What you need to know about hurricanes and their trends

Summary: Millions of words were expended reporting about Hurricanes Harvey and Irma, but too little about the science connecting them to climate change. Here are the details, contrasted with the propaganda barrage of those seeking to exploit these disasters for political gain. Let’s listen to these scientists so we can better prepare for what is coming. Failure to do so risks eventual disaster.

NASA photo of Hurricane Katrina on 28 August 2005
NASA photo of Hurricane Katrina on 28 August 2005.

(1)  A politically useful catastrophe: the Left speaks


The record-setting twelve-year long hurricane “drought” (no major hurricane landfalls on the US) was just weather. But the Left immediately boldly and confidently declared Harvey and Irma to be caused (or worsened) by anthropogenic climate change. Some of these screeds are mostly rational, just exaggerated or imbalanced. Such as “Harvey Is What Climate Change Looks Like” by Eric Holthaus at Politico — “It’s time to open our eyes and prepare for the world that’s coming.” And “Stop talking right now about the threat of climate change. It’s here; it’s happening” by Bill McKibben at The Guardian — “Hurricane Harvey, Hurricane Irma, flash fires, droughts: all of them tell us one thing – we need to stand up to the fossil fuel industry and fast.”

Many are simple political propaganda. “Irma Won’t ‘Wake Up’ Climate Change-Denying Republicans. Their Whole Ideology Is on the Line.” by Naomi Klein (activist) at The Intercept. Note this story is not labeled as an “op-ed”. “As Planet Rages With Fires and Storms, Ire Aimed at Murderous Climate Denialism” by Jessica Corbett (staff writer) at Common Dreams. “Climate Denialism Is Literally Killing Us” by Mark Hertsgaard (editor) at The Nation — “The victims of Hurricane Harvey have a murderer — and it’s not the storm. …It is past time to call out Trump and all climate deniers for this crime against humanity. No more treating climate denial like an honest difference of opinion.”

Many just assume the science says what they want it to say, without recourse to the IPCC, NOAA, or a similar authority. For example, Paul Krugman (professor of economics at Columbia, Nobel Prize 2008) says this at his NYT blog.

“The disaster in Houston is partly Mother Nature — natural disasters will happen sometimes whatever we did — but with a powerful assist from human action. Climate change definitely made such an event more likely …”

Similarly, Joseph E. Stiglitz (Professor of economics at Columbia, Nobel Prize 2001) in “Learning from Harvey” says this at Project Syndicate.

“It is ironic, of course, that an event so related to climate change would occur in a state that is home to so many climate-change deniers – and where the economy depends so heavily on the fossil fuels that drive global warming.”

Model of a hurricane.
Vapor visualization of a hurricane in the Weather Research & Forecasting Model. NCAR/UCAR.

(2)  Scientists tell us about hurricanes and global warming


Although many on the Left ignore, misrepresent, or exaggerate the science, there is well-established data about these matters. Here is a look at recent research (i.e., since the IPCC’s AR5 report), the foundation for the statement at NOAA’s website that concludes this section. Red emphasis added.

(a) Look at the trends in the number and intensity of hurricanes.

A good place to start is “Extremely Intense Hurricanes: Revisiting Webster et al. (2005) after 10 Years” by Philip J. Klotzbach and Christopher W. Landsea in Journal of Climate, October 2015. Abstract…

“Ten years ago, Webster et al. documented a large and significant increase in both the number as well as the percentage of category 4 and 5 hurricanes for all global basins from 1970 to 2004, and this manuscript examines whether those trends have continued when including 10 additional years of data.

“In contrast to that study, as shown here, the global frequency of category 4 and 5 hurricanes has shown a small, insignificant downward trend while the percentage of category 4 and 5 hurricanes has shown a small, insignificant upward trend between 1990 and 2014. Accumulated cyclone energy globally has experienced a large and significant downward trend during the same period.

“The primary reason for the increase in category 4 and 5 hurricanes noted in observational datasets from 1970 to 2004 by Webster et al. is concluded to be due to observational improvements at the various global tropical cyclone warning centers, primarily in the first two decades of that study.”

