UPDATE: This has been edited since first publication, and will continue to be revised until this sentence no longer appears. I’ve introduced mark-up in the charts, clarified the language and enhanced a number of explanations. This piece is part of a three-part set that together explains the climate crisis from “10,000 feet” — a very high-level view with the most basic elements explained for the interested lay reader.
The three parts of this set of posts are:
Thanks for reading.
In the first piece in our “view from 10,000 feet” we looked at global temperatures from the earliest explosion of life on earth (during the Cambrian Period, some 540 million years ago), and noticed that the first big temperature spike in the early days of the Cambrian is a match for the temperature spike we could well see in 2100 under the “do nothing about carbon” scenario. The Cambrian temperature spikes reached 7°C (12½°F) above pre-industrial (pre-1800) norms, which is also where we could be headed if we don’t stop.
We also saw that the entire period of time from the Cambrian Period until now is divided into just three geologic eras, or major divisions …
The Paleozoic Era — the era of life before the Age of Reptiles, 540–250 million years ago
The Mesozoic Era — the Age of Reptiles, 250–65 million years ago
The Cenozoic Era — the Age of Mammals, which we’re now in
… and that each of the first two eras ended in a major mass extinction event. Will a mass extinction end the Cenozoic Era, the Age of Mammals? If the earth warms enough, yes. This piece explains why and looks at the broad consequences for man under a couple of warming scenarios.
What does “major mass extinction” mean?
In order to discuss global warming and mass extinction, we need to look at mass extinctions in general to get a sense of the scale of these events and their effect.
Consider again the chart of extinctions since the Cambrian, 540 million years ago. (This chart was also presented here; source here.) The labels across the top — “Cm” and so on — are geologic “periods”. For your convenience I’ve added the larger divisions, the three geologic eras as well, and indicated where the current geologic period, the Quaternary, fits in.
“Extinction intensity” on the Y axis measures only marine extinctions. That’s for apples-to-apples comparison across the chart, since land plants evolved later, roughly 475 million years ago, and amphibians later still, roughly 375 million years ago. “Extinction intensity” doesn’t measure all species extinct, just countable ones based on the fossil record, but it’s still an excellent measure for showing the relative scale of these disasters.
As you can see, only a handful of mass extinctions grew to real size. The biggest one by far is between the Paleozoic and Mesozoic eras — at the Permian-Triassic boundary (between “P” and “Tr” above). That extinction is called the Great Dying, since over 90% of all marine species and 70% of land vertebrate species died out. It not only ended the Permian, it ended the whole Paleozoic Era.
There’s another major extinction at the Cretaceous-Paleogene boundary (between “K” and “Pg” above) in which half of all species went extinct. That extinction ended the Mesozoic Era, the Age of Reptiles. There aren’t many spikes of that intensity on the chart.
If James Hansen is right (see below), we’re about to create another one, an event that could kill off up to 50% of currently living species. Will that event end the Age of Mammals? That depends on the temperature that triggers extinctions, and also on what temperature the earth finally heats to before global warming levels off.
Let’s start with temperature and extinctions.
What temperature increase will trigger the next mass extinction?
This is a key question — what warming increase will trigger an extinction on the scale of those on the chart?
We know that global warming will cause some crises, since the warming we’ve experienced so far (0.8°C or so) is already a problem. But according to the Intergovernmental Panel on Climate Change (IPCC), the leading scientific organization studying this phenomenon, this is only the beginning of the warming we’re headed for. What temperature we should stop at if we don’t want to trigger another world-class extinction?
We also want to know the effect of these increased temperatures on man. As we showed in this post, the transition of homo sapiens from 200,000 years of hunter-gatherer life to what we call “civilization,” which occurred 12,000 years ago, coincides exactly with the stabilization of global temperature to its current narrow range.
To see that dramatically, look at the chart below. To see this chart full-size, click the image. To see the full-size unmodified original, click here; it’s large and interesting. (The chart was originally presented and discussed here.)
