Saturday, October 8, 2016

The Eye of the Storm: Hurricane Matthew, Attribution Science and Climate Change (Video)

It is widely accepted that warmer seas contribute to hurricanes but there are also a number of other factors that contribute to extreme weather events. Here is a review of the evidence linking climate change and Hurricane Matthew.

Matthew has already wreaked havoc in Cuba and the Bahamas and it has killed almost one thousand people in Haiti. Four Americans are known to have died due to the storm. There are currently states of emergency in effect in Florida, Georgia, South Carolina and North Carolina.

Although Matthew has been downgraded from a level 5 to a level 1 hurricane it still packs a punch.  It looks as though central Florida was spared the worst, but cities along the East Coast are bracing for the storm.

Florida is no stranger to hurricanes. In 2004, four major hurricanes (Charley, Frances, Ivan, and Jeanne) caused billions of dollars in damage in the state. These storms were followed by Hurricane Katrina a year later.

The cause of these extreme weather events was the subject of cautious reflection in the scientific community.

While we know that climate models predict more intense extreme weather events scientist are very careful about attributing any one storm to climate change. That is not to say there is no link, it merely reflects the cautious approach taken by scientists.

Until very recently there was inadequate data to make a conclusive pronouncement. We now have more reliable satellite systems, which allow better tracking.

Scientists are reluctant to assign any given weather event to climate change. Initially, hurricane Sandy was dismissed as being unrelated to climate change. However, scientists subsequently confirmed that climate change did play a role in Sandy.

The science connecting hurricanes and climate change has improved. Some scientific observations are obvious. One such recent observation corroborating the worsening hurricane trend came from the East Pacific basin which saw Hurricane Patricia generate the highest wind speeds ever recorded on Earth.

Here is a summary of what we know about the climate change/hurricane connection: Warmer oceans spawn more storms and more storms will increase rainfall and water vapor. These phenomena can fuel more intense hurricanes.

The Washington Post delved into the question of the science linking Hurricane Matthew to climate change. They quoted a recent scientific overview on the links between hurricanes and climate.-climate:

“While no significant trends have been identified in the Atlantic since the late 19th century, significant observed trends in [tropical cyclone] numbers and intensities have occurred in this basin over the past few decades, and trends in other basins are increasingly being identified. However, understanding of the causes of these trends is incomplete, and confidence in these trends continues to be hampered by a lack of consistent observations in some basins.”

The situation in the Atlantic is complex. To understand the relationship between climate change and hurricanes in the Atlantic we must factor natural cycles (such as the so-called Atlantic Multidecadal Oscillation). We must even evaluate the impact of reductions in atmospheric aerosols that occurred following tighter regulations on these emissions.

Even when we factor these phenomena it is still very hard to attribute an individual storm to a changing climate.

Still, there are reasons to believe that although other factors are involved, Matthew has been exacerbated by climate change.

Kevin Trenberth, a researcher with the National Center for Atmospheric Research in Boulder, Colo., told the Washington Post there seems to be a combination of the overall warming trend and natural variability, such as the El Nino-La Nina cycle, behind what we’re seeing: "The overall increase in moisture is about 5 to 6% from climate change, and in a hurricane that gets doubled because the storm intensifies and increases the convergence of moisture. But in the Atlantic, in the year following El Nino, the [sea surface temperature] tends to be higher in the subtropics (because with El Nino the winds are lighter and more sunny skies), and indeed in the subtropics east of Florida, Georgia and South Carolina, [sea surface temperatures] have been running 2 deg C (3 to 4F) above normal, and moisture 10 to perhaps 15% above normal. Indeed this was the region that fed the Louisiana floods (not so much the Gulf). So the potential has been there: a natural variability component on top of the global warming to produce a very strong storm."

Matthew had a very rapid rate of escalation to a category 5 and it is also very persistent. These statistically anomalous observations are consistent with climate change but no clear causal link can be drawn. While observations should not be confused with proof they are nonetheless consistent with scientific predictions.

As the world warms, sea levels are rising and this will worsen the impacts of storms. Even small increases in sea level can have a big impact on storm surges.

Climate change has influenced the strength and endurance of Matthew.  While the evidence may not be sufficiently rigorous for a climatologist, those who extrapolate and draw conclusions from existing evidence may prove to be more accurate than the understandably cautious claims of scientists.

It is important to note that the data in no way disproves the connection between Matthew and a changing climate. However, scientists require an abundance of evidence before they will say anything with any degree of confidence.

In a Think Progress article, the venerable  Joe Romm pulls no punches in connecting this hurricane and climate change. "Matthew has already set a number of records — and global warming is giving it a boost," Romm asserts.

He then goes on to review some of Matthew's record-breaking feats and squarely attributes them to climate change:
  • longest-lived Category 4 (or higher) Atlantic hurricane ever recorded. 
  • most accumulated cyclone energy of any Atlantic hurricane ever recorded in the eastern Caribbean
He then cites several papers that support a close relationship between hurricanes and climate change. One showing a 20 - 30 percent increase in category 4 and 5 hurricanes in recent years. A few other studies show an increase in hurricane intensity including in the North Atlantic.

These observations are corroborated by a host of other findings including the fact that the summer of 2016 was the hottest summer in recorded history and possibly the hottest in "thousands of years."

The oceans are warmer and we are also seeing a sharp rise in the most damaging storm surges.

One tropical cyclone expert just warned, "Category 4 and 5 hurricanes could double or triple in the coming decades."

Romm comes to the logical conclusion that in the face of the evidence, "We simply cannot cut carbon pollution fast enough."

Growing Levels of GHGs are Warming the Planet and Contributing to Disasters
Decades of Hot Data: The Harbingers of an Impending Climate Catastrophe
Extreme Weather and Existential Reflections on Life in the Anthropocene
Strong Body of Evidence for a Changing Climate 
Consistent Ongoing Heat is Rewriting the Record Books (April Temperature Update)
Review of Extreme Weather in 2015
Video - Climate Change and Extreme Weather: Prof. Jennifer Francis
Video - Climate Change Fueling Wilder Weather (Climate Commission)
Typhoon Haiyan and Climate Change
Video - Superstorm Sandy Climate Change and Extreme Weather
IPCC Report Predicts More Frequent and More Intense Extreme Weather
Extreme Weather and the Costs of Climate Change

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