Hypercanes
In the aftermath of the immense 2017 hurricane season, questions about the link between storm intensity and climate change and the future of storm intensity are being raised again.
I’m not an atmospheric modeler, but so far my impression is that we don’t have a good answer for what climate change will do to these storms. They might be commonly broken up by wind shear or there could be an increase in storm intensity due to warmer ocean waters if they hit the right spot. Those variables are strongly affected by local conditions, leaving it difficult to give coherent predictions of the future.
In earth science, when we’re unable to make a consistent prediction about the future, sometimes the thing we ought to do is look to the past, to the geologic record, for context. One of the most extreme climate changes in the geologic record occurred during the “Cryogenian” period, from about 800 million to 650 million years ago. This period is the time of the proposed “Snowball Earth” events.
Snowball Earth is a theory based on the albedo of the earth. Snow and ice are white; if something in the climate makes glaciers grow all the way to the tropics, the white snow and ice reflect so much sunlight that the Earth might well freeze shut. If this happened, the only way out of that state is CO2. As described originally by Dr. Kirschvink from Caltech, volcanoes keep pumping out CO2 even if the Earth is frozen shut. Eventually so much CO2 builds up in the atmosphere that global warming overwhelms the reflectiveness of the icecaps and they melt suddenly.
When that happens, the climate of the Earth is instantly far out of equilibrium. The planet would snap from being frozen solid to unbearably hot; the CO2 would remain in the atmosphere but the Earth would return to a normal albedo instead of being bright white.
The geologic record of these times is complex but it does tell an intriguing story. At the times of these glaciations, there are glacial sediments in the tropics, almost to the equator. Those glacial sediments are covered by “cap carbonates” with some incredible structures in them. You’re looking at a picture of one of those cap carbonates; this image shows the contact between the Beck Springs dolomite and the Noonday dolomite in Death Valley.
The cap carbonates would be produced from all that CO2 in the atmosphere. After the snowballs broke up, all that CO2 in the atmosphere would super-saturate the oceans, causing thick, global deposits of carbonate. The other interesting thing in this picture though is the structure of it. Look closely at the lower unit in this photo, it’s full of clasts.
This structure is called a “tempestite”, formed during tempests (major storms). I’d call those “rip-up clasts” as well. They’re formed when sediment deposited on the floor of the ocean starts to stick together, the beginnings of forming a rock. But, if a strong-enough storm occurs, that partially-lithified sediment can be ripped up and torn apart into chunks. The chunks stick together somewhat, but they can be tossed around like rocks and piled up.
Tempestites like this one are one hint that something incredible happened in the climate system when the cap carbonates were deposited. There are other structures, like very large ripple marks, which suggest these sediments and storm features were occurring in deep water, maybe over 100 meters deep. It takes an enormous storm to alter the ocean floor that deep; like nothing we’ve ever experienced.
It is in this context that I first heard the term “hypercane”. A hypercane is a proposed possible storm that could occur if waters were hot enough; maybe above 50°C, and could have wind speeds as much as 500 mph. We’re totally unfamiliar with the kinds of conditions that would produce this level of storm. It might take a giant impact that heats large areas of the ocean, or it could take CO2 levels thousands of times what they are today, but those conditions might have been created after the snowball events.
The geologic record doesn’t tell us very much about the frequency of large storms; doubling the rate of hurricanes today wouldn’t really leave a rock record we could interpret. But, the geologic record does make a strong argument that 650 million years ago, things were taking place in the climate system completely beyond the scale of what we conceive of today. We’re hopefully never going to see storms like the ones that produced these rocks. There might not be enough fossil fuels in the world to get CO2 levels that high, but one lesson from this part of the rock record is how crazy the climate can get when things are pushed far from the equilibrium we have today.
-JBB
Image credit: John N. (Reproduced with permission): http://www.flickr.com/photos/40179910@N00/6496814455/in/photolist-aU6SAk
More on “hypercanes”: http://onlinelibrary.wiley.com/doi/10.1029/95JD01368/abstract http://www.dailygalaxy.com/my_weblog/2009/04/is-a-mega-katri.html
