For 40 years now, satellites have provided reliable and comprehensive observational data on the warming of the oceans; before their introduction, researchers had to draw on sporadic temperature readings like those taken by merchant ships. “This comparatively long timeframe with good data shows a clearly recognisable warming trend in the oceans, which absorb the majority – more than 90 percent – of the additional heat in the Earth system produced by greenhouse gases,” says AWI oceanographer Hu Yang, the study’s first author. But this additional heat isn’t spread evenly throughout the world’s oceans: Whereas the subtropical ocean regions have warmed dramatically over the past four decades, in the tropical East Pacific and in the subpolar oceans – like those surrounding Antarctica – the surface temperature has barely climbed or even decreased slightly. “These observations contradict to a certain extent the reconstructed paleoclimates, which, during transitions to warm periods, show the most intensive warming in the high latitudes near the poles,” adds Prof Gerrit Lohmann, Head of the Paleoclimate Dynamics Section at the AWI.
“This pattern seen in the past 40 years is often explained as a natural climate variation, the so-called pacific decadal oscillation – or PDO for short,” Yang explains. “But model-based simulations point in a different direction.” According to the team’s findings, the warming trends are produced by circulation patterns in the oceans, in particular dynamic convergence and divergence. Convergence occurs e.g. in subtropical gyres, in which warm surface water flows together and is vertically transported to deeper sea (downwelling). In contrast, divergence takes place in subpolar gyres, where water masses flow apart and cold water rises to the surface (upwelling).
“But, this pattern is only a brief transitional state at the beginning of anthropogenic climate change,” says Yang. “It will only continue as long as the warming produced by rising CO2 concentrations in the atmosphere is limited to the upper layers of the oceans.” In the long term – as the experts’ computer model shows – this will change. The additional heat will gradually be transported to the deep sea. “Then the subpolar oceans of the high latitudes, which represent an important link between the near-surface layers and the deep sea, will warm far more intensively than the subtropical regions. And this warming will continue for centuries, even if we achieve carbon neutrality in the near future. The impacts will be massive. For instance, the trend could cause sea-level rise of more than 10 metres. As such, the dramatic warming of the oceans and its consequences are only the beginning.”