Adam Sobel

As offshore winds are again forecast to bring hot, dry air to the woodlands of western California, and wildfires threaten for the second time in a couple of weeks, Pacific Gas &amp; Electric warns that it may shut down power again. If it didn’t do this, and the sparking of its wires resulted in awful fires as they did last year, would it be culpable?

Or would climate be the cause — and if so, can we blame human-induced global warming or natural variability in the climate system that would occur anyway? Or is the real problem that people are now living in fire-prone places, where historically they didn’t, and where natural fires have been artificially suppressed, letting fuels build up?

The answer to all of these questions is yes.

A fire needs a spark. If PG&amp;E;’s wires provide that, then the company is responsible, just as I would be if I started a fire with a tossed cigarette or an unattended campfire. Californians are right to try to hold PG&amp;E; accountable, and the evidence suggests that it has been poorly managed, deferring crucial maintenance for years.

At the same time, the fire wouldn’t spread so far and be so catastrophic if the weather weren’t right for it. And while the proximate causes of the National Weather Service’s fire weather watches and red flag warnings lie in the more or less random fluctuations of day-to-day weather, there is compelling evidence that global warming has increased the risk substantially. Heat dries out what scientists who study the problem clinically refer to as “fuel” — that is, trees and shrubs and such — so that they burn more quickly and easily.

Then again, tree rings show that there have been droughts even worse than those in the past century and that fire has been a part of the natural history of California and the western United States for millenniums. So human greenhouse gas emissions are by no means solely to blame.

But they make the problem worse than it would otherwise be because the natural climate variability is already a given, in exactly the same way that bad habits, like smoking, increase the risks of diseases that one would already stand some risk of getting due to bad luck alone.

The same goes for human fire suppression and real estate development in the “wildland-urban interface.” Yes, these are part of the problem too — maybe bigger parts than climate change, so far. But these factors are not mutually exclusive, and the warming influence will only grow with time.

This situation is not specific to fire, either. It’s generic to a wide range of increasing climate risks. Disasters happen when a geophysical hazard meets people. Floods, for example, occur not just because of storm surges or heavy rains, but also because of unwise development in flood plains, or failures of infrastructure, or both. And global warming always acts as an extra influence on top of natural variability that, in the short-term view, is often more important.

The important distinctions are ethical as much as they are scientific. Natural climate variability is out of human control. Building homes in dangerous places isn’t. Neither are poorly maintained electric power lines. Or continued uncontrolled carbon emissions.

This article originally appeared in

Are Wildfires Caused by Utilities or Climate Change? Yes

(California Today)

Autumn is a time of transition. Summer heat persists while winter cold builds, and sharp contrasts can result, like the one in Denver last week: The temperature there dropped from 80 degrees Fahrenheit on Wednesday afternoon to 16 degrees Thursday morning.

Arctic air pushed into the city behind a cold front trailing a storm — the same storm that caused blizzard conditions in the upper Midwest two days later. That system was followed by the storm that developed into a “bomb cyclone,” lashing the Northeast with high winds and rain Wednesday and Thursday.

As Denver froze, the same Arctic air mass extended west to the Sierra Nevada, where it drove the downslope “Diablo” winds that blasted Northern California like a giant hair dryer. The hot, dry, gusty conditions there prompted Pacific Gas and Electric to cut power to millions of people, in the hope of avoiding catastrophic wildfires touched off by downed power lines, like those in 2018.

These weather events are predictable in the short term. Coloradans knew days in advance that the mercury would plummet, and accurate forecasts of severe fire weather in California let PG&amp;E; cut the power in time to be effective.

But do they tell us anything about what kind of winter will be coming? Probably not.

Individual weather events are not generally predictive of anything over the longer term. Bitter October cold snaps, early blizzards and roaring Diablo winds can occur as readily before a mild winter as a harsh one.

However, we do have forecast models that can tell us — in a general, broad-brush way — whether the upcoming season is likely to be different from an average winter, and if so, how. These models factor in what is going on not just in the atmosphere but also in the ocean, which changes more slowly and thus contains more “memory.”

The seasonal forecasts put out by the National Weather Service’s Climate Prediction Center include maps with broad areas colored to indicate the odds that temperatures or precipitation there will tend to run above (red) or below (blue) those of a normal winter.

Because the uncertainties in the data only permit a very fuzzy view of the future on these time scales, the odds in the forecast tend not to be very high. The center’s national forecast of precipitation for the next three months, for example, shows greater than average chances of wet (or snowy) conditions for the mid-Atlantic states, Florida and the northern Plains, with Oregon and northernmost California drier than average. But the map does not show any values higher than around 60%, meaning that the forecasters are only 60% confident that the forecast will prove correct.

That is not much better than a 50-50 coin flip, but even low-confidence forecasts can be valuable, as long as you do not expect them to be better than they are.

Everyone can make one basic seasonal forecast with great confidence, though — that the winter will be colder than the summer. We know this from experience, of course, but scientists also know the essential physical reason behind the change: Sunlight decreases from summer to fall to winter as the Earth’s travel through its orbit swings the Northern Hemisphere to tilt away from the sun. So as we pull out our winter jackets, we are making a prediction of a change in the climate that is caused by what scientists call “radiative forcing.”

