Wednesday, November 13, 2013

How come the connection between Haiyan and global warming is not being made?

What is the relationship between global warming and Super-Typhoon Haiyan which has destroyed so many Filippino lives?


First, global warming did not cause Haiyan. It certainly made the storm surge worse, because sea levels have been rising at 10mm/yr around the Philippines, compared to a global average of 3mm/yr, but it did not cause the typhoon per se.

But global warming did contribute to Haiyan's intensity, and will cause such extreme typhoons and hurricanes (generic name, Tropical Cyclones, TCs) to become more frequent in the future.
In fact, extreme TCs are already becoming more frequent.
The basic physics of TCs is pretty simple. Three main factors contribute to TC formation:
  • warm seas 
  • humid air 
  • low wind shear (changes in wind at different altitudes). 
Global warming promotes 1 and 2, and slightly inhibits 3.

So physical theory predicts that TCs will increase with warmer oceans and more humid air. 

Models are theories worked out in detail at a global level. They are useful but not perfect. Nothing is perfect.
 
Modelling studies by Knutson et al in 2010 conclude that models "typically project substantial increases in the frequency of the most intense cyclones"

Theory is borne out by observations. Here is a 2008 study published in Nature by Elsner et al which concludes "Our results are qualitatively consistent with the hypothesis that as the seas warm, the ocean has more energy to convert to tropical cyclone wind."

More interesting yet, here is a study by Judith Curry, who leans towards climate scepticism, which finds "A large increase was seen in the number and proportion of hurricanes reaching categories 4 and 5".

These are just two papers that bear out the expectation that extreme strength TCs like Haiyan will become more frequent as a result of global warming.

Why then, is the connection not made in the media, and in the under-reported COP19 talks taking place in Warsaw? Why has the recently issued IPCC report actually reduced its level of certainty about the connection?

One factor is that there are fewer scientists looking at this connection than look at climate in general, and of those that are, one or two are in the sceptic camp. The main man is Roger Pielke Jr.

On his blog, Pielke Jr says "I am declaring victory in this debate." and lists the statements in AR5 that he approves.

His technique is to create a narrative that can be summed up as - "There Is No trend".

To achieve this, he looks for data that shows no trend, and writes about it at length. He avoids information that shows trends. So he talks about frequency of average hurricanes, but does not look at frequency of high-intensity hurricanes.

This tendency to blank out the trends in major hurricanes is shown clearly in a Twitter exchange I had with Dr Ryan Maue, who collects hurricane statistics on his website. He agreed when I suggested that the PDI TC index shows a relation with the Pacific Decadal Oscillation, and pointed me to his paper on a historically low number of Pacific typhoons. Which again does not mention trends in major hurricanes.

I tweeted "Interesting, thanks. But it doesn't mention major TC trends. They are increasing are they not?"

He answered "Nope" and pointed me to this graph from his blog:




http://www.policlimate.com/tropical/index.html
Actually, the one on his link is October, not September, but the change is slight.

He is using this figure to deny that the trend in major TCs is increasing. 

However, if you look at the lower trendline, which is for major hurricanes, it is clearly increasing. Not a lot, but a definite increasing trend. In fact, given that the PDO is on a declining trend since 1985, we would expect the line to be declining.

I pointed this out to him, and asked him if we should therefore expect major TC intensity to increase when the PDO switches to positive in the coming decades. He did not answer, and has been twittering on matters other than Haiyan since then.
So those are the responses of two of the hurricane experts who seek to obfuscate the connection between warm sea surface temperatures and TCs.
As for social media, the sceptic delayers rely heavily on reciting past major TCs. This is standard fare for their response to all extreme weather events.


As @hurricanejim says: "Noting intense cyclones have always been part of nature does not address the question of are they getting stronger".

And the most risible response I saw was on Steve Goddard's blog:
Storms are driven by temperature, pressure and humidity gradients. If the Arctic warmed, these gradients would decrease and we would have fewer and weaker storms. If the Arctic was the same temperature as the equator, the Earth’s heat engine would turn off completely and there would be no storms, like on Venus.
Marvellous. But as Steve always says "just having fun."
You are Steve, you are.
Trouble is, the Filippinos are not.

2 comments:

Anonymous No. 6 said...

Nature has commented as follows

Are such storms getting worse in a warming world?
This is the million-dollar question, but there is not yet a scientific consensus on how to answer it.

Storms receive their energy from the ocean, so it would seem logical that they would get stronger, and perhaps also more frequent, as the upper layers of tropical oceans get warmer. The potential intensity of tropical storms does increase with warmer sea surface temperatures. However, the effect of warming seas could be counteracted by the apparent increase in the strength of shearing winds — winds blowing in different directions and varying in strength at different altitudes. Shearing winds tend to hinder the formation of storms, or tear them apart before they can reach extreme strength.

On balance, many climate researchers think that it is plausible that tropical-storm activity will rise as the planet warms. There is some evidence1 that storm intensity has increased over the last three decades, but reliable data are limited to the north Atlantic, where observations are most abundant. In other places, the evidence is not yet conclusive2.

The Intergovernmental Panel on Climate Change (IPCC) in its latest report cautiously summarizes the current state of knowledge:

“Time series of cyclone indices such as power dissipation, an aggregate compound of tropical cyclone frequency, duration, and intensity that measures total wind energy by tropical cyclones, show upward trends in the North Atlantic and weaker upward trends in the western North Pacific since the late 1970s, but interpretation of longer-term trends is again constrained by data quality concerns.”

What I would say, is don't overdo it Doc. It just gives people reasons and ammunition to fire at you. It's likely but there's just not the evidence yet.

http://www.nature.com/news/did-climate-change-cause-typhoon-haiyan-1.14139

Richard Lawson said...

Hi Anon 6
I accept your caution. I have been looking more deeply. It is clear that storms will become more intense, but the degree is said to be small. However, the field is somewhat dominated by those who lean towards the lukewarm position, and there are a couple of stray bits of information yet to be fed into the mincer. I am going to come back to this topic innabit.
Thanks for your helpful comment.
PS I wish I knew who you are. Email me? rlawson{at}gn.apc.org