2.3.2.2: Changes in radiative forcing
over the Industrial Era
As discussed in Section 2.3, changes in atmospheric
concentrations of GHGs produce a radiative forcing. The current understanding
of the radiative forcing effects of all important climate forcing agents over
the Industrial Era is synthesized in Figure 2.8.10 The following discussion
highlights major features of Figure 2.8, beginning with those agents causing
warming effects, followed by those agents causing cooling effects, and
concluding with a summary about the net forcing effects from human activity.
The main
warming agents, as indicated by bars extending to the right in Figure 2.8, are
CO2, CH4, N2O, and tropospheric ozone, with a few other gases contributing
small warming effects globally. These other gases include halocarbons —
synthetic industrial chemicals composed of carbon and a halogen, such as
chlorofluorocarbons. Together, GHGs have been by far the dominant positive
forcing agents over the Industrial Era. CO2 alone accounts for two-thirds of
the forcing (1.82 W/m2 [90% uncertainty range from 1.63 W/m2 to
2.01 W/ m2]) from all well-mixed GHGs (2.83 W/m2 [90% uncertainty
range from 2.54 W/m2 to 3.12 W/m2]). Increases in CH4 concentrations
have been the second largest contributor to positive forcing (0.48 W/m2
[90% uncertainty range from 0.43 W/m2 to 0.53 W/m2]). There is very
high confidence in these values, because the radiative properties of well-mixed
GHGs are well known and because historical concentrations of well-mixed GHGs
are also well known from ice cores and direct measurements.
Ozone is not
directly emitted but is formed in the lower atmosphere (troposphere) as a
result of both natural processes and emissions of air pollutant gases,
including CH4. The warming effect of increases in tropospheric ozone is
sizeable and known with high confidence. Ozone also forms naturally in the
upper atmosphere (stratosphere) as a result of chemical reactions involving
ultraviolet radiation and oxygen molecules. Stratospheric ozone levels have
decreased as a result of human emissions of ozone-depleting substances such as
refrigerants. The resulting cooling effect has slightly offset the warming effect
of increases in tropospheric ozone (Myhre et al., 2013).
Cooling
effects (as indicated by bars to the left in Figure 2.8) have been driven by
human emissions that have increased the levels of aerosols in the atmosphere
and by human changes to the land surface that have increased Earth’s surface
albedo. Aerosol forcing is divided into two components: direct effects, mainly
from absorbing or scattering incoming solar radiation, and indirect effects
from aerosol interactions with clouds. Most aerosols (e.g., sulphate and
nitrate aerosols) predominantly scatter (reflect) radiation. In contrast, black
carbon, an aerosol that is emitted as a result of the incomplete combustion of
carbon-based fuels, absorbs radiation. Black carbon is a strong warming agent,
although calculating the net effect of black carbon emission sources needs to
consider the warming and cooling effects of the other aerosols and gases
emitted with it during combustion (Bond et al., 2013; see Chapter 3, Box 3.3).
The direct effect of aerosols is therefore composed of a negative forcing
(cooling) from most aerosols and a positive forcing (warming) from black
carbon, for a net negative forcing of 0.45 W/m2 (90% uncertainty range
from a negative forcing of 0.95 W/m2 to a positive forcing of 0.05 W/m2)11
(medium confidence). The total aerosol effect in the atmosphere, including
aerosol–cloud interactions, is a strongly negative forcing, estimated with
medium confidence to be 0.9 W/ m2 (90% uncertainty range from
1.9 W/m2 to 0.1 W/m2). Although there continue to be large
uncertainties associated with the magnitude of aerosol forcing, overall there
is high confidence that the cooling effect of aerosol forcing has offset a
substantial portion of the warming effect of GHG forcing.