You can probably see where I am going with this. Predictions of global warming are inherently similar to the predictions of changing seasons that we all take for granted.

With global warming, the changes in incoming radiation caused by rising concentrations of greenhouse gases occur in the infrared part of the spectrum rather than the visible-light part, and the scale of the change is much smaller from year to year than the swings in the seasonal cycle. But the general idea is about the same, and so are the methods we use to predict its effects. Keep this in mind the next time you hear someone say, “If meteorologists can’t predict the weather more than a week in advance, how can they predict climate change?”

In fact, it is possible to see global warming even in the seasonal forecast. The climate center has issued temperature forecasts for every three-month period between now and a year from now, and all of the maps show only red or white, with no blue shading anywhere in the continental United States. That means all the forecasts are for conditions that are warmer, or at least no cooler, than the average for the past 30 years.

The confidence levels for the temperature forecasts are much higher than those for precipitation — as high as 80% in the Southwest for the coming three months. And the fact is, a forecast of higher average temperatures would be a pretty good bet even if it were for one, two, five or 10 years from now. The planet is warmer now than in any previous decade on record, it will continue to get even warmer, and the forecasting models and the scientists who use them know why.

Do Winter's Clues Blow in the Autumn Wind?

What does the weather we have been having tell us about the coming winter season?

On Friday, the computer models were in agreement on a landfall somewhere along Florida’s east coast early next week. Overnight, though, as Dorian continued its rapid intensification, the models blinked. Some of them still showed a Florida landfall, but the majority shifted their forecast tracks to the east.

On Sunday, these models showed a similar range of outcomes: landfall in Florida, Georgia or the Carolinas, or nowhere. And if Dorian does make landfall, its intensity at that point — whether it will still be a Category 5 or substantially weaker — is equally uncertain.

So nearly the entire southeast coast of the United States, from South Florida through the Carolinas, is at risk. Yet in the end, perhaps most people in these places may well see little or no effect. The storm can take only one of the possible tracks, and some of them are harmless. That said, a buzz saw along the coast, like Hurricane Matthew in 2016, is well within the range of possibilities for Dorian, and could affect a very large area. This could be particularly bad if Dorian were to make its way up the coast slowly, prolonging the heavy rains and storm surge. These are often the most damaging hazards, though they aren’t taken into account by the numerical category, which is measured only by wind speed.

How should all the people along this long coastline deal with this confusing, uncertain forecast?

The National Hurricane Center advises that Florida residents should “have their hurricane plan in place, know if they are in a hurricane evacuation zone, and listen to advice given by local emergency officials,” and that those in Georgia and the Carolinas should “continue to monitor the progress of Dorian.” That’s all good advice, but it’s frustratingly vague and generic.

Worse, recent memories of forecast threats that didn’t materialize may cause some people to make unwise decisions. Evacuations are unpleasant and difficult, and feel even worse when the storm turns out not to be so bad at the place left behind.

In 2017, as Hurricane Irma’s forecast tracks oscillated between the east and west coasts of Florida, some people actually evacuated away from places that ended up being safe, and right into the storm’s path.

Memories like that can make people disregard warnings that they ought to heed. In 2011, Hurricane Irene’s forecasts prompted evacuations in New York, but the storm didn’t end up causing a disaster there. When Hurricane Sandy came the next year, the experiences of Irene prompted some people directly in harm’s way to stay put — to their great regret when the 2012 storm did live up to its scary billing.

Can’t meteorologists be more precise?

Not really. The scientific information forecasters are charged with communicating is uncertain not because they are doing anything wrong. The information is inherently uncertain, because of chaos, also known as the butterfly effect. Undetectable small errors in our knowledge of today’s weather quickly grow, becoming big errors in forecasts of weather a few days hence. So while we know quite a bit about Dorian’s future, that knowledge can be properly expressed only in the language of probabilities.

And because not taking a dangerous storm seriously enough can have a much bigger downside (as in people dying) than taking a not-so-dangerous storm more seriously than turns out to be necessary (people evacuate who didn’t need to), forecasters have to talk about the worst-case scenarios more than they talk about the best-case ones.

And that guarantees that there will be false alarms sometimes.

The uncertainties will not go away until the future becomes the past, but they do get smaller as that storm we’re all watching nears. So “monitor the situation” is still the right advice now, frustrating as it may be.

A Storm Expert's View: Dorian's Damage Remains Impossible to Predict

Hurricane Dorian’s clear eye and near-circular symmetry, apparent in satellite images, signify a ferocious storm. The Bahamas are in serious, imminent danger. But for all we know about Dorian, which reached Category 5 on the Saffir-Simpson scale Sunday with maximum winds of 180 mph, one big question remains: Will it be a disaster for the United States?

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Adam Sobel

Articles written by the author

Are Wildfires Caused by Utilities or Climate Change? Yes

Do Winter's Clues Blow in the Autumn Wind?

A Storm Expert's View: Dorian's Damage Remains Impossible to Predict