Harvey commentary

Some further commentary on Harvey. Mostly (but not all) about the climate connection.

Global Warming’s Role in Hurricane Harvey (The Takeaway, NPR)

Why Harvey’s devastation is so severe (CNN)

Climate Change Didn’t Cause Hurricane Harvey, but It Made It Worse (Fortune)

Commentary by Columbia Scholars (not just me)

Meanwhile, Hurricane Irma is breaking records and looking truly fearsome in the Atlantic.


Mandatory Harvey evacuations are justified

Science can make predictions, but government has to act on them or they aren’t any use.

On Wednesday evening, Harvey was still just a tropical depression with winds of 35 miles per hour. The National Hurricane Center forecast that it would intensify to 40 miles per hour by Thursday morning, and reach its peak intensity two days later at 75 miles per hour – just barely crossing the threshold to be called a hurricane – just before landfall.

By 10 AM Central on Thursday, the picture had radically changed. The storm’s winds had already jumped up to 65 miles per hour, and the rapid intensification was forecast to continue, with Harvey reaching 115 miles per hour – category 3 on the Saffir-Simpson scale – before landfall. With these more intense winds came a prediction of 6-10 feet of storm surge for parts of the Texas coast, and an increase in forecast rainfall, with potential for up to two feet in some locations. Suddenly Harvey looked likely to produce a major disaster.

The machinery of government, media, and private sector began to engage, with emergency preparations and warnings ramping up to amplify the State of Disaster already issued Wednesday for 30 counties by Texas’ Governor.

Harvey was still just a tropical storm at this moment. All the emergency actions were predicated on forecasts of future changes in Harvey’s intensity. Intensity prediction is normally considered the weak point in the science of hurricane forecasting, and rapid intensification just before landfall, in particular, has often been called a “forecaster’s nightmare”. But the forecasters predicted it with confidence, and no one, as far as I know, questioned that prediction. The conditions were right, with sea surface temperatures in the Gulf very warm and vertical wind shear (the difference in the steering winds at different heights that can disrupt a hurricane’s circulation) very low. With landfall still somewhere between 36 and 48 hours away – not nearly as much lead time as would be ideal, but enough for many life- and property-saving actions – the NHC’s experts were able to sound the alarm effectively.

The forecasts quickly proved right. Harvey is closing in on the coast near Corpus Christi, and NHC a few hours ago declared it a category 3 Major Hurricane, with sustained winds now up to 125 miles per hour. A storm with winds of this magnitude has not made landfall in the US in over 11 years, but that “drought” will almost certainly end tomorrow.

These forecasts are a scientific success story, but their value depends on the actions of the state, local and federal governments.

Corpus Christi, a city of 325,000 people, is directly in Harvey’s sights. Much of the city – and all of its oil refineries – lie close to the water on a bay that can funnel and amplify the surge, now forecast to be between 6-12 feet (it’s already more than 4 feet as I write). The flooding will come with winds well in excess of hurricane force, and will likely be exacerbated and prolonged by heavy rains that could continue for days.

Yet on Thursday, the Mayor of Corpus Christi decided not to issue a mandatory evacuation order, but instead just to “encourage” evacuation of residents in the lowest-lying areas. I can’t understand this decision.

In a press conference this morning, the Mayor stood by it. When a reporter asked how many people might be left in the areas most at risk, the Mayor replied that he didn’t know, as there’s no way at this moment to take a survey. That’s probably true, but it would have been better if he could have said “we’re doing everything possible to get every one of those people out of harm’s way”. He couldn’t say that, because he hadn’t.

The motivation for avoiding a mandatory order appears to have been a fear of risking first responders’ lives by sending them in to enforce it. But there was no need to send them in. In the United States, a mandatory evacuation order is almost never truly mandatory in that sense. The Mayor could have simply said it was mandatory, and warned everyone in Harvey’s path that if they stayed in defiance of it, no one would be able to come and help them.

How many more people would have evacuated if the order had been mandatory? It’s impossible to know. Some won’t leave no matter what. But it’s hard to imagine that it wouldn’t have made a difference to some people if they’d heard their Mayor tell them they have to leave now, rather than just that it seems like a good idea.

