The radical "trillionth ton" meme

The more I read about the trillionth ton (or tonne for our non-American friends), the more intrigued I am by its power to change the way we approach the threat of global warming. I wrote last week about the idea, which represents a whole new way of thinking about carbon emissions, but I'd like to take another stab at it, in hopes of spreading the meme further than my last post managed.

The "trillionth ton" refers to the amount of carbon in the atmosphere. Before the invention of the steam engine and everything that followed since the middle of the 18th century, there was a certain amount of that fundamental element in the air. What humans managed to do over between 1750 and 2000 is transfer somewhere between 500 and 600 billion tons of the stuff from its long-term hiding place underground in the form of coal, oil and natural gas into the atmosphere.

There, in the form of carbon dioxide, methane, and few other molecules, it absorbs and re-radiates heat, warming the planet as it does. Figuring out just how much carbon results in how much warming has been a major focus of climatology for more than a century. The general agreement among those who are paid to calculate such relationships is that if you double the amount of carbon in the air, you raise the global average temperature by 3 °C (almost 6 °F). There is an important caveat here, but I'll get to that later.

Most climatologists suspect that dangerous climate change will kick at only 2 °C above pre-industrial levels and while this is somewhat arbitrary, it's warmer than the Earth has been for millions of years, and it's as good a guess as you're going to find in the literature. We've already raised the Earth's temperature by 0.7 or 0.8 °C. and there's upwards of another half a degree or so in the proverbial pipeline even if we stop burning everything now.

The question according to this new meme becomes, how much carbon can we afford to pour into the air before we hit 2 degrees of total warming? We're talking about the cumulative emissions of carbon, not a specific concentration or a rate of emissions, and we aren't even concerned about a time deadline, just a grand total of allowable emissions.

A pair of papers just published in Nature are in rough agreement that the Earth, with all its carbon sinks and oceanic heat-redistributive powers, might be able to handle a trillion tons of the stuff and stay below the extra 2 degrees. But we've already emitted more than half of that trillion, so we can only afford another 400 million tons or so before our only options become geo-engineering, sucking carbon out of the air and other schemes so expensive they make current plans to scale back emissions by 90 percent in a few decades look like a bargain by comparison.

But wait. Only part of our emissions comes in the form of carbon dioxide from fossil-fuel combustion. Most of the rest come from agriculture and forestry. We'll have to deal with those too, but it means that our allowable energy-producing carbon emissions quota is down to about 275 billion tons. Remember that our quota includes everything since the year 2000. And since then we've been emitting almost 10 billion tons a year, which means we've already used up nearly a third of that 275 billion tonnes.

Which leaves us with around 190 billion tonnes. That's it. Just 20 years at business-as-usual rates.

Our options are limited. We can continue on the current path, as Bjorn Lomborg and his acolytes would have us do, for another decade or so on the assumption that future carbon-free ways of producing electricity and moving our cars will be dramatically cheaper than they are today, and then make the entire switch in a year or two. But I think most will see the problems with that strategy.

Or we can start bringing our emissions down immediately, as fast as possible. Here's how a group of climatologists, including the lead authors or the "trillionth ton" papers explain the situation in a Nature Climate commentary:

A tonne of carbon is a tonne of carbon, whether released today or in 50 years time. Emitting CO2 more slowly buys time, perhaps vital time, but it will only achieve our ultimate goal in the context of a strategy for phasing out net CO2 emissions altogether.

At some point in the past few years, without any fanfare, we burned the half-trillionth tonne. Somewhere out there, in a coal seam, hydrocarbon reservoir or some as-yet-undiscovered exotic form of fossil carbon, lies the trillionth tonne. Its fate, perhaps more than any other consequence of climate-change policy, is inextricably linked to the risk of dangerous climate change. Where will it be in the twenty-second century?

But now to the caveat I mentioned. All of this trillion-ton talk is based on notion of the climate sensitivity of 3 °C for a doubling of carbon dioxide levels. Again, that's become an accepted relationship. But it's based only on "fast feedbacks" like the amount of water vapor in the air. (As the world warms, more water evaporates, and water vapor warms the world, too, resulting in a positive feedback.)

There are, however, long-term feedbacks, such as how much sunlight the Earth reflects back into space before it can trapped in the atmosphere. NASA's James Hansen, using ancient records of climate change, calculates that the real climate sensitivity over the long term is more like 6 °C. If that's true, then amount of carbon we can safely emit may be significantly lower than a trillion tons, and what remains of that quota proportionally smaller, too. We may even have already used it all up.

