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At what level must we stabilize atmospheric CO2 ?

september 2003

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There is no formal scientific answer to this question

Indeed, asserting that there is a precise threshold that we should not trespass for the atmospheric CO2 (for example 550 ppmv) supposes that we can prove that as long as we remain below this threshold we do not run any major risk now or in the future (and, in addition, we must define what distinguishes a major risk from a minor risk), but, as soon as we go over, we will experience a situation in comparison of which judgement day is a pleasant joke.

Well, nothing such can be inferred today from the scientific file, for the following reasons:

apart from the indications that we can get from climate models, there is no way to know preciseley what happens on a multidecennal scale when the atmospheric content in CO2 quickly goes above 280 ppmv (one ppmv = one part per million = 0,0001% ; in 2002 we are already at 370),

because of the very long residence time of the CO2 in the atmosphere, the consequences are not instantaneous: problems might follow the day we go over the limit by several decades or centuries, without any possibility to reverse the evolution of events afterwards,

But there are still two things that remain possible: the first one is to link the evolution of the emissions to the maximal concentration reached, and the other one is to link certain precise consequences to a rough threshold we should not go over. And of course it is probably easy to accept that the faster and the higher the CO2 concentration increases, the faster and greater the temperature will rise, and the more unpleasant consequences we will endure.

But let's start by some documented examples of thresholds:

it seems likely that corals cannot bear a permanent sea water temperature increase of more than 1 to 2 °C compared to the present level. Observations show that temporary temperature increases of that magnitude (for example during El Nino events) generate a reversible bleaching, but if this elevation becomes permanent the coral doesn't recover and dies. Avoiding the extinction of a large fraction of the corals implies that the eventual global temperature rise is restricted to less than 1 to 2°C (the sea surface temperature follows that of the air by a short delay, what is not true for deeper layers of the ocean). It happens that such a temperature increase is about what we should get in a century with a CO2 concentration stabilized at roughly 450 ppmv, the eventual temperature rise being probably 1.5 to 2 times greater. This suggests that if we do not implement very quickly a massive reduction of greenhouse gases emissions, corals and all ecosystems that depend on them all already condemned.

analyses indicate that it is possible that the "occidental ice cap" of Antarctica (the one that covers the "finger" that points to Chile, below the Cape Horn, plus a little continental land that surrounds it) has melted in the past with a global temperature only 2 to 3°C greater than the present one. In order to be sure that this does not happen again in the future (the melting of this little part of the Antarctic ice cap), we must also restrict the global warming to 2°C, no matter what time horizon is involved. This also requires to remain below 450 ppmv (rough figure, of course, nobody can say that 449 ppmv would be safe and 451 ppmv synonymous of a catastropha !). One should remember that the melting of this part of the Antarctica ice cap would result in a 6 meter rise of the sea level (inhabitants of La Hague, Plymouth, Brest, Bordeaux, New York, part of Los Angeles, and more generally speaking most coastal areas will have little problems, the only good news being that such a rise would take centuries...), with the possibility of a couple of upheavals included if part of the collapsing happens brutally.

at last the halt of vertical marine circulation (called thermohaline) could happen with 3°C of global temlperature rise, what suggests not to go over 550 ppmv of CO2 in the atmosphere.

So, even though we don't have a very good visibility, one can still keep in mind some rough figures concerning the CO2 concentration that should not be passed beyond in order to avoid some events for which a direct relation with the average temperature can be suggested, if not established. It is much harder to link events for which the average temperature - and its distribution over the surface of the planet - is not a direct determining factor : maximal intensity of hurricanes, maximal precipitation level or maximal length of droughts, etc.


After that, how can we link the maximal concentration with the emissions, in order to know how much CO2 we will get in the atmosphere depending on the path that emissions will follow ? This is actually pretty easy because CO2 is chemically inert once in the atmosphere: it is only removed from the air by the "sinks" (oceans and continental biosphere), so we have to deal with a banal "bathtub problem". This expression, typically french (?), refers to the kind of math problems that we used to have in elementary school, where the question was: suppose we have a bathtub with a tap pouring in water with a rate of so much and a plughole allowing a removal with a rate of so much, how much time will be necessary to fill - or empty - the bath ? Here the bathtub level is the amount of CO2 in the atmosphere, the plughole represents the sinks, and the tap corresponds to our emissions. And, just as for our good old bath, the amount of CO2 in the atmosphere - hence the CO2 concentration - is, on first approximation, a function of the rate of removal by the sinks and of the emissions.

