Multiple lines of reasoning in support of one claim

Earth Science, in my experience, tends not to work this way. Instead, many of the most bold and important claims in Earth Science have been built from many different forms of data and observations. Each data type leads to a chain of reasoning that "suggests" or "supports" or "is compatible with" the claim, like this:

For example, the data supporting the claim for the mass extinction at the Cretaceous/Tertiary boundary includes iridium isotope concentrations, shocked quartz, spherules, tsunami deposits, magnetic and gravity anomaly patterns, the pattern of cenotes (sinkholes) in the Yucatan Peninsula, etc.

Likewise, the data types supporting the claim for anthropogenic climate change include temperature measurements of the oceans and atmosphere, atmospheric carbon dioxide concentration, sea level from tide gauges, sea level from satellites, the dates of first flowering of various plants, pH measurements in the ocean, the thickness of tree rings, etc.

Ault (1998) makes the case that this structure of knowledge--in which independent, converging lines of inquiry have to be integrated and reconciled to produce the warrant for a claim--is a fundamental consequence of the historical and interpretive nature of geology.

Be that as it may, this structure of knowledge makes the claims of our field vulnerable to criticism. Each data set with its associated chain of reasoning lends support to the claim. But no single data set proves the claim, and any single data set taken by itself is vulnerable to criticism.

Critiques or skeptics can pick off the lines of reasoning one by one. The anthropogenic climate change claim has been particularly vulnerable to such sniping.

Somehow we have to convey to our students and the public that it is the assembled preponderance of the evidence that supports the claim, not any single line of reasoning taken by itself.

Although students may not be used to thinking this way in science class, they are used to thinking this way in life outside the classroom. Decisions of where to go to college, whom to marry, which car to buy, whom to hire, are often made through this same kind of holistic weighing of the evidence. Jury trials and search committees function through this same kind of reasoning, weighing multiple forms of evidence, comparing apples and oranges.

The process of weighing the scientific evidence, the various data types and their associated lines of reasoning, occurs at two scales: within an individual scientist and across the scientific community. Acceptance by the scientific community of both of the example claims cited above--the K/T boundary extinction and anthropogenic climate change--grew gradually over years to decades as new data types and more robust (less dashed in the diagram) lines of reasoning were placed upon the scale.

Conversation with Cathy Manduca inspired the diagram. Peter deMenocal's lectures in Frontiers of Science provided handy examples. Comments by participants in the Climate Education Workshop at West Point sharpened the argument.

Ault, C. R., Jr. (1998). Criteria of excellence for geological inquiry: The necessity of ambiguity. Journal of Research in Science Teaching, 35(2), 189-212.

See also Cleland, C. E. (2002). Methodological and epistemic differences between historical science and experimental science. Philosophy of Science, 69, 474-496, for a description of building of the claim of plate tectonics through the accumulation of multiple lines of evidence, and more detail about the K/T example.