Friday, April 26, 2019

Fire and Ice: Firenadoes and Snowball Earth

Two from Knowable Magazine:

Firenadoes and drifting embers: The secrets of extreme wildfires
Researchers probe the weather-like physics of deadly infernos
Flames begin to rise. Mike Heck jumps back. The tendrils lick upward, wavering in the wind, then coalesce into a vortex of flame, an incandescent tornado writhing in orange and red. “There it goes!” says one onlooker. Another whistles in astonishment.
But nobody is concerned. Heck set the fire deliberately, igniting a pan of liquid on the floor of a room lined with concrete blocks to contain the flames. A suction hood overhead prevents smoke from billowing into nearby classrooms.

Heck’s supervisor, fire scientist Michael Gollner of the University of Maryland in College Park, regularly conjures up such blazing pillars, known as fire whirls, in his lab. (Gollner and colleagues explore the science of these phenomena in the 2018 Annual Review of Fluid Mechanics.) From them, and from other fiery experiments, he aims to learn how flames intensify and spread as cities and landscapes burn. Gollner’s goal is to better understand what drives fire to leap its way from house to house and from tree to tree.

Gathering new insights into fire behavior has become increasingly urgent as wildfires have become more extreme, particularly in western North America. Starting in the mid-1980s, big wildfires suddenly became much more common in western US forests, especially in the northern Rocky Mountains. More recently, forests in the Pacific Northwest have seen the biggest increase in wildfire sizes, with a nearly 5,000 percent increase in burn area from 2003 to 2012 compared with the 1973–1982 average. Nationwide, the average acreage burned in the years since 2000 is nearly double the annual average for the 1990s.

And just in the last two years, several deadly infernos have incinerated parts of California. More than 5,600 buildings burned to the ground in and around Santa Rosa in October 2017. Last July in Redding, a towering plume of hot air and ash spawned a spinning “firenado” like the one in Gollner’s lab — but much bigger, and ferocious enough to kill a firefighter. The same month, fires burned vast acreage in Mendocino and three other counties. Four months later, 85 people died in the Camp Fire in Paradise, many of them incinerated while trying to escape the blaze in their cars....
Warning, very truncated charts ahead.
 (the fires of the 1800's. early 1900's, not shown, were orders of magnitude larger and/or deadlier)


And also from Knowable:

The story of Snowball Earth
Ancient rocks suggest that ice entirely covered our planet on at least two occasions. This theory may help explain the rise of complex life that followed.
The Earth has endured many changes in its 4.5-billion-year history, with some tumultuous twists and turns along the way. One especially dramatic episode appears to have come between 700 million and 600 million years ago, when scientists think ice smothered the entire planet, from the poles to the equator — twice in quick succession.
Drawing on evidence across multiple continents, scientists say these Snowball Earth events may have paved the way for the Cambrian explosion of life that followed — the period when complex, multicellular organisms began to diversify and spread across the planet. 
When Caltech geologist Joe Kirschvink coined the term Snowball Earth in 1989 — merging ideas that some geologists, climate physicists and planetary chemists had been thinking about for decades  — many earth scientists were skeptical that these cataclysmic events could really have occurred. But with mounting evidence in support of the theory and new data that help pin down the timing of events, more scientists have warmed up to the idea.

Paul Hoffman, a geologist at the University of Victoria in British Columbia, has helped pioneer Snowball Earth research over the past 25 years. Among other things, he amassed 50 months’ worth of fieldwork in Namibia, where he gathered evidence of ancient glacial activity in rocks that are interspersed with limestone. Since limestone tends to form in the warmest parts of the ocean, this sandwich-like pattern supports the idea that glaciers covered all of the Earth, cold as well as warm spots, during Snowball Earth episodes. Knowable spoke with Hoffman, who recounts his life work in the Annual Review of Earth and Planetary Sciences, about the evolution of the Snowball Earth theory and what questions remain. This conversation has been edited for length and clarity.

What did the planet look like during Snowball Earth?
The name describes its appearance from outer space — a glistening white ball. The ice surface is mostly coated with frost and tiny ice crystals that settled out of the cold dry air, which is far below freezing everywhere. Gale-force winds howl in low latitudes. Beneath the floating ice shelf, a dark and briny ocean is continually stirred by tides and turbulent eddies generated by geothermal heat slowly entering from the ocean floor.

What first tipped off geologists that this could have happened?
Geologists were struggling to understand what they saw in the geologic record — that not too long before the first appearance of complex life, there was unmistakable evidence of glaciation even in the warmest areas of the Earth. Geologists had a very difficult time understanding how this was possible.
The deposits that glaciers leave behind are very distinctive. They look like cement that has been dumped out of a cement truck. These Snowball ice sheets would have flowed from the continents out onto the ocean, so we have a lot of deposits that formed in the marine environment where you get what are known as dropstones: pebbles or boulders that are out of place. Very often, you see structures related to the impact, as if the stone was somehow dropped and then plunked into the underlying sediment. It’s difficult to imagine what, other than floating ice, could have possibly transported this debris; trees, which can carry soil and stones out to sea in their roots, had not yet evolved....MUCH MORE