The Oligocene to the Quaternary: climate and biology - Climate Change: Biological and Human Aspects - page 171

of global climate proxies. So if the Little Ice Age was a real global climate phe-
nomenon, and carbon dioxide change was not involved, what other factors might have
caused it?
Of the non-greenhouse contenders that could have generated the Little Ice Age
there are two more likely: volcanic eruptions (see Chapter 2 and Figure 2.1) and
variations in solar output, hence insolation on the Earth. Indeed, there were a number
of major volcanic eruptions during the Little Ice Age, including Billy Mitchell in the
south-west Pacific (approximately 1580), Huaynaputina in Peru (1600), Parker in the
Philippines (1641) and Long Island in New Guinea (approximately 1660). Then in
the second part of the Little Ice Age there was Laki in Iceland (1783), Tambora in
the Lesser Sunda Islands (1815) and Krakatau west of Java (1883). These can only
have served to cool, or further cool, the planet away from the late Holocene average.
Of these, Huaynaputina was one of the worst and the acid spike in the Greenland ice
cap is higher for 1600 than for 1883 (Krakatau). After Huaynaputina the following
few years saw very low temperatures. In Europe the summer of 1601 was cold with
freezing weather in northern Italy extending into July and overcast skies for much
of the year. In parts of England there were frosts every morning throughout June.
Less violent and more recent was the eruption of Laki in Iceland in 1783. It is
estimated that 80 Mt of sulphuric acid aerosol was released by that eruption. To put
it into a modern context, this is four times more than El Chichon (Mexico, 1982) and
80 times more than Mount St. Helens (USA, 1980). In terms of a vulcanologists'
perception of climate interactions, the Laki eruption illustrates that low-energy, large-
volume (14 km 3 basaltic lava and over 100 million t of sulphur dioxide for Laki) and
long-duration (8 months) basaltic eruptions can have climatic impacts greater than
quick, large-volume, explosive, silica-rich eruptions. The sulphur contents of basaltic
magmas are 10-100 times higher than silica-rich magmas. Immediately following
the Laki eruption the winter in the eastern USA was about 4.8 ◦ C below the 225-
year average, while the very broad estimate across the northern hemisphere was of
a1 ◦ C drop (to be treated with caution). What is clearer is that it took less than
2 years to recover from half the temperature drop and somewhere between 5 and
6 years for a complete recovery to occur. Another reason for viewing the effects of
the Laki eruption with a little caution is that the same year saw the eruption of a smaller
volcano, Asama, in Japan. Even so, whereas high-sulphur volcanic eruptions may cool
the planet for a few years, they by themselves cannot explain all of the Little Ice Age's
climate.
In 2012 Gifford Miller, Aslaug Geirsdottir, Yafang Zhong and colleagues suggested
that a series of possibly four volcanic eruptions over a period of 50 years could explain
the sudden expansion of Canadian and Iceland ice in 1275. They had 14 C-dated the
remains of vegetation that had been buried under an apparently sudden onset of ice in
the Canadian Arctic and, using climate models, the explanation that seemed to make
the most sense was that of sulphur-rich volcanic eruptions. Further, the temporary
cooling effect of this series of eruptions was sustained due to a critical transition
of ocean circulation causing a small (compared to glacial-interglacial transitions)
climate threshold being crossed (see section 6.6.8). However, the locations and
exact dates of these eruptions remain unknown and so have not been included in
Figure 2.1.