Our planet is now facing a long-term warming trend, the average global temperature has increased by at least 1.1° C since 1880, but how the Earth has responded to past, abrupt cooling episodes?  

The catastrophic volcanic eruptions in the 1810s

Tambora volcano is located at Sumbawa Island, Indonesia. The devastating eruption of Mt. Tambora in April 1815 was the largest explosive volcanic eruption recorded since AD 1400. The massive load of sulfate gases Tambora injected into the stratosphere produced an aerial dust cloud consisting of up to 100 km³ of debris. The aerosol particles stayed in the stratosphere for two years, blocking sunlight and causing abrupt cooling on Earth. This cooling effect also dampened the global hydrological cycle, causing anomalously dry conditions across the monsoon regions. Shortly before that, the undocumented 1809 eruption ranked as the third largest eruption of its kind. The two eruptions together made the 1800s-1810s the coldest decades during the past 500 years.

Processes in the Earth system related to volcanic eruptions (Source: Brönnimann et al., 2016).

How cold did the climate get?

Model simulations revealed that the average global land surface temperature in June decreased by -1.9 ºC in 1816, the so-called “year without a summer”, in Europe and North America. In historical documents, severe cold, frost damages and snow storms were reported during the spring and summer of 1815 and 1816 across southern China, where snow is rarely seen in winter.

The tightly linked Earth and human systems

The continuous cold anomalies of the 1810s also affected the biosphere, leading to poor crop harvests and food shortages in Europe, North America and Asia. Famine crisis prevailed worldwide. The year of 1817 in Germany was nicknamed “The Year of the Beggar”. Recent studies found that such a catastrophic event could still be a trigger for a global systemic disruption of the food system. A better understanding on how ecosystem resilience responded to such abrupt global cooling will help us improve resource management in anticipation of future explosive volcanic eruptions.

Perspective shift on using tree rings to assess post-eruption growth resilience

Tree rings contain long-term information on past environmental changes; thus, they are powerful tools to investigate the impacts of volcanic eruptions on forests. However, they were mainly used as proxies for detecting historic volcanic impacts on climate. Tree-ring width variability is also a direct indicator of ecological responses and resilience to such extreme events. As for this study, we aimed to study the ecological impacts of volcanic cooling on tree growth; thus, we gathered global tree-ring width data and analyzed changes in growth resilience by exploring the timing of growth departures from pre-eruption growth levels.

Tree ring samples of Picea balfouriana from Qamdo, southeastern Tibetan Plateau (These samples are provided by Dr. Haifeng Zhu).

How strong were the impacts of the post-eruption cooling on forest growth?

Through the study of tree rings, we found that the volcanic eruptions exerted strong negative influences on tree growth in boreal and mountain forests, and relatively mild influences on temperate forests. Both reginal mean growth levels and stability dropped significantly in the Russian Arctic and in western and central Europe for deciduous larch forests during 5 to 10 years after the 1815 eruption. The combination of reduced growth levels and stability suggests a loss of growth resilience during the 1810s lasting cold spell. In boreal and mountain forests, the probability of growth extremes even increased up to more than tenfold during the post-eruption period (i.e., 1809 to 1824) relative to the pre-eruption 50-year period.

The longest continuous influence time appeared in the Mongolian and Tibetan Plateaus, and the influence time extended longer in mid-latitude than in high-latitude regions. The delayed return time of tree growth to pre-eruption levels may due to the decline in precipitation linked to the volcanic cooling in mid-latitude regions. For example, significant reductions in spring and summer precipitation occur after large tropical volcanic eruptions and may last several years in the southern Tibetan Plateau, thus reducing tree growth.

The revelation: assessing ecosystem resilience to be better prepared for managing extreme climate events

The Tambora eruption has been perceived as a worst-case scenario for climate variability. Although the current and projected trajectory of climate change is toward climate warming, external forcing driven by explosive volcanic eruptions may cause sudden drops in global temperature. Probabilistic eruption forecasting suggested that Tambora-size volcanic eruptions may occur every 200 to 400 years. These forecasted eruptions have also been predicted to lead to similar or even stronger global cooling effects as those observed in the early 19^th century and may also amplify El Niño-like effects on some monsoon regions under climate change. Decreases in forest productivity and growth stability indicate an overall decline of ecosystem services to human society, which leads to an increased risk of social unrest. Therefore, assessing ecosystem resilience as done in this study can help mankind to better prepare for and cope with adverse impacts on natural resources of global cold spells by improving ecosystem management, especially in cold regions located at high latitudes and high elevations.