The threat of Earth’s oceans becoming more acidic

Abstract

Oceans, which cover over 70% of Earth¿s surface, contain about 97% of the Earth¿s surface water, represent over 99% of its living space, and provide approximately half of the oxygen that we breathe, are under pressure due to human activity. Ocean health is being disturbed by a range of stressors, three of them critical on a global scale. Global warming, now beyond doubt, was the first to be realized. Now, two additional stressors have been shown to be at play: Ocean acidification and ocean deoxygenation. In this talk, I will focus on the first, which is an insidious consequence of the anthropogenic rise in atmospheric CO2 due primarily to fossil fuel combustion. This phenomenon, often referred to as `the other CO2 problem¿, is the consequence of the marine absorption of a large portion of the cumulative anthropogenic atmospheric CO2 emissions, which has already acidified the surface oceans by about 0.1 pH units since preindustrial times. Models project that surface ocean pH will further decrease 0.3 to 0.4 pH units by the end of this century. This change will represent a 150% increase in the concentration of protons and a 50% decrease in carbonate ions with respect to pre-industrial values. In this presentation, we will take a tour through the different key aspects involved in this serious environmental problem, starting with a brief overview of the evolution of the CO2 levels in the air, the absorption of part of it by the oceans and the chemical changes that it causes, with examples on instrumentally measured data over the last decades in different parts of the ocean, which clearly reflect that the progressive acidification of the oceans can indeed already be detected. In addition, we will place into context these recent anthropogenic changes through an evaluation of paleoreconstructions at different temporal scales, where we will find that today surface oceans have already achieved levels of acidity without precedents over the last 20 million years and that the values that oceans will reach by the end of this century are unprecedented, at least, in 40 million years. More importantly, we will see how this rise in CO2 and lowering of pH is occurring at a rate not seen for hundreds of millions of years, so the oceans are entering an unknown territory possibly involving major ecosystem changes. We will discuss which are the consequences of acidification for marine organisms, with special emphasis on species that build skeletons of calcium carbonate such as corals, shellfish and important planktonic species. We will also list some examples where the existence of adverse effects on marine biota has already been detected in situ, and we will review some of the strategies of studying this phenomenon, with particular emphasis on pH-manipulation experiments in aquaria, instrumental measurements in cruises and at fixed stations, and paleoreconstructions