Ocean acidification, caused by the uptake of anthropogenic carbon dioxide, is a significant stressor to marine life. Ulf Riebesell charts the rapid rise in ocean acidification research, from the discovery of its adverse effects to its entry into the political consciousness.

When the German Chancellor Angela Merkel visited us at GEOMAR | Helmholtz Centre for Ocean Research in Kiel last year, the ‘other CO2 problem’ was well known. Not just to her, but also to the pack of journalists tailing her. One of the first questions Merkel asked was, “Will marine organisms be able to adapt to the acidifying oceans?” I couldn’t give a definitive answer, but her question gave me confidence that the message of ocean acidification posing a threat to marine life had reached our policymakers and communicators.

In 2007, when Nature Geoscience announced its first call for papers, the Intergovernmental Panel on Climate Change (IPCC) had just published its Fourth Assessment Report. In the 52-page synthesis, only 10 lines were devoted to the issue of ocean acidification. The report concluded that “while the effects of observed ocean acidification on the marine biosphere are as yet undocumented, the progressive acidification of oceans is expected to have negative impacts on marine shell-forming organisms (for example, corals) and their dependent species”.

How did the awareness of ocean acidification evolve from being a side note in the 2007 IPCC Report to grabbing the attention of Chancellor Merkel? The story of the rise in ocean acidification research actually begins three years earlier, at the first symposium on the Ocean in a High-CO2 World in Paris in 2004. The term ‘ocean acidification’ had just been coined, and possible impacts on marine life had barely been investigated. The symposium was, in fact, mainly set up to evaluate strategies to artificially enhance ocean carbon uptake to mitigate the rise in atmospheric carbon dioxide concentrations. By the end of the meeting, however, it was clear that oceanic uptake of anthropogenic carbon dioxide had the potential to change seawater chemistry sufficiently to endanger marine organisms and ecosystems ocean-wide, even when just natural uptake was considered. The symposium marked a turning point in our view of the priceless service the ocean provides by taking up large quantities of anthropogenic carbon dioxide.

On the heels of the Paris meeting, a remarkable scientific endeavour followed. A report on ocean acidification by the Royal Society of London in 2005 marked the starting point of what was to become an amazingly steep learning curve — yet it wasn’t until 2008 that research on ocean acidification gained substantial momentum in terms of funding. Since then, this field of research has become one of the fastest-growing areas in marine science. From barely 20 publications per year on the possible impacts of CO2-induced changes in ocean chemistry in 2004, research output rose to over 200 publications per year in 2010, and is expected to surpass 300 in 2012.

Important challenges remain. Although much knowledge has been gained about the effects of ocean acidification on individual organisms, in particular calcifying taxa, we still know little about responses at the community and ecosystem levels. We are just beginning to study the ability of organisms to adapt to an acidifying ocean. Moreover, ocean acidification does not happen in isolation. It occurs simultaneously with other changes such as ocean warming and a loss of oceanic oxygen, and interacts with other human-induced stresses such as over-fishing and eutrophication. Without proper knowledge of these interacting effects at the ecosystem level, it is difficult to project the biogeochemical consequences and climate system feedbacks resulting from ocean acidification, let alone the economic and social impacts.

Considering the evidence available today, the IPCC’s Fifth Assessment Report, due out in 2014, is likely to come to a different conclusion about the impacts of ocean acidification. Today we know that its adverse effects are not restricted to shell-forming organisms. Observed impacts range from shifts in microbial species composition to reduced egg production in zooplankton, and, further up the food chain, from delayed larval development to abnormal flight behaviour in fish. Moreover, impacts of acidification on marine biota have been well documented in the field. So too have some of the economic impacts, such as the collapse of shellfish production in oyster hatcheries related to the acidification of their feed-water on the west coast of the United States.

Even though many uncertainties about the impacts of ocean acidification on marine life remain, we now know enough to predict with reasonable confidence that significant changes in marine ecosystems and biodiversity will occur within our lifetimes, if the oceans continue to acidify at the current rate. And we know how to prevent it — by cutting back on carbon dioxide emissions.

Riebesell U., 2013. A steep learning curve. Nature Geoscience 6:12-13. Article.