Exploring the boundaries of climate change

In the public debate, references are regularly made to possible climate events with potentially severe consequences. The scientific literature on 13 of these climate eventualities has been reviewed to give an overview of their characteristics such as the timing, likelihood and level of scientific understanding. It is argued that insufficient information is available on these events, particularly on the possible impacts, to assess and compare the risks they represent. These climate eventualities can be seen as extra sources of uncertainty that have the potential to accelerate, and in some cases delay, developments associated with global warming.

The information gathered confirms that of the 13 climate eventualities, rapid disintegration of the Greenland and West Antarctic Ice Sheets are the most dangerous prospects. However, if the enhanced greenhouse effect can be mitigated and global warming remains at or near 2°C, most of these eventualities can be avoided. But developments already taking place, such as ocean acidification, slow release of methane from permafrost and ocean regions and rapid melt of Arctic sea ice in summer, would continue to have impacts. It is not clear whether and how rapid the Greenland Ice Sheet would disintegrate in a climate that is 2°C warmer. The possible considerable risk related to unknown tipping points or eventualities that we do not know about cannot be estimated. This ignorance is perhaps the greatest climate threat