Posts Tagged ‘Venus’

Venus battle resolved?

June 7, 2011

Jeff Id took issue with Chris Colose for bringing up the high surface temperature at Venus in his SkS post

CO2 is a strong greenhouse gas, and it is important in impeding how efficiently our planet loses radiative heat to space.  We don’t often think of CO2 as a “pollutant” on Venus, yet it still allows the planet to support temperatures well above the melting point of lead or tin.

Jeff replied at his blog:

Venus does have a more reflective atmosphere but it is also closer to the sun than the Earth.  For the thinking mind, it is difficult to ignore that the atmosphere is a ridiculous 90 times more dense. (…) In fact, if you just used Nitrogen alone at the same mass you would get a ton of heat just by the insulating properties of a gas.

On a subsequent post he reiterates his thought that

the reason for Venus surface temeprature being so high was the pressure and that any gas would create a huge warming effect 

However, during the discussion he seems to be backpedaling:

#15, Chris, (…)

My reply was that it was the pressure and amount of gas which caused the temperature more than the specific greenhouse effect of some particularly powerful gas. I pointed out that even N2 would cause a ton of warming with wording that clearly recognized there would be less warming and a paper was referenced where even the 96.5 percent N2 atmosphere had 80C of warming. I was also careful not to claim that all gasses would definitely cause a hot Venus and intentionally phrased even that as a question. In other words, you are making assumptions of a point I didn’t state.

To be fair, I admit that a pure nitrogen atmosphere had less warming than I would have (but did not) guess.

Is it just me, or does that indeed sound like he agrees that the majority of the >500 degrees greenhouse effect on Venus stems from the radiative properties of its atmosphere (~96% CO2) rather than from its density/pressure? The impression I got from his post was that the opposite though. Makes me wonder what the argument is really about. So I asked:

Jeff, 

It seems that you agree that the high temperature on Venus is due primarily to a strong CO2 greenhouse effect (few hundred deg) and secondarily (?) to the high surface pressure (the ~80 deg number that was mentioned upthread).

If so, then I don’t understand the beef you have with Chris’ take, where he uses Venus as an example that shows that CO2 is a greenhouse gas.  

Jeff replied:

Bart,

I object to its comparison to Earth as a scare tactic in general. That’s all.

Argument resolved. Both agree that the greenhouse properties of Venus’ atmosphere are primarily responsible for its high surface temperature, and they disagree as to whether or how this could be mentioned in a discussion about Earth’s climate.

Quick rundown on Venus’ climate:

Venus is closer to the sun than the Earth, but its higher reflectivity more than compensates for that. Without a greenhouse effect Venus would actually be colder than the Earth would be without a greenhouse effect. In reality Venus is about 500 degrees warmer than this so called black body temperature (the greenhouse effect on the Earth is about 33 degrees). This is primarily due to the inception of infrared radiation by its thick atmosphere of almost pure CO2. The high density also helps, but is of secondary importance.

More reading:

Realclimate on Venus

SoD’s Venusian Mysteries

Brian Angliss at S&R

Eric Wolff on areas of agreement and on the public debate about climate science

May 25, 2011

Dr. Eric Wolff is spot on (see also further below): 

as an outsider to the blogosphere, it surprises me that so many people, presumably mostly with even less knowledge and training than me, seem absolutely convinced they have mastered every area of climate science.

A peculiar line-up of speakers assembled recently at the Conference on the Science and Economics of Climate Change in Cambridge: Phil Jones, Andrew Watson, John Mitchell, Michael Lockwood, Henrik Svensmark, Nils-Axel Morner, Ian Plimer, Vaclav Klaus and Nigel Lawson. Bishop Hill reports that

Dr Eric Wolff of the British Antarctic Survey tried valiantly to find a measure of agreement between the two sides.

This proved an interesting exercise and resulted in a useful list (also reproduced by Judith Curry) on what we can all agree on (perhaps excluding those too far out on the fringes; links added by me):

  1. CO2 does absorb infrared radiation
  2. The greenhouse effect (however badly named) does occur in practice: our planet and the others with an atmosphere are warmer than they would be because of the presence of water vapour and CO2.
  3. The greenhouse effect does not saturate with increasing CO2
  4. The CO2 concentration in the atmosphere has risen significantly over the last 200 years
  5. This is because of anthropogenic emissions (fossil fuels, cement production, forest clearance). [Ian Plimer disagreed, but as Curry stated: "the anthropogenic contribution is (should be) undisputed"]
  6. If we agree all these statements above, we must expect at least some warming. [Bishop Hill noted “broad agreement” with this point]
  7. The climate has warmed over the last 50 years, [as is evident from] land atmospheric temperature, marine atmospheric temperature, sea surface temperature, and (from Prof Svensmark) ocean heat content, all with a rising trend.
  8. We probably don’t agree on what has caused the warming up to now, but it seemed that Prof Lockwood and Svensmark actually agreed it was not due to solar changes, because although they disagreed on how much of the variability in the climate records is solar, they both showed solar records without a rising trend in the late 20th century. [Excellent point and ironically hitting Svensmark with a stick of his own making. Note the distinction between variability and trend.]
  9. On sea level, I said that I had a problem in the context of the day, because this was the first time I had ever been in a room where someone had claimed (as Prof Morner did) that sea level has not been rising in recent decades at all.  I therefore can’t claim we agreed, only that this was a very unusual room.  However, I suggested that we can agree that, IF it warms, sea level will rise, since ice definitely melts on warming, and the density of seawater definitely drops as you warm it.
  10. Finally we come to where the real uncertainties between scientists lie, about the strength of the feedbacks on warming induced by CO2 [i.e. climate sensitivity]

