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Why did climate cool in the mid-20th Century?"It was the post war industrialization that caused the rapid rise in global CO2 emissions, but by 1945 when this began, the Earth was already in a cooling phase that continued until 1975. With 32 years of rapidly increasing global temperatures and only a minor increase in global CO2 emissions, followed by 33 years of slowly cooling global temperatures with rapid increases in global CO2 emissions, it was deceitful for the IPCC to make any claim that CO2 emissions were primarily responsible for observed 20th century global warming." (Norm Kalmanovitch) What the science says...There are a number of forcings which affect climate (eg - stratospheric aerosols, solar variations). When all forcings are combined, they show good correlation to global temperature throughout the 20th century including the mid-century cooling period. However, for the last 35 years, the dominant forcing has been CO2. Figure 1 compares CO2 to global temperatures over the past century. While CO2 is rising from 1940 to 1970, global temperatures show a cooling trend. This is a 30 year period, longer than can be explained by internal variability from ENSO and solar cycles. If CO2 causes warming, why isn't global temperature rising over this period?
The broader picture in this scenario is to recognise that CO2 is not the only factor that influences climate. There are a number of forcings which affect the net energy flux into our climate. Stratospheric aerosols (eg - from volcanic eruptions) reflect sunlight back into space, causing net cooling. When solar activity increases, the net energy flux increases. Figure 2 shows a composite of the various radiative forcings that affect climate.
When all the forcings are combined, the net forcing shows good correlation to global temperature. There is still internal variability superimposed on the temperature record due to short term cycles like ENSO. The main discrepancy is a decade centered around 1940. This is thought to be due to a warming bias introduced by US ships measuring engine intake temperature.
So we see that climate isn't controlled by a single factor - there are a number of influences that can change the planet's radiative balance. However, for the last 35 years, the dominant forcing has been CO2. Printable Version | Link to this page
Further readingOpen Mind has an interesting post Hemispheres that compares temperature records for both hemispheres over the past century and the significance for mid-century cooling. There have been no comments posted yet. Post a CommentPolitical, off-topic or ad hominem comments will be deleted. Comments Policy... You need to be logged in to post a comment. Login via the left margin or if you're new, register here. |
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