Anthropogenic and Natural
Radiative Forcing
Jón Egill Kristjánsson
Radiative Forcing and Temperature
Response
• RF: Change in net (down minus up) radiative flux (SW plus LW; in W
m-2)
• A sustained RF leads to a temperature change, and globally
averaged we may write:
T    RF
• where λ: Climate Sensitivity
• ERF: Change in net downward radiative flux after allowing
atmospheric temperatures, water vapor and clouds to adjust, but
with surface temperature unchanged
• What is missing in this simple conceptual model?
Different Ways to Calculate Radiative
Forcing
Instantaneous RF
RF
ERF
ERF
Response
Time Evolution of Anthropogenic and Biomass Burning Aerosols
Tropospheric Ozone: An
‘anthropogenic’ greenhouse gas
• Production: By-product of the oxidation of CO, CH4 and
other hydrocarbons, in the presence of NOx
• Limitation: Limited by the supply of HOx (OH, HO2) and
NOx (NO, NO2)
• Removal: Lost through photolysis
• Removal: Lost through reaction with HO2
• Lifetime: A few days in the tropical PBL; ~1yr in the
upper troposphere; Global mean: 23±2 days
• Radiative Forcing: +0.40 ± 0.20 W m-2
• Why is there such a large warming effect?
The Earth’s Greenhouse Effect (Natural
+ Anthropogenic)
Gases: + 125 W m-2
• H2O: 60%
• CO2: 26%
• O3: 8%
Clouds: + 30 W m-2
Kiehl & Trenberth (1997: BAMS)
Mass Budget of tropospheric O3
•
•
•
•
Globally averaged burden: 337 ± 23 Tg
Net chemical production: 618 ± 275 Tg yr-1
Ozone deposition: 1094 ± 264 Tg yr-1
Net influx from the stratosphere: 477± 96 Tg
yr-1
Tropospheric Ozone trends
Radiative Forcing of tropospheric and
stratospheric ozone
Net: + 0.35 W m-2
Why negative?
Estimates of RF from tropospheric and
stratospheric ozone
The three leading well-mixed
‘anthropogenic’ greenhouse gases
• CO2: RF = +1.82 ± 0.19 W m-2, increasing by 0.3 W
m-2 per decade, now contributes 80% of the RF
from well-mixed greenhouse gases
• CH4: RF = +0.48 ± 0.05 W m-2, lifetime ~12.4 ± 1.4
yrs; increases with decreasing OH (pos. feedback)
• N2O: RF = +0.17 ± 0.03 W m-2, lifetime 131 ± 10 yrs
• All well-mixed greenhouse gases:
RF = +2.83 ± 0.29 W m-2
What other gases contribute the remaining +0.36 W m-2?
Concentrations of the three leading
well-mixed ‘anthropogenic’
greenhouse gases
Radiative Forcing from well-mixed greenhouse gases
All the well-mixed
‘anthropogenic
greenhouse gases’
have positive
growth rates
RF due to Aerosol-Radiation
interactions (RFari)
RF due to Aerosol-Radiation
interactions and BC on snow/ice
Net: - 0.35 W m-2
RF due to anthropogenic Land Use
Changes
Net: - 0.15 W m-2
Why negative?
Measured Total Solar Irradiance (TSI:
Solar Constant) 1978 - 2012
Downward trend?
TSI record (World Radiation Center,
Davos)
Global Surface Temperature
1880-2015
NASA GISS
Reconstructed TSI since 1745 (only
measured since 1978!)
Volcanic Aerosol Optical Depth record
from volcanic reconstructions
Aerosol
Extinction vs
Height
Aerosol
Optical
Depth
Combining anthropogenic forcing and
natural variability
Anthropogenic
greenhouse
gases do not
necessarily
dominate on
time scales of a
decade or so!
Lean and Rind (2009: GRL)
Confidence Level of forcing estimates