Present climate and biological change - Climate Change: Biological and Human Aspects - page 198

records from other European cities. The summers were particularly consistent. What
signals the emergence from the Little Ice Age is the less-harsh winters. So, just as the
20th-century dust-bowl years in the midwestern USA are defined climatologically by
their hot dry summers, so the Little Ice Age is defined by harsher winters and shorter
summers that may have been a little wetter and only marginally cooler than those
in the 20th century. This resulted in a shorter thermal growing season with atypical
conditions at its beginning (sowing time) and end (harvest time). It is estimated that
in England during the coldest decades of the Little Ice Age the thermal growing
season was shortened by 3-4 weeks. As we shall see later in the chapter this is the
same order of magnitude of biological effect we are seeing early in the 21st century
compared with the mid-20th century due to global warming. The contrast is that the
effect is obviously the other way around, with an extension of the growing season (see
Chapter 6). As we shall also see in the next section, this shortening of the thermal
growing season in the Little Ice Age resulted in considerable human suffering.
Added to this change, in the late 18th and early 19th centuries there were also a few
volcanic eruptions that seem to have had a global effect, with the years following the
eruptions being even colder than the other years of the Little Ice Age. These included
Laki (1783), Tambora (1815), Cosiguina (1835), Cotopaxi-Awu (1856) and Krakatau
(1883; see Figure 2.1). A team of chemists from the USA and France, led by Jihong
Cole-Dai, in 2009 found compelling evidence of a previously little documented large
volcanic eruption that occurred in 1809. The following years were very cold; the
entire decade from 1810 to 1819 was one of the coldest of the past 500 years. Now,
we have long been aware that the eruption in 1815 of an Indonesian volcano Tambora,
which killed more than 88 000 people, had caused the global cold weather in 1816 and
subsequent years. Indeed, at a symposium in 2001, Marc Prohom, with co-authors
Pere Esteban, and Javier Martin-Vide from the University of Barcelona, presented the
case for a strong volcanic influence on the climate around this time. They noted that
cooling in the early part of the decade, before the Mount Tambora eruption, suggested
that Tambora alone could not have caused the climatic changes of the decade and
even cited an unknown eruption in 1809. Cole-Dai et al. in 2009 found a large amount
of volcanic sulphuric acid in the snow layers of 1809 and 1810 in both Greenland
and Antarctica, indicating that there had been a major volcanic incident at that time,
confirming Marc Phom's reference to an 'unknown' volcanic 1809 eruption.
Interestingly, in eastern Asia the 17th century was also a cold period, as it was
in Europe, especially around 1800. However, 19th-century eastern Asia did not see
years that were as cold as those in Western Europe or North America. The Little
Ice Age should therefore not be seen as a period of consistently uniform cold across
the planet. There were, though, a few short cold episodes, either across the northern
hemisphere or globally (which brings us to the ocean circulation changes discussed
in the previous chapter). These were the decades 1590s to 1610s, 1690s to 1710s and
1800s to 1810s. There were also a few slightly warm periods, including the 1730s and
1820s, before reaching the 20th-century warming, which commenced in the 1930s
(Burroughs, 1997).
The third quarter of the 19th century saw a brief easing of conditions, as signalled
by glacier retreat in Europe. This did not last and the century's final quarter saw