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Average ozone levels
The graph shows the average October ozone levels recorded by Farman's group at Halley Bay from 1957 through 1984.
Balanced reporting (1 of 2)
The public should be able to get information on all sides of an issue but that doesn't mean that all sides of the issue deserve equal weight.
Science is open to anyone and benefits tremendously from the expanding diversity of perspectives. However, science only works because the people involved with it behave "scientifically."
Building a scientific argument - cartoon 1
Scientific arguments involve three components: the idea (a hypothesis or theory), the expectations generated by that idea (frequently called predictions), and the actual observations relevant to those expectations (the evidence).
Building the argument for ozone depletion
Researchers needed to incorporate nitrogen dioxide into the hypothesis — but they weren't sure how it would affect the expectations generated by the hypothesis. Would we still expect to see significant ozone depletion?
Building the argument for rapid ozone depletion
Rowland, Solomon, and Garcia's work suggested a modification to the original hypothesis: CFCs cause significant ozone depletion — and they do it much more rapidly with the help of polar clouds.
Comparing ozone and chlorine monoxide levels
A plot of chlorine monoxide and ozone concentrations from data collected by an aircraft passing through the Antarctic ozone hole. Outside the hole (left side of graph), ozone levels are high and chlorine monoxide levels are low, while the reverse is true inside the hole (right side of graph) — just as the Molina-Rowland hypothesis would lead us to expect.
Competing ideas (1 of 5)
Scientists are more likely to put their trust in ideas that generate more specific expectations (i.e., are more testable).
Competing ideas (4 of 5)
Scientists are more likely to put their trust in ideas that are more consistent with well-established theories in neighboring fields.
A hydrogen atom has only a single proton in its nucleus, whereas deuterium, a rarer isotope of hydrogen, has a proton and a neutron.
Episodes of endosymbiosis
A cartoon illustrating Margulis' idea that early bacteria experienced several episodes of endosymbiosis.
Even theories change (1 of 4)
Science is always a work in progress and even theories change. For example, in the 1600s classical mechanics was the accepted explanation of the movement of objects both in space and on Earth.
Even theories change (2 of 4)
Then classical mechanics was one-upped by Albert Einstein's theory of special relativity because it explained more phenomena.
Even theories change (4 of 4)
General relativity seems up for a change. For example, it doesn't mesh with what we know about the interactions between extremely tiny particles. Will physicists develop a new theory that simultaneously helps us understand the interactions between the very large and the very small?
Examining the source (1 of 2)
When evaluating a media message about science, one of the first things to consider is where the information came from.
Percentage of organisms that have gone extinct over the past 200 million years, based on the fossil record.
How the base pairs match up
Given the correct forms for the bases, Watson was able to figure out how adenine-thymine and guanine-cytosine pairs matched up, and formed weak hydrogen bonds with one another. Watson and Crick originally suggested that there were two bonds between guanine and cytosine but later it was found that a third existed.
Hypothesis of ozone depletion
Molina and Rowland's over-arching hypothesis (that releasing CFCs into the atmosphere would cause significant ozone depletion) was based on many supporting hypotheses, backed by their own lines of evidence.
Image B 51
Franklin and Gosling's image confirmed the idea that DNA was helical in shape and that its bases were stacked pancake-style.
A simplified graph showing iridium content across the KT boundary as measured at Gubbio, Italy.
As new seafloor forms, the igneous rock records the current state of the Earth's magnetic field. Sedimentary rock layers forming at the bottom of the ocean may also record these magnetic flip-flops as sediment layers slowly build up over time.
Molecular components of DNA
The molecular components of DNA: phosphates, deoxyribose (a sugar), and the four nitrogenous bases, adenine, guanine, cytosine, and thymine.
Molecular structure of chlorine nitrate
In the presence of another molecule to serve as a catalyst (not shown), nitrogen dioxide (NO2
) and chlorine monoxide (ClO), a byproduct of the breakup of ozone molecules by CFCs, react to form chlorine nitrate (ClONO2
Multiple explanations for the extinction of dinosaurs
The evidence supports the idea that a giant asteroid struck Earth at the end of the Cretaceous, but additional explanations (volcanic activity, global climate change, sea level change, and tectonic movements) may also have played a role.
Northern hemisphere ozone change
Change in the amount of ozone in the Northern Hemisphere over a 17-year period. Note the large decline in ozone during the winter months and how the winter decline is greatest at higher latitudes.
Nuclear vs. mitochondrial DNA
Unlike nuclear DNA (left), mitochondrial DNA is only inherited from the maternal lineage (right) — a quirk which allowed Margulis to determine whether mitochondrial DNA encoded unique traits.
Ozone depletion - CFC phaseout
A graph showing world production of three major CFC types between 1970 and 1988. The dot marks the year (1977) that the US phaseout of CFCs was announced.
