Last week, our first guest speaker for the Climate Change in History Dream Course was Dr. Gregory Cushman, associate professor of international environmental history at the University of Kansas. Cushman reported on an article he is co-authoring with current and former graduate students Nicholas Cunigan, Patrick J. Klinger, and Adam Sundberg, all three experts in early-modern environmental history with interests in studying climate. Together, they offer an important challenge to conventional ways of understanding the Little Ice Age (often abbreviated LIA), a familiar topic in historical climatology, environmental history, and related fields.

The LIA was a period of significant global cooling that, conventionally understood, lasted roughly 1300–1850CE, in which average global temperatures dropped as much as 2 degrees Celsius. Climate historian Sam White nicely summed up its significance: “this cooling stands out as undoubtedly the most pronounced global climate anomaly of the past 8,000 years (until contemporary global warming).” The LIA is also a popular topic outside the academy, the subject of general-audience articles (for example here and here) and books. It has recently served all of us as the easiest go-to example of climate change to compare with our own predicament of global warming: it is a useful case study or precedent for helping us imagine climate adaptation today and tomorrow.

There are a few reasons why the LIA has become so well-known in climate history. First, geographically disparate studies have confirmed that societies across the northern hemisphere (from China to the Ottoman Empire, Europe, and North America) experienced unusual cold, worst in the 1600s, which caused significant socio-environmental stress—crop failure, famine, disease, declining population, war, rebellion, and political change. These findings match up well with the historians’ concept of the “General Crisis” of the 17th Century, a notion first theorized for Europe by renowned Marxist and generalist historian Eric Hobsbawm in the 1950s, and now increasingly applied more broadly, to the northern hemisphere or the whole globe. Second, these wide-ranging patterns of crisis and stress are confirmed by evidence from both the “archives of nature” (physical evidence) and “archives of society” (documentary evidence), which corroborate each other in useful ways across studies of the LIA. Sam White’s recent historical climatology of north American drought, which compares tree-rings to documentary records, spells this out very clearly. When the conclusions of interdisciplinary study are rather consistent across such broad swaths of space and time, we should pay close attention.

This is where Cushman and his coauthors powerfully intervene, because paying close attention to scholarly consensus can blind us to its limits. Cushman and company charge that the LIA consensus needs to be “decentered” in several ways, and replaced by a new notion of the Global Little Ice Age (GLIA).

• First, the LIA concept has historically been Eurocentric at worst, and Eurasian-centered at best, in ways that often excluded Africa, Australia, and the Americas.
• Second, even after recent studies showed that Eurasian patterns were echoed in North America, the concept has remained northern- or boreal-centered, ignoring much of the tropics and the southern hemisphere.
• Third, because the northern hemisphere holds 68% of the world’s landmass, existing understanding of the LIA is also land-centered, ignoring the important fact of climate science that atmospheric patterns are different over water than over land.

In addition, Cushman and his collaborators call for focusing LIA chronology more narrowly, on the 1600s, where most scholars locate the “peak” LIA and General Crisis. Recent climate science has shown that this narrower period (1570-1720 rather than 1300-1850) was not only the coldest of the LIA, but also the only time between 1300 and 1850 when the northern and southern hemispheres experienced the same cooling–in other words, the only period in which the stressful cooling trend was truly global. In fact, the GLIA was the only time during the past thousand years when the two hemispheres have been climatically synchronous, that is until today’s (post 1950) global warming.

What caused this global cooling around the 1600s? First, scholars have found increased volcanic activity that filled the atmosphere with cooling aerosols made of sulfur dioxide, hydrochloric acid, and ash. Second, scholars note reduced greenhouse gasses CO2 and methane. Finally, there was the Maunder solar minimum, which is physically measurable, but had an impact on global cooling 10 times less than greenhouse gasses and 100 times less than volcanic aerosols. These three factors are joined by complex questions about regional variation in dynamic atmospheric-oceanic interactions (such as the North Atlantic Oscillation, El Nino, and La Nina) whose circulation patterns are still not fully understood, especially in historical perspective. These atmospheric-oceanic interactions stand out as an area for further research, and point to the theme of climate variability.

Thus, the GLIA emerges from the work of Cushman’s team as a both globally synchronous (in its averages) and highly variable on a local level. For many societies in Africa, Asia, and Latin America the primary human challenge of the GLIA wasn’t cold, but either prolonged drought or unstable dry-wet oscillations which punished human societies with alternating drought and flood. Indeed, one team of researchers speaks of the “Little Drought Age” in colonial Mexico. The lesson about global climate change is clear: its effects vary with place, and so place matters. “Global” means everywhere, but not the same everywhere. Today, while California and Australia burn, the U.S. midwest and western Europe are soaked by floods–all of it synchronously intensified by global warming.

This has clear consequences for thinking about climate change today. Amid sharply rising global CO₂ levels and temperatures, we cannot assume that all parts of the globe will experience the same amount or kind of climate impact. What’s more, human habits and social evolution are based on climatic patterns. When climate rapidly shifts, becomes more variable, or enters the unprecedented territory we are in today, human adaptation becomes more difficult. Sometimes the problem with climate is not the extremes (too hot or cold, too dry or wet), but rather that it is unpredictable: it changes in ways that do not allow people to develop stable, workable environmental practices, but rather force humans to reconstruct their ecological niches repeatedly.

Thus, for example, Mark Carey’s work has shown how the high-altitude dams that store water in times of drought are beneficial until times of glacial melt or heavy rains, when they can collapse or burst, causing dangerous “breakout” floods that send deadly cascades of water, rock, mud and other debris down into mountain valleys, a bit like a downhill tsunami. Cushman’s lecture included the tale of the 1626 flood disaster that devastated the city of Potosi, Peru, home to the giant silver mine that fed the global economy in the early modern era of history. Perhaps the largest documented dam-flood disaster in history, it took as many as 4,000 human lives and released 19 tons of mercury into regional water and soil, in addition to countless other damages. When climate shifts, otherwise useful adaptations such as dams suddenly become a menace to lives and livelihoods. By no coincidence, both Carey and Cushman have looked to highland Peru as a useful place to detect the impacts of climate change, whether warming or cooling. (Another pattern in LIA studies is that locations at high altitudes suffered some of the worst impacts of cooling.)

Cushman’s talk was dynamic, engaging, and stimulating. It underscored the importance of the interdisciplinary collaboration that this blog champions. It also offered powerful reminders that even the most solid scholarly conclusions with the best evidence behind them (older LIA studies) leave room for revision, in ways that challenge what we think we know and drive us to learn more.