First at the left, notice the large orange temperature spikes. The biggest one, at the extreme left, reaches 7°C (or 12½°F) above the Y axis zero mark, the post-civilization, pre–Industrial Revolution “norm” shown at the right end.
Then look almost all the way to the right, at the 12,000-years-ago mark. That’s the start of the Holocene, “today” in geologic time. In the Holocene, Earth comes out of its last great ice age (an epoch called the Pleistocene) and global temperatures stabilize, going from about 1°C below the pre-industrial norm to almost flat, holding roughly in the range ±0.5°C. (To zoom in on just the Holocene temperatures, from 12,000 years ago until now, click here. The black line in both charts, this one and the linked Holocene chart, shows the average of eight regional temperature records.)
The flattening of global temperature in the last 10,000 years is remarkable. It also coincides exactly with civilized man, man emerging from hunter-gatherer status to build villages and farm land. It would be nice to keep the earth in that range, right?
Which brings us to James Hansen, head of the NASA Goddard Institute for Space Studies in New York City and a leading American researcher in this field. He’s been working on the problems of global warming and climate crisis since the 1980s (our discussion of Hansen’s earlier work is here.)
Hansen recently published a paper called Perceptions of Climate Change: The New Climate Dice (pdf), then authored several op-ed columns based on its conclusions. He thinks he has found the temperature number we’re looking for, the global warming increase that leads to a sizable mass extinction event. In his conclusion, he says this (my emphasis and paragraphing):
Although species migrate to stay within climate zones in which they can survive, continued climate shift at the rate of the past three decades is expected to take an enormous toll on planetary life.
If global warming approaches 3°C by the end of the century, it is estimated that 21-52% of the species on Earth will be committed to extinction (3). Fortunately, scenarios are also possible in which such large warming is avoided by placing a rising price on carbon emissions that moves the world to a clean energy future fast enough to limit further global warming to several tenths of a degree Celsius (29). Such a scenario is needed if we are to preserve life as we know it.
See the paper itself for the references. Footnote (3) refers to Climate Change 2007: The Physical Science Basis, volume 1 of the IPCC Assessment Report 4, the most recent assessment report from that group. You can read sections of IPCC AR4 or download the PDF here.
Hansen’s follow-up op-ed in the New York Times was just as stark (again, my emphasis and some reparagraphing):
Game Over for the Climate
… Canada’s tar sands, deposits of sand saturated with bitumen, contain twice the amount of carbon dioxide emitted by global oil use in our entire history. If we were to fully exploit this new oil source, and continue to burn our conventional oil, gas and coal supplies, concentrations of carbon dioxide in the atmosphere eventually would reach levels higher than in the Pliocene era, more than 2.5 million years ago, when sea level was at least 50 feet higher than it is now.
That level of heat-trapping gases would assure that the disintegration of the ice sheets would accelerate out of control. Sea levels would rise and destroy coastal cities. Global temperatures would become intolerable. Twenty to 50 percent of the planet’s species would be driven to extinction. Civilization would be at risk.
What Hansen is saying is what our chart above also says — that global warming of 3°C (5½°F) hasn’t been seen since the early Cenozoic, millions of years before the dawn of man. Check for yourself. Look at the second chart above, starting from the right edge (“now”), and trace back to the last time the earth was +3°C above our current temperature. It looks like over 10 million years ago to me, long before any species of man evolved.
Hansen also says +3°C — whenever it arrives — is a sure mass extinction point, potentially the start of a new geologic era. Hansen has since published other papers that indicate even +2°C could be too high. We’ll look at those papers later in this series. For now though, we can take +3°C as “game over.”
Why staying below 3°C global warming matters
We need to keep James Hansen’s 3°C warming number in mind in mind for two reasons. The first is that even if we stop global warming at “just” 3°C, it’s a disaster. Imagine living in a world in which the earth is so warm that 20–50% of species are going extinct. Imagine the chaos, the death from disease, starvation, drought, population migration and war. Now imagine that this all happens within the next 90 years. The compression is stunning.
