Abrupt Climate Change and Irreversibility
Future climate change poses particularly serious risks for human civilization in the case of abrupt climate change, which could result in environmental change happening more rapidly than civilization could adapt. Most possibilities for abrupt change have a very low probability of happening in the next century, but understanding how these abrupt changes have occurred in the past is critical in assessing future risk. Similarly, irreversibility implies serious, long-lived environmental consequences after crossing a climatic “tipping point,” and exploring examples of irreversibility in the paleorecord provides context for whether future tipping points will be crossed.
The abrupt climate change event highlighted in this chapter of the IPCC report is known as a Dansgaard-Oeschger (DO) event. This phenomenon refers to a cold phase followed by an abrupt transition to a warm phase, and this happened ~25 times during the last glacial cycle. Greenland ice cores indicate that warming as rapid as 16ºC in a few decades occurred during some of the DO events, and corresponding sea level rise was rapid as well. Some of the DO cold phases were also accompanied by massive export of icebergs to the ocean from continental ice (known as Heinrich events). The cause of these rapid changes is not perfectly known, but leading hypotheses include changes in Atlantic Meridional Overturning Circulation (a major current in the Atlantic Ocean that is driven by gradients in temperature and salinity and that has important effects on global climate), changes in atmospheric circulation, and changes in sea ice cover. Better constraining the cause of such events is vital in understanding whether similarly rapid warming events are possible in the future.
One important example of irreversibility is in the multiple equilibrium states of ice sheets with respect to atmospheric CO2 and temperature. Specifically, it has been shown that ice sheets will begin to grow at one CO2 level (when CO2 decreased to 600-900 ppm for the East Antarctic Ice Sheet), but they won’t begin to shrink until CO2 reaches a different, higher level (1200 ppm for the East Antarctic Ice Sheet). This hysteresis behavior is important to understand for modeling ice sheet growth and decay, along with other features in the climate system. Models that incorporate this phenomenon indicate that the West Antarctic Ice Sheet will eventually be completely lost if CO2 stays above 350–450 ppm for several millennia.