Could there be another abrupt climate change?

Our calculations indicate that considerably greater disturbances would be needed to disrupt today's Atlantic currents. This gives rise to the question whether man could upset the climate system to such an extent as to trigger another abrupt climate change.

This can be neither affirmed nor ruled out at present, nor will it be possible to make firm predictions in the foreseeable future. Global warming will probably weaken the formation of deep water in the Atlantic, because the ocean water in critical regions is diluted by increased atmospheric freshwater transport and by water from melting ice — this is already evident from observational data.

The question whether or when a critical threshold might be crossed and cause the current to cease completely is considerably more difficult to answer. Too much depends on regional circumstances which cannot be resolved by today's models, as well as on such uncertain forcing factors as the inflow of meltwater from Greenland.

Models can therefore provide no more than a rough indication. Regardless of how well we can already explain the mechanisms of climate change with our present understanding and computer models, the Greenland ice contains a clear warning: the climate system is by no means a sluggish, good-natured sloth — it can react very abruptly and violently.

In view of the uncertainty, what is needed is a risk assessment rather than predictions of abrupt climate change — rather like assessing the risks of a nuclear accident. Abrupt climate changes could be considered "accidents" in climate change.

In addition to the risk of a sudden change in ocean currents, there are other risks which must be considered — such as the risk of the West Antarctic Ice Sheet disintegrating due to global warming (raising the sea level by several metres), or the monsoon circulation changing, or large areas of rainforest drying out. Although the probability of such "climate accidents" is fortunately not very high, the risks need to be investigated in more detail.

Last but not least, we also need a broad public debate over what level of risk of abrupt climate change is considered acceptable. That is a question which cannot be answered by science alone.