BY MICHAEL E. MANN
CLIMATE
ON APRIL 22 (EARTH DAY) OF 1998, my co-authors and I published the now famous “hockey stick” curve. It was featured on the pages of the New York Times and other leading newspapers, helping it garner worldwide attention.
Here was a simple graph, derived from sources of “proxy” climate data such as tree rings, ice cores, coral, and lake sediment, depicting the average temperature of the northern hemisphere over the past six centuries. It resembled an upturned hockey stick, with the “handle” corresponding to the relatively constant temperatures over the preindustrial era, and the “blade” corresponding to the dramatic subsequent warming that coincided with the Industrial Revolution. A year later, we extended the graph back 1,000 years, conveying clearly the unprecedented nature of the warming taking place today. That made it a threat to carbon polluters, and it was subject to a crescendo of attacks by fossil-fuel companies and those doing their bidding. The hockey stick has nonetheless stood up to the scrutiny; indeed, other scientists have even extended it back two millennia.
The irony, in my view, is that some of the more important lessons we can learn from studying the climate of the Common Era (the period spanning the past 2,000 years) have been eclipsed by the almost single-minded focus of climate advocates and climate deniers alike on this one curve developed in the late 1990s. Now, the impacts of warming—the unprecedented heat waves, wildfires, floods, and storms of this past summer—are here. So it’s time to go beyond the hockey stick, and examine what else we can learn from the climate record.
THE HOCKEY STICK itself shows a single number for each year, which hides even larger regional episodes of warming or cooling that provide key insights. Consider the El Niño phenomenon—a natural warming of the eastern tropical Pacific, which comes and goes on timescales of four to six years, and has a profound impact on weather patterns around the planet. Reconstructions of past El Niño behavior based on climate proxy data bear out a controversial hypothesis linking explosive tropical volcanic eruptions and historical El Niño events. Such a relationship between climate drivers and El Niño is absent in most climate models today, suggesting that they might be wrong. The findings imply that greenhouse warming might lead to something akin to a more La Niña-like world (i.e., the opposite of El Niño conditions): an even greater-than-predicted increase in Atlantic hurricane