There is
also high confidence that human-caused land use changes (such as deforestation
and conversion of other natural landscapes to managed lands) have had a cooling
effect by increasing Earth’s albedo, with a negative forcing of 0.15 W/m2
(90% uncertainty range from 0.25 W/m2 to 0.05 W/m2). However, this
has been partially offset by decreases in Earth’s albedo due to black carbon
being deposited on snow and ice, darkening the surface and thereby increasing
the absorption of solar radiation. Black carbon deposition on snow is estimated
to have exerted a small warming effect of 0.04 W/m2 (90% uncertainty range
from 0.02 W/m2 to 0.09 W/m2) (low confidence) (Myhre et al., 2013).
The best
estimate of total radiative forcing due to human activities is a warming effect
of 2.3 W/m2 (90% uncertainty range from 1.1 W/m2 to 3.3 W/m2)
over the Industrial Era, composed of a strong positive forcing component from
changes in atmospheric concentrations of GHGs, which is partially offset by a
negative forcing (cooling effect) from aerosols and land use change. Forcing by
CO2 is the single largest contributor to human-caused forcing during the
Industrial Era.
This total
forcing from human activities can be compared with natural forcing from changes
in volcanic eruptions and solar irradiance. During the Industrial Era,
irregular volcanic eruptions have had brief cooling effects on global climate.
The episodic nature of volcanic eruptions makes a comparison with other forcing
agents difficult on a century timescale. Volcanic forcing is, however, well
understood to be negative (climate-cooling effect) with the strongest forcing
occurring over a limited period of about two years following eruptions (Myhre
et al., 2013; see Section 2.3.3). Changes in total solar irradiance over the
Industrial Era have caused a small positive forcing of 0.05 W/m2 (90%
uncertainty range from 0.00 to 0.10 W/m2) (medium confidence). Consequently,
there is very high confidence that, over the Industrial Era, natural forcing
accounts for only a small fraction of forcing changes, amounting to less than
10% of the effects of human-caused forcing.
2.3.3: Natural climate variability
Even when a strong
anthropogenic forcing drives climate change (see Section 2.3.2), signals of
climate change may be difficult to detect against a backdrop of a climate
system that is naturally chaotic – “noisy”. This chaotic behaviour is due to
internal climate variability and natural external forcings, which can be large
over short periods (for example, forcing by volcanic eruptions). Internal
climate variability, such as El Niño– Southern Oscillation (ENSO) (see Box
2.5), is variability that arises within the climate system, independent of
variations in external forcing.
Global mean
surface temperature (GMST), as calculated by a linear trend, has increased
significantly since the 1880s, especially since the 1950s (see Section 2.2.1).
However, the changes in GMST have been far from uniform, with substantial
variability among years, decades, and periods spanning several decades. These
short-term fluctuations are superimposed on an underlying externally forced
trend (see Figure 2.9) (Morice et al., 2012; Karl et al., 2015; Hansen et al.,
2010).
To analyze
the causes of the short-term fluctuations in GMST, we first need to be
confident that the observed variability is real and not an artifact, an error
introduced by the way the data were collected or analyzed. Longterm GMST time
series have been produced by a small number of scientific teams using data
collected from around the world. Values are reported as an anomaly: a departure
from the average over a reference period (1961–1990 for Figure 2.9).
Differences among the estimates are due mainly to differing choices made in
processing the underlying raw observations. For example, one estimate
(HadCRUT4.4) is an average for only those grid cells where observations exist,
whereas the other estimates (NOAA-Karl and GISTEMP) use infilling; if
observations are missing for certain locations, they are estimated based on
values for neighbouring locations. These estimates of GMST, and others, are
routinely updated as errors are identified and adjusted (see Box 4.1).
Correcting and updating long-term datasets with new observations as these
become available is imperative for tracking global change from year-to-year,
decade-to-decade, and century-to-century.
