Predictions for the 2018 Atlantic hurricane season, now officially under way, amount to a fairly typical forecast and hold no great surprises. Between projections from NOAA and from Colorado State University we’re pretty much looking at more than a dozen named storms, a half-dozen or so hurricanes and a few of those major ones.
Of course all it takes is one strategically-located, powerful storm to turn an average or even a below-average season prediction into a nightmare. Think hurricane Andrew in the accurately-predicted below-average year of 1992. Last year wasn’t supposed to have been all that terrible a season. Ask the victims of Maria, Harvey, and Irma how that worked out.
Hurricanes at best are difficult to predict at any stage of their development and lifespan. A lot of scientific data and new-age technology have improved our data collection ability and therefore our understandings. But assessing the impacts of climate change – they also difficult to predict – on hurricanes is very much a work in progress.
There is, however, beginning to be scientific consensus on a few key factors as noted in this recent report. But many more climate change/hurricane links remain to be determined.
Scientists know climate change is making the ocean surface temperature warmer, and they know too that it’s warming fastest in the north Atlantic, which is one of the areas we’re talking about here. They know that warm water is extending farther north, and that the water is staying warmer later into the season. And they clearly know that warm water is hurricane fuel.
Conversely, scientists know that cold sea water is a hurricane killer; and they know there’s still plenty of it under that warm surface layer; and that it can mix up all kinds of ways when a hurricane, or any other storm, comes through.
Well-established scientific evidence shows that climate change is melting the Arctic ice sheets and glaciers, and it’s clear that climate change is altering certain seasonal weather patterns.
We’ve seen stronger hurricanes and other kinds of storms in recent years. We’ve seen more intense precipitation and precipitation rates in hurricanes and other storms in recent years. A number of hurricane research studies pretty much indicate that precipitation rates are likely to increase even as the number of storms might actually decrease, though they are likely to be much stronger.
So, at the end of the day, what, if any, are the straight-line cause/effects between known climate change consequences and the hurricanes we’re told to expect?
No surprise that it’s probably less a line and more a series of chess moves – one leads to another and so on – when there is thought to be a connection. The best approach is to take the expertise of various respected researchers and piece them together to get anything that resembles a comprehensive view.
So here’s a sampling.
Jennifer Francis is a research professor in the Department of Marine and Coastal Sciences at Rutgers University in New Jersey. She studies effects of melting Arctic sea ice and glaciers and other climate changes.
The melting sea ice – for which she finds no explanation other than climate change – is creating colder than normal water off the coast of Greenland, amusingly referred to as the “cool blob.” The cool blob is backing up the Gulf Stream, essentially slowing it down like a blocked drain she says “is causing our warm water, which is connected to hurricanes.”
She says also that the rapidly melting Arctic is connected to winds. There are two mechanisms at work, both playing out through multi-step impacts.
One is that the north-south temperature difference is much smaller because of Arctic warming. That in turn weakens the west to east winds of the jet stream, which in turn makes it meander more. That meandering can result in the formation of atmospheric eddies – like those you would see in water. They spin off on their own and pick up systems and move them in strange patterns or block them. This is what happened during Sandy.
“There was a big eddy sitting near Newfoundland, and instead of the jet stream basically picking it up and taking it across the Atlantic as it normally would, it got caught in this return flow,” she explains. “The eddy was spinning clockwise creating east winds. Sandy came up coast and that’s what took it on a left-hand turn.”
The second mechanism involves the general worldwide warming that is resulting in earlier melting of snow cover, which in turn dries out and warms up the land earlier. That in turn pushes the jet stream farther north, which in turn leaves light wind patterns where the jet stream might otherwise have been. Francis said that’s what happened during Hurricane Harvey. It meandered in from the Gulf; there were no winds to move it; so it parked – right over Houston as it turned out – and just kept feeding off that nice warm water hurricane fuel.
“This is all very new merging research,” she says. “This is not something you can point to and say ‘this is an established fact and clear connection to climate change.’ But it’s all kind of making sense and fitting the kinds of behaviors we have expected to see more often.”
But she and others point to a missing link – what’s happening to the under-the-surface water temperature.
Scott Glenn is trying to figure that one out. He’s also at Rutgers; a distinguished professor in the Department of Marine and Coastal Sciences. He’s been looking at the water below the warming surface level in the mid-Atlantic – an area he says has one of the world’s largest temperature differences between top and bottom.
When storms come through, they suck heat out of the water; they stir up the cold and warm; and depending on how much of each is there and all other kinds of factors, that can have different impacts on a storm’s intensity.
Glenn uses underwater autonomous gliders that measure temperature to help determine how deep the warm water goes: how quickly a storm can change the composition of the water – pretty quick he says; and then what those factors do to a storm.
