A look at recent studies in climate science
- May 1, 2012
“Ugo… At least I should try. If we understand sufficiently the science story, we should teach and encourage others to enquire.
The importance of the non-condensing gases becomes clearer.
Through our own intellectual struggle we occasionally find a dawning reality.
The mental act of adding modern CO2 and CH4 numbers on to that figure of Hansen & Sato’s was such a moment for me.
It also prepared me for The Kilda Basin Conjecture.
We are becoming already an exhaling ‘Kilda Basin’?
This stuff got a ‘human reaction’ from me, which might be communicable.”
Recently Ugo Bardi raised the matter of methane and the fact that compared with geological history, the present level in the atmosphere of this potent ‘greenhouse-gas’ is exceptionally high. We see methane bubbling from the arctic margins. We know the present level is around 1800 parts per billion (1.8ppm); more than 2.5-fold the pre-industrial level. We know this rise has been sudden and that most of it occurred in the 20thC up to about year 1990, and that interestingly for a rapidly oxidised molecule, this high level has been sustained, and lately has begun to increase again. After a brief discussion with Ugo, I decided to attempt an update of my own knowledge. I needed also to integrate knowledge of methane with understanding the role of the chief non-condensing ‘greenhouse-gas’, carbon dioxide.
What I have experienced in the last few weeks has not been exactly a ‘Damascene’ moment, but as we all know, if we struggle hard enough intellectually then a new awareness of reality can dawn. Twenty and more years ago I had collected scientific papers that addressed the importance of atmospheric methane. This gas was already well understood to be part of the more general human-induced inflation of radiative forcing in the climate. We have dramatically increased the non-condensing ‘greenhouse’ gases in the earth’s atmosphere. It is a matter of fact that we experience extra radiative forcing (net trapped sunlight) because of these ‘trace’ gases released by industrialisation and in the case of methane also arising from the recent large extension of agriculture. We have for decades been able to watch the ongoing rise of carbon dioxide (CO2) measured continuously by NOAA Observatory in Hawaii. Methane (CH4), the second most important of the non-condensing gases was known to have increased even more dramatically from pre-industrial levels. All this we knew decades ago. And, already twenty years ago the ice and sediment records were beginning to tell their stories of past climates.
Where has the relevant science gone over the intervening 20 years? Can I interest you, the reader, in my recent journey of discovery, and particularly in what for me were the illuminating and I hope insightful moments?
I wrote a longer article in order to convince myself that I had sufficiently grasped the later scientific evidence and scientific arguments, and I used many quotes from and references to scientific papers: this longer article is available at the ASPO website if you want to engage more with the details. I would value additions, comments and corrections.
Toward the end of this present shorter article I draw your attention tothe ‘Kilda Conjecture’. Nisbet et al Nature 2009, see below, make the case that repeated exhalations of methane and carbon dioxide from an ancient ocean basin they name ‘Kilda’, could 55 million years ago, have kick-started the profound disorder in the carbon cycle seen in the geological record; a disorder known to have lasted about 100,000 years, the PETM. These authors draw parallels with the present extended exhalations of greenhouse gases from modern human society.
Firstly I familiarised myself again with the carbon cycle (‘sources and sinks’) and then with the way it has changed over geological time, so that I could better place in this context the vast “meta-stable” reserves of solid methane gas hydrates, otherwise known as ‘clathrates’. These are sequestered but potentially gaseous carbon deposits, which have been part of the earth’s carbon cycle for hundreds of millions years; maintained possibly continuously, if dynamically, over this unimaginably long history. More recently, clathrates have been part of a relatively stable, though oscillating, carbon cycle and climate. These oscillating cycles have been ‘normal’ for a million or more years. As the climate oscillates, so does the carbon cycle along with the consequent hydrological cycle. The earth during this period has oscillated from glacial era to part-glacial era and correspondingly the sea level has gone up and down by some 120 to 130m. Our kind has become used to the latest extended warm period since the sea level last rose by about 120m about 10,000 years ago.
We can ask, though, how the great stores of methane clathrates have interacted with climate changes not only in the last million years, but also much further back. What do we know from the records of longer geological time? Calculations have revealed that even a small fraction of the probable reserves if they were suddenly released into the atmosphere could overwhelm the photo-oxidation (OH’) capacity of the atmosphere and thereby persist for long enough to cause a great pulse of warmth from trapped sunlight. Indeed it was a long time ago, about 55 million years ago, but something like this actually seems to have happened. The result then was to initiate a disordered carbon cycle that lasted 100,000 years and a ‘thermal maximum’ climate we would not recognise – the PETM.