Here's how Stephen Schneider, in the same issue of Nature Climate, deals with the lack of consensus on this question:

Even if there's no inherent limitation on scientists' ability to figure out the climate's sensitivity, since it's proven so hard to home in on, learning to live with the uncertainty might be the safest bet. But it's not a reason for inaction, Schneider stresses. "Policy depends upon a generational transformation of basic energy production systems," he says. "You can't wait until you know. By that time it's way too late to do anything about it. That's not how anybody treats cancer, that's not how anybody makes investments, that's not how the military operates. And we are not entitled to this luxury."

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We may even have already used it all up.

Duh! What does James say.

I think we know really that were pass it, but dont want to accept this.

100ppm below 280ppm means 1 mile high ice over Boston.

So what do you think 100ppm over 280 means. Go figure. Who needs sophisticated models.

Which of those facts were shoddy, Jeff? Which of those goals impossible? And what right have you to revoke anyone's "science license" without presenting facts of your own?

I don't believe they know the sensitiviy. I don't think they can calculate it. In heat transfer and fluid flow we use temperatures in Kelvin. So a change of .6 C is a change of .2% Secondly, how would that temperature be measured and how do you determine the norms? Hansen has stated on the NASA website that they performed "QA" and removed data points in the dataset. So now we don't even have the original data. I am all for less polution. I am just not willing to accept poor scientific research because I believe we should pollute less.

Thanks for the thoughtful article. The meme is effectively reinforced in this reader of Nature. I haven't had a chance to read the papers yet but will do so with an enhanced interest.


Sadly this author like so may who find writing politically correct 'climate change' descriptions of the ill effects of CO2 utterly ignore the far more ominous direct effects of CO2. Indeed the trillionth ton itself is a figure that confuses as it seems to refer to the accumulated CO2 in the atmosphere not the emitted. More than a trillion tonnes of C02 has been emitted but abou 1/4 of that has been absorbed by earth and mostly oceans. CO2 remains in the air for centuries by the way which is why in 200 years oly 1/4 the emitted CO2 has been naturally mitigate.

The worst of CO2 is absolutely not climate change it is ocean change. As most CO2 ends up dissolving into the surface ocean where H20+CO2=H2CO3 carbonic acid, the CO2 bomb already airborne is more than sufficient to destroy life as we like it in the oceans. The oceans have already become 30% more acidic and studies show ocean life like shellfish are already disolving in the acidifying seas. Regardless of whether we stop 100% of new CO2 emissions from fossil fuels the deadl dose already airborne is more than sufficiently lethal to the oceans UNLESS we do something about yesterdays CO2.

The ONLY force capable of this are our allies the green plants the plankton of the oceans and the tree of the land. We must replenish and restore our SEAS and TREES to levels of natural abundance they and we enjoyed a few decades ago. ONLY the green plants, and mostly those phyto-plankton plants can compete for CO2 in the oceans and convert CO2 into plant life before it becomes acid death.

Fortunately teams of ocean scientists both private and public have spent the last 20 years and a quarter of a billion dollars researching and developing the very means to replenish and restore the ocean pastures back to not so distant levels of health and abundance.

We can replenish vital mineral dust to the oceans which our fossil fuel age CO2 emissions have denied the oceans. Mere tens of thosands of tonnes of natural iron mineral dust will restore ocean pastures to what they were 30 years ago when ocean plankton blooms competed for and converted 4-5 billion tonnes more CO2 into ocean life than is the case today. This isn't the entire burden of fossil fuels but it is half the problem of new emissions, perhaps more. If we can tackle the other half we might just survive with oceans full of fish and whales and sea birds instead of bacteria and slime.

There is no more time to waste. While climate change glacially slow effects with surly impact us over glacial time ocean change is happening in mere decades. Major science reports state that ocean CO2 tipping points are as near as 2030, 20 years off.

Restore the SEAS and TREES and do it now. Read more at

I wonder if "tonne" is used to mean metric ton? My dictionary is no help.