Update 1: A possible cause of the decline in major hurricane frequency during the past decade — “The role of Atlantic overturning circulation in the recent decline of Atlantic major hurricane frequency” by Xiaoqin Yan et al. in Nature Communications, 22 November 2017. Abstract:

“Observed Atlantic major hurricane frequency has exhibited pronounced multidecadal variability since the 1940s. However, the cause of this variability is debated. Using observations and a coupled earth system model (GFDL-ESM2G), here we show that the decline of the Atlantic major hurricane frequency during 2005–2015 is associated with a weakening of the Atlantic Meridional Overturning Circulation (AMOC) …”

Update 2: A new article looks at hurricanes in the South Pacific: “Extreme tropical cyclone activities in the southern Pacific Ocean” by Karl Hoarau et al. in the International Journal of Climatology, in press.

“This research concerning the South Pacific Ocean shows the 10-year and interannual variability of extreme cyclones (categories 4 and 5). The intensity of these cyclones has been reanalysed on the basis of GMS, GOES and NOAA satellite images with the Dvorak technique. In the period 1980–2016, 63 cyclones reached categories 4 and 5. During the decade 1980–1989, the intensity of cyclones of at least 115 knots over 1 min (100 knots over 10 min used by the meteorological centres in the South Pacific) was underestimated: we found 19 such cyclones, as opposed to 11 in IBTrACS, and 6 in the SPEArTC and in the Joint Typhoon Warning Center (JTWC) databases.

Between 1980 and 2016, the number of extreme cyclones did not show any tendency to increase. The 1983 season was the most active, with six cyclones of categories 4 and 5 that do not figure in the JTWC database for the South Pacific. Only one intense cyclone has been reported in the SPEArTC and five cyclones in IBTrACS in 1983. El Niño episodes concurred with a much higher number of cyclones of at least 115 knots than La Niña episodes. More than half of the category 5 (at least 140 knots) cyclones were observed during El Niño years.

“The South Pacific Ocean is the theatre of very intense cyclones, comparable to those in the western or eastern North Pacific. Thus, by reanalysing satellite images, an intensity of 170 knots was attributed to cyclone Hina (March 1985), which was probably one of the strongest in the southern hemisphere since 1980.”

Update 3: Australia’s Bureau of Meteorology agrees that there has been no increase in tropical cyclones in ocean around Australia during 1970-2017 (report here).

Update 4: “An Energetic Perspective on United States Tropical Cyclone Landfall Droughts” by Ryan E. Truchelut and Erica M. Staehling in Geophysical Research Letters, in press. Gated.

“We introduce an extended climatology of U.S. tropical cyclone activity based on accumulated cyclone energy (ACE) and use this data set to investigate variability and trends in landfall activity. The drought years between 2006 and 2016 recorded an average value of total annual ACE over the U.S. that was less than 60% of the 1900–2017 average. Scaling this landfall activity metric by basin-wide activity reveals a statistically significant downward trend since 1950, with the percentage of total Atlantic ACE expended over the continental U.S. at a series minimum during the recent drought period.”

(b)  What powers the trends in hurricane intensity?

Articles about hurricanes often say there is a strong and direct link to sea surface temperatures (SST). Reality is more complex. Philip Klotzbach explained his findings to me in more detail. Dr. Klotzbach is a research scientist at the Tropical Research Project at Colorado State U.

“Our paper found that the large increasing trends in Category 4-5 hurricanes observed in Webster et al. (2005) were primarily due to changes in observational technology at the various warning centers.  Most model projections predict a slight increase (on the order of 5-10%) in storm intensity, with perhaps fewer storms, over the next century.

“SSTs correlate tightly with Atlantic hurricane activity, due to other large-scale climate features such as sea level pressure and vertical wind shear. In the tropical Atlantic, warm sea surface temperature anomalies result in lower tropical and subtropical Atlantic pressure.  The associated weaker pressure gradient results in weaker trade winds, reducing vertical wind shear (since upper level winds blow out of the west in the tropical Atlantic).  The weaker trade winds cause less mixing, evaporation and upwelling of the sea surface, which then feed back into reinforcing the warm SST anomalies in the tropical Atlantic.

This wind-evaporation-SST feedback process in the Atlantic has been shown to be critical for the Atlantic Meridional Mode.  Generally, positive values of the Atlantic Meridional Mode are associated with warm SSTs, low sea level pressure, and reduced vertical wind shear.  The actual impact of the SST anomalies themselves is shown to be relatively small in partial correlation analysis.  This was first demonstrated two decades ago in “Atlantic Sea Surface Temperatures and Tropical Cyclone Formation” by Lloyd J. Shapiro and Stanley B. Goldenberg in Journal of Climate, April 1998.