And we’re halfway there if you count the warming that’s in the pipeline, inevitable — the warming that’s unavoidable as ice sheets continue to melt, summer arctic ice shrinks to nothing, summer oceans absorb the sun’s rays instead of reflecting it, and permafrost releases its frozen-for-millenia methane, also a greenhouse gas.
There’s no stopping the certain degree of roughness, of climate dislocation and crisis, we’ve already made for ourselves. Global warming is at 0.8°C now and headed inevitably for 1.5°C. There’s a great deal of consensus around that number as the minimum that’s inevitable. This is why world “leaders” want to stop at 2°C; they know stopping at any lower number is a lost cause.
In fact, if there’s disagreement at all among the unbought professionals in this field, it’s that some scientists now think our chance of keeping global temperatures below 2°C is already unlikely. Who will be proven right? I don’t know, but every time I look at the headlines, things are happening faster than anyone expected.
So the first bottom line for today is this. If you accept that …
… we’d be foolish not to heed Hansen’s warning. A 3°C warmer world by 2100 would be hell to manage.
Eventually we might be able to set up enclaves near the Arctic circle and preserve something that looks like civilization — some farming, some energy production, some manufacturing. But what are the human population numbers at that point, and what does the coming century look like during that transition? It won’t be a world anyone wants to live in.
Yes, a 3°C warmer world would be a challenge to say the least. But there’s a second bottom line for today that’s even more stark, and presents an even stronger reason to put on the carbon brakes now.
If we go to 3°C warmer, we may go to 7°C or beyond
For a reason I’ll discuss next time, if global warming is man-made — and few unbought scientists think otherwise — then 3°C warming may well be just the halfway point to the full disaster. By that I mean, because of the way the socio-political process works, the “never stop burning carbon” scenario could easily take us right past 3°C to a 7°C (12½°F) warmer world — in the worst case, by 2100 — and perhaps beyond.
That’s double the compression of Hansen’s 3°C scenario — it means 3°C warmer by the mid-2050s and 7°C warmer by the end of the century. The discussion of that outcome is also in the IPCC literature, the same literature Hansen used to make his mass-extinction prediction. This is their own worst-case scenario. It’s not a prediction, but it’s one of the possibilities. Yikes.
For a look at times when the earth was as hot as 7°C above pre-Industrial norms, you have to look at the Mesozoic Era and earlier (again, see the second chart above). In a 7°C warmer world, I’m not sure we’re a species. I’m not sure what it would take to exist even in the ice-free Arctic, much less live in a “civilized” way. I’ll expand that consideration next time.
So the second bottom line is this — If mankind’s carbon is driving the warming process, the process doesn’t stop until man does.
Man will stop spewing carbon (a) by intention, (b) by most of us going pre-industrial, or (c) by drastically reducing our population, perhaps to zero. When one of those three things is true and when all of the warming after-effects in the pipeline play out, global temperature will level off. Not before. Will that level-off point be after 2°C warming? After 3°C warming? After 7°C warming? If we don’t stop after +3°C, how will we ever stop?
Stopping at 2°C will take intention. The other stopping points imply an out-of-control process. We’ll look that aspect shortly. But for today, consider that it’s entirely possible that when 3°C warming is present, 7°C is in the pipeline, inevitable, thanks to the chaos caused by a +3°C world. If a 3°C warmer world doesn’t mark a new geologic era, 7°C warmer certainly will. Man as a species might survive a transition to the first. We won’t survive a transition to the second.
Why consider these scenarios?
I’m writing this series for just one reason — we can put ourselves on a different path whenever we want to. But we have to want to. So if you’re feeling some panic right now, good. When enough of us feel that panic, we’re halfway home. We just have to “hug the monster” and act. We have control.
To help us, world-wide resistance is coming, and that’s a good thing. In addition, the climate is making its own case. Last summer was all Arctic-ice news for more than a month. Expect more of the same this summer. The will to act can only get stronger. This isn’t hopeless, just very important.
The next big test in the U.S. is the Keystone Pipeline, by the way. To see how the resistance to warming is building, watch the resistance to Keystone.
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