I want to tell you a very unpleasant truth about climate change. It's a lot more serious than we've been discussing in public.

(Warning the first: This is not going to be a cheerful sort of post. It will, very likely, leave you feeling deeply unsettled. If you do not want this, you should stop reading now.)

(Warning the second: This is not a post on which to say "I don't believe climate change is happening!" or "this is a left-wing plot!" or "I don't believe there's adequate proof that humans are causing it!" There are times that I have the patience and interest to discuss what are essentially political arguments about science, and this is not one of them. If you believe this stuff, it's because you have a deep personal need to do so, and best of luck to you with that. But I'll just delete such comments on this post.)

The paper referenced here is one that +Larry Smarr shared. We have a new paper running several parallel models of Arctic ice collapse, and the one thing that even the most conservative models agree on is that the Arctic will be ice-free in summer within the next 20 years. This is pretty much right; anyone who's been watching the development of climate indicia and thinking about the positive feedback loops in ice melting has known it for a while.

But we don't really talk about positive feedback loops, much, and when we do, we stop soon afterwards because it's a bit too horrifying to think about. But climate systems are full of them; ice melting is a simple example. If you have a big sheet of ice, it reflects sunlight and stays cool. But if it melts, the top of it melts first, and then you have a puddle of water, which is great at storing heat and doesn't reflect as much sunlight, sitting on top of your ice. That puddle gets warmer, and melts the ice under it much faster than pure light would; lather, rinse, repeat, and ice melts fast. (You can test this out in your own backyard; take two pieces of ice and put them in the sun. If you keep draining the water from the top of one as it melts, it will melt much more slowly than the other one)

The best way to know that climate systems are a big hornking bag of positive feedback loops is to look at the Earth's climate record. (We can get this quite nicely from things like deep ice cores, tree cores, etc.) You can see a good summary graph here: http://www.scotese.com/climate.htm. What's distinctive are those sudden vertical spikes; these represent times when the climate suddenly and rapidly became a lot hotter, on time scales much shorter than any other variation. That sort of thing can only happen when you get a massive external driving force (think "giant comet") or a positive feedback loop.

What's important to understand about this is that, when you hit a loop like this, the consequences aren't measured in the ways that IPCC climate models talk, about so many degrees rise in mean temperatures, changes in the biomes of infectious diseases, crop failures, sea level rises making cities into ruins, giant storms wiping cities off the map. They change the entire ecosystem of the Earth -- the basic kinds of plants and animals which can live on it.

The last big spike like this was the Paleocene-Eocene Thermal Maximum, about 55 million years ago. Average temperatures rose by 6C over a period of 20,000 years -- which is enough to look like a giant, sharp spike on the history-of-the-entire-planet graph. During the PETM, the Earth looked like the inside of a giant greenhouse; hot, wet, tropical. Swamp cypress grew as far north as Ellesmere Island, the northernmost part of Canada. The large mammals of the Paleocene vanished, to be replaced by a huge variety of new species, mostly dwarf-sized. Many of our modern kinds of creature -- birds, ungulates, etc -- emerged in this period. Life before the PETM favored much bigger critters, like a snake the size of a school bus. (See below for a link)

As far as climate swings on the Earth, this one wasn't close to the biggest, although it was one of the fastest: the temperature rose by 0.0003C per year, enough to completely reset the biota of the planet. 

By comparison, since 1920, the mean temperature has been rising an average of 0.01C per year. Yes, that's about 30 times faster than the run-up to the PETM.

Some things you need to understand about these shifts.

* They've happened quite a few times. Look at that first graph I linked, and read about any of the times the climate changed sharply. Each of these was associated with a complete rewriting of the planet's biota -- which is a nice way of saying "almost all the life died out and was replaced by something completely different."

* Because of positive feedback loops, when a climate shift starts, it can speed itself up. There are a lot of different loops, ranging from ice melting to methane clathrates to ocean circulation pattern changes. Unfortunately, we don't understand these loops very well -- because if any of them had gone off full-force while we were around to study them, we wouldn't be around to study them. Because of loops like that, once something gets started it's not always possible to shut it off by reversing what you're doing, no matter how much you do so.

* When the biota of a planet get rewritten, the creatures that require the most delicate maintenance die first. This tends to mean really big creatures, that rely on large supplies of their foods; apex predators, which rely on the entire food chain beneath them; and "canary" species like many frogs, which are very sensitive and tend to be the first to die when something is going wrong. Historically, the cutoff for "large creatures" (that tend to not survive extinction events) seems to be in the ballpark of 20 pounds; things bigger than that just require the ecosystem to be too healthy.

So, yes, that includes you, it includes your dog, it includes most of the animals you eat. It probably also includes lots of the grains you eat, since large-scale agriculture is quite fragile as well. (As evidenced by the tremendous amount of work put in every year to keep crop yields high enough to feed humanity) 

* Technological methods of helping are actually more limited than you think, because so much of our technology stack is built on top of society being basically functional. Manufacturing microchips requires pretty much the full scope of human industry, from mining to power generation to transport logistics to chemical engineering. Growing the quantities of plants required to support humanity is, if anything, even more delicate. We're fairly robust against small perturbations because we can put in technological solutions -- but when problems start to knock out the basic infrastructure on which we can depend, the descent and collapse is fairly rapid.

Now, you may think that I'm writing this to make a political point, or to urge you to do something or other. Sometimes I would be, but this time I'm not: I'm just writing to show you a bit about the science, and give you an idea of just what the situation we're talking about could potentially entail. It's not clear where we are on the positive-feedback loop right now; it's very likely, for example, that the Arctic ice will collapse at this point, no matter what we do in the next 20 years. Whether we've gone far enough to trigger other catastrophes is still up in the air. 

But if it is, what we're looking at isn't a world where we all live like Bangladeshis, or a world where we're living in technological bubbles. It's a world where there are tropical rainforests going up to the poles, where there are millions of new and unfamiliar species... and we're simply dead.

If you want to know more about the history, here are some places to start:
Life in the Eocene: http://en.wikipedia.org/wiki/Eocene
Temperature change in the 20th century: http://en.wikipedia.org/wiki/Temperature_record
Titanoboa, and other giant creatures of the Paleocene: http://www.smithsonianchannel.com/site/sn/show.do?show=140671
One of the kinds of positive feedback loop that could be a problem for us: http://en.wikipedia.org/wiki/Clathrate_gun_hypothesis
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