Eric Wolff also chimed in with a substantial comment over at Bishop Hill’s, rebutting some commonly heard arguments and making some very spot-on remarks:

(…)

I should first state the rationale for the summary I made at the Downing event. The meeting was about the science and economics of climate change, and I was asked to lead a discussion that came between the science talks and the two economics talks. I therefore felt the most useful thing I could do was to try to summarise what we had heard, as a basis for the discussion of whether society should do anything in response to that, and if so what. In particular I did hear a surprising number of things on which almost everyone in the room could agree, and it seemed worth emphasising that, rather than rehearsing old arguments.

I notice in the thread here several comments about who sets the “terms of the debate”, and about the “context of the debate”. While such phrasings may make sense in discussing energy policy, it is a strange way to discuss the science. Our context is the laws of physics and our observations of the Earth in action; our aim as scientists is to find out how the Earth works: this is not a matter of debate but of evidence. I think some of the comments on this blog come dangerously close to suggesting that we should first decide our energy policy, and then tell the Earth how to behave in response to it.

Regarding Plimer’s proposal that volcanic emissions were more important than we thought, (…) if volcanoes were “causing” the recent increase, then around 1800 their emissions would have had to rise above their stable long-term rate, and then stayed high. This is however a rather hypothetical discussion because the change in the isotopic composition of CO2 in the atmosphere over the industrial period is not consistent with an increased volcanic source anyway.

Regarding the idea that the “temperature increase stopped in 2000”: my point is that we know there are natural variations (due eg to El Nino) that cause runs of a few years of temperature colder than the average or a few years warmer. Look on the record at the decade around 1910 for example when there was a long run of cold years on a flat background.

Such a period superimposed on a trend would look like the last decade (but more so). The point of my analogy is that you can’t determine the trend over several months by measuring the gradient in a run of a few days. Similarly, you simply can’t determine a multidecadal trend by measuring the gradient over a few years because all you get is the “noise” of natural variability.

Professor Morner claims that globally sea level has not risen at all; he dismisses the evidence, from both satellites and the global tide gauge network, that it has. (…) There are numerous reasons why a single site can show a sea level signal, either real or apparent, that departs from the global mean.

I am not the best person to discuss models and feedbacks in detail (see comment on expertise below). However, I could not let two issues pass. Firstly, when models are run out for a century into the future, they do indeed show runs of years with flat temperatures amidst a trend (…) (because of El Nino and other natural factors). I am therefore not clear why this is evidence that something is missing. Regarding positive feedbacks: a positive feedback implies amplification, but not a system out of control; this is only the case if the sum of the gain factors is greater than 1.

Finally, a few specific issues that interested or worried me.

(…) CO2 emits infrared as well as absorbing it. (…) indeed, this property is precisely why its effects do not saturate (but fall logarithmically), because it allows the height from which the emitted radiation finally escapes to rise into regions with less and less air.

Geckko accidentally made an important point. S/he did not like the statement: “We can agree that if it warms the sea level will rise”, because it was too simplistic. Well, as a scientist I always like to boil things down to a statement that my brain can grasp, but that contains the essential explanation of an observation or process. And this one does, for example being demonstrably what was observed in going from a cold ice age world, with sea level 120 metres below the present level, to the present. However, you are right: there are factors that could make this statement false, such as increased snowfall when it warms, adding more ice into ice sheets. As soon as several such competing processes have to be taken into account, our brains cannot predict the outcome, and so we have to resort to putting all the “millions of assumptions” into a numerical model and seeing which of them “win”. An argument for models?

Coldish made a good point about expertise, and this is where I am going to go into a slightly more challenging area. I freely admit that I am not an expert on all, or even most, aspects of climate. When I reach a topic that I have not previously studied, I go to those who are experts, either in person or by reading their work. I maintain scepticism about some of their conclusions, but my working assumption is that they are intelligent and that they have probably thought of most of the issues that I will come up with. Can I observe as an outsider to the blogosphere, that it surprises me that so many people, presumably mostly with even less knowledge and training than me, seem absolutely convinced they have mastered every area of climate science.

and more so, convinced that they are right and almost all of the experts are wrong. That must be the height of hubris.

However, Coldish specifically mentioned IPCC, and I think there is also an interesting point to make about that. At the Downing event, there seemed to be two IPCCs in the room. To some it was a huge plot, masterminded by some mysterious power that manipulates troublesome scientists. To me and the scientists in the room, it is (at least in WG1) simply a set of well-researched review papers, describing the present state of the peer-reviewed literature. I mention this only because I think the former view is a type of groupthink where, because people form an extreme opinion in their private space, they think it is widely held, or even true.

