Abrupt climate shifts since 1976
Had the 1997 El Niño lasted twice as long, the rain forests of the Amazon basin and Southeast Asia could have quickly added much additional carbon dioxide to the air from burning and rotting,[29] with heat waves and extreme weather quickly felt around the world (The "Burn Locally, Crash Globally" scenario.[30])
Most abrupt climate shifts, however, are likely due to sudden circulation shifts, analogous to a flood cutting a new river channel. The best-known examples are the several dozen shutdowns of the North Atlantic Ocean's Meridional Overturning Circulation during the last ice age, affecting climate worldwide.[11] But there have been a series of less dramatic abrupt climate shifts since 1976, along with some near misses.
The circulation shift in the western Pacific in the winter of 1976-1977[31] proved to have much wider impacts.
Since 1950, El Niňos had been weak and short, but La Niňas were often big and long, This pattern reversed after 1977.
Land temperatures had remained relatively trendless from 1950 to 1976, despite the CO2 rising from 310 to 332 ppm as fossil fuel emissions tripled. Then in 1977 there was a marked shift in observed global mean surface temperature to a rising fever of about 2°C/century.[32]
The expansion of the tropics from overheating is usually thought to be gradual, but the percentage of the land surface in the two most extreme classifications of drought suddenly doubled in 1982 and stayed there until 1997 when it jumped to triple (after six years, it stepped down to double).[33] While their inceptions correlate with the particularly large El Niňos of 1982 and 1997, the global drought steps far outlast the 13-month durations of those El Niňos.
There were near-misses for "Burn Locally, Crash Globally" in Amazonia in 1998, 2005, and 2007, each with higher flammability than its predecessor.[29][34]
There have also been two occasions when the Atlantic's Meridional Overturning Circulation lost a crucial safety factor. The Greenland Sea flushing at 75 °N shut down in 1978, recovering over the next decade.[35] Then the second-largest flushing site, the Labrador Sea, shut down in 1997[36] for ten years.[37] While shutdowns overlapping in time have not been seen during the fifty years of observation, previous total shutdowns had severe worldwide climate consequences.[11]
This makes abrupt climate shifts more like a heart attack than like a chronic disease whose course can be extrapolated.[30] Like heart attacks, some abrupt climate shifts are minor, some are catastrophic—and one cannot predict which or when. The recent track record, however, is that there have been several sudden shifts and several near-misses in each decade since 1976.