1st personal insight: comparability of the present day ‘trace’ gases with the remote geological past
During the PETM both CO2 and CH4 were maintained over millennia at very high concentrations; methane at perhaps 5 to 10-fold those of the recent pre-industrial concentrations. Numbers matter. To recapitulate; CH4 levels in the last few decades are sustained 2.5-fold higher than pre-industrial concentrations. I will return to the PETM but let me introduce another ‘moment’ that was for me one of increased clarity.
2nd: the importance of the non-condensing ‘trace’ greenhouse gases becomes clearer.
Snowball Earth and the non-condensing gases
There was, a very long time ago, a Snowball Earth; a period that ended around 635Ma. Gas hydrate releases are mentioned as one of putative positive feedback mechanisms that brought this phenomenon to an end.
[i] Hypotheses accounting for the abruptness of de-glaciation include ice albedo feedback, deep-ocean out-gassing during post-glacial oceanic overturn or methane hydrate destabilization.
Scientific discussion continues about this interesting period, but for our purposes it is worth noting the reasons why we do not have a snowball earth.
[ii] Ample physical evidence shows that carbon dioxide (CO2) is the single most important climate-relevant greenhouse gas in Earth’s atmosphere. This is because CO2, like ozone, N2O, CH4, and chlorofluorocarbons, does not condense and precipitate from the atmosphere at current climate temperatures, whereas water vapour can and does. Non-condensing greenhouse gases, which account for 25% of the total terrestrial greenhouse effect, thus serve to provide the stable temperature structure that sustains the current levels of atmospheric water vapour and clouds via feedback processes that account for the remaining 75% of the greenhouse effect. Without the radiative forcing supplied by CO2 and the other non-condensing greenhouse gases, the terrestrial greenhouse would collapse, plunging the global climate into an icebound Earth state (emphasis added).
Methane is only a transient ‘trace’ gas, but we know that in recent decades it supplies about 20% of the extra net radiative forcing that results from ‘our’ extra greenhouse gases in the atmosphere; a significant addition to the total greenhouse effect.
3rd: the enormity of the last few decades
Glacial and Inter-Glacial Periods over the last 800,000 years
Before our present Holocene interglacial there was the previous warmer Eemian (+1°C, 125,000 years ago), and before that the also warmer Holsteinian (400,000 year ago). Greenhouse gases in the atmosphere rose then to levels similar to recent pre-industrial Holocene levels.
Figure 1 800,000 years of CO2 and CH4 concentrationscorrespond with timing of glacial/interglacial temperature fluctuations; from Hansen & Sato, 2011; Reference iv below
Personally, I only get the enormity of what has happened in the last few decades if I superimpose present CO2 and CH4 concentrations (respectively 392ppm and approximately 1800ppb[iii]) on the end of the above figure (Hansen & Sato,2011[iv]). Methane immediately after the end of the Younger Dryas event was at ~700ppb; dropped to ~600ppb by 5000 years ago; climbed to >700 again by the year 1750.)
I encourage you to re-enact my mental process and superimpose your own visualisation.
4th: comparisons over 5 million years are valid enough
A mere 5 million years ago in the Pliocene the ocean was about 25m higher than today, but temperatures were not greatly higher than those in the inter-glacial Eemian 125,000 years ago, or those just now. However, CO2 levels back then in the Pliocene were higher than in the more recent one million year glacial period; i.e. higher than pre-industrial levels in our Holocene (280ppm), but probably comparable with those of the last 10 years at 380ppm. (See discussion in Hansen & Sato, 2011 ref. iv). Quote:
And regardless of the precise temperatures in the Pliocene, the extreme polar warmth and diminished ice sheets are consistent with the picture we painted above. Earth today, with global temperature having returned to at least the Holocene maximum, is poised to experience strong amplifying polar feedbacks in response to even modest additional global mean warming.
This is our world as it is emerging. ‘Our’ CO2, though, has the potential to go much higher than Pliocene levels, and is coupled at the same time with a sustained exceptional methane level.
…. ****** ….
