Present climate and biological change - Climate Change: Biological and Human Aspects - page 200

of 279 ppmv to 369.4 ppmv in the year 2000 (Worldwatch Institute, 2003) and 389.8
ppmv in 2010. 1
Apart from some wavering (which is mainly the result of natural variability), the
20th century was notably a century of warming. This was to such an extent that the
1990s saw a concentration of record-breaking hot years.

5.1.3 21st-centuryclimate

Such has been the anthropogenic injection of greenhouse gases into the atmosphere
since the Industrial Revolution, and especially during the 20th century, that there
is little doubt that global warming will continue throughout the 21st century. The
international scientific consensus on 21st-century warming has been determined by
the UN Intergovernmental Panel on Climate Change (IPCC). This will be covered in
the section on the IPCC Business-as-Usual scenario (section 5.3.1).

5.1.4 TheHoloceneinterglacialbeyondthe21stcentury

In looking at the likely future climate of the current interglacial (the Holocene) and
beyond, we need to consider how our perceptions have changed over the past few
decades. Since the 1970s, when ice-core analysis revealed the broad nature of the
glacial-interglacial cycle, there has been a general assumption that the length that
glacials last is the order of 100 000 years; conversely, interglacials are an order of
magnitude smaller and are about 10 000 years long. Furthermore, as noted in Chap-
ter 4, ice cores have shown that in the past the transition between glacial and inter-
glacial states has been sudden and not gradual. In the 1970s there was a concern that
as the Holocene was already more than 10 000 years old the Earth might be about
to enter a glacial period (see section 5.1.1). As noted in Chapter 1, global warming
and human-driven greenhouse concerns had long abounded and so the question for
the late 1970s and early 1980s focused on the balance between these two warming
and cooling forcing agents. The IPCC's first report in 1990 (see below) provided
an international scientific consensus, concluding clearly that anthropogenic global
warming factors superimposed on the natural climate forcing agents would dominate
through the 21st century and, by implication, decades beyond, if not more.
So, in light of recent record-breaking years, this leads us to the question of whether
these years are part of a longer-term change in the global climate system involving
climate and biosphere parameters other than temperature. It is a question to which we
shall return later when looking at the IPCC's conclusions (section 5.3) and climate
change's impact on biological systems (section 6.1). However, in the summer of 2007
the first signs of human influence on global precipitation patterns were announced by
a small group of researchers led by Xuebin Zhang and Francis Zwiers. They looked at
meteorological records for precipitation by latitude band throughout the 20th century
and then compared them with 14 climate models. They estimated that anthropogenic
forcing of the climate by the end of the century had contributed significantly to an
increase in precipitation in northern hemisphere mid-latitudes, drying in the northern

1

See the National Ocean and Atmosphere Administration website: www.esrl.noaa.gov/gmd/ccgg/trends/.