Ozone levels fluctuate so widely that it is difficult to detect subtle trends over a short-term period, as shown by these ozone measurements for the atmosphere over Switzerland taken between 1926 and 1975.
With the CFC ban in place, atmospheric levels of chlorine are beginning to decline.
Peer review of a scientific study submitted for publication provides assurance that someone who knows what they're doing has double-checked it.
Products of deuterium fusion
According to nuclear theory, deuterium atoms fuse and forming helium-4 which has a lot of energy — so much energy that it is unstable, so the atom discharges some of this energy by releasing a neutron, proton, or gamma ray.
Rutherford's argument (1 of 2)
In the early 1900s, Ernest Rutherford and his colleagues performed this experiment to test the hypothesis that an atom's mass and positive charge are spread diffusely throughout the atom and found that their expectations and actual observations did not match at all.
This checklist provides a guide for what sorts of activities are encompassed by science.
Scientific arguments (1 of 2)
Taken together, the expectations generated by a scientific idea and the actual observations relevant to those expectations form what we'll call a scientific argument.
Scientific arguments (2 of 2)
Though the elements of a scientific argument are always related in the same logical way, those elements may be assembled in different orders.
Scientists sometimes work alone and sometimes work together, but communication within the scientific community is always important.
People from all over the world from all sorts of different cultures and backgrounds are a part of the scientific community.
A scientific idea may require a lot of reasoning to work out an appropriate test, but to be scientific, an idea must be testable.
Testing an idea (1 of 3)
Psychologist Edward Tolman wanted to know how rats successfully navigate their surroundings — for example, a maze containing a hidden reward. Would they build mental maps of the maze as they investigated it or would they learn to navigate the maze through stimulus-response?
Testing an idea (2 of 3)
Tolman and his colleagues trained rats in a maze which offered them many different tunnels to enter first. One of the tunnels twisted and turned but consistently led to the reward, and the rats quickly learned to go down that tunnel.
Testing an idea (3 of 3)
When the entrance to the reward tunnel was blocked, most of the rats picked a tunnel that led in the direction of the food, supporting the idea that rats navigate using something like a mental map.
Testing hypotheses (1 of 2)
The atoll Eniwetok (Anewetak) in the Marshall Islands — an oceanic ring of exposed coral surrounding a central lagoon.
Testing scientific ideas
Scientific testing occurs in two logical steps: (1) if the idea is correct, what would we expect to see, and (2) does that expectation match what we actually observe? Ideas are supported when actual observations (i.e., results) match expected observations and are contradicted when they do not match.
Testing the endosymbiotic hypothesis
Testing the hypothesis: If some organelles evolved via endosymbiosis, we would expect to see a close genetic relationship between the organelles and bacteria.
Testing the supernova hypothesis
To further test the supernova hypothesis:If a supernova had occurred, it would have also released plutonium-244 as well as iridium. Original tests looked positive but replicating the analysis showed that there was no plutonium in the sample, contradicting the supernova hypothesis.
Testing the two hypotheses (4 of 4)
A group of scientists showed that if you remove tubule organelles from a eukaryotic cell, they can grow back. This discovery argued against the idea that these organelles evolved via endosymbiosis.
The argument for plate tectonics (1 of 6)
Powerful scientific ideas generate many different expectations, not just one. As an example, let's return to the idea that the continents as we know them today were once joined together into a supercontinent and have been moving apart ever since. This idea generates many different expectations; we would expect to find corresponding fossils on now distant continents.
The fusion cell (2 of 3)
A detailed diagram of Pons and Fleischmann's cold fusion cell from one of their published papers.
The fusion cell (3 of 3)
To really know how much heat is being produced by the fusion cell it is necessary to estimate how much heat is escaping from it.
The iridium anomaly
The results of the iridium analysis were surprising. The team found more than 30 times what they had expected.
The supernova hypothesis
An existing hypothesis proposed that a supernova at the end of the Cretaceous had caused the extinction of dinosaurs. Supernovas throw off heavy elements like iridium — so the hypothesis seemed to fit with the team's discovery of high iridium levels.
Thymine and guanine
The visiting American chemist, Jerry Donohue, provided a key piece of evidence when he revealed that the forms given for thymine and guanine in most textbooks were wrong. Note the changes, indicated by the glowing hydrogens.
Untangling media messages
Media representations of science and science-related policy are essential for quickly communicating scientific messages to the broad public; however, some important parts of the scientific message can easily get lost or garbled in translation.
Updating evolutionary theory
Margulis didn't overthrow any of the core ideas of evolution, but she did force some of them to move over and make room for endosymbiosis.
Watson and Crick's first model of DNA
Watson and Crick's model erroneously placed the bases on the outside of the DNA molecule with the phosphates, bound by magnesium or calcium ions, inside.
X-ray diffraction was a tool developed in the first half of the 20th century to infer atomic structure.