No 'Groves About It

Weather and climate vs. the environment in this year's Atlantic hurricane season

Oct 15, 2024

Hurricane Helene's path (left) went directly over the Big Bend region of Florida, which as you can see from the map in the middle is one of the lowest-density population area in the entire state. By contrast, Hurricane Milton's path (at right) across the I-4 corridor is densely populated.

Remember when American meteorologists kept commenting about 2024’s quieter-than-expected hurricane season? At the beginning of the summer forecasters predicted as many as twenty four named storms, but as of September 10th there had only been five, with just three reaching hurricane status (sustained winds greater than 74mph). A writer at Forbes magazine even suggested that this ‘disappointment’ proved climate change was not affecting hurricanes as much as people thought: “it’s important to maintain a balanced perspective,” Jim Foerster wrote, for “the long-term data reminds us that the relationship between warmer waters and hurricane activity is more complex than often portrayed.” While it is true that Atlantic hurricanes are influenced by the Atlantic Multidecadal Oscillation (discussed in Hot Tub Time Machine Part 2), he neglects to mention that sea surface temperatures have remained abnormally high independent of the AMO’s modality in recent decades, suggesting that other forces (ahem, climate change) are at play.

Avid readers of In Circulation will remember this graph from Knudsen et. al., discussed in a previous newsletter. Notice how the North Atlantic SST anomaly (Graph b) is decisively positive starting in 1980 — even though the AMO (Graph c) remained in its negative phase until 2000!

In his attempt to “[reveal] a more nuanced reality,” Foerster cites a ‘research’ study, which turned out to be a blog post discussing the supposed contradictions contained within the National Oceanic and Atmospheric Administration (NOAA)’s 2023 State of the Science Fact Sheet (I had to read the fine print, literally, to find a link to the original publication). The Fact Sheet is typical of scientific resources in that barely anything is stated definitively due to a “lack of scientific consensus.” However, the report did have several noteworthy conclusions, one of which being the “propagation speed,” or velocity, of storms has been decreasing over the past century. In other words, storms have been moving slower across land and sea, increasing the potential for rain and wind damage (by contrast, the ‘research’ study cites this metric as evidence that hurricanes will weaken because of climate change, which is simply incorrect). NOAA’s Fact Sheet also asserted that while the number of tropical storms would decrease overall, that of devastating Category 4 & 5 hurricanes would increase over time.

NOAA’s predictions turned out to be right: of the nine named hurricanes this season, half have been major hurricanes. On September 26th, Hurricane Helene made landfall on Big Bend, an area between the cities of Tallahassee and St. Petersburg on the state’s Nature Coast, and on October 9th Hurricane Milton came ashore just 250 miles down the coast. News stories have focused on the extreme strength of both storms, specifically their rapid intensification as they traveled across an abnormally-warm Gulf of Mexico. According to contemporaneous data published by NOAA, the Gulf is currently 2-3°F hotter than average, with sea surface temperatures (SSTs) sitting comfortably around 31°C. This has caused hurricanes to intensify: warm water carries more energy, and as it evaporates this energy is transported into the atmosphere, where it works to expand the scale and power of the storm.

Helene entered the Gulf of Mexico off the coast of Cancún on 9/25 and made landfall in northern Florida on 9/26. This gif from NOAA of sea surface temperatures shows the large pockets of 31°C water getting smaller as the week went on, possibly because of the large amount of evaporation

As usually happens with extreme weather stories, mainstream news sources often fail to differentiate between weather, climate, and the environment. One newscaster on MSNBC commented that the only silver lining of Helene was that it landed in one of the least-populated, marshiest areas of the entire Gulf Coast. He neglected to mention why this was important: coastal wetlands are incredibly resilient environments that work to protect humans from the brunt of hurricanes & floods. That these systems form a front-line defense, however, has made them increasingly threatened by human-induced climate change. As sea levels rise and ocean temperatures increase, coastal flora & fauna have moved further inland for survival; these vulnerable environments are therefore encroaching on developed areas, heightening tensions between humans and nature. This change has been extremely visible in recent years: researchers in Maine, for example, have warned the state could lose between 28-57% of its salt marshes by the end of the century because of sea level rise and coastal overdevelopment.

