While I was looking into Chris Landsea’s recent activities, I came across a new essay that is a pleasant change of pace from the Climategate Emails.
While it is definitely an opinion piece, and a wonderful example of how to disagree without being disagreeable, it’s also a great resource for our current understanding of hurricane hazards and activity over time. More than that – Landsea has some interesting attempts at adjusting the historical record to account for our increasing ability to spot hurricanes, even those that earn the title for less than a day.
Landsea agrees that the Earth has warmed over the last several decades, that greenhouse gases are to blame in some part, but acknowledges the sensitivity is not known. Excerpts follow, but I strongly recommend reading the full essay and images at http://www.aoml.noaa.gov/hrd/Landsea/gw_hurricanes/:
As a preamble, I definitely agree that global warming has occurred (around a degree F [or half degree C] in the last several decades at the earth’s surface).
Also there is substantial evidence – in my view – that mankind has caused a significant portion of this warming through greenhouse gas emissions such as carbon dioxide and methane. I do not know whether the human contribution toward the warming is relatively small (~a quarter) or large (~two-thirds), but do agree that there is quite a bit of evidence that mankind is altering the global climate and will continue to do so in the future.
… Thus there remains a large range of the amount of global warming to be expected in the future due to manmade changes in my view.) What does, then, a 1°F (0.5°C) ocean temperature change today and a potential 4-6°F (2-3°C) warming by the end of the 21st Century mean for hurricanes?
All climate models predict that for every degree of warming at the ocean that the air temperature aloft will warm around twice as much. This is important because if global warming only affected the earth’s surface, then there would be much more energy available for hurricanes to tap into. But, instead, warming the upper atmosphere more than the surface along with some additional moisture near the ocean means that the energy available for hurricanes to access increases by just a slight amount. Moreover, the vertical wind shear is also supposed to increase, making it more difficult (not easier) for hurricanes to form and intensify.
The bottom line is that nearly all of the theoretical and computer modeling work suggest that hurricanes may be slightly stronger (by a few percent) by the end of the 21st Century, even presuming that a large global warming will occur.
The climate models are also coming into agreement that the number of tropical storms and hurricanes will not go up and may perhaps even decrease (by around one-fourth fewer) because of the increased vertical wind shear.
… what does global warming imply for hurricane activity today? The ~1°F (~0.5°C) ocean temperature warming has likely made hurricanes stronger today by about 1%. Thus even for a Saffir-Simpson Hurricane Wind Scale Category 5 hurricane – like Hurricane Katrina over the Gulf of Mexico – the increase in hurricane winds are on the order of 1-2 mph (2-3 kph) today.
What Does the Observed Increase in Hurricane Damages Imply?
This section is hard to excerpt and has been covered well on WUWT. Landsea notes that hurricane damage depends on population density, per capita wealth, and coastal development. He notes that the Great Miami Hurricane of 1926 which cost $100 million in actual 1926 dollars would would normalize to about $165 BILLION today if it hit the same stretch of coast with today’s population and infrastructure.
Has There Been a Doubling in the Number of Tropical Storms and Hurricanes?
This starts with a couple studies that reached similar conclusions. “Overall, there appears to have been a substantial 100-year trend leading to related increases of over 0.78°C [~1.5°F] in SST and over 100% in tropical cyclone and hurricane numbers. It is concluded that the overall trend in SSTs, and tropical cyclone and hurricane numbers is substantially influenced by greenhouse warming.”
How Have the Ways We Measure Tropical Storms and Hurricanes Improved?
After comparing what’s available today to meteorologists to what was available a century ago, Landsea talks about the ongoing effort to reanalyze the hurricane database. While he says “I am fortunate to assist with,” I think he spearheaded the effort after years of working with William Gray on Colorado State University’s efforts to come up with seasonal predictions. The shortcomings with the historical record were a significant nuisance.
He considers the “lost hurricanes” of the Eastern Atlantic where storms can form and never come close to land and also the modern phenomenon he calls “shorties,” those annoying, count-wrecking short-lived tropical storms that modern tools can show meet the criterion for a day or so. From 12 shorties in the first 44 years of the 20th Century to four or so per year in the last decade, much credit must go to “new instruments such as the Advanced Microwave Sounding Unit (AMSU) and scatterometers from low-earth orbiting satellites, new methods for interpreting geostationary satellite imagery such as the Advanced Dvorak Technique, new observation techniques such as the Stepped Frequency Microwave Radiometer aboard the Hurricane Hunter aircraft and more oceanic moored buoys providing continuous measurements, and new diagnostic methods such as the Cyclone Phase Space analysis all have contributed – in my opinion – toward increased numbers of weak, short-lived tropical storms.”
Landsea offers graphs starting with unadjusted numbers (seven storms per year on average in the late 1800s to twelve now) to numbers after shorties are removed, and then including an estimate of storms that would have been missed in the past. The trend flattens out with new values nine to eight.
The remaining data still shows a variation in storm activity that Landsea, like Bill Gray before him, ascribes to the 60 year cycle of the Atlantic Multidecadal Oscillation (AMO). We entered a warm AMO phase in 1995 which coincided with the current period of high storm activity.
How May Hurricane Activity Change in the Future?
Landsea concludes the changes we can expect with significant warming are not major (the largest being a ~25% decrease in numbers of storms, offset by a ~3% increase in intensity – damagewise, I suspect that might be a wash).
He concludes with:
Knowing, however, when the cold phase of the Atlantic Multidecadal Oscillation will occur – and a distinct drop in major hurricane numbers – is unknown, but likely within the next decade or two. Is global warming a concern? Yes. We’re conducting an uncontrolled experiment where we really don’t completely know what the consequences will be. I’ve been particularly shocked about the drastic changes going on in the Arctic, with the huge ice cover loss in the summertime that may very well be related to manmade global warming. The biggest immediate worry I have is with the huge population increases of vulnerable coastal communities both in Florida, elsewhere in the U.S., and to our neighbors in the Caribbean. Such jumps in coastal residents are causing massive damage increases and, unfortunately, large losses of life such as the 10,000 deaths in Honduras and Nicaragua from 1998’s Hurricane Mitch and the 1200 people that drowned from Katrina in Mississippi and Louisiana. The confluence of more people and infrastructure with the current busy period for Atlantic hurricanes has me quite concerned today. But – in my opinion – the overall impact of global warming on hurricanes is currently negligible and likely to remain quite tiny even a century from now.