I have collected a number of up-to-date studies that look at abrupt (millennial scale) warm and cold climate events that occurred both during and at the termination of the last glacial maximum. These studies consider the raised level of methane (see again Figure 1, above), that accompanied both the earlier warmer excursions and, finally, the glacial termination. The studies include an assessment of the stability of marine clathrates and whether sudden release of methane might have initiated the warm periods. Details are in my longer article located here. Despite conjectures about the ‘Clathrate Gun’ (a sudden instability of very large clathrate deposits) having initiated positive feed-back changes and thus acted as a prompt cause of rapid climate warming events, marine hydrates actually appear to have been generally stable during the glacial and inter-glacial periods of the Pleistocene. Nevertheless, clathrates over this time have been to a degree dynamic, especially in the Arctic. They either form or are released in response to changing pressure/temperature combinations as the temperatures of both ground and ocean adjust to the prevailing cooling or warming trend and as the sea level falls or rises; … I quote from my longer article:
There is much of interest discussed [refs.], but the take-home point just now might be that although past thermal shocks must have gradually de-stabilised some CH4 gas hydrates, thus both increasing chronic methane release and adding to warming events during de-glaciations, these shocks did not cause sustained runaway temperatures during the subsequent inter-glacial periods. Further methane-induced positive feedback did not happen. Vast reserves of CH4 and other near-surface carbon still remained. For example; the previous Eemian inter-glacial 125,000 years ago achieved a greater global warmth (about +1°C with reference to year 2000, according to ocean cores, see Hansen & Sato above), high enough to entail a 5m higher sea level than at present, but did not provoke a self-stoking methane/CO2 release sufficient to prevent later re-glaciation. In the last very few decades, however, humanity is administering a powerful thermal shock to a still warm inter-glacial by inducing concentrations of non-condensing greenhouse gases that are higher by a margin not seen in the past 2 – 5 million or more years.
For those readers who are interested in arctic methane and the basis for future studies, there is also in my longer article an introductory discussion of a very recent publication: “Gas Hydrate Formation and Dissipation Histories in the Northern Margin of Canada”, 2012. I have even more recently read this paper “On carbon transport and fate in the East Siberian Arctic land–shelf–atmosphere system”, 2012, which makes a strong case for future monitoring of these processes. As a ‘lay person’ I heartily endorse the authors’ case. Earlier papers by Nisbet, 2002, and Archer, 2007, are also worth reading and links are in my longer article.
…. ****** ….
5th: atmospheric methane levels, and their impacts, depend on the rate of release not on reserves
In my longer article I comment in more detail on the calculations and thesis accompanying the ‘Kilda conjecture’ published in the journal Nature Geoscience; Nisbet, 2009 [Ref V below].
Recent calculations have assessed the quantities and the rate of release that would be needed for a sustained methane-induced thermal shock to the climate, large enough to lead to a runaway effect. The present dissipation of clathrates (or other near surface organic sources of methane) to the air, is more likely to remain chronic and will probably contribute to sustaining the high man-made level of atmospheric methane, rather than, on its own, initiate runaway ‘positive feedback’. (It can be assumed that in the absence of very high sustained ‘natural’ levels, future atmospheric CH4 levels would rapidly reduce if methane release from fossil fuels was to stop.)
[v]The period between gas release events (repeat time) needs to be comparable to, or shorter than, the atmospheric residence time of the warming gas, otherwise the warming effect of one release event will fade before the next event occurs. [Emphasis added.]
The snag, though, it seems is the continuing very large man-made releases of both CH4 and CO2, particularly from remaining fossil fuels, and the raised CO2 concentrations that will continue long after most fossil fuels have been burned.
6th: requirements for a disrupted carbon cycle and sustained climate disorder can be described; for example, the Kilda conjecture
A massive climate impact, such as the start of a disordered carbon cycle of the size-order of the Paleocene/Eocene Thermal Maximum, PETM, would require a very large and sustained release of greenhouse gases.
