## Why a reasonably stable climate?

Came across a nice paper today suggesting that Future climate forcing potentially without precedent in the last 420 million years, by Foster, Royer & Lunt (I say nice because I found it quite easy to understand, not because what it suggests is possible would be nice). The paper was mainly looking at why our climate (surface temperature) has been reasonably stable for a very long time (hundreds of millions of years).

The main factors that determine our climate are the amount of energy we get from the Sun (Total Solar Irradiance – TSI) and the greenhouse gas content of the atmosphere (mostly CO2). It is well known that a star like the Sun gets more luminous as it ages, with the TSI at time $t$, $F_s^t$, given by

$F_s^t = \dfrac{1}{1 + \frac{2}{5} \left(1 - \frac{t}{t_o}\right)} F_s,$

where $t_o$ is the age of the Earth, and $F_s$ is the TSI today. The change in solar forcing with time, relative to today, can then be written as

$\Delta F_{Sol} = \dfrac{(F_s^t - F_s) \times (1 - A)}{4} W m^{-2},$

where $A$ is the albedo (which is assumed to be constant). Since $F_s > F_s^t$, $\Delta F_{sol}$ is negative; i.e., the solar forcing is greater now than it was in the past.

However, despite the Sun actually getting more luminous, it’s thought that our climate was actually typically slightly warmer in the past than it is now; the ‘Faint Young Sun’ paradox. As this paper illustrates, the reason is probably because CO2 was higher in the past than it is now. The change in CO2 forcing is

$\Delta F_{CO_2}= 5.32 \ln \left( \dfrac{C}{C_o} \right) + 0.39 \ln \left( \dfrac{C}{C_o} \right)^2,$

where $C$ is the CO2 concentration at the time of interest and $C_o$ is the pre-industrial CO2 level.

Credit: Foster, Royer & Lunt (2017)

Okay, this is getting a little long. What the paper then did is construct a CO2 time series for the last 400 million years. The equation above can then be used to determine the change in CO2 forcing over that time interval, while the second equation in this post can be used to compute the change in solar forcing over the same time interval.

As can be seen in the figure on the right (top panel), the CO2 forcing has – on average – dropped over the last 400 million years, while the Solar forcing has increased. When combined (bottom panel), since these are the two dominant factors the control our climate, you see that the net forcing has – on average – decreased slowly over the last 400 million years. This both explains why we’ve been warmer in the past, despite the Sun being fainter, and why our climate has been reasonably stable (reductions in CO2 roughly balancing the brightening of the Sun).

Credit: Foster, Royer & Lunt (2017)

What the paper then did was to compare future changes in atmospheric CO2 and forcings with their estimates for past changes. As the top panel of the figure on the left shows, we have the potential to produce atmospheric CO2 concentrations that are comparable to, or potentially even greater than, the highest values in the last 400 million years. If we compare the combined CO2 and Solar forcing, the lower panel shows that even an emission pathway that might be quite likely (RCP6) could produce an increase in forcing that is comparable to the reduction that has occured over the last 400 million years. This would take us back to a climate similar to that during the Eocene. What is more, we have the potential to produce a change in forcing that could produce a change to our climate that is without geological precedent in the last half a billion years.

Anyway, I just found this quite an interesting paper, which gives a nice explanation for why our climate has been reasonably stable over geological timescales, why it was warmer in the past despite the “Faint Sun”, and also shows how we have the potential to make changes that could be unprecedented on geological timescales. It’s also interesting to consider why the change in CO2 forcing has almost exactly balanced the increase in Solar forcing. I guess that if this hadn’t been the case, we may not have been here to ponder this issue.

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### 66 Responses to Why a reasonably stable climate?

1. Poltsi says:

Foster, not Forster (in the image credit).

And yes, have to read the paper, sounds very interesting.

Poltsi

2. Thanks, fixed.

3. Magma says:

From the point of view of basic nuts and bolts climate/geoscience research, it will be important not to overlook the paper’s extensive new compilation of screened CO2 estimates for the past 420 million years (a total of 1241 using a variety of proxy methods, and available in the appendix).

