Workers tap into Marcellus natural gas at an active Hydraulic Fracturing drilling operation outside of Wellsboro, Pennsylvania operated by Shell. (Photo by Brett Carlsen for The Washington Post)

Last week, New York Democratic governor Andrew Cuomo banned the practice in his state of hydraulic fracturing or "fracking" -- blasting chemical laden water deep beneath the Earth at extreme pressures in order to crack rock and release natural gas. The move followed a report from the New York Department of Health, finding "significant uncertainties about the kinds of adverse health outcomes" that may be associated with the technology. It found the science on this question was uncertain but worrisome, and that was enough to put on the brakes.

Yet just a month ago, outgoing Maryland Democratic governor Martin O'Malley decided to let fracking go forward in the western part of his state. This, in turn, was based on a report from Maryland's departments of Environment and Natural Resources, which concluded that with adequate regulation,"the risks of Marcellus Shale development can be managed to an acceptable level."

So what's going on here? How can two states comprehensively assess the risks of hydraulic fracturing and then decide on very divergent policies?

Certainly, it's not that they were looking at radically different science. The reports came out within a month of one another, and given that these are professional state scientific agencies, they probably didn't miss much of significance to their assessments. And indeed, both reports acknowledge that there are risks from fracking, due to the potential for both water and air contamination -- although there is a great deal that we still don't know about the magnitude of these risks, or the long term effects.

So if the science didn't divide Maryland and New York, what did? Here are four factors:

The politics. First and most obviously, it is hard not to note that Maryland governor Martin O'Malley is on the way out, to be replaced by a strongly pro-fracking Republican, Larry Hogan. New York Governor Andrew Cuomo, on the other hand,was just reelected, which surely made his decision easier. So a ban on fracking in Maryland would, in all likelihood, have been a top priority for the state's new governor to reverse. In New York, the political context is totally different.

Which agency did the report. In Maryland, the Environment and Natural Resources departments did the study, whereas in New York it was the Department of Health, observes Stanford's Rob Jackson, who has published a number of influential studies on the link between fracking and groundwater contamination. "It's not surprising that a health department would frame the issue differently -- and reach a different conclusion," said Jackson. Environment and natural resources departments are more likely to balance health and environmental risks against economic promise -- but health departments primarily worry about protecting people.

Indeed, Kate Sinding of the Natural Resources Defense Council, which just released a report of its own on the health risks associated with air emissions from fracking-enhanced drilling, points out that Maryland also did a health-focused report of its own. It was conducted by the Maryland Institute for Applied Environmental Health at the University of Maryland, College Park, on behalf of the state. And that report found that in eight separate areas where fracking could have public health effects, the risks were "high" in four of them, "moderately high" in three, and "low" in only one area (earthquakes):

Source: "Potential Public Health Impacts of Natural Gas Development and Production in the
Marcellus Shale in Western Maryland," Maryland Institute for Applied Environmental Health
School of Public Health University of Maryland, College Park. July 2014.

"If you actually look at what the health professionals were saying in the two states, they’re pretty aligned," says Sinding.

The broader and final Maryland report acknowledged these risks, but nonetheless concluded that "best practices and rigorous monitoring, inspection and enforcement can manage and reduce the risks."

The amount of land at stake. Let's face it: Maryland also has less at stake, overall, than New York does, simply because a much smaller area is on the table for drilling and fracking. "The thing is, Maryland has a tiny bit of land in play, where New York has a huge amount of the Marcellus and Utica.  So, there is a very big difference in scale," says Alan Krupnick of Resources for the Future.

A simple look at a map proves the point -- we’re only talking about a slice of western Maryland, wedged between West Virginia and Pennsylvania, that’s promising for drilling. Here's a map from the U.S. Geological Survey of the extent of the Utica Shale (maps of the Marcellus Shale lead to a similar conclusion):

Source: USGS

This means the benefits, but also the risks, from fracking loom considerably larger in New York. If ten years from now, more science is in and the health risks look even more severe than they do now, there could be many more people at risk in New York.

The precautionary principle. The starkest difference, though, may be that unlike Maryland's research, the New York health report pretty clearly hews to an approach known as the "precautionary principle," which suggests that in the face of inadequate scientific information about risks, it is wise to pause and wait for more data, rather than allow potential harm to occur. The precautionary principle was on full display in a statement by acting New York health department commissioner Howard Zucker, who remarked,

I have considered all of the data and find significant questions and risks to public health which as of yet are unanswered. I think it would be reckless to proceed in New York until more authoritative research is done. I asked myself, 'would I let my family live in a community with fracking?' The answer is no. I therefore cannot recommend anyone else's family to live in such a community either.

It is important to recognize that the precautionary principle is not a purely scientific position -- nor is it an anti-scientific one. Rather, it represents a risk-aversive orientation towards scientific uncertainty -- a conscious decision that unknown risks are too serious to ignore.

That's why criticisms of New York's move on "scientific grounds" don't make much sense. For instance, one blog post at the pro-fracking site Energy in Depth sought to individually critique some of the health-related studies that fed into the New York report. But that's kind of missing the point: These studies don't need to be the unassailable "truth" in order for New York to justify its precautionary position. Rather, the state simply needs to be able to point to a body of evidence that, on the whole, raises concern.

Granted, if you were to act in a highly precautionary fashion towards every imaginable risk, nothing would ever happen in the world. A reasonable articulation of the precautionary principle, in contrast, is one in which risks must at least be plausible before they prevent an action -- like drilling and fracking -- that also has clear economic and other benefits. In this case, though, the risks are plausible, although highly uncertain, notes Stanford's Rob Jackson. "I do think there’s enough information on the air side and the water side at least to be concerned," he says -- though he emphasizes that there is a great need for longer term health studies, with much larger pools of research subjects.

What all this shows is that these two decisions on fracking, while draped in scientific language, were -- in fact -- probably not really scientific decisions at all.