Figure 8: Records of changes in atmospheric composition. (a)
Atmospheric concentrations of CO2, CH4 and N2O
over the past 1,000 years. 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 radiative forcing from these gases
is indicated on the right-hand scale. (b) Sulphate concentration
in several Greenland ice cores with the episodic effects of volcanic eruptions
removed (lines) and total SO2 emissions from sources in the US
and Europe (crosses). [Based on (a) Figure
3.2b (CO2), Figure 4.1a and b
(CH4) and Figure 4.2 (N2O)
and (b) Figure 5.4a] |
C. The Forcing Agents That Cause Climate Change
In addition to the past variations and changes in the Earth’s climate, observations
have also documented the changes that have occurred in agents that can cause climate
change. Most notable among these are increases in the atmospheric concentrations
of greenhouse gases and aerosols (microscopic airborne particles or droplets)
and variations in solar activity, both of which can alter the Earth’s radiation
budget and hence climate. These observational records of climate-forcing agents
are part of the input needed to understand the past climate changes noted in the
preceding Section and, very importantly, to predict what climate changes could
lie ahead (see Section F).
Like the record of past climate changes, the data sets for forcing agents are
of varying length and quality. Direct measurements of solar irradiance exist
for only about two decades. The sustained direct monitoring of the atmospheric
concentrations of carbon dioxide (CO2) began about the middle of
the 20th century and, in later years, for other long-lived, well-mixed gases
such as methane. Palaeo-atmospheric data from ice cores reveal the concentration
changes occurring in earlier millennia for some greenhouse gases. In contrast,
the time-series measurements for the forcing agents that have relatively short
residence times in the atmosphere (e.g., aerosols) are more recent and are far
less complete, because they are harder to measure and are spatially heterogeneous.
Current data sets show the human influence on atmospheric concentrations of
both the long-lived greenhouse gases and short-lived forcing agents during the
last part of the past millennium. Figure 8 illustrates
the effects of the large growth over the Industrial Era in the anthropogenic
emissions of greenhouse gases and sulphur dioxide, the latter being a precursor
of aerosols.
A change in the energy available to the global Earth-atmosphere system due
to changes in these forcing agents is termed radiative forcing (Wm-2)
of the climate system (see Box 1). Defined in this
manner, radiative forcing of climate change constitutes an index of the relative
global mean impacts on the surface-troposphere system due to different natural
and anthropogenic causes. This Section updates the knowledge of the radiative
forcing of climate change that has occurred from pre-industrial times to the
present. Figure 9 shows the estimated radiative forcings
from the beginning of the Industrial Era (1750) to 1999 for the quantifiable
natural and anthropogenic forcing agents. Although not included in the figure
due to their episodic nature, volcanic eruptions are the source of another important
natural forcing. Summaries of the information about each forcing agent follow
in the sub-sections below.
The forcing agents included in Figure 9 vary greatly
in their form, magnitude and spatial distribution. Some of the greenhouse gases
are emitted directly into the atmosphere; some are chemical products from other
emissions. Some greenhouse gases have long atmospheric residence times and,
as a result, are well-mixed throughout the atmosphere. Others are short-lived
and have heterogeneous regional concentrations. Most of the gases originate
from both natural and anthropogenic sources. Lastly, as shown in Figure 9, the
radiative forcings of individual agents can be positive (i.e., a tendency to
warm the Earth’s surface) or negative (i.e., a tendency to cool the Earth’s
surface).
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Figure 9: Global, annual-mean radiative forcings (Wm-2)
due to a number of agents for the period from pre-industrial (1750) to
present (late 1990s; about 2000) (numerical values are also listed in
Table 6.11 of Chapter 6).
For detailed explanations, see Chapter 6.13. The
height of the rectangular bar denotes a central or best estimate value,
while its absence denotes no best estimate is possible. The vertical line
about the rectangular bar with “x” delimiters indicates an estimate of
the uncertainty range, for the most part guided by the spread in the published
values of the forcing. A vertical line without a rectangular bar and with
“o” delimiters denotes a forcing for which no central estimate can be
given owing to large uncertainties. The uncertainty range specified here
has no statistical basis and therefore differs from the use of the term
elsewhere in this document. A “level of scientific understanding” index
is accorded to each forcing, with high, medium, low and very low levels,
respectively. This represents the subjective judgement about the reliability
of the forcing estimate, involving factors such as the assumptions necessary
to evaluate the forcing, the degree of knowledge of the physical/chemical
mechanisms determining the forcing, and the uncertainties surrounding
the quantitative estimate of the forcing (see Table
6.12). The well-mixed greenhouse gases are grouped together into a
single rectangular bar with the individual mean contributions due to CO2,
CH4, N2O and halocarbons shown (see Tables
6.1 and 6.11). Fossil fuel burning is
separated into the “black carbon” and “organic carbon” components with
its separate best estimate and range. The sign of the effects due to mineral
dust is itself an uncertainty. The indirect forcing due to tropospheric
aerosols is poorly understood. The same is true for the forcing due to
aviation via its effects on contrails and cirrus clouds. Only the “first”
type of indirect effect due to aerosols as applicable in the context of
liquid clouds is considered here. The “second” type of effect is conceptually
important, but there exists very little confidence in the simulated quantitative
estimates. The forcing associated with stratospheric aerosols from volcanic
eruptions is highly variable over the period and is not considered for
this plot (however, see Figure 6.8). All the
forcings shown have distinct spatial and seasonal features (Figure
6.7) such that the global, annual means appearing on this plot do
not yield a complete picture of the radiative perturbation. They are only
intended to give, in a relative sense, a first-order perspective on a
global, annual mean scale and cannot be readily employed to obtain the
climate response to the total natural and/or anthropogenic forcings. As
in the SAR, it is emphasised that the positive and negative global mean
forcings cannot be added up and viewed a priori as providing offsets in
terms of the complete global climate impact. [Based on Figure
6.6]
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