Present climate and biological change - Climate Change: Biological and Human Aspects - page 199

glacial expansions in Europe and Canada. In terms of high-latitude northern hemi-
sphere climate this quarter represented the harshest period of the Little Ice Age. The
Little Ice Age clearly came to an end in the early 20th century.
Although we do not know the exact nature of the Little Ice Age (whether it was
truly global and not just restricted mainly to the northern hemisphere) it may still have
been a period of the harshest climate on Earth since the beginning of the Holocene.
Certainly, European glaciers were at their most advanced during the Little Ice Age
compared with their decline early on in the Holocene. If the Earth were slowly
sinking back into a glacial (and it would be slow, given that Milankovitch forcing
will tend to promote an interglacial over the next 10 000 years; see section 5.1.4),
then the Little Ice Age might have been representative of a cooling episode as
part of a longer-term trend. If this is really the case then recent greenhouse cli-
matic forcing due to human emissions of greenhouse gases has had a tremendous
impact, for recent greenhouse warming would not only have to be compared against,
say, an early 20th-century standard, but also against a theoretical, cooler early 20th
century had the Earth's slow slide into a future glacial continued unaffected. This
perspective was supported in 2009 by work of Darrell Kaufman, David Schneider
and colleagues, who compiled a synthesis of decadally resolved proxy temperat-
ure records from poleward of 60 ◦ N covering the past 2000 years. This indicates a
pervasive Arctic cooling in progress from 2000 years ago that continued through
the Middle Ages and into the Little Ice Age. They then used those data coupled
with a research community climate model to discern what may have been happen-
ing. Their inference was that this long-term trend was caused by the steady Mil-
ankovitch orbitally driven reduction in summer insolation. The cooling trend was
reversed during the 20th century, with four of the five warmest decades of our 2000-
year-long reconstruction occurring between 1950 and 2000. The implication of this
research is that the generation of greenhouse gases by humans caused this Arctic
warming.
Looking at the Little Ice Age from a greenhouse perspective raises a question.
Carbon dioxide levels during that time showed no particular deviation from the norm,
not nearly sufficiently to account for the cooling that was seen. So, what caused
it? There are arguments that other factors played a part, such as the solar Maunder
Minimum (see Chapter 4) and the number of significant volcanic eruptions. What is
known (even if the exact extent beyond somewhere between half-a-degree to a degree
is still unclear) is that climate forcing from anthropogenic greenhouse emissions has
increased markedly since the end of the 19th century.
5.1.2 20th-centuryclimate
Greenhouse gas emissions from the burning of fossil fuels and land-use change
(mainly deforestation) increased dramatically in the 20th century. More carbon diox-
ide was emitted in the second half of the 20th century compared to the 100 years
leading up to 1950. Further, emissions from burning of fossil fuels were four times
higher by the end of the century (an increase of 300%) compared to 1950 (Cowie,
1998; we will return to the pattern of emissions later). As a result, the atmospheric
concentration of carbon dioxide rose from its pre-industrial (mid-18th-century) level