He’s gotten some pretty accurate information. As Irene approached New Jersey in 2011, he knew from the real-time data his gliders were sending back that there was intense mixing of cold and warm water, which he felt would de-intensify the storm as it hit land. That’s exactly what happened, sending what was largely a rainstorm inland.
With Sandy, the data showed the storm direction was pushing cold water away. The water in the storm’s path stayed warm, causing it to intensify as it hit land, with catastrophic results on the Jersey shore.
A potentially important forecasting tool, but not quite a direct link to climate change … yet.
“We know the water is warming in our area. We know that the surface water is changing. We know that the bottom cold water is changing even more rapidly. We know that it is undergoing that transition to that cold winter condition at later and later times each year,” he says. “How this warming and cooling is impacting the hurricanes is still kind of an unknown.”
Kevin Trenberth is a distinguished senior scientist in the Climate Analysis Section at the National Center for Atmospheric Research (NCAR) in Boulder, Colo. He is right onboard with the climate change impact of additional warming. He points out that atmospheric moisture over oceans is five to 10 percent higher than it used to be. Tropical storms can grab that moisture to invigorate themselves. They in turn become more intense, bigger, last longer, and then reach out and grab even more moisture.
But he is cautious: “I’m not saying the hurricanes themselves or the tropical storms are caused by climate change,” he says. “Hurricanes and tropical storms happen naturally and there’s a tremendous amount of natural variability.”
Most of that has to do with El Niño and La Niña. They are Pacific phenomena that tend to have oppositional impacts on the Atlantic and its hurricanes. Does climate change play a role in their formation? That’s unknown. They seem to be running through their longstanding cycles the way they always have.
To over-simplify – El Niño is marked by weaker winds, warmer temperatures and more rain over the ocean. La Niña has stronger winds, cooler temperatures and more rain over the land. They act in kind of a seesaw effect with the Atlantic. When the Pacific is more active in El Niño years, it suppresses action in the Atlantic. It’s the opposite with La Niña.
Last year El Niño did not materialize as expected and the Atlantic hurricane season was busier.
Trenberth and others also point out that hurricanes pull a lot of heat out of the water they pass over, which usually keeps hurricanes out of that area for several years. So if climate change is thought to have contributed to the formation of a big storm, arguably it could also be considered a contributor to the quiet years after that storm.
“There may actually be fewer storms,” he says. “One big storm can actually take a lot of heat out of the ocean and create a less favorable environment for the next storm. So one big storm can replace three or four smaller storms.”
Ethan Gutmann is a project scientist in the Research Applications Lab at NCAR. He figured out a way to show if and how conditions that could be related to climate change would affect hurricanes. He took 22 named hurricanes from 2001 through 2013 and simulated what they would be like in the future with factors expected from climate change.
Gutmann ran the data for every grid point and for every variable: temperature, humidity, wind speed, direction.
The storms reacted differently – but there were some consistencies, he says. “They all had more precipitation. On average across all of them, there was a statistically significant increase in maximum wind speeds. But there were individual storms that actually had a decrease in maximum wind speed,” he says.
They were also stronger and slower. Did climate change make a difference? It’s looking that way.
Thomas Knutson is a research meteorologist and climate scientist with NOAA’s Geophysical Fluid Dynamics Laboratory in New Jersey. He has done modeling that shows roughly the same thing – climate change would likely lead to fewer but stronger storms, and a lot more rain. He calculates that intensity would increase by two to 11 percent and rainfall rates by 10 to 15 percent by the end of the century
But that’s globally. However, he says in a summary: “There is at present only low confidence that such an increase in very intense storms will occur in the Atlantic basin.”
Not only that, he states in his findings: “It is premature to conclude that human activities – and particularly greenhouse gas emissions that cause global warming – have already had a detectable impact on Atlantic hurricane or global tropical cyclone activity.”
“Just coming into it from the outside you say ‘Oh, it must be obvious that things are going to be warmer. Ocean temperatures are going to get a lot worse in terms of hurricanes,’” Knutson says in an interview. “What we find though even in models is that the situation is a little bit trickier than that because it kind of depends. It’s not just the amount of warmth you have in a certain region, but also how much warmth you have relative to other regions.”
Knutson points out that it’s a lot harder to measure the characteristics of hurricanes than it is to figure out global mean temperature.
“We have some expectations from models,” Knutson says of hurricanes. “But we can’t quite back it up the same way we can for something like global mean temperature.
“We as climate scientists cannot draw a strong link between increasing greenhouse gases and every different type of phenomena. There’s some phenomena where we can more easily draw that link and other phenomena where it’s more difficult to draw that link. If you go to a short enough time scale and small enough spatial scale, you can find anything you want.”