{"id":17849,"date":"2021-10-05T09:48:55","date_gmt":"2021-10-05T07:48:55","guid":{"rendered":"https:\/\/bonpote.com\/ocean-acidification-and-climate-change\/"},"modified":"2022-06-27T12:30:40","modified_gmt":"2022-06-27T10:30:40","slug":"ocean-acidification-and-climate-change","status":"publish","type":"post","link":"https:\/\/bonpote.com\/en\/ocean-acidification-and-climate-change\/","title":{"rendered":"Ocean acidification and climate change."},"content":{"rendered":"\n

Ocean acidification is a largely forgotten consequence of climate change. It is certainly one of the less publicized issues. Hurricanes<\/a>, droughts<\/a>, heatwaves<\/a>or floods <\/a>all make for great headlines, with spectacular images and important human losses. Even climate deniers<\/a> have not invented some controversy about ocean acidification, which shows how little it is in the public eye.<\/p>\n\n

There is, however, a lot at stake. The food web, biodiversity loss and, more generally, life in several regions of the world are threatened by ocean acidification. There is no planet B, therefore there is no ocean B. That\u2019s probably what plankton, fish and corals would say, given that they are all threatened by extinction because of the combined effects of ocean acidification and other consequences of climate change\u2026 Unfortunately, they have no chance to speak out\u2026<\/p>\n\n

What role does – and will – climate change play in ocean acidification ? What are the consequences, and who is affected ? Will France be spared by this phenomenon? How can we avoid it? <\/p>\n\n

Jean-Pierre Gattuso<\/a>, a scientist at the CNRS, working at the Laboratory of Oceanography of Villefranche-sur-Mer, helped us to answer these questions.<\/p>\n\n

What is ocean acidification ?<\/h2>\n\n

The IPCC<\/a> defines ocean acidification as “A reduction in the pH of the ocean, accompanied by other chemical changes, over an extended period, typically decades or longer, which is caused primarily by uptake of carbon dioxide (CO2) from the atmosphere, but can also be caused by other chemical additions or subtractions from the ocean. Anthropogenic acidification refers to the component of pH reduction that is caused by human activity<\/em>“.<\/p>\n\n

Every year, the ocean and the atmosphere exchange large quantities of CO2<\/sub> with the atmosphere. Since the eighties, the oceans have absorbed over 25%<\/a> of all anthropogenic CO2<\/sub> emissions \u2013 around 40 billion metric tons of CO2<\/sub> \u2013 from the atmosphere each year.<\/strong> Without this CO2 intake global warming would be much worse than it is today. However, it\u2019s bad news for the ocean, as it contributes to its acidification.<\/p>\n\n

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Carbon emissions are divided between the atmosphere and terrestrial and oceanic carbon sinks. The “imbalance” between total emissions and total sinks is an active research area.Source : Global Carbon Projet<\/figcaption><\/figure><\/div>\n\n

When CO2<\/sub> dissolves in sea water a series of chemical reactions occur, thereby decreasing the ocean\u2019s pH, leading to ocean acidification<\/a>.<\/strong> Just a reminder, the pH scale goes from 0 to 14, with 7 being neutral. Anything higher than 7 is basic (or alkaline) and anything lower than 7 is acidic. Here\u2019s a quick chemistry lesson<\/a> to help understand acidification: <\/p>\n\n

As carbon dioxide (CO2<\/sub>) dissolves in sea water, it forms carbonic acid (H2<\/sub>CO3<\/sub>), a weak acid that breaks (or \u201cdissociates\u201d) into bicarbonate ions (HCO3<\/sub>–<\/sup>) and hydrogen ions (H+<\/sup>). A higher level of H+<\/sup> ions increases acidity (a decrease in pH). But that\u2019s not the only problem. Ocean acidification is slowed down because available carbonate ions (CO3<\/sub>2-<\/sup>) bond with excess hydrogen (H+<\/sup>), which forms bicarbonate. However, this buffering effect consumes CO3<\/sub>2-<\/sup> and thereby reduces the chemical capacity of the upper ocean to absorb more CO2<\/sub>.<\/em><\/p><\/blockquote>\n\n

\"Ocean
Ocean-atmosphere equilibrium that results in the dissolution of CO\u2082. When carbon dioxide dissolves in water, it hydrates to yield carbonic acid, H2 CO3. Carbonic acid quickly dissociates through the loss of hydrogen ions to form bicarbonate (HCO3-) and carbonate (CO32-) ions.Source : Guillaume Paris<\/figcaption><\/figure><\/div>\n\n

What causes ocean acidification ?<\/h2>\n\n

CO2<\/sub> emissions are by far the main driver of ocean acidification.<\/a><\/strong> No need to go into details here, we already know what causes these emissions: human activities.<\/p>\n\n

The second most important driver is agriculture,<\/strong> which emits large quantities of nitrogen compounds into the atmosphere, mainly N2<\/sub>O (nitrous oxide). When these nitrogen compounds dissolve in the ocean, they also have acidifying properties and contribute to ocean acidification. Compared to ocean acidification caused by rising CO2 <\/sub>emissions<\/a>, the impact of the nitrogen cycle is rather small, but it\u2019s raising more and more concerns, especially on coastlines where agricultural activities are important. <\/p>\n\n

