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Daniel Swain Nov 10
Replying to @Weather_West
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. (2/n)
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Daniel Swain Nov 10
Replying to @Weather_West
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. (3/n)
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Daniel Swain Nov 10
Replying to @Weather_West
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. (4/n)
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Daniel Swain Nov 10
Replying to @Weather_West
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. (5/n)
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Daniel Swain Nov 10
Replying to @Weather_West
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. (6/n)
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Daniel Swain Nov 10
Replying to @Weather_West
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. (Paper here: ) (7/n)
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Daniel Swain Nov 10
Replying to @Weather_West
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. (8/n)
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Daniel Swain Nov 10
Replying to @Weather_West
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). (Paper here: ) (9/n)
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Daniel Swain Nov 10
Replying to @Weather_West
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. (10/n)
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Daniel Swain Nov 10
Replying to @Weather_West
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... (11/n)
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Daniel Swain Nov 10
Replying to @Weather_West
...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. (12/n)
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Daniel Swain Nov 10
Replying to @Weather_West
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. (13/n)
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Daniel Swain Nov 10
Replying to @pyrogeog @climate_guy
But on top of these other factors, climate change is acting as a pervasive & growing " threat multiplier." , & I explored this confluence of factors in recent perspective piece. (14/n)
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Daniel Swain
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. (15/n)
Most of California’s hottest fires have burned since 2000—13 out of 20 total—as higher temperatures have made wildfires increasingly likely and destructive.
Popular Science Popular Science @PopSci
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Daniel Swain Nov 10
Replying to @Weather_West
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. (16/n)
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Daniel Swain Nov 10
Replying to @Weather_West
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. (17/n)
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Daniel Swain Nov 10
Replying to @Weather_West
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. (18/end)
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Jason 𝑻𝒉𝒆 𝑮𝒆𝒓𝒎 𝑮𝒖𝒚 Tetro Nov 10
Replying to @Weather_West
One question. Many models in Canada show precip coming sporadically in large amounts rather than consistent rain/snow periods. From what I hear, this does little to resolve drought-like conditions. Has this been seen in California more frequently as of late?
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Daniel Swain Nov 10
Replying to @JATetro @apuffycloud
Indeed. Check out our paper on increasing "precipitation whiplash" in CA. While avg precip not expected to change much, even in much warmer world, frequency of wet/dry extremes increases dramatically (as well as seasonality). See also 's work.
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Daniel Swain Nov 10
Replying to @climate_guy
Addendum: source for the initial maps in this thread is 's excellent climate toolbox:
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