'Look what you've done!! I'm melting, melting....'

Danni Schmidt is a Royal Society University Research Fellow at the University of Bristol is this weeks Smith lecturer. She writes
'I have always been deeply in love with the ocean (though I grew up in a rather land locked part of Germany) and hence studied Marine Geology at the University of Bremen and the Alfred Wegener Institute for Marine and Polar Research. During my time at AWI, I went to the Southern Ocean and knew that I want to continue studying the Ocean, its environments and how climate change is affecting life. During my PhD at ETH Zurich I studied the effect of biotic interaction and climate change on plankton evolution. Currently, my work is focussing on calcification and ocean acidification – past and future”. I am trying to inform policy decisons by contributing to the Marine Climate Change Impact Plan (UK) and by being a Lead author for the Working Group II of IPCC on the open ocean chapter. '



Schmidt will be talking about a hot new topic in Earth system science - ocean acidification. Another impact of raising atmospheric CO2 levels is the diffusion of atmospheric CO2 into water. About 1/3 of the CO2 we pump into the atmosphere dissolves in sea water (pCO2) where it dissociates into bicarbonate (HCO3) and carbonate (CO3) releasing H ions as it does so. Acidity or pH measures the number of H ions in water, as the number goes up, pH goes down (pH =-log10(H+) ). It is estimated that since the start of the industrial revolution (1750) ocean pH has dropped from 8.179 to 8.104.


Estimated change in annual mean sea surface pH between the pre-industrial period (1700s) and the present day (1990s). Δ pH here is in standard pH units.

The problem with this is that many marine organisms make their skeletons out of calcium carbonate (CaCO3). Different arrangements of CaCO3 molecules produce different minerals of calcium carbonate. For example aragonite is an arrangement that is physically strong and is favored by corals, but is chemically weak (dissolves easily), while calcite is chemically stronger and is favored by foraminifera and coccolithosphores. As pH drops these organisms will need to use more energy to build and prevent their shell from dissolving. The stress this causes can be seen in the misshapen shells/skeletons of these organisms.

Schmidt will be talking about other time periods in the past when CO2 levels were high. These intervals are perfect laboratories to examine the response of the ocean to higher pHs. Her research has shown that the change in ocean pH is happening faster than previous time intervals. Schmidt and her collaborators applied a model that compared current rates of ocean acidification with the greenhouse event at the Paleocene-Eocene boundary, about 55 million years ago when surface ocean temperatures rose by around 5-6°C over a few thousand years. During this event, no catastrophe is seen in surface ecosystems, such as plankton (the planktonic foraminifera and coccolithosphorids), yet bottom-dwelling organisms in the deep ocean experienced a major extinction. This is because the cold deep waters of the ocean can dissolve far more CO2 than the surface waters, making the deep ocean realm susceptible to rapid and extreme acidification. The occurrence of widespread extinction of these organisms during the Paleocene-Eocenegreenhouse warming and acidification event raises the possibility of a similar extinction in the future.