Chapter 3: causes of climate change P2 Flashcards
The resulting positive or negative changes in energy balance due to climate forcing are expressed as
radiative forcing
The resulting positive or negative changes in energy balance due to climate forcing are expressed as radiative forcing, which is used to
compare the relative importance of the various forcings of climate change
The radiative forcing is defined as
the (instantaneous) imbalance between the incoming and outgoing radiation at the top of the troposphere.
Radiative forcing, /_\ F which is the
difference between net incoming and outgoing energy
Radiative forcing, ΔF which is the difference between net incoming and outgoing energy, can be expressed as:
if /_\ F = 0
under equilibrium
For ΔF > 0
a positive radiative forcing, the incoming energy is higher than the outgoing. To counterbalance this forcing, the surface temperature has to increase by ΔT to produce a planetary radiative flux that is larger than the incoming flux.
For ΔF > 0, the required counterbalance
The required counterbalance (increase in temperature), assuming no changes in other factors affecting the climate, is represented by this equation. :
Climate Sensitivity
How sensitive is Earth ’s climate to change?
One way to measure sensitivity of Earth’s climate is to
measure the change in average surface temperature of the Earth due to radiative forcing.
Thus, climate sensitivity refers to
the change (increase/decrease) in the average surface temperature of the Earth for a given radiative forcing (positive/negative) .
the increase in surface temperature, ΔT can be obtained as:
If we know the radiative forcing due to various factors (forcings), we can calculate
the resultant change in the average surface temperature due to those factors
Fig‐a (equilibrium) radiative forcing
the globally averaged solar radiation intercepted by the Earth (the 100 units, equivalent to 342 W m−2)
positive radiative forcing
Assuming that the planetary albedo is unchanged (at 31%), an increase in the solar constant (Fig‐b) produces a positive radiative forcing: the rate at which the Earth‐atmosphere system absorbs solar radiation (69.69 units) is now greater than the rate at which it emits longwave radiation to space (69 units).
warming effect
negative radiative forcing
Conversely, a reduction in the solar constant (Fig‐c) produces a negative radiative forcing, which has a cooling effect.
According to the Figure above, what is the radiative forcing, in W m−2, associated with a ±1% change in the solar constant?
The radiative forcing is the difference between the rate at which the Earth–atmosphere system absorbs solar radiation and the rate at which it emits longwave radiation to space.
From parts (b) and (c) of Figure, the magnitude of the radiative forcing is:
(69.69 − 69) units or (69 − 68.31) units = 0.69 units, which is equivalent to:
(342 W m−2/100) × 0.69 = 2.4 W m−2 (to 2 significant figures).
The forcing is positive for a 1% increase in the solar constant (Fig‐b) and negative for a 1% decrease (Fig‐c).
The power of a process to change the climate by altering the earth’s energy balance is estimated by its
radiative forcing
Primary climate forcings graph
some climate forcings are positive, causing globally averaged warming, and some are negative, causing cooling.
Natural forcings include
changes in the amount of energy emitted by the Sun, very slow variations in Earth’s orbit, and volcanic eruptions
Since the start of the industrial revolution, the only natural forcing with any longterm significance has been a
small increase in solar energy reaching Earth.
However, this change is not nearly enough to account for the current warming.
Climate forcing can also be caused by human activities. These activities include
- greenhouse gas and aerosol emissions from burning fossil fuels and
- modifications of the land surface, such as deforestation.
Greenhouse gases are a …………….. climate forcing; that is, they
positive
have a warming effect. Carbon dioxide emitted from the burning of fossil fuel is presently the largest single climate forcing agent, accounting for more than half of the total positive forcing since 1750.
Burning fossil fuels adds
aerosols to the atmosphere
Burning fossil fuels adds aerosols to the atmosphere. Aerosols are
tiny particles in the atmosphere composed of many things, including water, ice, ash, mineral dust, or acidic droplets.
Aerosols can
deflect the Sun’s energy and impact the formation and lifetime of clouds. Aerosols are a negative forcing; that is, they have a cooling effect.
the emission from the atmosphere is ……………. to …………………
inversely proportional (i.e., if CO2 concentration increases emission decreases) to the logarithm of the concentration of CO2 in ppm, when other variables are held constant.
The baselines for comparison of the increases are usually taken as the
concentrations in 1750, at the beginning of the Industrial Revolution. For CO2, the baseline value, C0, is 278 ppm.
The slope of the line in the figure then gives the radiative forcing for C (another CO2 concentration):
