Remember when climate change seemed like the defining issue of the 2020 election? Believe it or not, it still is. Of every challenge faced by Americans, climate change may pose the most plausible existential threat to the future of humanity. It is a threat that can be greatly reduced over the next four years—or exacerbated to such a degree that we can no longer avoid catastrophic outcomes. In 2020, the future of the planet may hang in the balance.

Annual temperatures for the contiguous United States from 1895 to 2017. The color scale goes from 50.2°F (dark blue) to 55.0°F (dark red). Courtesy Climate Lab Book, ed. Ed Hawkins/CC BY 4.0

Climate change may seem like a quintessentially 21st-century problem, but it has a history in electoral politics. Presidents since Lyndon Johnson have been briefed on the threat posed by human greenhouse gas emissions, which already amounted to billions of tons annually in 1963. Environmental issues, particularly efforts to regulate or remove pollution, have featured prominently in every presidential campaign since 1977. Scientific consensus emerged around the human responsibility for warming in the early 1980s, when calls for action by prominent congressmen garnered national headlines. In 1988, at a time when some solutions still had bipartisan support, George H. W. Bush called for concerted action to confront climate change. Since then, a well-funded effort to discredit climate science has both polarized attitudes toward climate action and kept the issue from the forefront of electoral politics. Only during the 2020 Democratic primary did climate change receive the attention it deserves—until COVID-19 burst onto the scene. 

Yet the 2020 election should still focus on climate change, because the threat it poses is greater and more immediate than it may seem. To find out why, we must think like historians: we must consult the past. But we can’t do it alone, because the history of climate change begins with the formation of the Earth, long before there were any sources for us historians to interrogate. We must work with and learn from archaeologists, geographers, paleoscientists, and others who consider the past with different tools, interests, and assumptions than we do.

We can “reconstruct”—that is, piece together—the history of Earth’s climate using the same detective work that historians apply to the human past. Paleoclimatologists uncover aspects of the natural world altered by the influence of past climate change: rings in old tree trunks, for example, that were wider in wet years than dry ones; layers in ancient ice cores that contain more heavy oxygen isotopes when the Earth was hotter (and the oxygen in water therefore evaporated more readily). Modelers run computer simulations based on the physics of Earth’s climate, producing “hindcasts” that begin in the distant past and run forward toward the present. Historical climatologists scour the human record for evidence of long-term shifts in weather, from oral histories of lost ice to reports of winds and currents in mariners’ logbooks.

What do these sources and simulations tell us? They reveal that, in the 300,000-year history of our species, we have endured some staggering climate changes. For example, during the chilliest stretch of the Last Glacial Maximum—the last long period colloquially known as an “ice age”—Earth may have been nearly six degrees cooler than it is today. We have been deeply fortunate to build our civilization in the Holocene, a long and globally stable warm period between glaciations. Yet our good fortune has always had an expiration date; the Earth, we now know, changes abruptly with just a little nudge—a little “forcing,” as climatologists call it—that alters the amount of solar energy that reaches us or escapes into space.

The threat climate change poses is greater and more immediate than it may seem.

Now we are not so much nudging the Earth as pushing it off a cliff. Today’s warming is faster, more uniformly felt across the globe, and different in origin than anything complex human societies have encountered in 10,000 years. Our best bet is that, barring dramatic action, we will warm the Earth by around three degrees Celsius (relative to the 20th-century average) over the next 80 years, by doubling the concentration of carbon dioxide in the atmosphere.

Those of us who work on climate toss around such numbers as though it’s obvious what they mean, but for most people, it isn’t. While most of us scarcely notice a three-degree difference in temperature, the Earth does. A world that is three degrees warmer than our own will be a fundamentally different planet. All of the world’s ice sheets beyond East Antarctica, for example, may eventually melt, raising sea levels until present-day coastlines are entirely overwhelmed. Extreme heating, especially along the equator, will likely combine with fundamental changes in the circulation of the atmosphere and oceans to transform the distribution of life on Earth, including human life.

