New paper finds prior estimates of global aerosol forcing off by ± 20-50%, which models "depend critically" upon

From the Annals of Settled Science:

A paper published today in Nature Climate Change updates records of large volcanic eruptions from Antarctic ice cores and finds 

"Assessments of climate sensitivity to projected greenhouse gas concentrations underpin environmental policy decisions, with such assessments often based on model simulations of climate during recent centuries and millennia. These simulations depend critically on accurate records of past aerosol forcing" 

"that prior to the year 1500 the reconstructions [of volcanic eruptions] were either previously overestimating global aerosol forcing by 20–30% or underestimating it by 20–50%. This has implications for estimates of climate sensitivity" to CO2. 

"Simulations of climate impacts after large eruptions predict stronger cooling than is found from temperature reconstructions, although the reasons for the mismatch are widely debated."

Perhaps the reason why ""simulations of climate impacts after large eruptions predict stronger cooling than is found from temperature reconstructions" is because, as Willis Eschenbach has shown, the climate really cares little about radiative forcing at the top of the atmosphere, and the real control knob is the emergent thermodynamics of the lower atmosphere.


Summary of the paper from Nature Climate Change:

Historical aerosol forcing from large volcanic eruptions are reconstructed from sulphate deposition measured in ice cores. This study updates these records by using a more extensive collection of Antarctic ice cores, which provide new records and accurate dating of published records. The results show that prior to the year 1500 the reconstructions were either previously overestimating global aerosol forcing by 20–30% or underestimating it by 20–50%. This has implications for estimates of climate sensitivity.

Nature Climate Change 4 693 doi: 10.1038/nclimate2293

Insights from Antarctica on volcanic forcing during the Common Era

• Michael Sigl,
• Joseph R. McConnell,
• Matthew Toohey,
• Mark Curran,
• Sarah B. Das,
• Ross Edwards,
• Elisabeth Isaksson,
• Kenji Kawamura,
• Sepp Kipfstuhl,
• Kirstin Krüger,
• Lawrence Layman,
• Olivia J. Maselli,
• Yuko Motizuki,
• Hideaki Motoyama,
• Daniel R. Pasteris
• & Mirko Severi

Assessments of climate sensitivity to projected greenhouse gas concentrations underpin environmental policy decisions, with such assessments often based on model simulations of climate during recent centuries and millennia^1, 2, 3. These simulations depend critically on accurate records of past aerosol forcing from global-scale volcanic eruptions, reconstructed from measurements of sulphate deposition in ice cores^4, 5, 6. Non-uniform transport and deposition of volcanic fallout mean that multiple records from a wide array of ice cores must be combined to create accurate reconstructions. Here we re-evaluated the record of volcanic sulphate deposition using a much more extensive array of Antarctic ice cores. In our new reconstruction, many additional records have been added and dating of previously published records corrected through precise synchronization to the annually dated West Antarctic Ice Sheet Divide ice core⁷, improving and extending the record throughout the Common Era. Whereas agreement with existing reconstructions is excellent after 1500, we found a substantially different history of volcanic aerosol deposition before 1500; for example, global aerosol forcing values from some of the largest eruptions (for example, 1257 and 1458) previously were overestimated by 20–30% and others underestimated by 20–50%.