Figure 2: Long records of
past changes in atmospheric composition provide the context for the influence
of anthropogenic emissions.
(a) shows changes in the atmospheric concentrations of carbon dioxide (CO2),
methane (CH4), and nitrous oxide (N2O) over the past 1000
years. The ice core and firn data for several sites in Antarctica and Greenland
(shown by different symbols) are supplemented with the data from direct atmospheric
samples over the past few decades (shown by the line for CO2 and incorporated
in the curve representing the global average of CH4). The estimated
positive radiative forcing of the climate system from these gases is indicated
on the right-hand scale. Since these gases have atmospheric lifetimes of a decade
or more, they are well mixed, and their concentrations reflect emissions from
sources throughout the globe. All three records show effects of the large and
increasing growth in anthropogenic emissions during the Industrial Era.
(b) illustrates the influence of industrial emissions on atmospheric sulphate
concentrations, which produce negative radiative forcing. Shown is the time history
of the concentrations of sulphate, not in the atmosphere but in ice cores in Greenland
(shown by lines; from which the episodic effects of volcanic eruptions have been
removed). Such data indicate the local deposition of sulphate aerosols at the
site, reflecting sulphur dioxide (SO2) emissions at mid-latitudes in
the Northern Hemisphere. This record, albeit more regional than that of the globally-mixed
greenhouse gases, demonstrates the large growth in anthropogenic SO2
emissions during the Industrial Era. The pluses denote the relevant regional estimated
SO2 emissions (right-hand scale).
[Based upon (a) Chapter
3, Figure 3.2b (CO2); Chapter
4, Figure 4.1a and b (CH4) and
Chapter 4, Figure 4.2
(N2O) and (b) Chapter 5, Figure
5.4a]
