (e.g. Hoegh-Guldberg et al 2007;Silverman et al 2009). High-latitude reefs are of particular concern as they are already considered to be a marginal reefal environment (Veron 1995;Kleypas et al 1999a;Guinotte et al 2003).…”