These types of relationships do not necessarily occur in other basins.  For example, the NW Pacific has just had its 2nd Cat. 3+ typhoon this year, while the average to date is 4.5.  All of this despite record warm SST anomalies in their Main Development Region.  Circulation features are a far more critical driver of typhoon activity than SSTs, since they are always plenty warm to support intense activity.”

(c)  Cliff Mass describes the relationship of global warming to hurricanes.

See “Global Warming and Hurricane Harvey” by Cliff Mass at his website. He gives a rebuttal to those articles asserting a clear link between Global Warming and Hurricane Harvey. Dr. Mass is a professor of atmospheric sciences at U Washington. See his bio, presentations and papers. Opening…

“Before the rains had ended, dozens of media outlets had published stories suggesting that global warming forced by humans (mainly by emissions of CO2 into the atmosphere) played a significant role in producing the heavy rainfall and resulting flooding associated with Hurricane Harvey.

“Most of the stories were not based on data or any kind of quantitative analysis, but a hand-waving argument that a warming earth will put more water vapor into the atmosphere and thus precipitation will increase.  A few suggesting that a warming atmosphere will cause hurricanes to move more slowly.

“This blog will provide a careful analysis of the possible impacts of global warming on Hurricane Harvey.  And the results are clear:  human-induced global warming played an inconsequential role in this disaster. …”

(d)  Roger Pielke Sr. tells me about an important but often ignored point.

“Model projections of hurricane frequency and intensity are based on climate models. However, none have shown skill at predicting past (as hindcasts) variations in hurricane activity (or long term change in their behavior) over years, decades, and longer periods. Thus, their claim of how they will change in the future remains, at most, a hypothesis (i.e. speculation). When NOAA, IPCC and others communicate to the media and public, to be scientifically honest, they should mention this.”

Dr. Pielke Sr. is a Senior Research Scientist in CIRES and Emeritus Professor of Atmospheric Science at Colorado State U. See his bio.

(e)  Judith Curry discusses the link between global warming and Hurricane Harvey.

From her post about Hurricane Irma at Climate Etc.

“Ever since Hurricane Harvey, the global warming – hurricane hysteria has ratcheted up to levels I haven’t seen since 2006. NOAA GFDL has written a good article on Global Warming and Hurricanes. {See below.} …I much prefer {NOAA’s} model-based quantitative estimates (but they need some serious uncertainty estimates, including structural uncertainty), relative to hysterical arm waving by Mann and Trenberth using undergraduate basic thermodynamics reasoning.  There is nothing basic or simple about hurricanes. …

“{See} my 2010 post Hurricane Katrina – 5 years later, particularly relevant given the cool SST values that Irma formed and intensified.”

Dr. Curry is a professor emeritus at the Georgia Institute of Technology and President of Climate Forecast Applications Network (CFAN). See her bio.

NOAA logo

(f)  NOAA gives their verdict.

The bottom line comes from NOAA’s Geophysical Fluid Dynamics Laboratory: “Global Warming and Hurricanes – An Overview of Current Research Results.” Journalists should consider this definitive. But few of them mention it.


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. …”

A. Statistical relationships between SSTs and hurricanes.

… The Power Dissipation Index (PDI) …is an aggregate measure of Atlantic hurricane activity, combining frequency, intensity, and duration of hurricanes in a single index. …

This is in fact a crucial distinction, because the statistical relationship between Atlantic hurricanes and local Atlantic SST shown in the upper panel of Figure 1 would imply a very large increases in Atlantic hurricane activity (PDI) due to 21st century greenhouse warming, while the statistical relationship between the PDI and the alternative relative SST measure shown in the lower panel of Figure 1 would imply only modest changes of Atlantic hurricane activity (PDI) with greenhouse warming. In the latter case, the alternative relative SST measure in the lower panel does not change very much over the 21st century in global warming projections from climate models, because the warming projected for the tropical Atlantic in the models is not very different from that projected for the tropics as a whole. …

B. Analysis of century-scale Atlantic tropical storm and hurricane records.

To gain more insight on this problem, we have attempted to analyze much longer (> 100 yr) records of Atlantic hurricane activity. If greenhouse warming causes a substantial increase in Atlantic hurricane activity, then the century scale increase in tropical Atlantic SSTs since the late 1800s should have produced a long-term rise in measures of Atlantic hurricanes activity.

