Twenty-five years ago, when I was a postdoctoral researcher at the University of Massachusetts Amherst, my coauthors and I published the now-famous hockey stick graph. Based on climate “proxy” data such as ice cores, corals, sediments, and tree rings, the graph laid bare the unprecedented warming spike coinciding with industrialization and became a central exhibit in the case for concern about human-caused planetary warming.
That concern was certainly borne out this summer with extreme flooding in New England towns and villages and wildfire smoke that originated in drought-stricken Canada filling the air in Boston and other East Coast cities.
The recent near miss of Hurricane Lee, which veered to the east just in time to spare New England from most (but not all) of its impacts, is a reminder that hurricanes are another climate-related threat. How great is the threat? That’s difficult to say, as it is uncertain how El Niño might change in a warmer world.
El Niño — when the eastern tropical Pacific undergoes a natural warming that impacts wind patterns around the world, including the tropical Atlantic — tends to be a moderating impact on Atlantic hurricanes. But warmer tropical ocean waters due to climate change favor greater hurricane activity — and these two factors are currently battling it out this hurricane season.
It turns out that the same sort of climate “proxy” data used in the hockey stick graph can help provide insights into how El Niño changed in the past in response to natural climate drivers.
While at UMass, I was involved in another paleoclimate project with other researchers. We analyzed sediments from ancient lakes that had formed in the wake of the retreating Laurentide Ice Sheet, which covered all of New England and large swaths of North America at the height of the last glacial maximum about 21,000 years ago, as the ice age came to an end between 12,000 and 18,000 years ago.
There existed a near continuous 4,000-year record of sediments from the ancient lakebeds, from southern Connecticut all the way to northern Vermont. But there was one crucial missing segment. It turns out, it corresponded to the time when the retreating ice sheet edge was making its way through western Massachusetts. And the optimal location for drilling? It turned out to be a UMass soccer field, which sits above the ancient lakebed (yes, the soccer coach grudgingly permitted us to drill the core).
With this segment in hand, we had a 4,000-year record of summer temperatures in New England as the most recent ice age was coming to an end. The record indicated there were fewer El Niño events. A warming climate, in this case, seemed to cause a dissipation of El Niño.
Might this relationship hold additional evidence? As it turns out, the early centuries of the past millennium displayed the flip side of El Niño, La Niña, when the eastern equatorial Pacific was warmer than normal. This was a time of relative natural heating with high levels of solar output and relatively few cooling volcanic eruptions. In other words, a warmer overall climate in this case was once again associated with a general absence of El Niño.
Some years later I would collaborate with researchers at UMass and Woods Hole Oceanographic Institution to reconstruct how hurricane systems have changed over the past millennium based on coastal sediment deposits (a particular type of sediment known as an “overwash” deposit that is uniquely associated with powerful landfalling storms). We found that the overall warmth of the Atlantic Ocean in the earlier centuries (favorable to hurricane development) combined with the La Niña conditions (also favorable to hurricane development) yielded more frequent major hurricanes at that time.
If there’s an analogous relationship between human-caused warming from fossil fuel burning and increases in carbon pollution continue, we may expect an ever-greater coastal threat from landfalling Atlantic hurricanes as long as global warming continues. Combined with evidence that landfalling hurricanes may be tending to drift northward from the mid-Atlantic coast to the Northeast with a warming climate, it is cause for concern for New Englanders.
Uncertainty is often cited as a reason for climate inaction, but we should see it as just the opposite. For as we learn more about the potential consequences of continued human-caused warming and reduce the uncertainties, we’re learning that some unpleasant surprises may lurk. It is all the more cause to take concerted action to reduce carbon emissions and prevent further warming of the planet. It is not yet too late to preserve our fragile moment.
Michael E. Mann is director of the Center for Science, Sustainability and the Media at the University of Pennsylvania. His latest book is “Our Fragile Moment: How Lessons from Earth’s Past Can Help Us Survive the Climate Crisis.”