Geoscience Reference
In-Depth Information
10
RADIATIVE FORCING OF
CLIMATE CHANGE
The following three chapters are concerned with climate change. They build
on the background presented in the previous chapters and discuss how and
why climate changes, with a focus on contemporary climate change issues.
We begin with the pure radiation side of the issue, examining current changes
in the atmosphere's chemical composition and the effects of those changes on
the flow of shortwave and longwave radiation through the atmosphere in the
absence of any response of the climate system to the changes in radiation. In
chapter 11,
we focus on the climate system's response to these changes in radia-
tive fluxes. Finally, we discuss climate change prediction using climate models
in
chapter 12
.
Earth's climate has changed throughout its 4.5 billion year history in as-
sociation with
radiative
forcing factors
, including changes in insolation and
in the chemical composition of the atmosphere. On the longest time scales,
up to billions of years, changes in solar luminosity force climate change. The
sun is evolving and is about halfway through its 10-billion-year life as a
main
sequence
star, that is, a star fueled by the conversion of hydrogen into helium.
Over the first 4.6 billion years of the sun's life its luminosity has increased by
about 25% and, according to models of stellar evolution, it will continue to
increase for another 5 billion years until all its hydrogen has been converted
to helium.
The development of life on the planet, along with early periods of intense
volcanic activity and outgassing, altered the chemical composition of the at-
mosphere and also contributed to climate change on time scales of millions
and billions of years. Plate tectonic processes modify the land/sea distribution
and build mountains and, as a result, climate adjusts on time scales of mil-
lions of years. On time scales of tens of thousands of years, changes in the
earth's orbital parameters (see
chapter 3)
force the glacial-interglacial oscilla-
tion that has characterized climate during the Holocene (the most recent 1.2
million years).
And now, climate is changing due to human activity. Since the Industrial
Revolution in about 1750, burning of fossil fuels (coal, oil, and natural gas) has
released CO
2
into the atmosphere. A host of other atmospheric constituents
are increasing as well, including powerful greenhouse gases such as methane
and nitrous oxide. The atmosphere's particulate (aerosol) concentration is also
changing, along with land surface features such as albedo and surface rough-
ness due to altered plant distributions, paving, and buildings.
