|
6.2. Radiative Forcing and GWP Concepts
6.2.1. The Concept of Radiative Forcing
The most useful assessment of the impact of the aircraft fleet on climate would
be a comprehensive prediction of changes to the climate system, including temperature,
sea level, frequency of severe weather, and so forth. Such assessment is difficult
to achieve given the current state of climate models and the small global forcing
of climate attributable to the single sector of aviation chosen for this special
report (see discussion in Sections 6.1 and 6.5).
Following IPCC (1995, 1996), we choose a single measure of climate change: radiative
forcing (RF), which is calculated directly from changes in greenhouse gases,
aerosols, and clouds, and which allows ready comparison of the climate impact
of different aviation scenarios.
The Earth's climate system is powered by the sun. Our planet intercepts 340
W m-2 of solar radiation averaged over the surface of
the globe. About 100 W m-2 is reflected to space, and
the remainder-about 240 W m-2-heats the planet. On a
global average, the Earth maintains a radiative balance between this solar heating
and the cooling from terrestrial infrared radiation that escapes to space. When
a particular human activity alters greenhouse gases, particles, or land albedo,
such activity results in radiative imbalance. Such an imbalance cannot be maintained
for long, and the climate system-primarily the temperature and clouds of the
lower atmosphere-adjusts to restore radiative balance. We calculate the global,
annual average of radiative imbalance (W m-2) to the
atmosphere-land-ocean system caused by anthropogenic perturbations and designate
that change radiative forcing. Thus, by this IPCC definition, the RF of the
pre-industrial atmosphere is taken to be zero. (Although the term "radiative
forcing" has more general meaning in terms of climate, we restrict its use here
to the IPCC definition.)
As an example, burning of fossil fuel adds the greenhouse gas CO2
to the atmosphere; this burning is responsible for the increase in atmospheric
CO2 from about 280 ppmv in the pre-industrial atmosphere
to about 360 ppmv in 1995. Added CO2 increases the infrared
opaqueness of the atmosphere, thereby reducing terrestrial cooling with little
impact on solar heating. Thus, the radiative imbalance created by adding a greenhouse
gas is a positive RF. A positive RF leads to warming of the lower atmosphere
in order to increase the terrestrial radiation and restore radiative balance.
Radiative imbalances can also occur naturally, as in the case of the massive
perturbation to stratospheric aerosols caused by Mt. Pinatubo (Hansen et al.,
1996).
Because most of the troposphere is coupled to the surface through convection,
climate models typically predict that the land surface, ocean mixed layer, and
troposphere together respond to positive RF in general with a relatively uniform
increase in temperature. Global mean surface temperature is a first-order measure
of what we consider to be "climate," and its change is roughly proportional
to RF. The increase in mean surface temperature per unit RF is termed climate
sensitivity; it includes feedbacks within the climate system, such as changes
in tropospheric water vapor and clouds in a warmer climate. The RF providing
the best metric of climate change is the radiative imbalance of this land-ocean-troposphere
climate system-that is, the RF integrated at the tropopause.
When radiative perturbation occurs above the tropopause, in the stratosphere
(as for most HSCT impacts), this heating/cooling is not rapidly transported
into the troposphere, and the imbalance leads mostly to changes in local temperatures
that restore the radiative balance within the stratosphere. Such changes in
stratospheric temperature, however, alter the tropospheric cooling; for example,
warmer stratospheric temperatures lead to a warmer troposphere and climate system.
This adjustment of stratospheric temperatures can be an important factor in
calculating RF and is denoted "stratosphere-adjusted."
All RF values used in this report refer to "stratosphere-adjusted, tropopause
RF" (Shine et al., 1995). For primarily tropospheric perturbations (e.g., CO2
from all aviation, O3 from subsonic aircraft), this quantity
can be calculated with reasonable agreement (better than 25%) across models
used in this report (see Section 6.3). For specifically
stratospheric perturbations (e.g., H2O and O3
perturbations from HSCT aircraft), the definition of the tropopause and the
calculation of stratospheric adjustment introduce significant sources of uncertainty
in calculated RF.
The concept of radiative forcing (IPCC, 1990, 1992, 1995) is based on climate
model calculations that show that there is an approximately linear relationship
between global-mean RF at the tropopause and the change in equilibrium global
mean surface (air) temperature (łTs ). In mapping RF to climate change, the
complexities of regional and even hemispheric climate change have been compressed
into a single quantity-global mean surface temperature. It is clear from climate
studies that the climate does not change uniformly: Some regions warm or cool
more than others. Furthermore, mean temperature does not provide information
about aspects of climate change such as floods, droughts, and severe storms
that cause the most damage. In the case of aviation, the radiative imbalance
driven by perturbations to contrails, O3, and stratospheric
H2O occurs predominantly in northern mid-latitudes and
is not globally homogeneously distributed (see Chapters 2,
3, and 4), unlike perturbations
driven by increases in CO2 or decreases in CH4.
Does this large north-south gradient in the radiative imbalance lead to climate
change of a different nature than for well-mixed gases? IPCC (Kattenberg et
al., 1996) considered the issue of whether negative RF from fossil-fuel sulfate
aerosols (concentrated in industrial regions) would partly cancel positive RF
from increases in CO2 (global). Studies generally confirmed
that global mean surface warming from both perturbations was additive; that
is, it could be estimated from the summed RF. Local RF from sulfate in northern
industrial regions was felt globally. Nevertheless, the regional patterns in
both cases were significantly different, and obvious cooling (in a globally
warming climate) occurred in specific regions of the Northern Hemisphere. Such
differences in climate change patterns are critical to the detection of anthropogenic
climate change, as reported in Santer et al. (1996). As a further complication
of this assessment, aviation's perturbation occurs primarily in the upper troposphere
and lower stratosphere, and thus may alter the vertical profile of any future
tropospheric warming. Therefore, the patterns of climate change from individual
aviation perturbations (e.g., CO2, O3,
contrails) would likely differ, but we take their summed RF as a first-order
measure of the global mean climate change (see also the discussion in Section
6.5).
The equilibrium change in mean surface air temperature (łTs(equil)) in response
to any particular RF is reached only after more than a century because of the
thermal inertia of the climate system (primarily the oceans) and is calculated
with long-term integrations of coupled general circulation models (CGCM). A
climate sensitivity parameter (l) relates RF to temperature change: łTs(equil)
= l RF. Provided that all types of RF produce the same impact on the climate
system (in this case measured by mean temperature), the climate sensitivity
parameter derived from a doubled-CO2 calculation can
be used to translate other RFs, say from ozone or contrails, into a change in
global mean surface air temperature.
For doubled CO2 relative to pre-industrial conditions
(+4 W m-2), surface temperature warming ranges from 1.5
to 4.5 K, depending on the modeling of feedback processes included in the CGCM.
The recommended value in IPCC (1996) of 2.5 K gives a climate sensitivity of
l = 0.6 K/(W m-2). With limited feedbacks (e.g., fixing
clouds and surface ocean temperatures), the sensitivity parameter is smaller,
and most models produce similar responses. In contrast, when all feedbacks are
included, model results are quite different, as a result (for instance) of alternative
formulation of clouds. The obvious limitation of this approach is that we get
no information about regional climate change. The sensitivity parameter for
aircraft-like ozone perturbations is discussed in Section
6.5.
In spite of all these caveats, the radiative forcing of an aviation-induced
atmospheric perturbation is still a useful index that allows, to first approximation,
the different atmospheric perturbations (e.g., aerosols, cloud changes, ozone,
stratospheric water, methane) to be summed and compared in terms of global climate
impact.
|