Thursday, March 27, 2014

New paper finds climate sensitivity is independent of the forcing. What are the implications?

A paper published today in the Journal of Geophysical Research Atmospheres finds climate models say that climate sensitivity "is independent of the forcing," i.e. a change of 1 W/m2 forcing from the Sun, clouds, volcanic eruptions, CO2, any greenhouse gas or other forcing should have identical effects upon the climate feedbacks and Earth's energy budget. In addition, the models predict a rapid change in cloud cover to a change in solar irradiance [due to evaporation], which is another self-regulating mechanism of climate, but the models continue to have "persistent uncertainties in cloud feedback mechanisms."

The paper is interesting in light of ERBE satellite observations showing that large volcanic eruptions have essentially zero effect upon surface temperature. Assuming these observations and this new paper are correct, then radiative forcing from CO2 and other greenhouse gases would also have essentially no effect on Earth's energy budget or surface temperature.

How could this be? The barometric formulas explain why the Earth surface temperature has little to do with radiative forcing, and is primarily determined by solar insolation plus thermodynamics [convection, phase change, pressure], not radiative forcing.

1. The surface temperature, as well as the entire atmospheric temperature profile, is entirely explained by solar insolation plus the behavior of adiabatic gases in a gravity field, which establishes the wet and dry adiabatic lapse rates.

2. The dry adiabatic lapse rate equation: dT/h = -g/Cp [g is gravity, Cp is heat capacity of the atmosphere] does not have a term for radiative forcing and is independent of radiative forcing.

3. Addition of GHGs increase the heat capacity Cp, which causes a decrease in the lapse rate, as is observed: the dry lapse rate is much steeper than the wet. A decrease in the lapse rate causes a cooler surface.

4. The entire 33K “greenhouse effect” is entirely explainable by the average adiabatic lapse rate i.e. the observed average lapse rate = 6.5K/km * 5 km = 33K. The 255K equilibrium temperature with the Sun at the top of the atmosphere [TOA] + 33K due to the lapse rate sets the surface temperature at 288K or 15C.

5, Thus, a large volcanic eruption or large change in radiative forcing from CO2 doesn’t change surface temperature, because there is no effect on the average lapse rate, and no change of the equilibrium temperature with the Sun at the top of the atmosphere [TOA]. 

6. The troposphere is in radiative/convective equilibrium, and any increase in surface temperature from radiative forcing is easily overcome by an equal and opposite cooling from increased convection:

Illustration of an electrical circuit analogy to radiative-convective equilibrium in a planetary atmosphere. Pressure and heat capacity set the resistance [opacity] to infrared transmission illustrated as the resistor Rc above. GHGs set the resistance [opacity] to infrared transmission illustrated as the resistor Rt above. As noted, "Resistance Rc corresponds to convection "shorting out" the radiative resistance Rt, allowing more current [analogous to heat in the atmosphere] to escape. If the resistance [IR opacity] of Rt increases due to adding more greenhouse gases, the resistance [IR opacity] of Rc will automatically drop to re-establish balance and thus the current through the circuit remains the same, and analogously, the temperature of the surface of the planet remains the same and self-regulates. Source

The barometric formulas have been known since the 1800's and published in many thousands of papers. Here is a paper reviewing the history and physical derivations, which are completely independent of "radiative forcing":

To determine the entire atmospheric temperature profile and surface temperature of any planet

1. Determine the equilibrium temperature with the Sun. For Earth, 255K

2. To determine the "greenhouse effect," determine the height of the tropopause [h] from eq40 in the above paper. T
he tropopause height is where P=0.1 bar, which has been known for many years establishes the radiative/convective boundary on all planetary atmospheres, and is where the atmosphere is too thin to support convection, allowing radiation as the only heat transfer mechanism. At pressures above 0.1 bar, convection easily dominates radiative transfer.

The "greenhouse effect" is then determined by

dT=h*g/Cp = 33K on Earth

3. The surface temperature is thus 255+33 = 288K

Thus, the entire greenhouse effect, troposphere temperature profile, and surface temperature have all been derived from the 1st and 2nd laws and ideal gas law, completely independent of "radiative forcing" from greenhouse gases.

The wet adiabatic lapse rate is only one-half of the dry, which proves increased water vapor has a negative feedback cooling effect. Water vapor increases Cp in the above equation, therefore decreases the lapse rate. CO2 has essentially zero effect on atmospheric mass, any of the barometric equations, or lapse rate and thus essentially zero effect on the surface temperature.

Forcings and feedbacks in the GeoMIP ensemble for a reduction in solar irradiance and increase in CO2

Nicolas Huneeus et al

The effective radiative forcings (including rapid adjustments) and feedbacks associated with an instantaneous quadrupling of the pre-industrial CO2 concentration and a counterbalancing reduction of the solar constant are investigated in the context of the Geoengineering Model Intercomparison Project. The forcing and feedback parameters of the net energy flux, as well as its different components at the top-of-atmosphere (TOA) and surface, were examined in ten Earth System Models to better understand the impact of solar radiation management on the energy budget. In spite of their very different nature, the feedback parameter and its components at the TOA and surface are almost identical for the two forcing mechanisms, not only in the global mean, but also in their geographical distributions. This conclusion holds for each of the individual models despite inter-model differences in how feedbacks affect the energy budget. This indicates that the climate sensitivity parameter is independent of the forcing (when measured as an effective radiative forcing). We also show the existence of a large contribution of the cloudy-sky component to the shortwave effective radiative forcing at the TOA suggesting rapid cloud adjustments to a change in solar irradiance. In addition, the models present significant diversity in the spatial distribution of the shortwave feedback parameter in cloudy regions, indicating persistent uncertainties in cloud feedback mechanisms.

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