Reconstructions reveal that even the slightest temperature change has profound implications for the Earth. Therefore, we need to dramatically reduce the carbon emissions forcing Earth over the climatic edge, and then actively absorb the carbon we have already emitted. Presidential candidates have proposed cuts before, but Congress has rarely cooperated. Now dramatic emissions reductions will be needed to limit global warming to two degrees Celsius.

Even if we act aggressively to limit carbon emissions, we will still need to adapt to unprecedented climate change. Reconstructions permit historians, archaeologists, geographers, and natural scientists to pursue “climate history”—the study of how past climate changes influenced human ideas and actions. The Holocene was, for the most part, globally warm and stable, yet regional climates could still change dramatically for decades, even centuries. For example, the best-studied period of pre-industrial climate change, the Little Ice Age of (arguably) the 13th to 19th centuries, reduced global temperatures by just tenths of a degree Celsius in its chilliest centuries, but likely cooled some regions by one degree or more in select decades.

Climate history suggests that some communities and societies were highly sensitive to these changes. Traditionally, climate historians have focused on examples of societal crisis and “collapse” that unfolded as subsistence strategies attuned to one climate unraveled with the arrival of another. The Maya, for example, are alleged to have abandoned their great cities from the 8th through the 10th centuries amid prolonged drought; so too the many denizens of 15th-century Angkor Wat. The Norse settlements of western Greenland disappeared as unstable cooling undermined pastoral ways of life and weakened connections to hunting grounds and trade routes. A “fatal synergy” of cooling, harvest failure, malnutrition, disease, and warfare supposedly claimed a third of the world’s population during a 17th-century “global crisis” that coincided with the chilliest stretch of the Little Ice Age in the northern Atlantic. Waves of cooling and drought may have even delayed European colonization of North America.

Climate history suggests that some communities and societies were highly sensitive to these changes.

Climate historians have mostly sought climatic explanations for disasters previously blamed on other causes, and so they have tended to overlook examples of populations that survived or even thrived amid climate changes. Yet a new wave of research reveals that climate change did benefit some societies or spurred development of “resilient” cultures and economies. Diverse societies that traded widely and provided for their poor may have been especially resilient, although such resilience may have belied, or even exacerbated, the vulnerability of communities and individuals within them.

Here is another lesson for the 2020 election. Most societies likely will survive a three-degree warming, but only by changing, and only at great cost. Adaptation in the face of climate change has, until the development of the Green New Deal, largely entered public and political discourse through proposals to build new infrastructure, develop new technology, or encourage new practices in personal or professional life. Yet for individuals and communities to broadly share in climate change resilience, policymakers must think bigger: they must begin to reform every aspect of society that makes some communities more vulnerable than others.

While many climate historians differentiate between more or less direct social responses to climate change, most agree that shifts in temperature or precipitation reverberated through every aspect of past societies, affecting everything from military strategy to daily diets. This is doubly true today, when all of us, visibly and invisibly, both respond and contribute to climate change in everything we do.

This means, of course, that climate change is connected to every other issue on the ballot this November—sometimes in surprising ways. Republican voters who care deeply about the waning relative strength of the American military, for example, would do well to remember that climate change could fundamentally threaten the capabilities of the Department of Defense. Democrats should consider how the makeup of the Supreme Court could affect the viability of environmental laws. All voters should remember that stimulus bills present a priceless opportunity to reward or fund transitions to lower-carbon lifestyles and industries. They also offer a chance to build wealth and resilient infrastructure in predominantly Black, Hispanic, and Indigenous communities left vulnerable to heat waves or rising seas by generations of structural racism.

We were asked to explain what Americans should know about climate change as they head to the ballot box in 2020. Simply, it is this: climate is the foundation upon which all other policies stand, and a changing climate could transform everything. No matter what issues you hold dear, keep the trajectory of our warming world in mind this November.