Emissions of greenhouse gases like carbon dioxide from human activity cause ocean warming, acidification, oxygen loss, and other physical and chemical changes that are affecting marine ecosystems around the world. At the same time, natural climate variability and direct human impacts, such as overfishing and pollution, also affect marine ecosystems locally, regionally and globally. These climatic and non-climatic factors mutually reinforce each other.
Tipping points – a threshold beyond which an abrupt or rapid change in a system occurs – that have already been reached in ocean systems include the melting of sea ice in the Arctic, thermal bleaching of tropical coral reefs and the loss of kelp forests.
To prove that the oceans are changing because of human-induced climate change, scientists use various methods. With the help of paleo records, they reconstruct connections between climate, evolutionary and ecological changes in the geological past; with experiments, they investigate current climatic and ecological reactions, for example to higher CO2 levels in the water, or test in models what relative influence certain factors have.
