The CO2 concentration graph: Guesses solicited for its future shape and consequences

[intention]

@SentientMeat

Well, real life has caught up with me again so I’ll be saying au revoir to the SDMB once more.

SentientMeat, go well, stay well. Many thanks for your ideas, comments, questions, and particularly your pleasant and collegial tone.

@SentientMeat

There were 3 ways my concerns for the future might have been allayed here. To summarise:
[ul]The CO2 concentration is rising almost entirely naturally[/ul]
This is clearly not the case, as attested to by Blake’s resounding silence since I set forth the argument in the way he requested a few days ago. Human emissions are right there in front of us in volumes easily vast enough to explain the current level and the current annual increase, such that to even seek another explanation is a grave violation of Ockham’s Razor.
[ul]Some natural process will prevent CO2 levels rising past about 500 ppmv[/ul]This is what I have been discussing with intention. Yes, the annual emission rate due to humans might rise further, confounding any hope of a natural limiting mechanism. However, even if this rate stays constant at around 8 Gt CO2 per year, I consider the possibility of a roughly 500 ppmv level-off to be remote. intention has now identified what I see as the problem here:
Calling this the “sequestration rate” is unhelpful and confusing, IMO. If the emission rate is measured in Gigatons per year, so should the sequestration rate. And since the excess is increasing so dramatically, then proposing a ‘constant’ sequestration rate of 2.5% *of the excess * per year actually implies a dramatic increase in the sequestration rate. I simply cannot see how this will continue to happen indefinitely, given the obvious limiting mechanisms which prevent ever more CO2 being sequestered as atmospheric CO2 rises: soil fertility, ocean acidification and so on, all in addition to the tropical deforestation and desertification which continue apace. In intention’s graph, the orange line will have to become as steep as the blue line - ie. 2.5% of excess will have to be sequestered every year even when that excess has increased dramatically as the steeper blue line continues to pull away from the orange.

As you point out above, there are theoretical reasons to think that the sequestration of ~ 2.5%/yr. should be dropping. However, those same reasons have been going on for at least a century, and there has been no change in the e-folding time, the 2.5% has remained constant.

Given a discrepancy between theory and evidence, I go with the evidence. The evidence says that the changes you mention above have not changed the sequestration rate. Why? We don’t know. But new carbon sequestration mechanisms are discovered all the time.

@SentientMeat

I might be optimistic enough to accept a small increase in the sequestration rate ove the next few decades, but I cannot see how it can continue to eat up 2.5% of a dramatically increasing excess. Of course, I hope intention is right in this regard, but I hope he would also consider my concerns to be reasonable.

Well, when I’m concerned about something, it’s generally something for which I have evidence. I know of no scientific paper, study, or data that says that the e-folding time for the sequestration of CO2 is changing. To date, the evidence to support your concern simply doesn’t exist.

@Sentienmeat

Finally:
[ul]CO2 concentrations of 500-800 ppmv won’t have particularly negative impacts anyway[/ul]If there is hope, it lies here. Maybe intention’s increased cloud cover will kick in and reflect the heat which the extra CO2 would otherwise have captured. If so, well, those extra clouds better start measurably forming pretty soon. Maybe the climate is sufficiently insensitive to extra IR-absorbing gases that large temperature rises are not forthcoming. If so, it would have to be the bare minimum sensitivity allowed for the Greenhouse Effect to exist at all, and even a modest sea level rise would surely have highly negative impacts on countries like Bangladesh.

But both of these are possible, even though in my admittedly amateur eyes they don’t seem all that likely. So I guess these are what I’ll have to Believe in, since the consequences of large temperature rises do not bear thinking about. Is this mere positive thinking on my part, or positively wishful thinking? Again, I’m not sure I wish to examine that question too closely lest my optimistic bubble bursts.

Many thanks to all who contributed here, and all who spent time reading those contributions.

Sea level went up about a foot in Bengladesh over the last century … where are the “highly negative impacts”? In fact, the current land area of Bengladesh has increased by about 20 sq. km. per year for the last 32 years, despite the rising sea. Why? Because it’s a river delta, and megatonnes of soil are added to it each year.

As I have said before, climate is often counter-intuitive. The sea has gone up, and despite that, the land area of Bangladesh has increased … go figure.

Nor, despite a century of warming, is there any indication of any increase in sea level rise … like I say, when there is such evidence, I’ll be concerned. Until then, people in the world are starving, I have other things to be concerned about than fantasies about what might possibly have a chance of happening in Bangladesh in fifty years.