by Judith Curry
Part I addressed the mid-20th century surface temperature “bump” (peaking circa 1940). The IPCC AR4 states in the figure caption for FAQ3.1, Figure 1:
From about 1940 to 1970 the increasing industrialisation following World War II increased pollution in the Northern Hemisphere, contributing to cooling, and increases in carbon dioxide and other greenhouse gases dominate the observed warming after the mid-1970s.
In Chapter 9, a more detailed analysis is provided:
Differences between simulations including greenhouse gas forcing only and those that also include the cooling effects of sulphate aerosols (e.g., Tett et al., 2002) indicate that the cooling effects of sulphate aerosols may account for some of the lack of observational warming between 1950 and 1970, despite increasing greenhouse gas concentrations, as was proposed by Schwartz (1993). In contrast, Nagashima et al. (2006) find that carbonaceous aerosols are required for the MIROC model (see Table 8.1 for a description) to provide a statistically consistent representation of observed changes in near-surface temperature in the middle part of the 20th century. The mid-century cooling that the model simulates in some regions is also observed, and is caused in the model by regional negative surface forcing from organic and black carbon associated with biomass burning. Variations in the Atlantic Multi-decadal Oscillation (see Section 3.6.6 for a more detailed discussion) could account for some of the evolution of global and hemispheric mean temperatures during the instrumental period (Schlesinger and Ramankutty, 1994; Andronova and Schlesinger, 2000; Delworth and Mann, 2000); Knight et al. (2005) estimate that variations in the Atlantic Multi-decadal Oscillation could account for up to 0.2°C peak-to-trough variability in NH mean decadal temperatures.
Skeptical Science provides a detailed explanation for the aerosol-driven mechanism for the mid-century cooling. It concludes with:
So what caused the mid-century cooling to end? The main cause of the sudden shift in global temperature trends was the passage of Clean Air Acts by various countries in response to air pollution and acid rain. The USA, for example, first passed its Clean Air Act in 1970, with amendments in 1977 and 1990. Coincidentally, the US Supreme Court (inMassachusetts v. EPA) and EPA (in an endangerment finding) also recently decided that greenhouse gases qualify as ‘air pollutants’ in the Clean Air Act and must be regulated accordingly.
Smith et al. paper
There is a new paper out that is very relevant to this issue (see also discussion at WUWT):
Anthropogenic sulfur dioxide emissions: 1850-2005
S.J. Smith, J. van Aardenne, Z. Klimont, R.J. Andres, A. Volke, and S. Delgado Arias
Abstract. Sulfur aerosols impact human health, ecosystems, agriculture, and global and regional climate. A new annual estimate of anthropogenic global and regional sulfur dioxide emissions has been constructed spanning the period 1850– 2005 using a bottom-up mass balance method, calibrated to country-level inventory data. Global emissions peaked in the early 1970s and decreased until 2000, with an increase in recent years due to increased emissions in China, inter- national shipping, and developing countries in general. An uncertainty analysis was conducted including both random and systemic uncertainties. The overall global uncertainty in sulfur dioxide emissions is relatively small, but regional uncertainties ranged up to 30%. The largest contributors to uncertainty at present are emissions from China and interna- tional shipping. Emissions were distributed on a 0.5◦ grid by sector for use in coordinated climate model experiments.
The graphs show a broad peak from about 1970-1990, then a 10-15% decrease from 1990 to 2000 (with a slight increase between 2000 and 2005). Eyeballing the graph shows emissions in Gg:
- 1910-1940: 25,000 – 50,000 Gg
- 1940-1970: 50,000 – 125,000 Gg
- 1970-2000: 125,000 – 110,000 Gg
The drop in global sulfate emissions after 1970 just isn’t all that large; while there were large decreases in the U.S. and Europe, developing economies (esp Asia) were rapidly increasing. So it it seems to be more accurate to state that there was a leveling off around 1970, with a small decrease in the 1990’s, then an increase beginning ca 2000.
So how does the aerosol explanation hold up for the mid century cooling? There is a marked change in the slope of emissions at 1950; hence the aerosols don’t seem to be a convincing explanation for the large cooling observed in the 1940’s.
And how does the aerosol explanation hold up for the resumption of warming ca 1970? Again, there is a marked change in slope ca 1970, where the emissions level off until 1990; there is no global reduction in total emissions between 1970 and 1990.
JC’s conclusion: It seems implausible to attribute the mid century cooling and the resumption of warming to an increase in sulfate emissions following WWII and then a decrease ca 1970 following the Clean Air Acts. There may be some sort of complicated lag that may be evident to support the 1970-2000 warming (from the increase during the period 19501970), but the large cooling from 1940-1950 cannot be explained by aerosol forcing.