Existing records of past Atlantic tropical storm or hurricane numbers (1878 to present) in fact do show a pronounced upward trend, which is also correlated with rising SSTs (e.g., see blue curve in Fig. 4 or Vecchi and Knutson 2008). However, the density of reporting ship traffic over the Atlantic was relatively sparse during the early decades of this record, such that if storms from the modern era (post 1965) had hypothetically occurred during those earlier decades, a substantial number would likely not have been directly observed by the ship-based “observing network of opportunity.”

We find that, after adjusting for such an estimated number of missing storms, there is a small nominally positive upward trend in tropical storm occurrence from 1878-2006. But statistical tests reveal that this trend is so small, relative to the variability in the series, that it is not significantly distinguishable from zero (Figure 2).

In addition, Landsea et al. (2010) note that the rising trend in Atlantic tropical storm counts is almost entirely due to increases in short-duration (<2 day) storms alone. Such short-lived storms were particularly likely to have been overlooked in the earlier parts of the record, as they would have had less opportunity for chance encounters with ship traffic. …

“While major hurricanes show more evidence of a rising trend from the late 1800s, the major hurricane data are considered even less reliable than the other two records in the early parts of the record. Category 4-5 hurricanes show a pronounced increase since the mid-1940s (Bender et al., 2010) but again, we consider that these data need to be carefully assessed for data inhomogeneity problems before such trends can be accepted as reliable.”

E. Summary for Atlantic Hurricanes and Global Warming.

In summary, neither our model projections for the 21st century nor our analyses of trends in Atlantic hurricane and tropical storm counts over the past 120+ yr support the notion that greenhouse gas-induced warming leads to large increases in either tropical storm or overall hurricane numbers in the Atlantic.

(g)  About “Hurricane Harvey’s Rainfall and Global Warming“, January 2018.

Analysis by Christopher W. Landsea, the Science and Operations Officer at the National Weather Service’s National Hurricane Center. Excerpt from his conclusions.

  • “Hurricanes (and Tropical Storms) have been associated for millenniums with extreme rainfall and freshwater flooding. There is nothing that one can do to prevent these storms from occurring, hitting land, and impacting people.
  • “Massive flooding and catastrophic impact from tropical storms and hurricanes occurs when the system moves slowly over a major city. This is precisely what happened because of Harvey as a tropical storm over Texas. …
  • “Linking hurricane rainfall to global warming today (and even decades from now) based upon such a tiny contribution is misleading. Moreover, such a fixation can delay steps that can be taken now to better mitigate the effects of extreme flooding from hurricanes.”

(3)  See the trends for yourself in hurricane energy and frequency

Graphs from Ryan Maue (click to enlarge). He also notes that the “Southern Hemisphere 2016-17 tropical cyclone season was weakest/quietest in 50-years since reliable records (sort of) exist.” His dataset has 4,137 named global Tropical Storms since January 1970. Of those, 2242 has a period of hurricane level force (54%).

Global frequency of tropical cyclones.

Global Tropical Cyclone frequency

Global accumulated cyclone energy (ACE) of tropical cyclones.

Global Tropical Cyclone total ACE

(4)  About those wildfires!

Wildfire Earth


The National Interagency Fire Center shows year-to-date statistics for wildfires in the US. This year ranks third in the past eleven years. The total acres burned per year have been in a flat range since 1999 (details here).

In the 20th century forests were managed by Smokey the Bear — “only you can prevent forest fires” — in the mistaken belief that forest fires must be prevented. This made the western US forests into tinderboxes. The Left blames the resulting massive fires on climate change. Too see why that is not correct, read “Human Activity, more so than Climate Change, Affects the Number and Size of Wildfires” — Testimony of David B. South (Professor Emeritus of Forestry, Auburn U) before the Senate Subcommittee on Green Jobs and the New Economy, 3 June 2014. It has a wealth of information and useful graphs.