To a lesser extent, coastal pollution also leads to acidification, particularly through eutrophication<\/strong>, i.e. the excessive input of nutrients into the water which leads to plant proliferation, oxygen depletion and an imbalance in the ecosystem. This phenomenon is more local than the first two.<\/p>\n\n

How has ocean acidification changed over time ?<\/strong><\/h2>\n\n

This may be the only argument that climate deniers<\/a> will use. “Oceanic acidity has already been high in the past. And humans have always adapted”<\/a>. <\/p>\n\n

A pH level at a given moment does not provide much information. However, its evolution<\/strong> is a crucial indicator. In less than 200 years, oceanic pH went from 8.2 to 8.1. This may not look like much, but make no mistake: the pH is logarithmic. Which means that a small difference of 0.1 corresponds to a 30% increase in oceanic acidity.<\/strong> <\/p>\n\n

How can we observe this evolution ?<\/h3>\n\n

Sustained observations provide much needed data and understanding not only about ocean warming and water cycle reorganization (e.g., salinity changes), ocean eutrophication, and ocean deoxygenation, but also about changes in ocean chemistry.<\/span> <\/p>\n\n

To give an example: consistent variations in surface seawater CO2<\/sub>-carbonate chemistry are documented by seven independent CO2 <\/sub>time series that provide sustained ocean observations collected for periods from 15 to 30 years (see image above). In seven different parts of the world, the pH is consistently decreasing<\/strong>:<\/strong><\/p>\n\n

\"Ocean
Montage : Bon PoteSource : Bates & al_2014 <\/figcaption><\/figure><\/div>\n\n

Has this already been observed in the past ? Yes. And yet, the alterations of the marine carbonate system observed since the industrial revolution are unprecedented in the last 65 million years<\/a>.<\/strong> Is it a coincidence that this shift started when people began to burn coal on an industrial scale?<\/p>\n\n

Here are 2 graphs: pay attention to the time scale. The first is expressed in millions of years: <\/p>\n\n

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Source : Ho\u0308nisch & al_2012<\/figcaption><\/figure><\/div>\n\n

The second, in decades. The observed drop in pH is simply spectacular:<\/p>\n\n

\"AR6
Multi-decadal trends in surface layer pH (complete scale) at various ocean sites and global distribution of annual mean pH adjusted to the year 2000.Source : AR6; Figure 5.20<\/figcaption><\/figure><\/div>\n\n

How is past and present ocean acidification measured ?<\/h2>\n\n

We now know that oceans absorb carbon dioxide and that they are becoming more acidic (and pH is decreasing) as a result to oceanic measures taken during field missions.<\/a>They provide broad spatial coverage over a short period of time as well as automatic or manual oceanic carbon measurements on stationary moorings, producing long-term, high-resolution data from a single location.<\/p>\n\n

Up to a million years ago <\/h3>\n\n

These records can be extrapolated using so-called \u201cproxies\u201d that provide an indirect measure of marine carbonate chemistry. An indicator is a measurement taken from natural archive (ice cores, corals, tree rings, marine sediments, etc.) and is used to infer past environmental conditions. <\/p>\n\n

For instance, by analyzing the chemical composition of tiny fossil shells found in deep ocean sediments, scientists have established records of oceanic pH levels in ancient times when pH measurers didn’t yet exist. <\/p>\n\n

Also, because the chemical balance between surface water and the atmosphere is relatively stable, it\u2019s possible to measure historical oceanic pH levels from atmospheric carbon dioxide concentrations. These are obtained from Greenland and Antarctic ice cores, which contain trapped air bubbles providing information on past climates. These data indicate that current atmospheric carbon dioxide concentrations and oceanic pH levels are unprecedented going back at least 800,000 years.<\/p>\n\n

And even more ancient\u2026<\/h3>\n\n

By going back even further in Earth’s history, to the Paleocene-Eocene boundary about 55 million years ago, scientists have found geochemical evidence of a massive release of carbon dioxide leading to a significant warming and dissolution of shallow carbonate sediments in the ocean.<\/p>\n\n

Although similar to what we observe today, this release of carbon dioxide occurred during the course of thousands of years, at a much slower rate than what we are observing today, thereby giving the oceans time to partially adapt to the damage. Geological records show that, during periods of rapid environmental change, species have acclimated, adapted or disappeared. Corals have suffered mass extinctions in the past <\/strong>(such as the Permian extinction around 250 million years ago), and new coral species have evolved to take their place, but it has taken them millions of years to recover to previous biodiversity levels.<\/p>\n\n

\"The
Source : Ho\u0308nisch & al_2012 <\/figcaption><\/figure><\/div>\n\n

If there\u2019s one lesson to learn,it\u2019s that in the past, oxygen loss coupled with global warming and ocean acidification has systematically led to mass extinction.<\/span><\/strong> There were five of them, and wouldn\u2019t it be great to avoid another one?<\/p>\n\n

What role does ocean acidification play in the future ?<\/h2>\n\n

Thanks to the work of group 1 of the 6th IPCC report<\/a>, we know that: <\/p>\n\n