Beyond The Hockey Stick: What We’re Missing When We Talk About Climate Change

On April 22 (Earth Day) of 1998, the warmest year that had yet been observed, 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 pre-industrial 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. The millennial hockey stick, published on the dawn of a new millennium, conveyed 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 teams of 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)—a few of which I discuss below—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.

Two and half decades later, while the signal of planetary warming had emerged from the noise, the impacts of that warming were still subtle. Now, they’re staring us in the face in the form of the unprecedented heat waves, wildfires, floods, and storms of this past summer. So it is time to go beyond the hockey stick, and examine what else we can learn from the climate record of the common era when it comes to the climate crisis we now face.

The hockey stick itself shows a single number for each year, representing the entire northern hemisphere. That hides even larger regional episodes of warming or cooling that provide key insights of their own. 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. The El Niño brewing right now is adding to the record global heat, and causing extreme weather around the planet.

Reconstructions of past El Niño behavior based on climate proxy data provide an important opportunity, for example, to revisit a controversial hypothesis linking explosive tropical volcanic eruptions and historical El Niño events—a hypothesis that has profound implications for the impact climate change may have on drought in the desert southwest and Atlantic hurricane activity.

The reconstructed El Niño chronology bears that hypothesis out, showing a rough doubling of the likelihood of an El Niño event following a large tropical volcanic eruption. Lest this seem an arcane and academic matter, such a relationship between climate drivers and El Niño is absent in most climate models today, suggesting that these models might not be correctly predicting how the El Niño phenomenon will change in response to ongoing human-caused warming. Flipped on its head, the observed relationship between volcanic cooling and El Niño implies that greenhouse warming might lead to the opposite, La Niña climate state, associated with colder waters in the eastern tropical Pacific. The reasons are somewhat complicated, but it's tied to the same factors responsible for El Niño in the first place, the complex interrelationship between the tropical atmospheric circulation, the strength of the trade winds and the upwelling of cold, deep waters induced by those winds. A more La Niña-like world would mean an even greater-than-predicted increase in Atlantic hurricane activity and western U.S. drought—two climate threats that have loomed large in recent years.

Moving on, what can we learn about other key climate phenomena? If you’ve watched the 2004 movie The Day After Tomorrow, you’ve seen a caricature of what would happen if global warming led to the collapse of the “great ocean conveyor,” a current that warms the mid-latitudes of the North Atlantic and neighboring regions of North America and Europe. In reality, Los Angeles won’t get destroyed by an outbreak of mega-tornadoes, and neither will another ice sheet form over North America. But this scenario would cause a decrease in the health of food webs and fish populations in the North Atlantic—one of the world’s great natural fisheries—at a time when we’re already struggling to meet the nutritional needs of a growing global population. And, for reasons that have to do with oceanographic physics, it would mean even greater sea level rise along the east coast of the U.S. than models currently predict.

Climate models project this ocean current system to slow down later this century. But an analysis of paleoclimate proxy data spanning the common era suggests that the slowdown has already taken place this past century, likely because the Greenland ice sheet is melting earlier than expected. So, we may be well ahead of schedule when it comes to this unwelcome climate system “tipping point,” a reminder that uncertainty is not our friend when it comes to the unfolding impacts of human-caused warming.

Last but not least, what do data and simulations of the common era tell us about how near we are to the threshold of truly dangerous planetary warming? The conventional estimate is that we must reduce carbon emissions by 50% by the end of this decade to avoid 1.5°C (~3°F) warming over pre-industrial levels, where we’re likely to see far worse climate consequences. When we examine climate model simulations and climate proxy data spanning the common era, however, we find that the conventional estimates may be missing a few tenths of a degree’s worth of early human-caused warming that took place prior to the mid-19^th century—when the historical temperature record began. If so, we might have as much as 40% less carbon to burn than the current models suggest. And if that’s the case, it means countries across the globe need to substantially ratchet up commitments to avoid climate catastrophe.

The hockey stick emphasizes the relative stability of the global climate over the common era, the period during which much of our civilizational infrastructure was developed. But evidence is growing that suggests we are rapidly leaving this era of climate stability—we find now ourselves in what I’ve termed our “fragile moment.” There is still time to preserve that moment, but only if act with the urgency the climate crisis demands. Among other things, we need to demand more ambition from our elected leaders when they gather to meet next month at the COP28 international climate summit in Dubai, possibly the last opportunity to negotiate the emissions reductions necessary.

Adapted from Mann’s most recent book, “Our Fragile Moment: How Lessons from Earth's Past Can Help Us Survive the Climate Crisis.”