By now, it is probably too late for anyone to get out who hasn’t already. I hope not many are left, and for those who are, I hope Harvey’s impacts turn out less catastrophic than predicted. Corpus Christi may get some luck yet – as I write, the latest forecasts have the center coming in very close to the city, which could put the strongest winds and surge just to the east. Several of the closest counties on that side have issued mandatory evacuation orders; some have not.


False Uncertainty

The New York Times wants to promote dialogue between left and right on climate. This is a worthy impulse. And the person they have hired towards this end, a conservative journalist with a history of climate denial, states in his first piece for the Times that he accepts as “indisputable” the human influence on the observed warming of the earth since the late 19th century. Good.

The rest of Stephens’ piece, though, is a stew of innuendo and misdirection, in service of no apparent goal beyond a smug scolding of some unnamed others’ perceived “100% certainty” on climate.

Who and what is Stephens talking about? The entire climate science literature, including the 2014 IPCC Report (which Stephens cites), is full of statements about uncertainty about virtually all aspects of future climate change. What climate scientist or credible policy maker has ever claimed anything is 100% certain? It would be great if a conservative writer who accepts the science on climate would engage, seriously and sincerely, with real arguments made by credible climate scientists and advocates of climate action on the pages of the Times. Stephens, instead, takes the lazy route, arguing with straw men.

Stephens shows no interest in making constructive arguments in defense of any actual position. He doesn’t articulate any specific view on climate policy, which is what really ought to matter to Times readers of any political persuasion. But one is left to assume he thinks we don’t need to do anything about global warming. Why? Because nothing is 100% certain.

This passes for substantive argument in some circles. Though his acceptance of scientific fact – at least in this piece, if not his prior work – distinguishes him from the climate deniers at the Heartland Institute and Heritage Foundation, Stephens’ strategy is nonetheless the same cynical one as theirs: promote doubt, nothing more. He seems to be saying that liberals (or someone) should be more tolerant of conservative “skepticism” on climate, but that skeptics have no need to justify their views (or even to have any), because it’s ok just to be generally skeptical.

And Stephens is uninterested in applying any of this skepticism to the claims of those opposed to climate action. Presumably referring to (unnamed) proposals for reducing greenhouse gas emissions, he writes that “Demanding abrupt and expensive changes in public policy raises fair questions about ideological intentions.” Leaving aside the crass culture-warring tone – saying those who are in favor of climate action are “demanding” while those opposed are just “raising fair questions” is particularly absurd under the current regime in Washington – how well do we know how expensive any proposed climate policy is? Predictions of the economic impacts of climate policies are far more uncertain than climate projections. The price of renewable energy is dropping much more rapidly than nearly anyone expected a few years ago. Isn’t there some excessive certainty here that could benefit from self-examination?

Perhaps what Stephens means by “100% certainty” is the passion among many for the view that the evidence of a human influence on climate is sufficient to merit a serious policy response. This passion does not require absolute certainty on any aspect of the science, just an intellectually honest view of risk. Because what the genuine uncertainty on climate really means is that while there is a chance future climate change won’t be as bad as the projections say is most likely, there is also a chance it will be much worse.

In no other area of life do we respond to risks by simply assuming the best case scenario. We take out insurance on our homes not because we are 100% certain they will burn down, but because there is a chance they might. Climate should be no different – except that the situation now is more like one in which a wildfire is already spreading towards our house, and we’re just debating whether it will singe the house a little or consume it entirely. How is skepticism a justification for inaction, as long as one accepts at least some possibility that the science is right?

This is the fundamental dishonesty of Stephens’ piece, and of all those who justify inaction by claims of skepticism. It makes no sense to be opposed to any action to mitigate global warming just because there is uncertainty in climate projections. The only way it makes sense is if you’re certain the science is entirely wrong. Or, though maybe you don’t want to say it, you just don’t care, because you’re pretty sure it won’t affect you personally. That is the true false certainty, masquerading as skepticism.

Thanks to Emmanuel Vincent of Climate Feedback for asking me to write this.