So, if we know the rate of removal by the sinks, the atmospheric CO2 concentration can be deducted from the emission curve (figures below).

Various emission scenarios included in the 1995 IPCC assessment report . The vertical axis gives the CO2 emissions in billion tonnes of carbon equivalent.

The names of the curves are the ultimate CO2 concentrations that will be reached. For example, S750 means that if the emissions follow the path described by the curve, the concentration will eventually stabilize at 750 ppmv (reminder: one ppmv = one part per million = 0,0001%).

It can be seen that whatever the ultimate concentration reached, stabilization requires that emissions decrease below half the 1990 level "sometime".

Corresponding evolutions of the CO2 concentration in the atmosphere, in ppmv. Each curve here refers to the curve with the same name on the left figure.

For those who like maths, these curves correspond to the integrals of the curves on the left figure, minus the integral of the absorption by the sinks, which, in first approximation, is a constant function with a value equal to 3 Gt/year (actually the value of the sink depends on the CO2 concentration, but the value remains around a couple Gt carbon equivalent per year, while the emissions could go way over).

From IPCC, 1996

The curves above can be read the following way: for a given maximum value of the atmospheric CO2 eventually reached (right figure), various corresponding emissions curves have been calculated (left figure). The link between curves is done with the same abreviations (S450, S550, etc). The origin of the figure is the 1995 situation. The thick black curve on the left (IS92a) refers to a "catastropha scenario" where emissions never decrease during the 21st century;

These curves allow to draw an important - if not essential - conclusion:

the simple fact of stabilizing the atmospheric concentration of CO2, whatever level is reached, requires that the world emissions of this gas returns one day below half of the 1990 level at most (and even below one third later on),

If we wish to stabilize the CO2 at "only" 450 ppmv (S450), value over which major troubles seem likely in a near or remote future (but nobody can say that below we will not have some trouble also), we need to engage into a reduction of the emissions as soon as 2010-2020 to bring them below a fat third of the present emissions before 2100.

The only thing that we can say, when looking at these curves, is that the eventual concentration earth will experience is heavily dependant on the time at which we will engage in the decrease of emissions, and therefore the sooner the (much) better. Hence, if Kyoto is obviously not an ultimate objective, it has the great merit of attempting to put in motion as fast as possible the decrease of the emissions of industrialized countries, without provinding any warranty that global emissions will not increase, that is true.

But preventing a global increase seems pretty unlikely as long as we consider legitimate that "developping" countries should come as close as possible to our way of living, because in a world where 85% of the energy consumed is of fossil origin, the level of material consumption is closely linked to the amount of greenhouse gases we emit.


Decreasing emissions will anyway be a major change

Energy consumption is definitely on a rising trend right now. Does that mean that we can't do anything to prevent the atmospheric CO2 to increase ? No, of course, because we can turn to "CO2 free" sources, and there are mitigation possibilities for the other greenhouse gases. And anyway, as the world is finite, emissions will eventually decrease "some day".

But it is certain that nothing wished and planned will happen if decreasing emissions does not become an explicit and prioritary objective.

Explicit: this means that this objective is always in the list of the elements taken into account by the collectivity when it decides of something (building - or not building ! - an infrastructure, fiscal system, advantages attributed to this or that industrial activity, etc). It means that this objective is not something that "we will adress later", that most of all that citizens expressly wish it and force political leaders to implement such an objective (but at the same time citizens must know and explicitely accept the counterparts, of course).

Prioritary: this means than whenever there is an choice to be made between an orientation that is compatible with decreasing the emissions and one that is not compatible with it (for example keeping low prices for fossil fuels as long as possible, or encouraging an ever growing consumption for each of us as long as it is possible), we must arbitrage in favor of the reduction of emissions (what is generally never the case presently).

Without being able to know if we have already lost the game or not, one thing is certain: the more we delay the reduction, and the more ourselves or our descendants are at risk. Emissions scenarios show that our emissions in a couple decades could tremendously vary depending on the choices we will make today.

Of course this poses the problem of who should take the lead and show the way, and how we can implement such an objective. Wouldn't organizing a referendum at the world level, or at least at the european level, be an interesting thing to do ?


An interesting article to know more: "Dangerous climate impacts and the Kyoto protocol, Brian C. O'Neill & Michael Oppenheimer, Science, vol. 296, 1971-1972, june 14, 2002


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