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Politics : Formerly About Advanced Micro Devices -- Ignore unavailable to you. Want to Upgrade?

To: combjelly who wrote (936859)	5/25/2016 6:34:11 PM
From: Wharf Rat	Read Replies (1) \| Respond to of 1575400

So we passed clean air laws, and it warmed up. Why did climate cool in the mid-20th Century? What The Science Says: The average global surface temperature decreased slightly from 1940 to 1975. The main reason behind this mid-century cooling was global dimming due to anthropogenic sulfate aerosol emissions. Figure 1: Global temperature anomaly and distinct linear trends over the 21st century (Open Mind) Climate scientists believe that the primary cause of this mid-century cooling was an increase in atmospheric aerosols due to anthropogenic emissions (primarily from the burning of fossil fuels). Aerosols have a complex effect on the climate, because they have both direct and indirect impacts.... What happened since 1975?Clearly since about 1975, global surface temperatures have trended rapidly upwards (at a rate of nearly 0.2°C per decade). 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 (in Massachusetts 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. Under the Clean Air Acts, sulfate emissions were regulated, and as a consequence their rapid atmospheric increase was stabilized right around 1975: Figure 6: Global anthropogenic sulfur emissions ( Pacific Northwest National Labs) Meanwhile anthropogenic greenhouse gas emissions have continued to increase. Since 1975, the atmospheric CO2 concentration alone has increased from 331 to 392 parts per million by volume, which corresponds to a temperature increase of about 0.7°C, though we've only seen about 0.55°C warming over that period due to the thermal inertia of the oceans and 'warming in the pipeline'. Mid-century cooling was primarily anthropogenicTo sum up, anthropogenic sulfur emissions appear to be the main cause of the mid-century cooling. These emissions decreased the mean global surface temperature by approximately 0.5°C during this period, while anthropogenic greenhouse gas emissions caused a warming of approximately 0.4°C. Therefore, even though greenhouse gas emissions continued to have a warming effect during this period, it was more than offset (hidden) by anthropogenic aerosol emissions, until those emissions were brought under control by government intervention while greenhouse gas emissions continued to increase unabated. In other words, the mid-century cooling is actually an expected result based on our current understanding of climate science, and is successfully hindcasted by climate models ( Meehl 2004). Figure 7: Anthropogenic plus natural vs. just natural radiative forcing temperature change vs. observed global surface temperature increase ( Meehl 2004) However, the overall impact of sulfate aerosols, particularly due to their indirect effects via cloud formation, remain a significant source of uncertainty. Despite this uncertainty, they remain the likely dominant cause of the slight mid-20th century cooling. skepticalscience.com