Warn on the hazard

In this article, Andrew Freedman of Mashable argues that the Saffir-Simpson scale should be changed because it doesn’t measure rainfall, and yet rainfall causes disasters in many hurricanes – including Matthew in North Carolina just now. The Governor of North Carolina has been saying the same thing. The National Weather Service disagrees, and so do I.

In fact the Saffir-Simpson scale used to be based on not just wind, but also pressure and storm surge. It was simplified to just wind a while back because the different variables often wouldn’t match for a single storm – e.g., Sandy would have been a cat 1 due to wind but a cat 3 due to surge. There is no way to make a single number capture all the different hazards without being confusing. You wouldn’t know just from the number what the reason for the number was, and would still have to read the fine print in the forecast to find that out.

In addition, heavy floods can happen from storms that are not tropical cyclones at all. E.g., Louisiana a couple months ago. How would a change to the Saffir-Simpson scale help with those?

The rational approach is to warn on the hazard. That is, de-emphasize the Saffir-Simpson category, and issue warnings about each specific hazard separately. This is in fact the direction the National Weather Service (including the National Hurricane Center) has been moving. For example, they have recently introduced Storm Surge Watches and Warnings.

The NWS system can surely still be improved – apparently, the NWS forecasts of heavy rain didn’t reach everyone in North Carolina, even though they did exist and were quite accurate. So it would be reasonable, for example, for NHC to issue a formal Heavy Rainfall Watch or Warning (or whatever one wants to call them) when it is warranted in association with a tropical cyclone.

But multiple hazards from a tropical cyclone will remain multiple hazards. Combining them into one number wouldn’t eliminate the need for people in harm’s way to understand that. It would just disguise the problem, and disguising problems isn’t usually the best way to solve them.

You climate scientists and your models, part 2

My previous post was about the notion that models don’t constitute “evidence” for propositions in climate science. In this post I write about the criticism, also expressed to me in comments after my New York Times op-ed, that climate models (or the tools and methods of climate science more broadly) have been disproved by observations, so we should ignore them.

The people who wrote this to me didn’t spell out what disagreement they were talking about. But I assume that these comments were stimulated by the way I framed the op-ed piece, in that I wrote at the start about the major hurricane “drought” in the US over the last decade. The commenters seemed to be implying that since climate scientists had predicted stronger hurricanes in the past, the fact that the last ten years of Atlantic hurricane seasons have been relatively quiet disproves climate models, or climate science generally.

I set up the piece that way to be provocative, and to make a connection between current events in the US and the broader questions addressed in our Science paper that motivated the Times piece. But the point of the whole op-ed piece, as well as the Science paper, was that our recent history is, in fact, consistent with our current projections of long-term trends towards stronger hurricanes worldwide. My most vehement critics didn’t seem to seriously consider the arguments I made, as I didn’t get any responses which refuted them in any substantive way. But let’s elaborate a bit here on the two basic issues which our Science paper raises, one at a time, and then come back to the issue of whether the models, or the predictions made by climate scientists overall, are any good or not.

The first issue, very well known by the scientific community, is that there is large natural variability in hurricane activity. Hurricane seasons in different parts of the world are doing different things at different times, and they all fluctuate due to natural causes. So if you want to see climate change trends, you have to wait a while for them to become clear. Think about investing in the stock market for your retirement. Let’s say you start investing in your twenties and plan to retire at 65. Is the market going up or down over your working lifetime? You can’t tell by what happens from one day, one year, or even one decade to the next. That long term is what climate change predictions are about.

And you have to wait longer to see trends in hurricanes than you do to see trends in other variables. Take temperature for example. There are many more measurements of temperature than there are of hurricanes, since there aren’t many hurricanes while temperature can be measured anywhere. So statistical analyses of temperature data are much easier than those of hurricane data, because the sample is larger. Looking for temperature trends is like looking for stock market trends in a broad index like the S&P 500, made up of many stocks, while looking for hurricane trends is more like looking at an average of just a few stocks. The average of just the few will be more subject to the vagaries of what’s going on with those few companies day-to-day, and will have much more volatility, while the S&P 500 will show broader market trends more clearly. Looking at the hurricanes in just a single basin, like the Atlantic, reduces the sample size still further and makes the data even less representative of global trends. Continuing our stock market analogy, looking at the Atlantic only is like looking at just a single stock. It may be important, if you’re interested in that particular one, but it doesn’t necessarily tell you much about the bigger picture of the whole market.

