Yesterday a confluence of bad weather that included Hurricane Sandy brought sustained winds of 80 miles per hour, threw a large 13-foot storm surge against New Jersey, and spanned a thousand miles. It forced the cancellation of over 10,000 flights, has left millions without power, and has killed at least 17 people. One early estimate puts the cost of the storm at about $20 billion. The intensity and sheer size of the storm have meteorologists declaring that they’ve never seen anything like this before.

Would this intense, widespread storm have happened if it weren’t for climate change?

To state the obvious, extreme weather is nothing new. Scientists have discovered traces of periods of storm activity in the Northeast U.S. since the last ice age that were far worse than storms we’ve seen recently. We’ve have had to deal with intense storms long before we started burning fossil fuels in huge amounts and increasing levels of heat-trapping gases such as carbon dioxide in the atmosphere. There’s a huge amount of natural variability in the weather: storms can collide, as just happened in the Northeast. They can, in turn, be exacerbated by weather patterns such as El Nino. Because of these sources of natural variation, it’s impossible to attribute any single storm to climate change.

But as greenhouse gases accumulate, the surface temperature of the ocean has been gradually increasing, leading to moister air over the ocean that is likely making some storms far worse. The loss of sea ice in the Arctic may also be contributing to more severe storms by contributing to changes in the jet stream.

“All weather events are affected by climate change because the environment in which they occur is warmer and moister than it used to be,” writes Kevin Trenberth, a senior scientist at the National Center for Atmospheric Research, in the journal Climatic Change. He adds that the effect is especially pronounced in the most extreme weather events.

How does this happen? In the same paper, Trenberth offers one of the most succinct explanations I’ve read of how greenhouse gases can affect the weather, in some places promoting droughts and elsewhere leading to intense rainfall. It’s worth a read in its entirety, but here’s a summary of some of the key points.

As heat it trapped in the atmosphere, it increases evaporation, drying out some areas of land, and potentially increasing the number and intensity of droughts (see “Is Climate Change to Blame for the Current U.S. Drought?”).

Increased evaporation, especially over the oceans, adds to the amount of moisture in the air. Winds carry that moisture to places where storms are most likely to develop, and storms pull in water vapor from a large area around them to produce rain. “In weather systems, convergence of increased water vapor leads to more intense precipitation and the risk of heavy rain and snow events,” Trenberth writes.

These storms dump a lot of rain all at once, which can actually have the effect of increasing the amount of time between rain storms. “The result is longer dry spells, as observed in the United States,” Trenberth writes. “A warmer climate therefore increases risks of both drought—where it is not raining—and floods—where it is—but at different times and/or places.”

Putting an end to burning fuels immediately wouldn’t stop this effect because carbon dioxide levels will persist for a long time, which has led to increased calls for research into geoengineering—using technology to purposefully counter the effects of greenhouse gases (see “Geoengineering Could Be Essential to Reducing the Risk of Climate Change”).

Continuing to add more greenhouse gases to the atmosphere might exacerbate the impact, but because climate is complicated, no one knows by how much. There’s a lot about how the planet adapts to increasing levels of carbon dioxide (and particulates from pollution and other anthropogenic effects) that scientists don’t know.

But some insurance companies, are getting worried. A major reinsurer, Munich Re, recently issued a report on weather disasters, noting in a press release that,

“Nowhere in the world is the rising number of natural catastrophes more evident than in North America. The study shows a nearly quintupled number of weather-related loss events in North America for the past three decades, compared with an increase factor of 4 in Asia, 2.5 in Africa, 2 in Europe and 1.5 in South America. Anthropogenic climate change is believed to contribute to this trend.”

The study is meant to help insurers and re-insurers assess risk. According to Munich Re, “All stakeholders should collaborate and close ranks to support improved adaptation [to climate change]. In addition, climate change mitigation measures should be supported to limit global warming in the long term to a still manageable level.”

Tony Kuczinski, CEO of Munich Reinsurance America, comments in the release,

“What is clearly evident when the long term data is reviewed is that losses from weather events are trending upward. To simply say that this trend is a statistical anomaly or part of a long-term cycle of activity misses the point of these efforts – we must set aside our biases and continue a meaningful dialogue in search of answers to mitigate the losses that we are experiencing.”