These facts are routinely ignored by climate skeptics, who prefer to focus on the singularity of severe weather events, or exploited by climate activists, who weave a narrative of irreversible environmental destruction. And yet, the science tells a slightly different story. This week on In Circulation, we’ll use Florida as a case study to examine the dynamics of coastal wetlands and how they are affected by both extreme weather and our changing climate.

What’s happening to our wetlands?

Globally, wetlands are responsible for storing 700 billion tons of carbon, with 11.52 billion of those sequestered deep in US soil, according to a 2016 study by scientists Nahlik and Fennessy. A separate study showed that wetlands can shift from sink to source in as little as 61 years; if these environments disappear, a catastrophic amount of greenhouse gases would be released into the atmosphere, compounding the effects of climate change. This explains why wetland preservation initiatives have become a popular commodity in the global carbon credit market. Companies seeking to offset their CO2 emissions (like the one that probably funded Nahlik and Fennessy’s research) can buy ‘stock’ in projects that reduce atmospheric carbon. While some may interpret this as a new form of capitalistic land exploitation, many of these offsetting organizations take the time to educate clients about that in which they’re investing. Yet even the most well-intentioned companies gloss over the diversity and complexity of wetland ecosystems, which cover about 75.5 million acres, or around 4% of the US’s total land area. The main types are outlined below:

Bogs and fens are areas with low-lying shrubs growing on top of peat, which is a layer of decomposed plant matter that acts as a natural fertilizer and preservative. While fens are deep enough to be fed by groundwater, bogs are kept moist solely from precipitation (and occasionally local river runoff). Both are generally found in the northern United States, especially areas that were previously covered with ice during the Last Glacial Maximum 20,000 years ago. These environments are uniquely primed to trap and store methane (CH4), a gas emitted during the decomposition process, within layers of peat. However, excess greenhouse gases have begun to inhibit these environments’ ability to sequester CH4: a 2014 study of northern peatlands found that fens will actually become carbon sources, not sinks, because hotter air temperatures are drying them out!

Salt marshes are low-lying, open plains of grasses with little creeks and pools that are intermittently flooded by rising tides. They’re found along coastlines of oceans and large lakes, and according to NOAA, roughly half of the US’s salt marshes are located along the Gulf Coast. In addition to providing food and shelter to over 75% of fisheries species, these environments also protect shorelines from erosion by buffering wave action. They reduce flooding by slowing and absorbing precipitation, filtering runoff, and trapping excess nutrients into their mud. As mentioned above, salt marshes are threatened by the rapidity of sea level rise in recent years, rendering them vulnerable to submergence and invasive species.

There are several different types of swamps, which are environments that are seasonally flooded yet retain moisture year-round. Swamps can house either low-lying shrubs like Buttonbush or large trees like the Black Mangrove, Red Maple or Bald Cypress. These environments are found in the broad floodplains of the Northeast, Southeast, and south-central US and receive water from nearby rivers & streams. However, the presence of moist soil and tall vegetation does not always signify a swamp — ghost forests are former deciduous forests that have become inundated with water from rising sea levels. These pop up in coastal environments or where inland flooding from large lakes and rivers has become permanent.

An example of a Red Maple swamp in Florida (left) versus a ghost forest in the mid-Atlantic USA. Notice how swamp plants are thriving despite the high water levels, whereas the oak and birch trees shown in the photo on the right are dead and slowly being decomposed.

Weathering The Storm(s)

Because wetlands are migratory ecosystems, when one foundational species moves out of the area, another is likely to replace it. Sometimes this is viewed as a negative ecosystem change, like how the Phragmites australis grass has taken over Cape Cod’s salt marshes because it is taller than the native Spartina alterniflora, rendering it better suited to survive higher levels of flooding. But a 2014 study by Williams et. al. observed that the increasing presence of mangroves in drowned salt marshes along Florida’s Big Bend coast was beneficial for the environment: although mangroves now grow in places they ‘weren’t supposed to’, the wetland ecosystem was able to continue, albeit in a different form.

