SECOND IN A SERIES
The Science of Climate Change
By John Brodman
As we approach the upcoming global conference on climate change set to
begin in Paris this November, we can expect a media storm of increased coverage
about climate change. What are we to make of so many conflicting claims? This is
the second part of a series of articles about global warming. Part I in last month’s
Shoreline explained the components of the United Nations Framework Convention
on Climate Change (UNFCCC) and its processes. The third and final part will
appear next month and will explore the expected outcomes of the Paris meeting.
Simplified overview. The earth’s climate is a chaotic system with multiple
variables and non-linear feedback loops that are constantly changing,^interacting
with each other and producing the wide range of variability we observe in the
weather and climate over different periods of time. We know that natural variations
in the sun’s energy, changes in the earth’s orbit and tilt on its axis, volcanoes,
earthquakes, asteroids, forest fires, currents, ocean oscillations (like el Nino) and
ocean/atmosphere interactions can all have a significant impact on the earth’s
climate over time frames ranging from a few months to millennia. While the earth’s
climate is a dynamic system that is always changing, many scientists believe this
natural variability in the climate system alone cannot explain the observed warming
of the planet during the past century. Mainstream climate science attributes most
of the observed warming during the last 100 years to increases in greenhouse gas
concentrations caused by human activity (anthropogenic global warming). Other
scientists believe that we don’t know enough about all the forces at work to separate
the “natural variability” in our climate from human influences. This is what climate
science is all about.
The greenhouse effect is the term often used to describe the impact of rising
concentrations of greenhouse gases (GHGs) in the atmosphere on the radiative
balance of the earth. As the sun shines, it radiates short wave energy to the earth.
Some of the sun’s short wave energy is immediately reflected back into space, and
some is absorbed by the earth (land and water) and radiated back into space in the
form of long-wave, infrared radiation. The term “albedo” refers to the percentage
of solar radiatibn that gets immediately reflected back into space, and the earth as
a whole has an albedo of about 30% to 35%, depending on cloud cover and some
other factors. This means that about 70% of the sun’s energy that reaches the earth
is initially absorbed by our land, water and atmosphere, and is then reradiated back
in the form of long-wave infrared radiation.
Greenhouse gases in the atmosphere absorb someof this long-wave energy
preventing the loss of heat to space. These gases effectively trap the heat, like a
blanket, and warm the earth’s atmosphere. Under normal circumstances, these
radiative forces are pretty much in balance, creating more or less stable temperatures
over long periods of time. In a balanced system, GHGs, which by definition are gases
that are able to absorb infrared radiation, trap enough of the sun’s heat to make the
earth a habitable place. Without them, scientists estimate that the earth’s average
temperature would be 60 degrees cooler than it is, so having some amount of these
gases in our atmosphere is a good thing. We couldn’t survive without them.
Emissions and concentrations. Concentrations of GHGs in our atmosphere
have been accumulating at an increasing rate as a result of human activity since
the beginning of the industrial era. Annual emissions of GHGs now exceed the
earth’s natural ability to remove them, causing concentrations of these gases in
our atmosphere to rise. A common analogy is the water level in a bathtub. If the
faucet (annual emissions of GHGs) is allowing water to run into the tub faster
than the drain lets it run out, the water level in the tub rises (increasing GHG
concentrations). Increasing concentrations of GHGs in the atmosphere trap more
of the outgoing infrared radiation (heat), causing the planet to warm. Attempts to
model this process have produced a wide range of results in the timing and amount
of warming we can expect with a given increase in GHG concentrations.
Greenhouse gases. The main greenhouse gases include water vapor (H2O),
carbon dioxide (CO2), methane (CH4), nitrous oxide (NO2), ozone (O3) and
halocarbons (CFCs, HFCs, HCFCs, PFCs and SF^, together called the “F” gases).
In addition, there are particles, like dust, soot, sulfur compounds and ground level
smog in the atmosphere (called aerosols), that can both absorb and reflect some
of the sun’s energy, depending on where they are in the atmosphere, having both
warming and cooling effects. Not all these GHGs are the same; some are more
powerful than others, they interact with each other to form new compounds and
.they remain in the atmosphere for different periods of time. For the purposes of
accounting, scientists convert measures of each of these gases into their carbon
dioxide equivalents (C02e), based on their “global warming potential” (GWP).
The GWP of a gas is the warming over a 100-year period caused by the emission
of a given amount of the gas relative to the warming caused by emission of the same
amount of CO2. The GWP of CO2, by definition is 1, but methane has a GWP of
25, and nitrous oxide, a very powerful GHG, has a GWP of 298, meaning that a ton
of NO'2 has 298 times the warming impact as a ton of CO2, Why CO2 equivalents?
Because, other than water vapor (H2O), carbon dioxide is the most prevalent GHG,
accounting for approximately 80% of all GHG emissions on an equivalent basis. CO2
and C02e are often used interchangeably in the popular press, even though they are
quite different. AU GHGs have different GWPs for different periods of time.
GHG concentrations in our atmosphere are higher than they have been at any
time during the last million years or so, and they are still rising. Concentrations of
CO2 in the atmosphere have risen 40% from around 280 to 300 parts per million
(ppm) in the 1850 to 1900 period, to just over 400 ppm today. Methane, a very
powerful GHG in our atmosphere, is up 250% from the pre-industrial era. Nitrous
oxide concentrations are up about 20% from pre-industrial levels. The “F” gases
(halocarbons) are a relatively small part of overall GHG concentrations, but they
are very powerful and they remain in the atmosphere for a long time. Aerosols are
a bit of a mystery, as they can react with water vapor and other components of the
atmosphere to form clouds and a range of compounds that both absorb and reflect
energy At the planetary level, these aerosols are thought to offset about 20% to 35%
of the warming caused by the other GHGs.
CO2 is absorbed and emitted naturally as part of the carbon cycle, which
involves plant and animal respiration, decay and growth, volcanic activity, forest
fires and ocean/atmosphere interactions. CO2 emissions have risen from about
five gigatons per year in the pre-industrial era to about 37 gigatons per year
today Eighty percent of the extra (human related) CO2 entering our atmosphere
(equivalent to two-thirds of all GHGs) comes from the production and use of fossil
fuels, and the other 20% comes about as a result of changes in land use patterns,
deforestation and agricultural practices. Methane enters the atmosphere from
natural sources like wetlands, decaying plant and animal materials, natural leakages
of methane from oil and gas deposits and coalbeds. Human sources of methane
include landfills, energy production, processing and transportation, mining, animal
feedlots and other agricultural practices. Nitrous oxide comes primarily from fossil
fuel burning and the use of fertilizers.
GHGs remain in the atmosphere for a long time. About one-third of the GHGs
emitted today will remain in the atmosphere 100 years from now. About 45% of the
GHGs we have emitted so far this century will remain in the atmosphere in 2100,
so even if we immediately ceased emitting all GHGs right now (an impossibility),
our past emissions will go on warming the climate for several hundred years
before a new equilibrium is established. Annual GHG emissions have risen from
27 gigatons C02e in 1970, to about 55 gigatons C02e in 2014 (there are ranges
associated with all these figures). Total concentrations of GHGs in the atmosphere
are now estimated to be close to 465 ppm C02e (with a range of 440 to 485 ppm,
without adjustment for the effects of aerosols).
Tipping points. The UNFCCC has agreed to try and limit future warming to
no more than two degrees Celsius (C) by 2100, as an “insurance policy” against
the largely unpredictable consequences of global warming. While no one knows
for sure what the impacts will be, the fear is that rising temperatures may create a
(Continued on page 27)
26 The Shoreline
September 2015