Highlights of 2010: Definitive statements: a new trend?

by Megan Sever
Friday, January 20, 2012


“This is the way it was.” Or: “This is what is happening.” Hmmm. Scientists don’t usually make such definitive statements, given that in science, there are almost always caveats. Yet in the last year, such statements have been issued by several large groups of scientists who have come together to support a certain point of view. Are scientists feeling the need to dig in their heels because of public pressures? Or are we actually reaching some consensus?

In particular, one wonders what brought about the sudden interest in making such blanket statements on two of arguably the most contentious issues in the geosciences: mass extinctions and climate change. Scientific societies have been making those consensus statements for a couple of years now about climate change, in an effort to present a united front to naysayers — so that’s not entirely surprising. As for extinctions, large groups of scientists have made several definitive statements this year: that the end-Cretaceous extinction event was caused by an asteroid impact; and then groups came down on both sides of the debate about whether a comet caused the Younger Dryas cold spell.

In the March 5 issue of Science, a team of 41 scientists from institutions around the world published a paper that reviewed all the available evidence over the past 30 years and concluded that an asteroid that struck the Yucatán Peninsula in Mexico 65 million years ago killed off half of all species on the planet, including the dinosaurs. The impact theory is the “only plausible” explanation of the end-Cretaceous mass extinction, they wrote.

The asteroid impact theory was first published by Luis and Walter Alvarez and colleagues in Science in June 1980. They hypothesized that a 6- to 14-kilometer-wide asteroid struck Earth with a force a billion times more powerful than the atomic bomb that hit Hiroshima. The impact sent out a ball of fire and tremendous amounts of dust and material into the atmosphere, choking out life and collapsing the food chain.

Since that theory was presented, others have also been floated — most notably that massive volcanic activity caused the extinction. The Deccan Traps in India formed during an eruption of some 1.1 million cubic kilometers of basalt over a period of about 1.5 million years. Researchers have reported that this eruption would have cooled the atmosphere and produced enough acid rain to choke out life.

However, the March paper in Science stated that all of the geological evidence suggests that the extinction event occurred too rapidly to have been caused by the Deccan Traps erup­tion. Plus, the scientists wrote, models and data agree that the amount of poisonous gases released by the eruption would not have caused enough widespread and long-lasting damage to have created a rapid mass extinction.

Will this “consensus statement” end the debate about what killed the dinosaurs? Not a chance. But on the 30th anniversary of the original theory’s presentation, it’s nice to know a lot of people do still agree with it.

Over the past couple of years, scientific meetings of the American Geophysical Union (AGU) and the Geological Society of America (GSA) have hosted multiple sessions — often highly contentious — on the cause of the Younger Dryas cold period and the megafauna extinctions, both of which occurred roughly 12,900 years ago. The issue at hand is whether a comet hitting the planet or exploding in the atmosphere could have caused the cold period and the extinctions, or whether it was something else (the leading thought is that melting ice sheets produced a catastrophic discharge of freshwater into the Atlantic Ocean, shutting down ocean circulation).

Some researchers say they have definitively found all the telltale evidence of an impact: nanodiamonds, microspherules, “fullerenes” with extraterrestrial helium and elevated iridium levels, and charcoal, black mats and other signs of widespread fires. Meanwhile, other papers have discounted each piece of that evidence.

Both sides have presented strongly worded statements supporting their points of view.

Who’s right? No one knows for sure. But as freelance writer David B. Williams wrote in the June EARTH, only one thing was clear after multiple sessions on the topic at last year’s AGU and GSA meetings: No one had changed anyone else’s mind.

But if you want to get into a real battle, bring up climate change. Assessments by the National Academies of Science in 2005, the National Research Council in 2006 and the Intergovernmental Panel on Climate Change in 2007 found that the global climate is changing and that human activities are at least in part to blame. In 2008, AGU released a similar consensus statement. In April 2010, GSA published its own consensus statement, also stating that the evidence suggests that human activities account for most of the warming since the mid-20th century. The American Association of Petroleum Geologists issued its statement in 2007, noting that although its members were “divided on the degree of influence that anthropogenic carbon dioxide has on recent and potential global temperature increases,” the members still supported more research on the topic. In May, the National Research Council issued three more reports emphasizing why the U.S. should act now to reduce greenhouse gas emissions and why the U.S. needs to “develop a national strategy to adapt to the inevitable impacts of climate change.”

Several researchers undertook a study this year to determine whether consensus statements actually change anyone else’s mind, and to understand why the public remains sharply divided even when expert scientists largely agree. Law professors Dan Kahan of Yale University and Donald Braman of George Washington University worked with political science professor Hank Jenkins-Smith of the University of Oklahoma to study topics as varied as climate change, nuclear waste disposal and gun control. They found that people’s own cultural values trumped “expert opinions,” they reported in September in the Journal of Risk Research. No matter how strong the scientific consensus is, it doesn’t seem to change people’s minds if it conflicts with personal beliefs.

“The problem won’t be fixed by simply trying to increase trust in scientists or awareness of what scientists believe,” Braman said in a press release accompanying the paper. “To make sure people form unbiased perceptions of what scientists are discovering, it is necessary to use communication strategies that reduce the likelihood that citizens of diverse values will find scientific findings threatening to their cultural commitments.”

Perhaps “extinctions” and “climate change” have joined “religion” and “politics” on the list of topics “not to be discussed in mixed company.”

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