As for the flashy main hypothesis of the paper, what better way to test whether the climate system has nonlinear responses than to drive it hard? And it’s so tedious to listen to paleontologists speculate away about what caused mass extinctions. Let’s try one of our own, just to put the debate to rest.

Stages of denial, #9.
We’re not climate change deniers, we’re climate experimentalists.

4. lerpo says:

According to this pop-sci piece, the paper suggests that we’ll warm by 16-25F by the end of the century. That seems unlikely (I hope).

5. John Hartz says:

As noted above, summaries of the Foster et al paper are popping up all over the place. one of the better ones that I have seen to date…

No, the headline is not a typo. Current carbon dioxide levels are unprecedented in human history and are on track to climb to even more ominous heights in just a few decades.

If carbon emissions continue on their current trajectory, new findings show that by mid-century, the atmosphere could reach a state unseen in 50 million years. Back then, temperatures were up to 18°F (10°C) warmer, ice was almost nowhere to be seen and oceans were dramatically higher than they are now.

The Climate Could Hit a State Unseen in 50 Million Years by Brian Kahn, Climate Central, Apr 4, 2017

6. angech says:

John Hartz says:
‘” carbon emissions continue on their current trajectory, new findings show that by mid-century, the atmosphere could reach a state 18°F (10°C) warmer, ice almost nowhere to be seen and oceans dramatically higher . The Climate Could Hit a State Unseen in 50 Million Years by Brian Kahn, Climate Central, Apr 4, 2017″
33 years, 18 degrees F.

7. John Hartz says:

What concerns me personally about the Foster et al paper is that, as someone who will celebrate their 74th birthday this year, I won’t be around for decades to witness what transpires. That is why I tend to focus on more short-term analyses such as…

Four years of current emissions would be enough to blow what’s left of the carbon budget for a good chance of keeping global temperature rise to 1.5C.

That’s the conclusion of analysis by Carbon Brief, which brings the Intergovernmental Panel on Climate Change’s (IPCC) carbon budgets up to date to include global CO2 emissions in 2016.

Our infographic above shows how quickly the budgets for 1.5C, 2C and 3C will be used up if emissions continue at the current rate. For 1.5C, this could be a soon as four years’ time.

Analysis: Just four years left of the 1.5C carbon budget, Carbon Brief, Apr 5, 2017

8. Francis says:

Query– Foster aka Tamino?

9. Poltsi says:

No, Gavin is another, equally talented, Foster.

27 November 2015
Accepted:
06 February 2017”

At least the pseudoskeptics can’t claim that this was rushed through the prosess.

Poltsi

10. Michael Hauber says:

The last time we had climate forcing as high as we may be in a century or three temps may have been 10 C higher. But that doesn’t mean we will get to 10 C higher. Once Co2 peaks there will be a large amount of warming still to come over centuries and even thousand years as long term feedbacks, and ocean inertia play their roles. During this time period significant drawdown of Co2 will occur, so by the time all these feedbacks play out, and the ocean inertia is overcome the total climate forcing will be significantly lower.

11. Marco says:

lerpo, I don’t think they suggest it will warm by that much by the end of this century. It is at the very least a difference between ECS and ESS that matters here.

12. Francis,
No, a different Foster.

Michael Hauber,
Yes, a good point. When we eventually stop emitting, CO2 will start being drawn down and we would not expect to eventually warm to the equilibrium of the peak CO2 concentration (the transient response might be a more reasonable estimate). Of course, that is assuming that we don’t get to the point where carbon cycle feedbacks start to be significant.

13. Michael Hauber’s comment gives me an opportunity to point out this paper. It is correct that if we stopped emitting that atmospheric CO2 would probably start dropping and that a reasonable approximation is that there is little committed warming (i.e., we would warm/settle at the transient response to the peak CO2 concentration, not the equilibrium response to that peak). This paper, however, suggests that there is probably some committed warming and that it depends on the emission trajectory and how much we’ve emitted.

