Between 12,800 to 11,500 bp (before present) there was a cold period in North America called the Younger Dryas – named after the dryas flower whose pollen is a good marker for such cold periods. During this time the megafauna of North America, including the Mammoth, largely died out. Along with them went the Clovis culture – a big game hunting culture with distinctive stone points.

What caused this period of climate change and mass extinction?

This is a genuine scientific controversy. One group of scientists believe that the melting glaciers dumped fresh water into the northern Atlantic, temporarily shutting down the ocean currents that bring warm water to North America. Another group think that a comet impact is to blame.

There is no smoking gun of a comet or meteor impact at this time, however. This is partly why the debate continues. However, there is significant inferential evidence, and as this evidence grows we may be getting to a tipping point where the comet theory becomes dominant.

There are two new published papers which the authors claim support the comet theory. I don’t know if this will be the tipping point, but the evidence is starting to seem pretty compelling. We’ll have to see how the other side reacts.

The Big Burn

So far the evidence presented for a comet impact is indirect but some scientists find it compelling. Mostly it consists of finding exotic materials in the geological layer that corresponds to the Younger Dryas boundary.

These exotic materials include spherules, nanodiamonds, melt glass, and a platinum spike. Comet impact proponents argue that these materials resulted from the impact. Critics claim that they are either erroneous or result from other causes, such as forest fires. And in any case – without an impact crater, the case for an impact is thin. Meanwhile the case for glacier melt is strong.

Now the comet proponents are firing another salvo – evidence from ice cores and other studies showing that right at the Younger Dryas boundary layer there is evidence for massive biomass burning. Further, there is a CO2 spike at the same time, enough to suggest that as much as 10% of the plant life on Earth at the time burned.

What this could mean is that there wasn’t a single impact, but that essentially the Earth passed through the cloud of a comet and was hit by a swarm of smaller pieces, igniting forest fires around the world.  Smoke from these fires could have then resulted in an impact winter – blocking out the sun reducing photosynthesis and reducing temperatures.

This conveniently explains away the absence of a large impact crater, while also explaining the presence of exotic impact-related materials, and also the products of massing burning, all at the Younger Dryas boundary layer.

This sounds like a reasonable case for the comet theory. Will it be compelling enough, however, to win over former critics?

Scientific Controversies

That is often how these scientific controversies work. Different factions make their case in the absence of definitive evidence, and also point to future evidence that will support or refute their hypothesis. The various sides duke it out, until eventually the evidence builds enough on one side that the majority of scientists come over.

There is no magic threshold, but at some fuzzy point we may have a consensus built on evidence that has been thoroughly debates by proponents of various hypotheses. A consensus does not mean 100%, no further evidence needed, the science is over. It just means that the evidence is strong enough in support of one side that the vast majority of scientists now accept that as the most likely answer.

There is always room for minority opinions, and sometimes surprising lines of evidence may disrupt the debate.

We see this process with many current scientific controversies. Is floresiensis (the Hobbit) a unique species or a diseased human? Did the dinosaur extinction result entirely from an impact, or did other sources of climate change play a role?

For a time there was serious debate about whether or not dark matter existed or were observations explainable by modifying Newtownian gravity. We have now reached a consensus that some form of dark matter probably exists – but until we identify exactly what it is, there is room for dissent.

There are countless such debates, big and small, in the world of science. That is how science works and progresses. For every question, there is a spectrum from unknown, to genuine controversy, to strong consensus, to rock solid. Science communicators should always strive to put any new study or bit of evidence into the context of where the scientific question is on this spectrum.

Further, there are practical implications for properly characterizing the state of a scientific controversy. What do we teach in classrooms? What research do we fund? Was medical treatments are acceptable, and should be reimburse with public funds? And what scientific conclusions are sufficient to base public policy on?

There is no formula. This takes judgment, a thorough understanding of the science, and an individual decision. Conclusions also have to remain open to revision as new evidence and theories come to light, but we also have to make decisions in the meantime.

The obvious controversy to which all this applies is global warming, because it is such a hot political topic. The prevailing scientific opinion is that there is a strong-enough consensus that global warming is happening and is due to human activity to form a basis for policy. That does not mean the science is over, and there are always error bars on every scientific conclusion. But we have a practical consensus. It would be folly to ignore it, or to focus myopically on uncertainty in order to manufacture paralysis.

It is useful to examine non-political scientific debates and controversies, to see how they work, and how they are resolved. Not many people are going to have a strong ideological position on the Younger Dryas, and I don’t expect to see it in the platforms of major political parties. That is why it is a great way to see how science works. Those lessons can then be applied to more politically controversial topics.