Ibid ref V a recurrent release of greenhouse gases is therefore required to explain the much longer-term warming in the PETM. …
Even a large release from a single deep ocean clathrate deposit, perhaps if it occurred because of volcanic action unrelated to climate change, would not be enough to firstly interrupt and then promote self-sustaining disorder of the carbon cycle. I quote from my own longer article:
“In particular, single event methane releases have been examined [by Nisbet et al. ref V] as putative trigger events for a cascade leading to sustained high levels of atmospheric non-condensing gases. Single releases from sources such as ocean floor hydrates were/are not, individually, sufficiently large, nor did they recur frequently enough, to act as trigger events for subsequent self-sustaining high atmospheric concentrations, and these sources are rejected as explanations for the ‘PETM trigger’. The authors, though, identify one possible singular source of methane, the geologically brief Kilda Basin 55Ma. This basin apparently has no large modern parallel although some modern Rift Valley conditions provide qualitative parallels. The ancient Kilda Basin could have provided a single source large enough to suddenly overwhelm the atmospheric OH’ oxidising sink and thus prolong for many decades the atmospheric residence time of a massive methane release. Hence, the release could have been big enough to promote a subsequent very prolonged period of both high CO2 and CH4 concentrations. (It is possible that the Kilda Basin might have produced recurrent exhalations). Plausibly the trajectory to the inevitable PETM was begun in this way. The authors speculate:
Ibid ref V Unlike other suggested triggers, bursts of methane and carbon dioxide from Kilda could have been large enough, and could have been repeated frequently enough, to initiate the persistent global warming throughout the PETM. Could the comparable injection of modern anthropogenic emissions induce the same response from the planet? [Emphasis added.]
Thus, for now, my remaining query will be: Are ‘we’ the modern ‘Kilda Basin’?
Could ‘we’ be an initiating trigger like Kilda?
There are already signs of a disrupted carbon cycle as we lower the pH in the ocean.
Modern rising CO2 levels are rising more rapidly and changing the ocean more quickly than the slow changes recorded for the Pliocene a mere 5 million years ago when CO2 was last near 390ppm in the atmosphere. [See footnote and ref[vi]]
The configuration of the continents, mountain ranges and ocean connections are different from those 55 million years ago. The PETM took (several) thousands of years to reach a maximum. We can hope our descendents are spared.
Personally I do not wish to even think about a future PETM equivalent, even if it is not imminent for a thousand years. The current human-induced mass extinction of biota and the emergence of a ‘New Climate’ are bad enough to contemplate, even with scientific caveats about uncertainty. There was a symposium in London at the Royal Society of Chemistry, Burlington House, November 2-3, 2010, and abstracts are available on-line. Presentations reviewed past Carbon Isotope Excursions, CIE’s, particularly the Palaeocene Eocene thermal maximum (PETM, 55Ma), when discussion centred on these past ‘greenhouse worlds’ and mass extinction events as analogues for future events and ecologies. I refer you to the set of symposium abstracts and leave you with the safety instructions for Burlington House displayed prominently at the end of the programme ’flyer’;
If you hear the Alarm
Alarm Bells are situated throughout the building and will ring continuously for an evacuation. Do not stop to collect your personal belongings.
Notes 1 – 5 were added 2012; links are not functional 2020
 In remote geological time carbon became sequestered in very large persistent sinks of carbonaceous rock and in petroleum and gas deposits. Weathering, tectonic movement and volcanic activity release carbon from rocks, and seepage occurs from trapped “fossil fuels” and buried organic material, but since the last 10s of millions of years, the earlier sequestration has had the net ongoing effect of a reduced carbon gas level maintained in the atmosphere. Thus, more recent geological ages have experienced much lower levels of free CO2 and CH4 than those remote epochs when the largest ancient carbon stores were laid down.
 PETM: Palaeocene/Eocene Thermal Maximum. Configurations of continents mountain ranges and oceans have changed since then and the world now could have a different reaction to ‘trigger events’.
References i to vi The link to Hansen & Sato is valid. I am indebted to the authors for the use of the image in Figure 1 above: atmospheric CO2 & CH4 over 800k years. The link in Ref. vi to the Abstract is also valid
[i] Snowball Earth termination by destabilization of equatorial permafrost methane clathrate;
Kennedy M, Mrofka D, von der Borch C. Nature, 2008 May 29; 453(7195):642-5.
[ii] Atmospheric CO2: principal control knob governing Earth’s temperature; Lacis A.A. et al. Science. 2010 Oct 15;330 (6002):356-9.
[iii] Global atmospheric methane: budget, changes and dangers; Dlugokencky EJ, et al. Philos Transact A Math Phys Eng Sci. 2011 May 28; 369(1943):2058-72.
[v] Kick-starting ancient warming; E. G. Nisbet et al.; 2009, Nature Geoscience 2, 156 – 159 (2009)