Furthermore, we find significant ZEC for high CO2 concentration scenarios (4 × CO2 ZEC = 0.2 °C – 0.9 °C, RCP8.5 ZEC = 0.2 °C – 0.6 °C) but small ZECs for lower concentration scenarios (2 × CO2 ZEC = 0 °C – 0.1 °C, RCP4.5 ZEC = 0.07 °C – 0.2 °C). This implies that for low concentration scenarios no additional warming may need to be taken into account when estimating the cumulative CO2 emissions compatible with climate targets, whereas additional warming may have to be considered for high concentration scenarios.

ZEC stands for Zero-emissions Warming Committment.

14. Andrew Dodds says:

There is a reasonable geological explanation for the CO2 ‘thermostat’ effect; More warming means increased erosion rates, means more CO2 drawdown, means a cooler climate. And so on.

This is a very long term effect, and it’s modulated by mountain formation; times where we have a lot of big mountain ranges (like now) have higher erosion rates for a given temperature, hence lower CO2 levels. And times of lower mountain building – such as the Cretaceous to Eocene – see higher temperatures. All a bit hand-wavy, admittedly.

It also shows the importance of plate tectonics and active geology; a planet without active mountain building like Mars would presumably see a long term build up of CO2 as all surface geological sinks were filled, and hotspot vulcanism continued to supply it.

15. There was a claim on Twitter that this post missed the point because the reason for us having a stable climate is that there is life. Does anyone know if there is any merit to this claim (I realise that life plays a big role in the carbon cycle, but my understanding is that geological processes play a much bigger role in setting the atmospheric CO2 concentrations).

16. Andrew Dodds says:

It’s hard to see life as a stabiliser on such timescales; the timescale of evolution of the main photosynthetic organisms is <1ma, but the climate takes large excursions over periods up to 100ma.

Although life does sequester some CO2 – Limestone, Coal et al – this would presumably be an equilibrium process on geological timescales, with these rocks being recycled through the mantle.as fast as they were formed.

17. David B. Benson says:

Don’t discount the Lovelock & Margulis Gaia hypothesis that life regulates the climate so as to be conducive to life. Don’t ask me to explain how this presumed feedback is to work; I don’t understand it.

18. chris says:

Don’t think there’s evidence that life is the reason for a stable climate. in fact life is responsible for some of the extreme swings in surface temperature – e.g. the evolution of photosynthetic bacteria that released oxygen, precipitated the iron from the oceans and oxidised methane were v. likely responsible for the “snowball earth” episodes – the massive drawdown and burial of CO2 in plant matter during the laying down of what became fossil fuels during the Carboniferous.

I guess that without the huge drawing down of atmospheric CO2 especially in the Carboniferous CO2 levels might still be a lot higher but that’s probably debatable – tectonic processes might have reduced CO2 levels towards current levels by now already.

You can’t consider the earth’s atmospheric composition and surface temperature history without taking the massive influence of life into account – however it’s not obvious (Gaia notwithstanding) that life has stabilised the climate – current dominant Earth-life (aka us) certainly isn’t doing so…

19. Chubbs says:

Interesting paper. Regarding the impact of life, the paper states that expansion of the terrestrial biosphere over the past 400 million years and silicate weathering combined to keep climate forcing relatively constant.

20. Chubbs,
Thanks, I missed that bit.

21. JCH says:

You’re forgetting that 500,000 years ago total extinction was averted when the unicorns found a fuel source we’ve never heard of, sucked it all out of the ground with their horns, and burned all of it. That’s the secret to how it all works. It’s going to be okay.

22. Andrew Dodds says:

Of course, you can invoke the Anthropic principle a different way at the end of the post.. it could be that out of 100 billion earth like planets in the universe, we are the ONLY one that’s lasted 4.5 billion years without freezing solid or boiling dry.. the climate could be long term unstable and us just very, very lucky.

23. Andrew,
Indeed, that is a possibility. Fermi paradox and all that.

24. The problems with this post are lack of perspective.
Climate is not, as so corrupted by the issue of CO2, global average temperature.
Clearly glacials and interglacials are much more significant climate events, as indicated by temperature extremes, temperature gradients, precipitation, wind, and circulation, than climate change from CO2, which in many ways moderates “climate”.

