19 April, 2011

Confusing Cause & Effect

Matt Ridley writes “When Scientists Confuse Cause and Effect”, in the Wall Street Journal: 
Scientists like to remind us not to confuse cause and effect.  But they’re not immune from making that mistake themselves.  [...] 
Whole districts of Freudian theory are confused about cause and effect.  [...]  Nor is medicine immune.  Some years ago epidemiologists found that women taking hormone replacement therapy had fewer heart attacks, but controlled trials found that HRT caused more heart attacks.  It turned out that the women taking HRT in the epidemiological study were from higher socio-economic classes, so they ate and exercised better.  Class caused both HRT and fewer heart attacks. 
Even climate science has encountered cause-effect confusion.  When in 1999 Antarctic ice cores revealed carbon-dioxide concentrations and temperature marching in lockstep over 400,000 years, many [...] found this a convincing argument for attributing past climate change to carbon dioxide.  (About 95% of carbon dioxide in the atmosphere is natural, coming from the exhalations of living things.  In the past, carbon-dioxide levels rose as the earth warmed at the end of ice ages and fell as it cooled at the end of interglacial periods.) 
Then four years later came clear evidence from finer-grained analysis of ice cores that temperature changes preceded carbon-dioxide changes by at least 800 years.  Effects cannot precede their causes by eight centuries, so temperatures must drive carbon dioxide, chiefly by warming the sea and causing carbon dioxide dissolved in water to “out-gas” into the air. 
Climate scientists fell back on a “feedback” hypothesis, arguing that an initial change, probably caused by variations in the earth’s orbit that affect the warmth of the sun, was then amplified by changes in carbon-dioxide levels.  But this made the attribution argument circular and left the reversal of the trend after a period of warming (when amplification should be at its strongest) still harder to explain.  If carbon dioxide is still driving the temperature upward but it falls instead, then other factors must be stronger than expected.

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