When are extreme events part of natural climate variability and when are they due to climate change? It’s important to ask—no matter where you stand on the role of humanity’s impact on the environment.

A group of international scientists decided to address this question, focusing on a dozen or so extreme events from 2012. Their results were published last week in the Bulletin of the American Meteorological Society. (The findings are also available in a downloadable report.)

And the results, were, well, variable.

The researchers did not look at Hurricane Sandy, but they did examine the flooding and the inundation it caused. Because of sea-level rise (a direct result of climate change), the researchers determined that the superstorm did far greater damage than it would have with oceans at normal levels.

The team also determined that heavy rains in the United Kingdom, Japan, and China were not due to global warming, and Australia’s above-average rainfall was due to a La Niña event (or short-term climate variability).

However, a deluge in New Zealand was due to climate change. From Wired:

Total moisture available for this extreme event was 1% to 5% higher as a result of anthropogenic greenhouse gas emissions.

And Arctic sea ice melt? The cap of sea ice covering the North Pole shrunk to its smallest extent last summer. The cause? Climate change.

What about last year’s devastating drought in the Midwest? Scientists judged that climate variability was to blame—not global warming.

However, Stanford researchers did find that the extreme heat that came with last summer’s drought could be attributed to climate change. They also found strong evidence that the high levels of greenhouse gases now in the atmosphere have increased the likelihood of severe heat.

In addition, their findings indicate that extreme weather in the north-central and northeastern United States is more than four times as likely to occur than it was in the pre-industrial era.

The Palo Alto scientists hope the results from these studies can help to quantify the true cost of emissions to society, since the cost of the disaster is measurable.

“Knowing how much our emissions have changed the likelihood of this kind of severe heat event can help us to minimize the impacts of the next heat wave, and to determine the value of avoiding further changes in climate,” says lead author Noah Diffenbaugh, a Stanford associate professor of environmental Earth system science.

Image: Theresa L Wysocki/Flickr

But what about the opposite? The US Geological Survey had a conference and issued a press release this month about preparing for severe flooding in our state. Flooding that could cause more damage and be even deadlier than a large earthquake.

What the what? How is this possible?

Atmospheric rivers, my friend, atmospheric rivers.

According to NOAA:

Atmospheric Rivers are narrow corridors of concentrated moisture in the atmosphere and are a key process linking weather and climate. When atmospheric rivers strike land, they produce flooding rains that can disrupt travel, induce mud slides, and cause catastrophic damage to life and property.

The USGS goes on to say that atmospheric rivers are:

a hurricane-like phenomenon that occurs on the west coast.

And the Sacramento Bee defines the phenomenon this way:

a focusing of high-powered winds that drag a fire hose of tropical moisture across the Pacific Ocean, pointed directly at California for days on end.

The big, wet storms we had in northern and southern California in December were due to a series of atmospheric rivers, but apparently only a tame series.

It can get much worse, says USGS,

Beginning on Christmas Eve, 1861, and continuing into early 1862, an extreme series of storms lasting 45 days struck California. The storms caused severe flooding, turning the Sacramento Valley into an inland sea. A storm comparable to that of 1861-1862 could occur again.

“We think this event happens once every 100 or 200 years or so, which puts it in the same category as our big San Andreas earthquakes,” stated Lucy Jones, chief scientist of the USGS Multi-Hazards Demonstration Project, in the recent press release.

To prepare for a storm of this magnitude and greater, USGS has created a model scenario called ARkStorm. The scenario estimates that California’s flood protection system would be overwhelmed and more than $300 billion in damage would result. That could be worse than a large earthquake.

Being prepared is the key.

“The time to begin taking action is now, before a devastating natural hazard event occurs,” said USGS Director, Marcia McNutt. “This scenario demonstrates firsthand how science can be the foundation to help build safer communities. The ARkStorm scenario is a scientifically vetted tool that emergency responders, elected officials and the general public can use to plan for a major catastrophic event to help prevent a hazard from becoming a disaster.”

Image of Sacramento in early 1862 courtesy of the Center for Sacramento History