The global news about wildfires looks better than America’s, more good news that journalists don’t report. See “Global trends in wildfire and its impacts: perceptions versus realities in a changing world” by Stefan H. Doerr and Cristina Santín in Philosophical Transactions B, 23 May 2016. Excerpt from the abstract…

“{G}lobal area burned appears to have overall declined over past decades, and there is increasing evidence that there is less fire in the global landscape today than centuries ago. Regarding fire severity, limited data are available. For the western USA, they indicate little change overall, and also that area burned at high severity has overall declined compared to pre-European settlement.”

(5)  Results from the propaganda campaign


Much of the propaganda about Harvey and Irma has been directed at Trump. How has his job approval levels changed — an instant measure of their success? Harvey made landfall in Texas on August 26. Trump’s job approval numbers began to improve on September 1 and have remained flattish since September 3 (graph as of Sept 15). Lots of firepower expended on Trump to no visible effect.

Gallup - poll of Trump job approval

(6)  A better lesson from these hurricanes (bitter if we wait too long)

“We don’t even plan for the past.”
— Steven Mosher (of Berkeley Earth), a comment posted at Climate Etc.

The debate about the best US public policy response to climate change has run for three decades, with Left and Right relying on misinformation and exaggeration to influence the public. We should be able to agree on the need to prepare for the inevitable repeat of past weather — like category 3, 4, and 5 hurricanes hitting the east coast.

It is pitiful that a rich nation like America has hysterics from events so commonplace as a cat 3 hurricanes. We should be prepared for the on average six major hurricane landfalls per decade (see the average return period for each section of the East Coast.

Eventually a cat 5 will hit the center of a major city. Then perhaps we will take some simple steps to build a more resilient America.

(7)  For More Information

To learn more about the matters discussed here.

If you liked this post, like us on Facebook and follow us on Twitter. For more information see all posts about the IPCC, see the keys to understanding climate change and these posts about the politics of climate change…

  1. How we broke the climate change debates. Lessons learned for the future.
  2. Important: climate scientists can restart the climate change debate – & win.
  3. Ignoring science to convince the public that we’re doomed by climate change.
  4. A leaked memo about climate change explains why we’re unprepared.
  5. Irma might defeat the skeptics and end the climate wars – a thought experiment.


21 thoughts on “What you need to know about hurricanes and their trends”

  1. Why the earthquake? I don’t get that. Is the idea that fracking is involved, or is this just “God is sending a message!!”

  2. Living in the San Francisco Bay Area, I’m interested in trend of California’s wildfires. Here is some news about a new paper about this..

    See a summary of this story: “Over the last 40 years, there is a surprising trend with California wildfires” in the Orange County Register.  Great graphs.

    The bottom line: fewer fires but more acreage burned.

    The paper discussed is “Different historical fire–climate patterns in California” by  Jon E Keeley and Alexandra Syphard in the International Journal of Wildland Fire, Vol 26 nbr 4, 2017.

  3. Wildfires also seem to be decreasing at the global scale, despite global warming and changes to fire seasons. As with hurricanes, this is a complex problem. Here’s a good review: “Global trends in wildfire and its impacts: perceptions versus realities in a changing world” by Stefan H. Doerr and Cristina Santín in Philosophical Transactions B, 23 May 2016. This article is part of themed issue ‘The interaction of fire and mankind’. Abstract:

    “Wildfire has been an important process affecting the Earth’s surface and atmosphere for over 350 million years and human societies have coexisted with fire since their emergence. Yet many consider wildfire as an accelerating problem, with widely held perceptions both in the media and scientific papers of increasing fire occurrence, severity and resulting losses. However, important exceptions aside, the quantitative evidence available does not support these perceived overall trends.

    “Instead, global area burned appears to have overall declined over past decades, and there is increasing evidence that there is less fire in the global landscape today than centuries ago. Regarding fire severity, limited data are available. For the western USA, they indicate little change overall, and also that area burned at high severity has overall declined compared to pre-European settlement.

    “Direct fatalities from fire and economic losses also show no clear trends over the past three decades. Trends in indirect impacts, such as health problems from smoke or disruption to social functioning, remain insufficiently quantified to be examined.

    “Global predictions for increased fire under a warming climate highlight the already urgent need for a more sustainable coexistence with fire. The data evaluation presented here aims to contribute to this by reducing misconceptions and facilitating a more informed understanding of the realities of global fire.”

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