Some of the
ups and downs over time shown in Figure 2.9 are associated with the ENSO, the
fairly periodic internal variation in sea surface temperatures over the
tropical eastern Pacific Ocean, affecting much of the tropics, subtropics, and
some areas outside the tropics, including Canada (Box 2.6). The warming phase
is known as El Niño and the cooling phase as La Niña. ENSO events can be
powerful enough to be recorded as significant signals in GMST. The 1997/1998 El
Niño was regarded as one of the most powerful El Niño events in recorded
history, resulting in widespread droughts, flooding, and other natural
disasters across the globe (Trenberth, 2002). It terminated abruptly in
mid-1998 and was followed by a moderate-to-strong La Niña, which lasted until
the end of 2000 (Shabbar and Yu, 2009).
Some other
ups and downs shown in Figure 2.9 are associated with natural external forcing
agents, such as large volcanic eruptions. The 1991 eruption of Mount Pinatubo,
in the Philippines, was the second largest terrestrial eruption of the 20th
century. It ejected a massive amount of particulate matter into the
stratosphere and produced a global layer of sulphuric acid haze. GMST dropped
significantly in 1991–1993 (McCormick et al., 1995). Similarly, the 1982
eruption of El Chichón, the largest volcanic eruption in modern Mexican
history, ejected a large amount of sulphate aerosols into the stratosphere
(Robock and Matson, 1983). The cooling impact of the of El Chichón eruption on
GMST from 1982 to 1984 was partly offset by global warming associated with a
very strong El Niño event during this time (Robock, 2013).
Naturally
occurring variations in GMST, whether internally generated or externally
forced, should be viewed in the context of global mean radiative forcing caused
by human activities (Fyfe et al., 2016). The combined radiative forcing from human
activities has increased over time (see Figure 2.9) (Meinshausen et al., 2011).
The periods in Figure 2.9 labelled “big hiatus” and “warming slowdown”
correspond to times when the dominant mode of internal decadal variability in
the Pacific — the Interdecadal Pacific Oscillation (IPO) — was in its negative
(cold) phase. In addition, during the “big hiatus” period, radiative forcing
increased relatively slowly, owing to cooling contributions from increasing
tropospheric aerosols, as well as stratospheric aerosols from the Mount Agung
eruption in 1963 (e.g., Fyfe et al. 2016). In the intervening period, the IPO
was in its positive (warm) phase. A given phase, warm or cold, of the IPO
typically lasts from 20 to 30 years, much longer than the timescale associated
with ENSO. Recent computer models (Meehl et al., 2013; Kosaka and Xie, 2013;
England et al., 2014) and studies based on observations (Steinman et al., 2015;
Dai et al., 2015) indicate that the IPO plays an important role in changes in
GMST over time.
Finally, the
“warming slowdown” — a slowdown in the rate of increase of GMST observed over
the early 2000s — has been much debated (Karl et al., 2015; Lewandowsky et al.,
2015; Rajaratnam et al., 2015). Observations indicate that the rate of global
mean surface warming from 2001 to 2015 was significantly less than the rate
over the previous 30 years (Fyfe et al., 2016). It is now understood that both
internal variability and external forcing contributed to the warming slowdown
(Flato et al., 2013; Fyfe et al., 2016; Santer et al., 2017). The contribution
from external forcing has been ascribed to: 1) a succession of moderate
volcanic eruptions in the early 21st-century (Solomon et al., 2011; Vernier,
2011; Fyfe et al., 2013; Santer et al., 2014; Ridley et al., 2014; Santer et
al., 2015); 2) a long and anomalously low solar minimum during the last solar
cycle (Kopp and Lean, 2011; Schmidt et al., 2014); 3) increased atmospheric
burdens of sulphate aerosols from human activity (Smith et al., 2016); and 4) a
decrease in stratospheric water vapour (Solomon et al., 2010). In the last
several years, GMST has warmed substantially (e.g., Hu and Fedorov, 2017), with
an exceptionally strong ENSO in 2015/2016, suggesting that the warming slowdown
is now over.
No comments:
Post a Comment