Lesson 8 Summary

In this lesson, we further examined potential anthropogenic climate change influences on a host of climate and meteorological phenomena. We found that:

• Arctic sea ice extent is projected to continue to decrease, and it is likely that summer sea ice will disappear if the globe warms up beyond 2°C. Observations suggest that the rate of decline is greater than what is projected by the models;
• Mountain glaciers and small outlet glaciers around the world are projected to continue their decline under further warming, but there is greater uncertainty regarding the rate and magnitude of decline of the two major continental ice sheets—Greenland and Antarctic;
• While we may commit to enough warming to set in motion the ultimately disappearance of the Greenland and West Antarctic ice sheets within decades and given only modest additional warming, current generation ice sheet models suggest that the scenario could take many centuries or even millennia to play out, owing to the intrinsically slow nature of key governing processes;
• Actual observations of ice mass suggest that both Greenland and West Antarctic ice sheets already have entered into a regime of negative mass balance, and it is quite possible that physical processes not well represented in current generation ice sheet models, such as ice shelf buttressing, could allow for a much faster retreat;
• There are several different components expected to contribute to global sea level rise, including the expansion of seawater with warming, the contribution of melting mountain glaciers, and ice loss from the major ice sheets; the latter component is both likely the largest and the most uncertain;
• Semi-empirical models which attempt to account for all contributions to global sea level rise suggest the possibility of a meter of sea level rise or more by 2100, and perhaps as much as 5 meters by 2300;
• Anthropogenic climate change is projected to lead to further increases in the incidence of specific types of extreme weather events. These include intense rainfall and snow events and flooding, and the incidence of high temperature extremes and heat waves. While the shifts in extreme weather thus far have been subtle, the projected increases are likely to be perceptible in the pattern of day-to-day-weather;
• Climate change projections have traditionally neglected potentially important carbon cycle feedbacks which may serve to further accelerate anthropogenic climate change. For a given emissions pathway, ${\text{CO}}_{\text{2}}$ concentrations might increase more than indicated by the nominal ${\text{CO}}_{\text{2}}$ concentration scenarios because of feedbacks which tend to further increase levels of atmospheric ${\text{CO}}_{\text{2}}$. The impact is quite uncertain, but probably about 20-40 ppm additional ${\text{CO}}_{\text{2}}$ for each degree C of warming, which has non-trivial consequences for the magnitude of future warming;
• The traditional so-called "Charney" concept of equilibrium climate sensitivity may not be appropriate for assessing the full extent of anthropogenic climate change, because it neglects slow feedbacks like changes in vegetation and land surface properties, and the retreat of the continental ice sheets, which tend to further amplify the projected changes; the importance of these long-term feedbacks is suggested by the relationship between ${\text{CO}}_{\text{2}}$ levels and global warmth in the geological climate record. The so-called Earth System Sensitivity, which attempts to incorporate the amplifying effects of the slow-feedbacks, suggests a modestly (50%) greater amount of warming and associated climate change than the traditional notion of climate sensitivity;
• There are a number of phenomena, including the behavior of the cryosphere, the thermohaline circulation of the ocean, and certain carbon cycle feedbacks related to, e.g., currently frozen methane stores, which may behave in a threshold-like manner. Rather than exhibiting a steady trend in time, these phenomena may exhibit abrupt transitions in behavior in response to ongoing anthropogenic climate forcing. Other potential examples of systems that might exhibit tipping point behavior are ENSO and the Asian summer monsoon.

Reminder - Complete all of the lesson tasks!

You have finished Lesson 8. Double-check the list of requirements on the first page of this lesson to make sure you have completed all of the activities listed there before beginning the next lesson.