By Gaius Pubius, a professional writer living on the West Coast. Follow him on Twitter @Gaius_Publius. Cross posted from AmericaBlog
As you may know, we’ve restarted our climate crisis writing here at La Maison, beginning with this piece, a global warming picture “from 10,000 feet”:
There we took the long view and noticed that the big temperature spike in the early days of the Cambrian, some 540 million years ago when life on earth was exploding in number and diversity of species, is a match for the temperature spike we could very well see in 2100 under the “do nothing” carbon scenario. The Cambrian temperature spikes reached 7°C (12.5°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 (again, about 540 million years ago) until now is divided into just three geologic eras, or major divisions …
The Paleozoic Era — the era of life before the Age of Dinosaurs, 540–250 million years ago
The Mesozoic Era — the Age of Dinosaurs, 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 we hit a warm enough temperature, 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 presented in slightly different form 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 Quarternary, fits in.
“Extinction intensity” on the Y axis measures only marine extinctions. That’s for apples-to-apples comparison across the chart. Land plants evolved roughly 475 million years ago, and amphibians 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 (“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 (“K” and “Pg” above) in which half of all species went extinct. That extinction ended the Mesozoic era, the Age of Dinosaurs.
Look at the chart again and locate the most recent extinction, the one that ended the dinosaurs. That spike killed off 50% of all species. 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, a 25–50% species-extinction event. Will it end the so-called Age of Mammals? That depends on the effect of temperature on extinctions, and also the temperature the earth finally heats to before 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 of the scale of those on the chart?
We know that global warming will cause some crises, since the little 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. What’s the temperature we should stop at if we don’t want to cause 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. I presented an unmodified version here; the version below has been enhanced with names of relevant geologic eras and periods, and also information about the appearance of human species, including our own. (To see this chart full size, click the image. To see the full-size unmodified original, click here; it’s large and interesting.)
First at the left, notice the large orange temperature spikes. The biggest one, at the extreme left, reaches 7°C (or 12.5°F) above the Y axis zero mark, the post-civilization, pre-industrial “norm.”
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 ice age 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 Holocene chart, shows the average of eight regional temperature records.)
The flattening of global temperature in the last 12,000 years is remarkable. It also coincides exactly with civilized man, man emerging from hunter-gatherer status. 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 lead 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 has the temperature number we’re looking for, the global warming increase that leads to a sizable mass extinction event. In his conclusion, he says (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. You can read sections 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 paragraphing):
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 (or 5.5°F) hasn’t been seen since the early Cenozoic, millions of years before the dawn of man. He also says 3°C — whenever it arrives — is the mass extinction point, potentially the start of a new geologic era.
Why Staying Below 3°C Global Warming Matters
We need to keep James Hansen’s number 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 and war, the global population migrations. Now imagine that this all happens in the next 90 years. The compression is stunning.
And we’re halfway there if you count the warming that’s in the pipeline and 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.
There’s no stopping a certain degree of roughness 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 All the Way to 7°C
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 (12.5°F) warmer world — in the worst case, by 2100.
That’s double the compression of Hansen’s 3°C scenario — 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 absolute 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 chart above). In a 7°C warmer world, I’m not sure we’re even a species. I’m not sure what it would take to exist in the arctic, much less live in a “civilized” way.
I’ll expand that consideration next time. But the short-form is this: If mankind’s carbon is the big driver in the warming process, then 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 numbers, perhaps to zero. When one of those three things is true, all of the warming after-effects in the pipeline will play out, and global temperature will level off. Not before.
Will that level-off point be after 2°C? 3°C? 7°C? Stopping at 2°C will take intention. The others imply an out-of-control process. We’ll look that aspect shortly. This is however your second bottom line for today. It’s entirely possible that when 3°C warming is present, 7°C is in the pipeline. If a 3°C warmer world doesn’t mark a new geologic era, a 7°C warmer world 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. I’m trying to help us 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.
In addition, world-wide resistance is coming, and that’s a good thing. We still have some time — I’ll lay out some what-happens-when scenarios in a bit — and the will to act can only get stronger. This isn’t hopeless. It’s just very important.
To follow or send links: @Gaius_Publius