25. TE,
I rather fail to see how that provides any perspective. I also think that some of what you’ve said is simply wrong. For example, the rate at which we are changing the climate today is greater than most other periods of climate change (AFAIA, at least). I, however, suspect we’ve been over this type of stuff before (and I think I’ve pointed out how your absolute certainty seems rather unjustified) so I can’t see much point in going over it again.

26. izen says:

Reasonably stable climate, compared to what?

The Earth has a reasonably stable climate compared to its neighbours. It has managed to maintain liquid water on the surface for a few billion years.
Life has made radical changes to the composition of the atmosphere and altered the carbon cycle. Much of the terrestrial carbon is in rock cycling through the mantle. a significant proportion of that is derived from limestone, carbonates sequestered by biological processes. Oxygen triggered a snowball Earth and mountains of iron oxide.

The Gia idea that Life has a role in making the climate reasonably stable is the latest version of uniformitarianism. The belief that the climate is reasonably stable because some benign force ordains a climate conducive to our existence. That is not supported by the geological evidence of climate change on million year timescales.

But within Human history a reasonably stable climate is the only thing we know. ~7000 years of the Holocene has been uniform and stable enough to enable the emergence of agriculture, domestication and civilisation.

The paper gives a geological context to the present. In deep time the climate is constrained, but NOT reasonably stable and benign in the way it has been during the recent past. That may make it too distant to overcome the cultural sense that the climate is uniform. Say since 4004BC. The unquestioned assumption that the climate will always be reasonably stable may have deep roots.

27. -1=e^iπ says:

The effort put in to generate a decent long term CO2 time series over 400 million years, with error appropriately estimated, is greatly appreciated.

Though it would have been nice if the effect of CH4, NO2 and Albedo forcing were taken into account. I understand that there are no decent proxies for CH4 and NO2 that goes back this far, however, CH4/NO2 and CO2 are extremely correlated over the period we have decent data (especially over the Pleistocene) so that could have been used. For Albedo forcing, climate models combined with knowledge of the geographic history of Earth could give us a reasonable idea of the effects.

If these 3 forcings where taken into account, some of the conclusions of the paper wouldn’t have been as strong.

28. -1=e^iπ says:

I meant N2O not NO2. Sorry for the mistake.

29. Nathan Tetlaw says:

WRT life assisting climate stability, it may be because living organisms greatly accelerate the weathering process; weathering simply by interacting water and rock is very slow.

30. -1,

If these 3 forcings where taken into account, some of the conclusions of the paper wouldn’t have been as strong.

You seem very certain. I’m not surprised.

31. -1=e^iπ says:

“You seem very certain. I’m not surprised.”

N2O and CH4 have gradually decreased over the past 400 million years, similar to CO2. Albedo forcing has generally decreased over the past 400 million years if you look at the position of the continents and the fact that we have massive ice caps now.

If you take those into consideration, then the combined forcing now has a much more negative slope that just taking solar irradiance and CO2 into account, so forcing isn’t as stable as this paper might suggest. This means that you would need a larger forcing change to get a forcing state not seen in the past 400 million years than what is suggested in the paper.

Are you suggesting any of what I just wrote is controversial?

32. Andrew Dodds says:

EyePie

Both methane and nitrous oxide are mainly biological products as I understand it, and therefore their concentrations will be primarily driven by biological activity, which will itself be a function of climate. As such they cannot be seen as drivers of climate on these timescales.

Albedo does change – although reconstructions would be speculative at best past 250ma – and more importantly continental positions – but as the graphs show, there is any awful lot of wiggle room over the phanerozoic. As we expect; climate has not been constant. However, on the timescales under consideration, even continental drift averages out.

33. -1,

Are you suggesting any of what I just wrote is controversial?

No, I’m suggesting that you might consider adding some terms like “probably” etc. It’s just, in my view, good practice. You don’t need to, of course.

34. verytallguy says:

Clearly glacials and interglacials are much more significant climate events, as indicated by temperature extremes, temperature gradients, precipitation, wind, and circulation, than climate change from CO2

Given that Earth System Sensitivity is quoted as roughly 2x ECS, and the change in global temperature between now and the last glacial maximum is about 5 degrees C, precisely the opposite conclusion applies:

It is unlikely that glacials and interglacials are much more significant climate events, as indicated by temperature extremes, temperature gradients, precipitation, wind, and circulation, than climate change from CO2

35. vtg,
Indeed, according to Figure 2 here, the change in GHG forcing between a glacial and an inter-glacial is comparable to the change in anthropogenic forcing we’ve already produced.