By Jim Thomerson (not verified) on 05 May 2009 #permalink

Isn't "Most climatologists suspect..." just a bit loosey-goosey? For the past 20 years I believed global warming was caused by CO2. Now, after many months of research, I'm not so sure. It's looking more and more to be a natural phenomenon to me. My interest in energy policy is so great I launched where I take a common sense look at global warming and energy policy. I've waded through the wellspring of global warming theory, the United Nation's IPCC reports, and conclude they lack the 'smoking gun' that proves global warming is man-made. Moreover, I've come to realize that man-made global warming theory cherry picks facts and ignores contradictory evidence from reliable studies.

I'm dismayed that my own party, the Democrats, the thinkers, have turned a scientific issue into religious zealotry where faith trumps facts. I'm also dismayed that the American press seems content with publishing hearsay, without backing up conclusions and presumptions with facts and evidence.

Before we effectively add more taxes to energy with cap-and-trade, I believe it's imperative that the United States establishes a non-political, scientific commission to review all facts and evidence surrounding global warming. The UN, a political organization, should not be making policy for us. The stakes are huge. If we respond to global warming incorrectly, our children and grandchildren will likely lead lives of increasing hardship and desperation.

Hi James,

Interesting post, and glad you seem to quite like the basic idea behind the trillionth tonne paper. I'm working with various folks to follow it up in terms of thinking about other gases, tracing through the economic theory, etc.

Regarding equilibrium climate sensitivity (ECS): we don't make the assumption that ECS~3. The likelihood-based approach we use permits some model versions with high ECS (well in excess of 6 degrees). See the Supplementary Info for a discussion of what we constrained and what we used to do so, but the main point I think that's relevant here is that we don't actually estimate a distribution for ECS in the Allen et al paper (though we did in Meinshausen et al), but the implied distribution for ECS is presented in figure S1(c) of the supplementary information. Subjective estimates based on additional constraints (eg Hegerl et al., 2006) would presumably tighten this and actually reduce the uncertainty in the response.

But avoiding the need to worry too much about ECS is actually one of the advantages of this approach, in my view:
this approach avoids exposure to the slow feedbacks, since the forcing is at its peak value for a short-time compared to the response time of the system. So those high ECS worlds never come into equilibrium, which thins the tail of responses, compared to ECS. [There's a GRL paper at which describes this: "[t]he basic problem with sensitivity is that high sensitivities take a long time to come into equilibrium. This requires that forcings remain constant (or very nearly so) for a long time, possibly hundreds of years. This seems environmentally and economically unlikely."]

Regarding the emissions scenarios - we treated the emissions of CO2 as net emissions (in our future scenarios) and didn't try to partition them by source. We tried to steer clear of the conversations regarding negative emissions - sequestration by forestry or other means... to me the nice things about the Allen et al paper are that (1) it find a better-constrained relationship between emissions and peak temperature; (2) which exploits the fact that long timescale processes, which are responsible for much of the uncertainty in ECS, can be ignored if the forcing is short compared to those long timescales; (3) and this results in a nice "exhaustible resource" reframing of the problem which; (4) happens to be pretty tractable for economists.


By Dave Frame (not verified) on 06 May 2009 #permalink

A Tonne is metric, it's 1000Kg, which is .98 UK imperial tons and 1.1 US imperial tons. It's a happy coincidence that these units are on a practical level the same.

My problem with the Trillionth Tonne argument is that what actually contributes to global warming (and sea acidification) is the concentration of CO2 in the atmosphere, and the how long the high levels of CO2 remain, (in maths terms this is the integral of CO2 concentration with respect to time).

It is clear that if we nuked the Russian permafrost and burnt all our oil reserves as tomorrow but then stopped all emissions as soon as we got to 1T Tones, this would be worse than reaching the same total emissions by 2100 as in the former case the atmospheric concentration would have been much higher for longer. As an analogy, if an industrial plant discharges a low level of pollutant every day over 10 years the river's eco system will be damaged but life in the river will continue. If the plant dumps 10 years worth of pollutants in the river in one day, all life in the river will be killed off taking years for it to return to normal.

Also if we keep to the recent level of emissions but decimate the carbon sinks (AKA rain forests) then we will be even worse off as the natural rate of removal of CO2 will drop leading to further increases atmospheric CO2 concentration, but the total level of emissions us unchanged in these two scenarios.

If I had to recommend a carbon "meme", I prefer the 350 ppmv target as this uses concentration of atmospheric CO2 as the yard stick

Isn't "Most climatologists suspect..." just a bit loosey-goosey? For the past 20 years I believed global warming was caused by CO2. Now, after many months of research, I'm not so sure. It's looking more and more to be a natural phenomenon to me.