Monday, September 16, 2013

New paper finds reduction of soot caused ~17 times more warming than alleged from CO2 over past 20 years

A new paper published in the Journal of Geophysical Research-Atmospheres finds that reduction of soot over the past 20 years in California has had a surface warming effect about 17 times larger than claimed for CO2 over the same time period. 

According to the authors, "[Soot] reduction over the last two decades may have caused a surface brightening of 1.5–3.5 W/m2." [average 2.5 W/m2] By way of comparison, the IPCC-alleged surface warming effect of increased CO2 over the last two decades is about 0.5 W/m2* at the top of the atmosphere, and 0.5/3.7 = 0.14 W/m2 at the surface, about 17 times less surface warming effect than the reduction of soot. 

The larger role in climate for carbonaceous aerosols would necessarily imply a much lower climate sensitivity to CO2 than previously believed. Note also that there has been no statistically-significant warming for the past two decades, therefore, radiative forcing from both carbonaceous aerosols and CO2 levels are either overwhelmed by natural forcing or are much less than thought. 

*  5.35*ln(393.82/356.38) = 0.5 W/m2

Estimating the radiative forcing of carbonaceous aerosols over California based on satellite and ground observations

Yangyang Xu et al

Carbonaceous aerosols have the potential to impact climate directly through absorption of incoming solar radiation and indirectly by affecting cloud and precipitation. To quantify this impact, recent modeling studies have made great efforts to simulate both the spatial and temporal distribution of carbonaceous aerosol's optical properties and associated radiative forcing. This study makes the first observationally constrained assessment of the direct radiative forcing of carbonaceous aerosols over California. By exploiting multiple observations (including ground sites and satellites), we constructed the distribution of aerosol optical depths and aerosol absorption optical depths (AAOD) over California for a ten-year period (2000–2010). We partitioned the total solar absorption into individual contributions from elemental carbon (EC), organic carbon (OC), and dust aerosols, using a newly developed scheme. Our results show that AAOD due to carbonaceous aerosols (EC and OC) at 440 nm was 50%–200% larger than natural dust, with EC contributing the bulk (70%–90%). Observationally constrained EC absorption agrees reasonably well with estimates from global and regional chemical transport models, but the models underestimate the OC AAOD by at least 50%. We estimated that the top of the atmosphere (TOA) forcing from carbonaceous aerosols was 0.7 W/m2 and the TOA forcing due to OC was close to zero. The atmospheric heating of carbonaceous aerosol was 2.2–2.9 W/m2, of which EC contributed about 80–90%. We estimated the atmospheric heating of OC at 0.1–0.4 W/m2, larger than model simulations. EC [Elemental Carbon or soot] reduction over the last two decades may have caused a surface brightening of 1.5–3.5 W/m2.

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