36. tlsmith says:

A minor quibble, I thought the major greenhouse gas was water vapour not C02.

They assume that the albedo remains constant, which seems pretty dubious to me. I would expect variations in cloud cover and ice cover would have pretty major effects on albedo. Variations in the positions of land masses could also have an important effect on average temperatures over those timescales I would guess.

37. tlsmith,

A minor quibble, I thought the major greenhouse gas was water vapour not C02.

Water vapour precipitates and so the major greenhouse gases that can actually induce long-term warming/cooling are the non-precipitation greenhouses gases, mainly CO2. Water vapour acts to amplify the CO2 driven warming.

They assume that the albedo remains constant, which seems pretty dubious to me. I would expect variations in cloud cover and ice cover would have pretty major effects on albedo.

Yes, but they do say that they’e assumed it to be constant. I’ve also been trying to find some info about how we might expect albedo to change and it seems that it might be relatively small (a few percent) given that most is due to the atmosphere and clouds, not the surface.

38. Actually, something to bear in mind that the albedo today will also change as we increase anthropogenic forcings – it’s a feedback, rather a forcing. So, I think their assuming a constant albedo doesn’t really influence the comparison of the change in forcings. Ignoring some of the other non-precipitating greenhouse gases may well mean that they’re slightly estimating the past changes in forcing, but CO2 is, almost certainly, the dominant non-precipitating GHG.

39. Willard says:

> Are you suggesting any of what I just wrote is controversial?

I’d rather say a counterfactual. A more negative slope may or may not impact much on the authors’ conclusion.

40. Chubbs says:

Note that per the chart above, its going to be a challenge to maintain any climate within the past 20M years, given current CO2 levels and emissions. In other words delay is quickly precluding any climate within the past 20M years.

41. Andrew Dodds says:

Well.. I suspect a major component of albedo change would be an expanding sea surface area; high sea levels will increase the proportion of the planet’s surface that is water, and water has a very low albedo. Since sea level tracks temperature to a degree, on this scale albedo is also regulated by temperature and hence CO2.

42. -1=e^iπ says:

“you might consider adding some terms like “probably” etc.”

Okay, fair point.

43. Chris says:

N2O and CH4 have gradually decreased over the past 400 million years, similar to CO2.
………
Are you suggesting any of what I just wrote is controversial?

Not so much “controversial” as probably wrong. Neither methane nor N2O have so far found useful proxies in the sedimentation records, and so pre-ice core records of these gasses can only be assessed by modelling based on current understanding of methane (especially) production from plants (mostly wetlands and peat in pre-human times).

It would be helpful to know the sources of the assertions you consider non-controversial. The published data on modelling of Phanerozoic atmospheric methane concentrations supports the conclusion that current and pre-industrial methane levels are/were higher than those of 400 million years age (Mya), and that methane levels may have risen somewhat since around 200 Mya to pre-industrial levels. There was likely a very large spike of CH4 during the long period ~ 100 Mya of the Carboniferous.

see e.g. Bartdorff et al. (2008) Phanerozoic evolution of atmospheric methane Global Biogeochemical Cycles vol 22 GB1008, doi:10.1029/2007GB002985

44. BBD says:

CH4 and N2O co-vary with temperature, and if we accept that CO2 is an efficacious forcing, then they co-vary with CO2.

45. -1=e^iπ says:

@ Chris – thanks for the reference/link

46. Patrick Hackett says:

there is always the idea that with the increase in solar intensity over hundreds of millions of years, warmer and more rain (than would be the case for a given CO2 level) will encourage weathering of silicate rocks to increase removing CO2 faster than would occur for a given CO2 level….i.e. a long term negative feedback …leading to ” a reasonably stable climate”

47. angech says:

“The main factors that determine our climate are the amount of energy we get from the Sun (Total Solar Irradiance – TSI) and the greenhouse gas content of the atmosphere (mostly CO2).”

