The American Geophysical Union, meeting this week in San Francisco for their annual conference, released a report discussing the potential for abrupt climate change and the likely impacts it would have on the United States.
The study, based on the latest scientific data and observations, updates the research of recent reports from key agencies and institutions, including the 2007 assessment from the Intergovernmental Panel on Climate Change (IPCC).
Many earlier projections of the impacts of climate change have been conservative, the report concludes, such as retreating glaciers, decaying ice sheets, and loss of Arctic sea ice. On the other hand, some potential impacts may not pose as immediate a threat as previously thought, including the rapid release of methane, a much more potent greenhouse gas than CO2, or an imminent shift in the ocean current known as the Thermohaline that helps keep Europe warm (a reason why "climate change" is often a better term than "global warming" since the rise in average global temperature could potentially make some regions, like northern Europe, much colder).
Key concerns in the report are the dramatic and unexpected acceleration of sea ice melt, rising sea levels, and the potential for a permanent state of drought in the American Southwest.
What appears both more certain and immediate are rapid changes to the Greenland and West Antarctic ice sheets, diminishing sea ice, and changes to the hydrological patterns over North America and the global subtropics. All of which will persist and get worse with continuing global warming.
Peter Clark, an Oregon State professor of geosciences, says,
Our report finds that drying is likely to extend poleward into the American West, increasing the likelihood of severe and persistent drought there in the future. If the models are accurate, it appears this has already begun. The possibility that the Southwest may be entering a permanent drought state is not yet widely appreciated."
Climate change is a recurring pattern in Earth's history, usually occurring slowly over the course of hundreds or thousands of years. There have been times, however, when dramatic shifts in climate have happened abruptly, within decades. Which is of particular concern for human society.
Abrupt climate change presents potential risks for society that are poorly understood," the report states.
The research by the AGU focused on four mechanisms of rapid climate change that have happened prehistorically and evaluated the risks of such abrupt changes happening now. They include:
- Glaciers, ice sheets and sea level
- Widespread changes to the hydrologic cycle
- Abrupt changes in the "Atlantic Meridional Overturning Circulation," or AMOC, an ocean current pattern
- Rapid release to the atmosphere of methane trapped in permafrost or on continental margins
Conclusions of the report in terms of these mechanisms are:
- The edges of the Greenland and West Antarctic ice sheets are thinning and flow is accelerating. The speed of some glaciers has more than doubled. These observations are not included in current climate models and "changes in ice dynamics can occur far more rapidly than previously suspected".
- Once these changes in ice dynamics are included in climate models, the extent of sea level rise will "substantially exceed" the approximately two-foot rise now projected by the end of the century. However, data is not yet adequate to determine the exact amount of sea level rise.
- The American Southwest and subtropical areas around the world will likely become more arid, with an increasing risk of persistent and severe drought. If models are correct, this is already underway and is "among the greatest natural hazards facing the United States and the globe today".
- The Atlantic Multidecadal Oscillation (AMOC) ocean circulation pattern that keeps Europe warm may weaken by as much as 30% due to increases in greenhouse gases in the atmosphere, but will probably not collapse altogether - however the possibility can't be be completely excluded.
- Emissions of methane will accelerate due to climate change, both from hydrate sources and wetlands. It is likely that the increase will double within the next century, but it is unlikely that a catastrophic release will occur within that time.
The "overarching" recommendation from the report is the need for a sustained commitment to monitor these climate forces that have triggered sudden climate change in the past and could therefore do so again. Better observing systems are needed, better forecasting of droughts should be developed, there should be a more comprehensive understanding of the AMOC circulation pattern, and methane levels should be systematically monitored.