Florida is unique because it is the northernmost location in the USA where mangroves can grow, so it should be a prime location to study the northward migration of mangroves into low-lying marshes. According to Florida’s Department of Environmental Protection, this is happening because of global warming: the trees typically require a mean annual temperature of 66°F or higher. However, more recent studies have shown that, in other parts of Florida, mangrove encroachment into salt marsh territory is aided by frequent hurricanes. Researchers at Florida International University discovered that storm surges caused by major hurricanes actually helped fertilize mangrove forests by redistributing phosphorous-rich sediments from the ocean floor to coastal lands. Yet their study, and others like it, focused on populations present elsewhere along the Gulf Coast, such as in Everglades National Park or near the Mississippi River Delta.

There are only two parts of the world where salt marshes (Black lines) and mangrove swamps (Red lines) overlap: the Nile River Delta and the Gulf of Mexico. This is because the northernmost extent of mangrove forests (25°N) overlaps with the southernmost extent of marshland.

In fact, there is a surprising dearth of academic research on the marsh-mangrove ecotone in Florida’s Gulf Coast, which is one of the only parts of the world where both mangroves and saltmarsh plants naturally occur. Only a handful of scientists have published studies on whether salt marsh-mangrove cohabitation affects resiliency to hurricanes. Armitage et. al. examined the effect of 2017’s Hurricane Harvey on mangroves and marsh grasses along the Texas Gulf Coast; they discovered that even a handful of mangroves growing alongside marsh grasses helped reduce erosion by 100% compared to areas with only saltmarsh plants (0.5m of coastline lost versus 5m)! Given mangroves’ unique and sturdy root structure, it makes sense that their presence stabilizes coastal areas. One would assume, then, that the conservation goals of the state of Florida would reflect a desire to protect mangrove forests. However, their current management strategy is to build levees all along the coast; while these structures temporarily protect homes and roads, they are constricting the natural expansion and diversification of wetlands. Scientists concur that living shorelines are far better than man-made ones, so permitting the horizontal expansion of mangrove forests will better protect vulnerable coastal areas from sea-level-rise-induced erosion and hurricane-induced flooding.

According to the National Wildlife Federation, red mangroves are sometimes called “walking trees” because the shape of their roots make them look like they’re walking on water!

Conclusion: Putting the “Model” in “Role Model”

Although Florida’s Gulf Coast is usually spared from severe weather — prior to Helene and Milton, the area had not seen direct impact from a major hurricane since 1921 — climate change is altering the trajectory and strength of modern storms. Admittedly, the lack of hurricane impacts has made it hard to study the resiliency of wetlands in the field, but I was surprised to find almost no papers that used computer modeling to do so. In recent years, many researchers have relied on weather and climate models to simulate the effect of sea level rise or 100-year floods, allowing scientists to better envision our uncertain future. Yet when I searched for “the effect of hurricanes on mangroves climate modeling” in Google Scholar, all of the recent papers discussed merely the best ways of accounting for mangrove resilience when modeling storm surge and erosion, not the direct impact of severe storms on these environments.

Since the media tends to take its lead from the scientific community, it shouldn’t be a surprise that no news articles have been published (yet) documenting the extent of recent storm damage to coastal wetlands. In this case, one cannot fault the journalists: many of these swamps and marshes are hard to reach in the best of times, let alone after two fierce hurricanes have swept through. Yet it is during times like these that ‘science’ seems to move at a frustratingly slow pace — even Armitage’s study, which was conducted as promptly as possible, was not published until 3 years after Hurricane Harvey had hit. While the peer review process is a tedious one, if an environmental journalist had accompanied these scientists during their research, perhaps the results would have caught the eye of local governments along the Gulf of Mexico, who could have worked to prepare their environs for the inevitable.

Jim Foerster wrote in his Forbes article that when it comes to hurricanes, “the only certainty is uncertainty.” This may be true from a meteorologist’s perspective, but making this claim in an article about climate change is irresponsible, especially from an American Meteorology Society-certified consultant. He falls into the trap of conflating weather with climate, as many environmental journalists do nowadays (with those at The New York Times being the most frequent offenders). While the joke used to be how ‘the weatherman is always wrong,’ advanced computer modeling and increasingly accessible fact sheets have improved how research is conducted and communicated to the public. If more journalists followed the science from start to finish instead of waiting for research to be published, it would ensure one doesn’t explain only part of the truth to their readers.

In Circulation

No 'Groves About It

Weather and climate vs. the environment in this year's Atlantic hurricane season

What’s happening to our wetlands?

Weathering The Storm(s)

Conclusion: Putting the “Model” in “Role Model”

Discussion about this post