Outline of the Scientific Method

1. Outline of the Scientific Method Dr. Carlson’s Description of the Scientific Method • Observation or other source of inspiration • Form two or more hypotheses • Discard those contradicted by previous experience • Design experiment • Perform experiment and record data • Analyze data • Check for errors and repeat steps 4 – 6 as needed • Report your results for critical review by others • Repeat step 2 – 8 • Once it has survived several cycles it becomes a theory • Repeat in a wide variety of situations over time • Once it has survived a large variety of tests, it becomes a law

2. Examining Alternative Hypotheses Repetition is a Good Thing • Generally, surprising scientific results are not accepted by the scientific community until they have undergone scrutiny by multiple experimenters • Often, the results ultimately prove to be caused by something unexpected • Example from my field – the 17 keV neutrino • There were perhaps six experiments that saw the effect • And perhaps four that saw no effect • But most of these were low statistics • It was noted that four of the positive experiments used something called a baffle • It was proposed that the baffle was causing the anomalous results • The experiment was redone without the baffle – and the effect went away • One of the remaining experiments increased statistics – the effect went away • The one remaining experiment was low statistics – and no one believed it

3. Sometimes We Go Down the Wrong Track • Papers get published based on results that ultimately prove incorrect • But in the long run, we tend to catch the errors

4. A Positive Example from My Field • Back in the 1970’s, it was discovered that the number of neutrinos coming from the Sun was smaller than predicted • Many explanations proposed • In the 1980’s, one such explanation – the so-called MSW effect – came to the fore • The theory predicted that the neutrinos were being changed from one type to another invisible type • It was predicted that other neutrino detectors – most importantly including the Sudbury Neutrino Observatory – would be able to accurately count all the neutrinos • This and several other experiments all confirmed the MSW hypothesis to be true • We would now regard the MSW hypothesis as a theory

5. Searching Out Alternative Hypotheses • Once experiments seem to verify a hypothesis, it is still good to consider whether alternatives can explain the pattern • Can we explain the trends seen as being due to random chance? • Could we be confusing cause and effect? • Could the correlations seen have a different cause entirely? • Once we have ruled many of these hypotheses out, the hypothesis has survived a test of time, and becomes a theory

6. Anthropogenic (Man-Caused) Global Warming Example of How the Scientific Process Works • In the 19th century, it was realized out that carbon dioxide (CO2) is a greenhouse gas • It allows visible light through, which warms the Earth • But it doesn’t let infrared light out, so heat can’t escape • In 1896 ArvidHogbom suggested that human emission of CO2 due to industrial activity would lead to long-term warming of the Earth • But he thought it would be very slow (CO2 production was much lower then) • By 1950’s scientists suspected thatanthropogenic CO2 production would besufficient to cause significant climate change • By 1960 the first accurate measurements ofCO2demonstrated that it was rising • These measurements continue today • By 1965 it was recognized that carbondioxide was likely to cause global warming

7. A Warming Earth • By the 1980’s it was most scientist’s opinion that human activities would cause warming of the Earth • By this point, satellites capable of measuringmean global temperatures were built, so thatreliable measurements of the effects of globalwarming were possible • These measurements continue today • In 1988 the International Panel on ClimateChange (IPCC) was established to monitorand study global warming • This panel has stated with increasingconfidence that • The Earth is getting substantially warmer • Anthrogogenic causes are the primary driver of this warming

8. Alternative Hypotheses (1)? • One can always come up with alternative hypotheses • For example, there is evidence that Earth’s climate has fluctuated substantially in the past, even within the time frame when humans were around • Glacial periods (most recent ? – 80th century BCE) • Medieval warm period (10th – 13th century CE) • “Little Ice Age” (16th – 19th century CE) • These were not caused by human activity • We now believe that small variations in Earth’s orbit are responsible for the ice ages • Milankovich cycles • Changes in the past 1000 years are too rapid to be accounted for by these cycles • The Sun’s output of energy varies over time • Solar cycle – eleven year cycle where the Sun brightens and dims • Longer term variations of unknown cause • But we can now directly measure (with satellites) this variation in the Sun’s output • These changes probably do account for the Little Ice Age and the Medieval warm period

9. Alternative Hypotheses (2)? • One can similarly question the data itself • Could it be due to random chance? • Mean temperatures do fluctuate from year to year • But the 10 hottest years of all have all been in last 20 years • Also, much early data relied on temperature stations • Many of these areas have become more urbanized over time • The process of urbanization causes local temperatures to rise • Concrete is hotter than vegetation, for example • “Heat Island effect” • It has been found, however, that when corrected for this effect,the Earth is still getting hotter • Bottom line – No plausible alternative hypothesis has been found • Most (~95%) climate scientists accept anthropogenic global warming Hottest • 2016 • 2015 • 2017 • 2014 • 2010 • 2005 • 2013 • 2009 • 1998 • 2007

10. Global Warming: Hypothesis, Theory, Law • I think most scientists would now say that the idea that carbon dioxide in the atmosphere causes global warming started out as a hypothesis • Although it made sense, there were a lot of unknowns • Size of effect • Relative importance of CO2 vs. other contributors • Feedback effects • Etc. • Many of these uncertainties have been eliminated • We can now label this as a theory • To be a law, it must be tested in a large variety of circumstances • We don’t know to what extent CO2 played a major role in the heating of the Earth in the past • We have a little bit of evidence that it is an effect on other planets • Venus is hot due to thick CO2 atmosphere • But since we don’t know how it all fits together, it’s still not a law