Climate Variability
Contents |
Radiative Forcing
Radiative Forcing is as the change in the net energy flux into the surface/atmosphere system. This is measured at the top of the atmosphere or tropopause, the lowest part of the atmosphere known as the troposphere and is defines as the amount of energy coming in to the system causing a postive forcing. Currently Solar radiation = 342 watts/meter squared.
What can alter that balance?
Suppose, as an example, that the output of the Sun increases so that an extra 4 units of solar radiation is received by the atmosphere – a positive radiative forcing of 4 units acting to warm the climate. Initially the incoming energy exceeds the outgoing energy, so there is now an energy imbalance, and the system must change in response. The Earth/atmosphere system will respond by warming up until the outgoing long wave emission once again balances the incoming energy; a new equilibrium will be established with the Earth’s surface at a warmer temperature.
Natural changes include volcanoes erupting and changes in solar radiation.
Human examples include chnages in concentration of active gases due to burning of fossil fuels, aerosol cans, land use such as deforestation. The biggest impact on radiative forcings have been greenhouse gasses
The biggest warming climate forcings are Halocarbons The biggest cooling climate forcings are Stratospheric ozones The median warming climate forcings are fossil fuels The median cooling climate forcings are biomass burning and sulphate Low warming forcings are sun radiation and aircraft Low cooling forcings are land use (Albedo only)
Climate Feedbacks
These amplify (positive) or counteract (negative) Climate forcings. For instance the warming of the climate creates water vapor which is a greenhouse gas and warms the climate further. The melting of snow and ice does the same. The negative feedback is the creation of clouds, especially low ones. Note if the climate forcing is cooling then the positive forcing is further cooling.
Natural and Anthropogenic variability
- Natural External - Sunspots
- Natural Internal - El Nino/El Nina
Anthropogenic - the burning of fossil fuels since 1750 has led to an increase of CO2 in the atmosphere quicker than it can be removed.
The net solar (S) and long wave (L) radiation at the top of the atmosphere together with the corresponding surface temperature at the bottom. Initially solar and long wave radiation are in balance at 236 Wm-2 each, with a corresponding surface temperature of 15 °C. When the atmospheric concentration of CO2 is doubled, the initial perturbation is such that more long wave radiation is absorbed by the atmosphere so the outgoing long wave (L) radiation at the top of the atmosphere is reduced to 232 Wm-2, whilst the surface temperature remains at 15 °C. The atmosphere/surface system responds to this change by heating up until S and L are once again in balance at 236 Wm-2 each – to achieve this the surface warms by about 1.2K. The final panel of the figure shows that when feedbacks are included this surface warming is amplified to about 2.5K. They are back in balance ut at a higher temperature.
Sulphate aerosols act to backscatter solar radiation back to space, thus leading to a cooling effect on climate. This is the direct effect of aerosol. Some other types of aerosol, however, behave in a different manner. Black carbon, for example, is of the right molecular composition to absorb solar radiation, rather than backscatter it, and thus leads to a climate warming.
Aerosols also have two indirect effects on climate by influencing the brightness and lifetime of clouds.
Aerosols provide surfaces on which water vapour can condense to form clouds – so-called ‘cloud condensation nuclei’. In the presence of aerosol, the total available water vapour condenses onto more surfaces than it would if no aerosol were present, so the presence of aerosol makes the cloud droplets smaller. Smaller cloud droplets make the cloud brighter, so it reflects more solar radiation back to space, and thus via this effect aerosols have a cooling effect on climate. This is the first indirect effect of aerosol.
In addition, there is evidence that aerosols can lengthen the lifetimes of clouds, which increases the time over which sunlight is reflected back to space and provides yet another climate cooling mechanism. This is the second indirect effect of aerosol.
The human perturbation and the movement of carbon among the various reservoirs.
- Burning of fossil fuels and cement production both act to move carbon from geological reservoirs into the atmosphere. Cement production releases carbon dioxide to the atmosphere both directly (because of the chemical processes involved) and indirectly (because of the production of the energy required by the process).
- Changes in land use such as deforestation and biomass burning affect the balance of carbon between land and atmosphere. Changes in the amount of vegetation affect the uptake of carbon dioxide from the atmosphere, while biomass burning releases carbon dioxide into the atmosphere.
- The ocean naturally takes up carbon dioxide from the atmosphere, and in the process it becomes more acidic (lower pH).
Here are some of the forcing mechanisms which showed their associated radiative forcings. Changes in these all lead to changes in climate of various sizes and signs.
- Increases in well-mixed greenhouse gases (CO2, CH4, N2O and halocarbons)
- Anthropogenic
- Increases in tropospheric ozone
- Anthropogenic
- Decreases in stratospheric ozone
- Anthropogenic
- Sulphate aerosol from volcanic eruptions
- Natural
- Black carbon from fossil fuel burning
- Anthropogenic
- Solar radiation, or sunlight
- Natural
- Sulphate aerosol from fossil fuel burning
- Anthropogenic
- Aerosols from biomass burning
- Anthropogenic
- Wind-blown mineral dust
- Natural
- Land use change
- Anthropogenic
Activity
Here are some of the forcing mechanisms that appeared in Figure 6, which showed their associated radiative forcings. Changes in these all lead to changes in climate of various sizes and signs. Decide whether you think each forcing is natural or anthropogenic in origin, then click the button to check if you are correct.
Increases in well-mixed greenhouse gases (CO2, CH4, N2O and halocarbons)
Answer
Anthropogenic
Increases in tropospheric ozone
Answer
Anthropogenic
Decreases in stratospheric ozone
Answer
Anthropogenic
Sulphate aerosol from volcanic eruptions
Answer
Natural
Black carbon from fossil fuel burning
Answer
Anthropogenic
Solar radiation, or sunlight
Answer
Natural
Sulphate aerosol from fossil fuel burning
Answer
Anthropogenic
Aerosols from biomass burning
Answer
Anthropogenic
Wind-blown mineral dust
Answer
Natural
Land use change
Answer
Anthropogenic
Activity
Here are, again, some of the forcing mechanisms which have had an effect on climate over the last 250 years. Decide whether each is a positive forcing (climate warming) or negative forcing (cooling), then click the button to check whether you are correct.
- Well-mixed greenhouse gases
- Positive
- Stratospheric ozone
- Negative
- Sulphate aerosol
- Negative
- Black carbon from fossil fuel burning
- Positive
- Organic carbon from fossil fuel burning
- Negative
- Aerosols from biomass burning
- Negative
- Aviation-induced contrails
- Positive
- Aviation-induced cirrus (very high, wispy cloud that is composed of ice)
- Positive
- Land use (albedo change only)
- Negative
- Solar
- Positive
