An introduction to climate change - Climate Change: Biological and Human Aspects - page 6

Table1.1 Summaryoftheprincipalgreenhousegases(withtheexceptionoftroposphericozone,
O 3 ,duetoalackofaccuratedata).Atmosphericlifetimeiscalculatedascontent/removalrate

Greenhouse gas . . . CO 2 CH 4 CFC-11 CFC-12 N 2 O

Atmospheric concentration
Late 18th century

280 ppm 0.7 ppm 0 0 288 ppb

2010 388 ppm 1.809 ppm 240 ppt 533 ppt 323 ppb

Atmospheric lifetime (years) 50-200 12 45 100 114

ppb, parts per billion; ppm, parts per million; ppt, parts per trillion (all by volume).

topic's first edition was written) it had topped 380 ppm and by 2011 (when drafting
the second edition) it had reached more than 392 ppm and was still climbing.
If this rise is because of the addition of carbon dioxide to the atmosphere (and it
is) then it would be useful to get an idea as to how much carbon is needed to raise
the concentration by 1 ppm. Well, the Earth's atmosphere weighs around 5.137 10 18

kg, which means that a rise of 1 ppm of CO 2 equates to 2.13 Gt of carbon (or 7.81
Gt of CO 2 as carbon dioxide is heavier than carbon). 1
Over the time since the Industrial Revolution the Earth has also warmed. The
warming has not been as regular as the growth in greenhouse gas but, from both
biological and abiotic proxies (which I will discuss in Chapter 2) as well as some
direct measurements, we can deduce that it has taken place. Furthermore, we now
know that Tyndall was right. With less greenhouse gas in the atmosphere the Earth
cools and there are ice ages (glacials); as we shall see in Chapter 3 we have found that
during the last glacial period, when the Earth was cooler, there was less atmospheric
carbon dioxide.
Nonetheless, there has been much public debate as to whether the current rise in
atmospheric carbon dioxide has caused the Earth to warm. An alternative view is
that the warming has been too erratic and is due to random climate variation. To
resolve this issue the United Nations (UN), through the UN Environment Programme
(UNEP) and World Meteorological Organization (WMO), established the IPCC. Its
four main reports, or assessments (IPCC, 1990, 1995, 2001a, 2007), have concluded
that the emissions of greenhouse gases and aerosols due to human activities continue
to alter the atmosphere in ways that are expected to affect the climate.
The current rise in atmospheric greenhouse gases (over the past three centuries to
date) is well documented and is summarised in Table 1.1.
As we shall see, each of the above greenhouse gases contributes a different pro-
portion to the human-induced (anthropogenic) warming, but of these the single most
important gas, in a current anthropogenic sense, is carbon dioxide.

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1

You may have noticed that some estimates of CO 2 emissions seem to be about three-and-a-half times as
large as others. This is because numbers are sometimes expressed as the mass of CO 2 but are in this topic
mainly expressed in terms of the mass of the carbon (C). Because carbon cycles through the atmosphere,
oceans, plants, fuels, etc., and changes the ways in which it is combined with other elements, it is often
easier to keep track only of the flows of carbon. Emissions expressed in units of carbon can be easily
converted to emissions in CO 2 units by adjusting for the mass of the attached oxygen atoms; that is,
by multiplying by the ratios of the molecular weights of carbon dioxide and carbon respectively, 44/12,
or 3.67.