The article states,
“The majority (∼75%) of the greenhouse effect is due to the warming effects of water vapour and clouds, with the non-condensing greenhouse gasses (predominantly CO2 and CH4) accounting for the remaining 25% ”

It also gives one of the few accessible direct statements on the earth’s actual average temperature
” the observed surface temperature of the Earth (+14.0 °C or 287.1 K is the 1961–1990 mean)”

Counter intuitive though it is, the idea of something giving out more heat as it gets older seems explicable. The notion of CO2 decreasing with time is at odds with longer time scales such as snowball earth where CO2 elevation is supposed to be the way that the snowball unfroze. The problem being that an explanation for lack of change depends on CO2 production changing in the last 400 million years from the way it had behaved in the previous 800 million years. Sort of fitting the facts to explain the results.

48. -1=e^iπ says:

@ Chris – If you take those results and fit a line of best fit through the past 400 million years, it would have a negative slope. So CH4 has been on average decreasing over the past 400 million years according to those results. So it seems to support my earlier claim to a small extent.

49. angech,
The very next sentence says

However, at the temperatures and pressures typical of the Earth’s surface, water vapour and clouds act as feedbacks rather than drivers of the greenhouse effect, with CO2 and CH4, and the other non-condensing GHGs (for example, N2O) determining the overall strength of the greenhouse effect.

50. angech says:

missed that .sorry. disagree with it but as you say the comment is there.

51. What do you mean “disagree with it”?

52. Magma says:

Counter intuitive though it is, the idea of something giving out more heat as it gets older seems explicable. The notion of CO2 decreasing with time is at odds with longer time scales such as snowball earth where CO2 elevation is supposed to be the way that the snowball unfroze. The problem being that an explanation for lack of change depends on CO2 production changing in the last 400 million years from the way it had behaved in the previous 800 million years. Sort of fitting the facts to explain the results. — someone

Anyone searching for a textbook example of argument from personal incredulity can stop now. Your short, easy search is finally over.

53. John Hartz says:

Speaking of clouds and manmade climate change, here’s a handy reference document recently published by the WMO…

Humanity has a primordial fascination with clouds. The meteorological and hydrological communities have come to understand through decades of observation and research that cloud processes – from the microphysics of initial nucleation to superstorms viewed from satellites – provide vital information for weather prediction, and for precipitation in particular. Looking at clouds from a climate perspective introduces new and difficult questions that challenge our overall assumptions about how our moist, cloudy atmosphere actually works.

Clouds are one of the main modulators of heating in the atmosphere, controlling many other aspects of the climate system. Thus, “Clouds, Circulation and Climate Sensitivity” is one of the World Climate Research Programmes (WCRP) seven Grand Challenges. These Grand Challenges represent areas of emphasis in scientific research, modelling, analysis and observations for WCRP and its affiliate projects in the coming decade.

Understanding Clouds to Anticipate Future Climate by Sandrine Bony, Bjorn Stevens & David Carlson, Bulletin nº Vol 66 (1) – 2017

54. BBD says:

Why is Eye-Pie ignoring my comments again?

Is is because I pointed out the CH4 and N2O are positive feedbacks to CO2?

Could that be it?

55. BBD says:

angech

The notion of CO2 decreasing with time is at odds with longer time scales such as snowball earth where CO2 elevation is supposed to be the way that the snowball unfroze. The problem being that an explanation for lack of change depends on CO2 production changing in the last 400 million years from the way it had behaved in the previous 800 million years. Sort of fitting the facts to explain the results.

I’m sorry, but I don’t understand this at all.

It is thought that CO2 eventually overcame the huge albedo of snowball earth states but these all predate the 400Ma horizon. They are also unique climate states where ice seals off all the carbon sinks but volcanism continues to add CO2 to the atmosphere for millions of years, hence the eventually massive buildup.

56. John Hartz says:

“We have met the enemy and he is us.” – Pogo.

Leading climate change scientist warns that it isn’t only oil companies undermining hopes of meeting targets to limit global warming.

LONDON, 6 April, 2017 – The world is “meandering into a failed future” because of its unwillingness to take decisive action on climate change, a leading UK academic has warned.

