Climate Scientist Explains the Cause of Northern California’s Megafires

Following the Carr Fire, we tackled the “new normal” of devastating California wildfires in an article called The Wildfire Debate: What’s the Cause of these Massive NorCal Fires? where we outline the numerous factors attributed to these destructive and sometimes deadly fires.




In a thread of tweets, UCLA Climate Scientist Daniel Swain (also author of the Weather West blog) gets down to the pure science of the fires, offering his explanation for the devastating fires.

“If Northern California had received anywhere near the typical amount of autumn precipitation this year (around 4-5 in. of rain near point of origin), explosive fire behavior & stunning tragedy in would almost certainly not have occurred,” he started in a thread of tweets. He then went on in detail to explain the causes of the fires with scientific explanations and graphics.

“Rainy season has started late this year in California…again. While autumn precipitation isn’t usually huge fraction of overall annual average, it’s hugely important to ecosystems & in bringing “fire season-ending” moisture. This yr, autumn precip was <20-30% of avg.”




“Objective indicators of vegetation dryness and potential fire intensity were at record-high levels for the date this week in vicinity of –and would have been very high even for peak summer levels–at a time of year when the rainy season is usually ramping up.”

“Strong downslope winds were a key factor in the devastation of by the . But strong winds in damp forest simply aren’t going to drive the same kind of wildfire. The extreme, summer-like dryness of vegetation clearly matters.”

“In this part of California, summer 2018 was warmer than any prior to 2014 (4 of the 5 warmest on record have occurred in the past 5 years). Cumulative effect of warmth over many months also helped to dry out vegetation more than would otherwise have been the case.”

“You may notice that warming is part of a strong long-term trend. And it’s exactly what it looks like: California, like the rest of the world, is warming due to climate change. Fire season, along with the rest of the year, is getting warmer.”




“But what about the low autumn precipitation? Is that part of a trend, too? Well, yes: we recently found that autumn is not only warming across all of California, but also drying in recent decades.”

“But what’s causing that trend? Is it just bad luck? While the exact level of dryness in a particular year is somewhat random, less precipitation in autumn & spring–California’s “shoulder seasons”–has long been a projected outcome of climate change.”

“We replicated this earlier finding in work earlier this year, finding a large projected “concentration” of California’s precipitation into core rainy season months in the heart of winter (at the expense of autumn and spring).”

“CA wildfire folks will immediately understand significance of this: rainy months essentially define beginning/end of fire season. Dry autumns, in particular, are risky as they mean that summer-like vegetation dryness persists longer into “offshore wind” season.”

“This is just *one example* of how a changing climate has affected key risk factors during what has already become California’s most destructive wildfire in history. Does this mean that climate change “caused” the fire? No, of course not. But… that’s just a testament to how poorly framed the question of “Did climate change cause X extreme event?” really is. It misses the most essential point: all disasters are compound events, w/many contributing factors. But sometimes, climate can play starring role.”




“Wildfire risk is a key example of this complexity. In many cases, human factors like human encroachment/urban development in high fire-risk wildlands is at least as important as climate change. In other cases, forest and fuels management is also key consideration.”

“This thread comes partly as result of personal frustration & sadness surrounding what is happening in California. Last few years have been very tough for millions of people who have been directly affected by astonishing multi-year fire siege.”

“I grew up in California. My family & friends live there. And after last few years, almost everyone has story to tell. For some, it’s struggling to breathe in smoke-choked air. For others, it’s nightmarish escape from walls of flame in darkness of night.”




“What is abundantly clear is that we have big, rapidly accelerating problem–both in California & elsewhere. And my point is not that it’s all due to climate change. But we have to start having more nuanced conversations on societal risk. Clearly, status quo is not working.”

“From my perspective as a climate scientist, the increasingly profound changes we’re bearing witness are a big part of reason why. When you just look at the numbers, sometimes these changes seem subtle, incremental. But on-the-ground reality is that they’re anything but.”

See his entire thread here:

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