Carbon dioxide and acidification

Figure 2.4 Global mean sea level estimated by different methods of averaging coastal tide gauge data (purple, green and blue lines – all held by the Permanent Service for Marine Sea Level). The pale purple envelope shows the estimated uncertainties of the purple line. The brown line shows altimeter data, which are independent of tide gauge data.

The oceans play an important role in reducing the contribution of carbon dioxide (CO2) to climate change, by taking up more CO2 than they release, which substantially reduces the rate of increase in the atmosphere. However it also makes the oceans more acidic and potentially reduces their capacity to take up CO2 in the future. Continental shelf seas play a key role in this global CO2 uptake. Changing the pH of seawater alters the balance of and rate of conversion between different nitrogen compounds, changing their availability to support the growth of phytoplankton and hence eutrophication. Biogeochemical and ecosystem processes affected include planktonic calcification, carbon and nutrient assimilation, primary production and physiology; many marine animals have planktonic larval stages that are likewise vulnerable. Organisms such as bivalves and tube worms may have difficulty forming shells in lower-pH waters. Changes in pH also affect the availability of trace metals, which may be necessary for plankton growth, or may in some cases be toxic. We assessed the state of CO2 uptake and acidification in UK waters using models of the sea, inverse modelling of atmospheric concentrations and validation with evidence from direct measurements.

We found that the north-west European continental shelf is a net absorber of atmospheric CO2, but that its capacity to do so is highly variable. More widely, the North Atlantic apparently reduced its net uptake of CO2 by more than 50% from the mid-1990s to 2005. However, this may be part of a natural cycle rather than a one-way trend.

Since the industrial revolution, ocean acidity has already increased by a third (or decreased by 0.1 in pH units).

Because there are currently no baseline measurements of pH against which changes in UK waters can be judged, it will be some time before we can make accurate judgements about the rate of acidification relative to natural annual and interannual cycles of pH. We also need a better understanding of the physical, chemical and biological processes controlling the ocean’s ability to absorb CO2.