Background

Sourced from Forster et al 2007 FAQ2.1, Figure 1. Atmospheric concentrations of important long-lived greenhouse gases over the last 2,000 years. Increases since about 1750 are attributed to human activities in the industrial era. Concentration units are parts per million (ppm) or parts per billion (ppb), indicating the number of molecules of the greenhouse gas per million or billion air molecules, respectively, in an atmospheric sample. © Intergovernmental Panel on Climate Change (IPCC).

As one of the main greenhouse gases, carbon dioxide (CO₂) plays a crucial role in regulating the Earth's temperature. Since the beginning of the 20th century, the rapid rise of CO₂ in the atmosphere has been directly linked to observations of global warming at the Earth's surface. Atmospheric concentrations of carbon dioxide are currently 40% greater than pre-industrial concentrations and at their highest levels in human history. Their rise is directly due to to human activities such as the burning of fossil fuels (e.g. coal, petrol) and large scale changes in land use (e.g. deforestation).

It is known that the oceans act as a buffer against the accumulation of CO₂ in the atmosphere. Since 1800, they have absorbed about one half of the CO₂ emissions released by human activities. As a result the ocean is helping to moderate the impact of rising CO₂ emissions on the Earth's climate. Understanding how the oceans' capacity to absorb CO₂ may change in the future is fundamental to improving our ability to predict climate change. This task is made more difficult due to the influence of many factors.

The oceans absorption of CO₂ is driven by the difference in concentration (measured as partial pressure, pCO₂) between the atmosphere and the ocean (termed ΔpCO₂). This difference is determined by a number of interactions

1. increases in atmospheric pCO₂ will directly lead to increases in surface ocean pCO₂
2. rising sea surface temperatures caused by global warming will decrease the solubility of CO₂ in seawater which may lead to a slowdown in the absorption and removal of CO₂ from the atmosphere
3. phytoplankton in the oceans take up CO₂ when they grow and then sink to the bottom when they die, locking up a small amount of this carbon in ocean sediments

Although the capacity of the oceans to absorb CO₂ is good news for tempering global warming in the short term, it also introduces new risks to marine life. This is because the more CO₂ absorbed, the more acidic the oceans become. Increases in ocean acidity can be measured as a decrease in ocean pH, which is estimated to have declined by 0.1 units (about a 30% increase in acidity) since pre-industrial times.

Our understanding of the full impacts of changing pH on marine ecosystems is limited and requires further research. However, one of the better-known consequences is that organisms containing calcium carbonate shells (e.g. some corals, shellfish, zooplankton and phytoplankton) will find it harder to make calcium carbonate.

More information about CO₂ and climate change is available in our references and further reading section.