Kevin Anderson, a professor of energy and climate change at Manchester University, UK, criticises oil companies for pushing an agenda of complacency.

But, more unusually, he also blames academics, journalists and even some green groups for encouraging a belief that renewable power or other simple solutions can counter global warming.

In an interview with Climate News Network, Anderson denies he is being alarmist or hardline.

But he argues that there is a 95% chance that climate change action will not be robust enough to hold the growth in the Earth’s warming below the 1.5°-2°C target agreed at the UN climate conference in Paris in 2015.

Signs of misguided thinking in the UK, he says, include the government’s support for shale gas drilling and for building a third runway at Heathrow, London’s main airport.

Complacency threatens climate change action by Terry Macalister, Climate News Network, Apr 6, 2017

57. John Hartz says:

ATTP: Please correct the error in my priior post. “Climate Home News” should be “Climate News Network.” Thank you.

[Mod: Done.]

58. Chris says:

“If you take those results and fit a line of best fit through the past 400 million years, it would have a negative slope. So CH4 has been on average decreasing over the past 400 million years according to those results.”

I can’t imagine why one would want to fit a line of best fit through the past 400 million years (MYa) of atmospheric [CH4]! [CH4] was lower 400 MYa than current and pre-industrial levels according to the model and a “line of best fit” through [CH4] over the last ~ 240 MYa would have a positive slope; apart from that there was a very large long-lived 100 MY spike during the main part of the Carboniferous.

This isn’t that big a deal, but I’m curious why you are pursuing the notion! After all the fact that CH4/NO2 and CO2 may be “extremely correlated…..(especially over the Pleistocene)” doesn’t mean that they are inherently correlated. We’re pretty sure that [CH4] was very low when [CO2] levels were very high 400 MYa ago and earlier (and can understand why), and that [CH4] levels remained very high as [CO2] levels were dropping to very low levels in the late Carboniferous – compare, for example, the Figure at the top of this thread from Foster, Royer and Lunt ([CO2]) with Figure 7 from Bartdorff et al (2008) [CH4] linked in my earlier post and which I gather you’ve looked at.

59. angech says:

BBD says: April 8, 2017 at 5:46 pm
“angech I’m sorry, but I don’t understand this at all.”
My fault, apologies, trying to make an argument without enough thought/direction in it.

“It is thought that CO2 eventually overcame the huge albedo of snowball earth states but these all predate the 400Ma horizon. They are also unique climate states where ice seals off all the carbon sinks but volcanism continues to add CO2 to the atmosphere for millions of years, hence the eventually massive buildup.”

This is part of the problem. On a short! time scale we argue for a balanced climate range while at the same time putting up opposing concepts of a fainter sun and a falling CO2 level which coincidentally balance out. Then we have times of snowball earth theory states where CO2 can continually build up just to melt these snowballs.
Never mind that the further back the earth goes the warmer it probably was with more vulcanism and CO2 than today but still able to stay in a temperature range that supported the development and evolution of life for say 3 to 4 billion years.
I do not disagree with the views in the article and your good explanation above. I am just saying that things like the level and causes of CO2 production are not and may not be fully understood for the past.

60. Windchaser says:

Never mind that the further back the earth goes the warmer it probably was with more vulcanism and CO2 than today but still able to stay in a temperature range that supported the development and evolution of life for say 3 to 4 billion years.

Sure, though obviously a sufficiently stable climate to sustain life was a prerequisite for this conversation to occur. (If that hadn’t happened, we wouldn’t be here to talk about it). So it’s hardly a coincidence.
But, yeah, sure, the stable climate will still have physical mechanisms, of course.

I guess I don’t understand what your argument is. CO2 built up in the atmosphere *because* of early vulcanism.

61. John Hartz says:

Windchaser: Spock was a Vulcan. 🙂

62. -1=e^iπ says:

@ Chris – Here is a paper that suggests higher levels of CH4 and N2O during the Eocene and Cretaceous. http://www.pnas.org/content/108/24/9770.full.pdf
I was referring to past 400 million years because that is what ATTP and Foster et al. were using.