Last phase of the Little Ice Age forced by volcanic eruptions

Abstract

16 17 2 During the first half of the 19th century, several large tropical volcanic eruptions occurred 18 within less than three decades. Global climate effects of the 1815 Tambora eruption have 19 been investigated, but those of an eruption in 1808 whose source is unknown and the 20 eruptions in the 1820s and 1830s have received less attention. Here, we analyse the effect 21 of the sequence of eruptions in observations, global three-dimensional climate field 22 reconstructions, and coupled climate model simulations. All eruptions were followed by 23 substantial drops of summer temperature over the Northern Hemisphere land areas. In 24 addition to the direct radiative effect, which lasts 2-3 years, the simulated ocean-25 atmosphere heat exchange sustained cooling for several years following these eruptions, 26 affecting the slow components of the climate system. Africa was hit by two decades of 27 drought, global monsoons weakened, and the tracks of low-pressure systems over the 28 North Atlantic moved south. The low temperatures and increased precipitation in Europe 29 triggered the last phase of advance of Alpine glaciers. Only after the 1850s the transition 30 into the period of anthropogenic warming started. We conclude that the end of the Little 31 Ice Age was marked by the recovery from a sequence of volcanic eruptions, which makes it 32 difficult to define a single pre-industrial baseline. 33The period between around 1350 or 1450 and 1850 is often termed the "Little Ice Age" (LIA). 34In several regions the LIA was accompanied by glacier advances 1,2 . It might have been 35 initiated by volcanic eruptions 3 , but the relative contributions of solar and volcanic forcing 36 remain unclear. Given the regional differences 4 and uncertainties in the mechanism involved, 37 the onset of the LIA is still highly debated 5 . 38Importantly, the transition from the LIA into the period of anthropogenic warming is also not 39 well understood. After a rather warm phase around 1800, global climate cooled again in the 40 3 early 19th century 6 for several decades, accompanied by pronounced glacier advances in the 41 Alps. Recent work therefore dated the start of anthropogenic warming back to the early 19 th 42 century 7 . However, the fact that several major tropical volcanoes erupted between 1808 and 43 1835 (note that there is still large uncertainty -the 1808/09 eruption remains unknown 8 and 44 the attribution of the 1831 eruption has recently been questioned 9 ), including the well-studied 45 1815 Tambora eruption, makes the separation between volcanic and anthropogenic 46 contributions difficult. Based on attribution results, a small drop in greenhouse gas levels 47 109 ( Fig. 3a). According to both data sets the region remained dry during most of the first half of 110 the 19 th century. 111 We further analysed other monsoon regions and compared the palaeo-reanalysis with 112 independent observations 25,26 . We find a weakening of all India monsoon rainfall (Fig. 3b) 113 and of the strength of the Australian mons...

Causes of climate change over the historical record

“…Some of the coldest observed periods followed in the two years after the powerful eruption of Mount Tambora in 1815 (Raible et al 2016). After that, temperatures began to show a slow rise, interrupted by cooling induced by volcanic eruptions in the 1830s (Brönnimann et al 2019a) and then the Krakatoa eruption in 1883 (see figure 1). Global temperatures rose particularly rapidly over the early 20th century, showing anomalous warming from the 1920s through the 1940s (see figure 1; and Hegerl et al 2018), before plateauing in the 1950s and 60s, and beginning their strong ongoing increase.…”

“…It has been argued that volcanic eruptions only cause shortterm cooling. However, climate model simulations show an extended cold period following the 1809/ Tambora period, with no single year in model simulations with HadCM3, for example, reaching the average of the 20 years prior to the eruptions up to the 1830s (Schurer et al 2014), when another period of volcanism kept temperature low until the 1840s (Brönnimann et al 2019a). Equally, climate models simulate long-term warming in periods with little volcanic forcing, such as the early 20th century (Hegerl et al 2018).…”

“…However, volcanism is important over much of the 19th century as well: figure 1(d) illustrates that in models, the warming period up to the eruption of Krakatoa was in large parts a relaxation from a period of heavy volcanism around the Mount Tambora eruption (Brönnimann et al 2019a). From the 50 year trend centred around 1860 onwards (ending around 1885, figure 1(d)), climate models indicate that the warming trend originating from the recovery after heavy volcanism is exceeded by the warming trend caused by greenhouse gas increases.…”

author response to reviewers comments

“…. ., including reduced solar activity during the late 17th century (Shindell et al 2001), increased volcanic activity during the early 19th century (Brönnimann et al 2019) and reduced Gulf Stream flow (Lund et al 2006)." P20, Lines 430-433…”

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“…For instance, during the LIA, temperatures of areas like eastern China or the Northern Andes did not experience a significant reduction. Nonetheless, past climate changes have been different compared to the recent one, as the climate changes have not been recorded worldwide unlike the current (Neukom et al, 2019a,b;Brönnimann et al, 2019).…”