The level of heat-trapping carbon in the atmosphere has passed the dangerous 400 part per million (ppm) global warming milestone.
Even though this is just an “odometer reading” event — like when your car goes from 99,999 miles to 100,000 — this global warming milestone is a big number and a big deal. The earth has not seen levels like this in at least three million years.
The news: We just crossed 400 ppm atmospheric carbon dioxide
First the news from the New York Times (my emphasis and paragraphing):
The level of the most important heat-trapping gas in the atmosphere, carbon dioxide, has passed a long-feared milestone, scientists reported Friday, reaching a concentration not seen on the earth for millions of years.
Scientific instruments showed that the gas had reached an average daily level above 400 parts per million — just an odometer moment in one sense, but also a sobering reminder that decades of efforts to bring human-produced emissions under control are faltering. The best available evidence suggests the amount of the gas in the air has not been this high for at least three million years, before humans evolved, and scientists believe the rise portends large changes in the climate and the level of the sea. …
Carbon dioxide above 400 parts per million was first seen in the Arctic last year, and had also spiked above that level in hourly readings at Mauna Loa. But the average reading for an entire day surpassed that level at Mauna Loa for the first time in the 24 hours that ended at 8 p.m. Eastern Daylight Time on Thursday. …
Carbon dioxide rises and falls on a seasonal cycle, and the level will dip below 400 this summer as leaf growth in the Northern Hemisphere pulls about 10 billion tons of carbon out of the air. But experts say that will be a brief reprieve — the moment is approaching when no measurement of the ambient air anywhere on earth, in any season, will produce a reading below 400.
Here’s what that looks like starting from 1960 — this is the famous (to scientists) Keeling curve, in a snapshot taken just last month:
Ralph Keeling, in fact, is quoted in the Times article: “It means we are quickly losing the possibility of keeping the climate below what people thought were possibly tolerable thresholds.”
Do David Koch (or Barack Obama) care? Not so you’d notice. But maybe they’ll care later, after it’s too late.
The historical context
Here’s a taste of the context for this news, since the meaning of “400 ppm” is probably not immediately apparent. (I’m preparing a larger piece on the correlation between the gigatons of carbon (GtC) we’ve been emitting, the resultant concentration of CO2 (ppm), and historical temperature increases since 1800. All three measurements are used, and it can get confusing.)
According to the lastest IPCC Assessment Report (2007), pre-industrial (pre-1800) levels of atmospheric carbon were 280 ppm. The 2005 measurement was 380 ppm. “Today” we’re at 400 ppm according to the news above. In addition, the increase is accelerating — meaning the rate of growth is growing. Before 1995, the increase was about 1.4 ppm/yr. Now it’s about 1.9 ppm/yr.
The IPCC estimates that we’ll see 450 ppm in 2030, so that average growth rate of 1.9 ppm/yr is going to increase as well. And I’ll bet they’re way low, since these days the scientists are consistently way low. (Doubt me? Even the scientists admit that. Download the Copenhagen Diagnosis (pdf), an executive summary that updates the science from 2007 to 2009, and search for the phrase “faster than”.)
350 ppm is the maximum safe number
For comparison, 350 ppm is the highest level that James Hansen says (pdf) we can tolerate if we want to “preserve a planet similar to that on which civilization developed and to which life on Earth is adapted.” More Hansen (my emphasis):
If humanity wishes to preserve a planet similar to that on which civilization developed and to which life on Earth is adapted, paleoclimate evidence and ongoing climate change suggest that CO2 will need to be reduced from its current 385 ppm to at most 350 ppm, but likely less than that….
Decreasing CO2 was the main cause of a cooling trend that began 50 million years ago, the planet being nearly ice-free until CO2 fell to 450 ± 100 ppm; barring prompt policy changes, that critical level will be passed, in the opposite direction, within decades.
Here’s what that all looks like in a handy data table:
|Year||Atmos. Carbon||Est. Warming||In Pipeline?|
|“Max safe”||350 ppm|
|2030||450 ppm||2°C (maybe)||3.5°–4°C (maybe)|
There are uncertainties here, and they don’t necessarily benefit us. First, note in Hansen’s quote above the figure “450 ± 100 ppm.” That’s a lot of plus or minus, and it could go either way. Second, in the third column (“Est. Warming”) of the table, the 2°C warming associated with 450 ppm is widely taken as a given, but as 350.org argues here, it shouldn’t be. There’s no guarantee that we’ll get only 2°C at 450 ppm. It could easily be worse.
And finally, it’s admitted everywhere that the carbon-temperature effect works both ways — each affects the other. From pdf p. 49 of the Copenhagen Diagnosis:
In climate history, didn’t CO2 change in response to temperature, rather than the other way round?
It works both ways: CO2 changes affect temperature due to the greenhouse effect, while temperature changes affect CO2 concentrations due to the carbon cycle response. This is what scientists call a feedback loop.
With that in mind, look again at that “in the pipeline” column. Most scientists agree that we’ll see warming of 1.5°C (a little under 3°F) regardless. Even if we stopped carbonizing the air completely today, we couldn’t stop that amount of warming. Some are now saying we’re beyond the 1.5°C point, and that 2°C (about 3.5°F) is now inevitable. That’s the range of current opinion, and that’s what the table expresses.
Me, I’m personally more pessimistic, since in the last 10 years, every prediction has been exceeded to the fast side. We’ve been consistently wrong to the slow side, and that needs to be taken into account (click to see my personal climate model).
So what happens when 450 ppm is indeed a fact? Do we get “just” 2°C warming (about 3.5°F) on the ground, or something worse? And at that point, what’s in the pipeline, unavoidable? Remember, as we noted here, according to the same James Hansen, 3°C forces a mass-extinction scenario, or as he said in the New York Times, “game over for the climate.”
For comparison, I think 3°C is a foregone conclusion (an “in the pipeline” number) in the next 5–10 years at the earliest unless we force our “leaders” to stop. A narrow window, but still, a window.
The good news
The good news — and there is good news — is that even the most pessimistic estimates say it’s not over yet. But we do have to move on this, and now. Time for a little action in the streets, and some civil disobedience? Chris Hayes for one says yes.
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