Hurricane Florence: More than just the weather — climate change, too

With early estimates of Hurricane Florence’s damage at $17–$22 billion, questions are again being raised on the role of climate change in extreme weather. Did climate change cause this large storm, or was it just bad weather and bad luck? The answer is — both.

Hurricane Florence made landfall in North Carolina on September 14, 2018 as a Category 1 hurricane. The storm made slow westward movement across the Carolinas, dropping record amounts of rainfall across a wide swath of eastern North Carolina. Coastal property was damaged from storm surges of up to 10 feet, but the damage extended far inland. Rivers in both North and South Carolina set record flood levels, and the riverine flooding is expected to last for weeks.

As with any extreme weather event, it’s not accurate to say that climate change exclusively caused the event. Climate is the long-term average temperature and precipitation conditions of a certain place. It is not the day-to-day variability in those conditions – that’s the weather. However, this distinction is often lost in the popular debate about climate change and hurricanes. This post will clarify the nuance around this debate and discuss an important new advance in understanding the climate connection.

How does weather cause hurricanes?

Hurricanes (or, more generally, tropical cyclones) form over warm ocean water near the equator. To grow a storm from an initial atmospheric disturbance to the well-formed structure we’re familiar with, the right weather conditions have to be in place. The surface ocean waters have to be warm, and the winds have to be weak and not change much with height.

These conditions can and do happen at any point in the year, but September is the month with the most hurricane activity, since that’s when Northern Hemisphere ocean waters are their warmest.

Still, tropical cyclones are rare events with about 11 forming in the Atlantic Ocean in a normal year. About 6 of these will reach hurricane strength, as defined by windspeed. Landfalling hurricanes are even rarer, with the U.S. historically experiencing a landfalling hurricane about 18 times each decade (note this statistic is for hurricanes, not all tropical cyclones).

How does climate change influence hurricanes?

Assessment reports, such as the Fourth National Climate Assessment (NCA4), provide robust conclusions of the latest scientific understanding of how climate change influences tropical cyclones and hurricanes. Instead of just relying on a single study, assessment reports review the entire body of literature for a certain topic to understand the confidence and certainty around various findings. If 20 studies all point to the same conclusion, there is more confidence in that result than if only 2 studies do or if there is disagreement between the conclusions of different studies.

Assessment reports use terms like ‘low confidence’ and ‘extremely likely’ in very specific and defined ways. For example, NCA4 ascribes low confidence to a finding when there is “inconclusive evidence (limited sources, extrapolations, inconsistent findings, poor documentation and/or methods not tested, etc.), disagreement or lack of opinions among experts.” A finding supported by a single study would be ascribed low confidence. This does not imply anything about the quality of the study or the chances that the finding will hold up over time. It just means that we’re early in our understanding of the phenomenon of study. Similarly, a finding in NCA4 with high confidence is something that has been studied by many researchers for many years and is supported by a large body of evidence.

What do assessment reports tell us about tropical cyclones and climate change? Unfortunately, they don’t tell us much with high confidence. The NCA4 assesses trends in the number of tropical cyclones, how strong they get (as measured by windspeed), and the tracks they follow. While these trends do exist and have been measured in the literature, there is still low confidence in the trends because there is a limited body of evidence supporting them.

Assessment reports do, however, have high confidence that heavy precipitation events are getting more intense and that sea levels are rising for most parts of the United States. This high confidence level is rooted in part in the extensive and robust observational record of rain events and sea levels. In contrast to tropical cyclones, for which the best datasets only go back 40 years or so, reliable data exists for rainfall and sea level for 100 years or more. In fact, tide gauges in the Carolinas show sea level rise of around one foot over the last century. Increasing rainfall and higher seas both cause the consequences of hurricanes to be greater than in the past, all else being equal.

It is also worth stating that the other key factor in the destructiveness of hurricanes is how much property and infrastructure lies in a hurricane’s path. As coastal populations grow, even small storms can cause large damages.

A new technique for understanding the climate change influence

The science of climate change attribution takes advantage of computer models and statistical analysis to determine the extent to which anthropogenic climate change is making extreme weather events more likely. Typically, researchers using this approach will run many simulations of weather conditions, with and without climate forcing, to ask the question “what’s the return period of an event like _____?”.

For example, this kind of study on Hurricane Harvey found that the chance of 1 meter of rain falling on Houston, roughly what Harvey dropped on the area, is 3 times more likely because of climate change.

A team of researchers at Stony Brook University, Lawrence Berkeley National Laboratory, and the National Center for Atmospheric Research developed a new technique to ask a slightly different question. They asked, “how strong would Hurricane Florence get if it happened in a pre-industrial world?”.

This new technique is different in an important way. It side-steps the debate of whether it’s appropriate to discuss the climate change influence on a weather event. It explicitly acknowledges the specific weather conditions that led to the formation of this specific hurricane. The researchers spun up their model to the actual meteorological conditions on September 11, 2018 (3 days before Florence’s landfall) and then ran forecasts of Florence’s storm track. They ran two sets of forecasts – one reflecting current conditions, and one where they removed the warming that has already occurred. The team found that in current conditions the forecast was for a bigger, stronger storm with up to 50 percent more rain, compared to what would have been forecast in a pre-industrial world.

The new study doesn’t deal with the statistics of hurricanes like Florence – it deals with Florence itself. So, the attribution is really just looking at the impact of increased air temperature, increased water temperature, and increased air moisture, given that the correct weather conditions were in place to create Hurricane Florence.

There is a robust body of evidence linking a warmer, wetter atmosphere with heavier rain; and also linking a warmer surface ocean with stronger, bigger hurricanes. The new insight that this methodology provides is to attempt to quantify these factors for an individual storm.

How can policymakers use this scientific understanding?

The irony is that the more storms we have like Florence, the more evidence we’ll have that climate impacts are making them bigger and stronger, and the stronger the case will be to both build resilience against such storms and reduce our emissions to slow the warming responsible for intensifying their impacts. While the scientific understanding of past changes in hurricanes is still growing, there is higher confidence about the future. The scientific assessment is that we’ll see more destructive storms in coming years. Fortunately, we already have many cost-effective strategies to reduce the threat from hurricanes. We just need to leverage our scientific understanding into action.