Now, there are several credible studies by very established researchers that do, in fact, find that increasing trends in hurricane activity are already evident in recent historical data, either in individual basins or globally. But there is some disagreement between studies on the of statistical significance of these trends. So one argument is that the long-term trends are there, but that we need to wait longer to see them rise further above the noise so that everyone agrees they are there and significant.

Regardless, though, the notion that the hurricane drought in the Atlantic has somehow disproved the consensus projections of climate science is wrong, because the drought is still a relatively short-term fluctuation in a single basin, while the projections are for long-term global trends.

The second issue raised in our Science paper (now available free, see bottom of this post) is that perhaps there shouldn’t yet have been substantial long-term trends in hurricane intensity – whether we would be able detect them above the natural variability or not – because until the last couple of decades, aerosol cooling effects on hurricanes have been counteracting the effects of greenhouse gas warming. Though not a new idea either, this is one that’s perhaps a bit less understood than the one about natural variability. It has been raised before mostly with regard to the Atlantic, while to my knowledge our paper may be the first to show evidence from the IPCC climate models that aerosol effects have been important for hurricanes globally. It’s a subtle argument, because aerosol cooling has clearly been less than greenhouse warming – if not, the planet wouldn’t have gotten warmer over the last century. But it appears that aerosol effects on hurricane intensity are disproportionately stronger than their effects on the climate overall, because they reduce incoming sunlight while greenhouse gases act through longwave (infrared) terrestrial radiation, and changes in sunlight have a bigger influence on tropical cyclone intensity than changes in infrared radiation do.

To the extent the aerosol cooling estimates in the climate models are accurate, potential intensity theory (discussed in the previous post) implies that hurricane intensities shouldn’t have started to increase at all until the 1980s or 1990s, even though by that point the planet had warmed quite a bit. This would further delay time when the increases would be detectable above the noise of natural variability. The aerosol cooling in the models may be wrong to some extent, but is almost certainly in the ballpark enough so that the aerosols have compensated for the greenhouse gas effects on hurricane intensity to a significant degree, even if the exact degree is different than what’s shown in our paper.

So, to summarize: the projection that a world warmed by greenhouse gases – absent other compensating factors – will have stronger hurricanes still holds, but a) large natural variability means that it may take a while to see the trends, and the Atlantic “drought” is just part of that variability, and b) aerosol cooling can compensate the greenhouse effect on hurricane intensity, and has done so for much of the historical record until recently, slowing down the intensity increases. The role of natural variability in obscuring long-term trends has been understood (at least broadly) for a long time. The role of aerosols has emerged over the last decade and is still not fully appreciated.

Back to the broader issue of whether climate models have been disproved, though: the questions I’ve just written about are all about how climate change affects hurricanes, and not about the basic fact of human-induced climate change itself. Because hurricanes are so rare, and their natural variability so large, they are among the least clear indicators of climate change. There are plenty of much clearer ones, from increasing surface temperature (the global warming itself), to melting of ice on land and sea, to the long-term cooling of the stratosphere, increasing intensity of heavy rain events, etc.

When we consider all these climate change predictions as a whole, there is now a good track record. Climate scientists have been making predictions for several decades. The evidence shows that not only have these predictions come true, but they have tended to be conservative, understating the rapidity, extent and impacts of human-induced climate change. There are still many uncertainties, but those are reasons to be more concerned, not less, since they mean the reality could just turn out worse than we expect just as easily as better, and that’s what seems to have been happening so far.

PS: If you want to read our Science paper, it is now available for free – legally – via links on my academic publications page. (I can’t put the link directly to the paper here, as Science gives us this free link on the condition that it only can be used from a single referring page.) It’s currently at the top of the list of papers on that page, but in any case look for Sobel, Camargo, Hall, Lee, Tippett, and Wing, 2016: Human influence on tropical cyclone intensity. Science, 353, 242-246, DOI: 10.1126/science.aaf6574.