"...we determine the global volcanic aerosol forcing since 2000 to be −0.19 ± 0.09 W/m2. This translates into an estimated global cooling of 0.05 to 0.12°C. We conclude that recent volcanic events are responsible for more post-2000 cooling than is implied by satellite databases that neglect volcanic aerosol effects below 15 km."The study examined the years 2000-2013, so the reduction in the trend would be 0.04 to 0.09°C/decade of cooling. That's could be up tonearly half the expected (from models, with no natural variability) 0.2°C/decade.
Here's their time series of the stratospheric aerosol optical depth (SAOD) since shortly after the 1991 large eruption of Mount Pinatubo washed out of the atmosphere:
After putting this all together, they get the following impacts on total radiative forcing and on temperature:
1 comment:
A radiative forcing is a CHANGE in the flux reaching and leaving the earth. Before 2000, we didn't the ability to accurately measure SAOD (sulfate aerosol optical depth much below 0.005 OD units. Now that we have somewhat better measurements, we can begin to see changes that would previously be undetectable or noise. For a forcing to be a forcing, we have to have accurate before and after measurements. The average forcing since 2000 is supposed to be −0.19 ± 0.09 W/m2, which is equivalent to 0.0075 AOD units. Can you clearly see an 0.0075 AOD change between the pre-2000 mean (unspecified, but probably 1995-2000) and the post 2000 mean - FOR THE MEASUREMENT METHODS THAT COVER BOTH PERIODS?
Now look at the uncertainty in the forcing +/-50%. As best I can tell, this is a 25-75% confidence interval. In that case the 95% confidence interval includes 0. In other words, the forcing may not even be statistically significant by normal standards!
Now, do you see a convincing case that the variations in AOD being measured after 2000 actually correlate with volcanic eruptions? In some cases, yes and others no.
If you look at the Sato and Venier records of AOD, you'll see they tell a somewhat different story.
http://www.columbia.edu/~mhs119/StratAer/
http://onlinelibrary.wiley.com/doi/10.1029/2011GL047563/full
No one knows what the average volcanic forcing was before 2000 accurately enough to say how much cooling the increased weak volcanic activity has caused during the hiatus.
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