Alas, Wolff’s forray into the blogosphere was short, as is evident from a short comment a while later:

Just in case anyone thinks they are addressing me with their remarks:

I thought this might indeed be a chance for a civilised discussion, and some of the respondents seem happy to have that. However there are also a lot of remarks on here that are frankly rude and aggressive, and I won’t be returning. Now I remember why I hate the blogosphere.

Reactie op Le Pair

April 27, 2009

Kees le Pair heeft op zijn website gereageerd op mijn korte reactie in het Nederlands Tijdschrift voor Natuurkunde. Helaas blijft hij steken in allang ontkrachte argumenten, die in de wetenschappelijke discussie al even lang niet meer meedoen. Wetenschap gaat door, op zoek naar nieuwe onzekerheden (die er nog volop zijn, natuurlijk, maar dan in de details; niet in de vraag of de mensheid verantwoordelijk is voor de extra CO2 in de lucht, of dat meer CO2 tot opwarming zal leiden. Dat weten we intussen wel.) De bijdragen van le Pair en mijzelf zullen ook gereproduceerd worden op de extreem “skeptische” webite klimatosoof. Wordt vervolgd…

 

Verzadiging van CO2 absorptie

De (zelfvernoemde) bron van de grafiek die le Pair aanvoert, Hans Erren (eveneens een klimaat-“skepticus”), spreekt hem tegen in dit opzicht: Het CO2 gordijn breidt zich zijwaarts uit. Bovendien leidt een toenemende mate van verzadiging van CO2 aan het aardoppervlak ertoe dat de infrarood straling vanaf grotere hoogte naar het heelal wordt uitgestraald. Aangezien de temperatuur op grotere hoogte kouder is, verlaat daarmee minder energie het aardsysteem, en dus warmt de planeet op.

 

Venus

Venus heeft een dikke atmosfeer van bijna puur CO2. Vanwege verzadiging van de CO2 absorptiebanden aan het oppervlak vindt uitstraling naar het heelal plaats op grote hoogte, waar het veel kouder is dan aan het oppervlak. Bij die lage temperatuur kan via uitstraling maar weinig energie ontsnappen, en daarom wordt het oppervlak zo warm: Gemiddeld 467 graden Celsius.

 

Dit is een stuk warmer dan op basis van de kortere afstand tot de zon verklaard kan worden. Mercurius (nog dichter bij de zon) heeft nagenoeg geen atmosfeer, en is daarom een stuk koeler dan Venus (167 graden Celsius). Hier is een aardig schoolproject over het temperatuurverschil op deze drie planeten. Kortom, “Venus is het mooie voorbeeld dat de concentratie CO2 in principe een heel eind kan toenemen voordat je echt tegen natuurkundige limieten aanloopt.”

 

In één ding heeft Le Pair gelijk: Fysische wetten zijn overal in het heelal hetzelfde.

 

Bijdrage van de mens aan CO2 toename

Hier haalt Le Pair twee dingen door elkaar: De emissie van CO2 door menselijke activiteit mag dan misschien klein zijn in vergelijking met de natuurlijke emissies, maar zij is wel degelijk verantwoordelijk voor de toename van de hoeveelheid CO2 in the atmosfeer. Ook hier wordt le Pair terecht door mede-“skepticus” Hans Erren terug gefloten.

 

De natuurlijke emissie en opname van CO2 door de biosfeer zijn normaalgesproken namelijk met elkaar in evenwicht (over tijdschalen langer dan een paar jaar). De extra emissies door menselijke activiteit hebben dit evenwicht verstoord en geleid tot een gestage stijging van de concentratie. Sterker nog, de natuur heeft als buffer gediend, door een deel van de door ons uitgestoten CO2 op te nemen (in de oceaan en in de biosfeer). Dit is door metingen duidelijk aangetoond.  

carboncycle_rev3d 

Figuur 1. Natuurlijke hoeveelheden (pre-industrieel) in het zwart; antropogene hoeveelheden in het rood.

 

Bovendien is de isotopische samenstelling van fossiele koolstof anders dan die van huidige biologische oorsprong. Hierdoor kan duidelijk worden aangetoond dat de toename van de CO2 concentratie in de atmosfeer voor een groot deel van fossiele oorsprong is: verbranding van kolen, aardolie en aardgas. 

 

Zeespiegelstijging

Le Pair haalt hier verschillende tijdsschalen door elkaar. De zeespiegel is versneld aan het stijgen sinds ongeveer 1900, na niet noemenswaardig veranderd te zijn in de paar duizend jaar daarvoor. Le Pair merkt correct op dat nog langer geleden de zeespiegel wel degelijk perioden van grote verandering kende: Tegen het einde van de laatste ijstijd steeg de zeespiegel over een periode van ongeveer 12.000 jaar met meer dan 100 meter.

 

post-glacial_sea_level 

recent_sea_level_rise

Figuur 2: Zeespiegelstijging over beide tijdsschalen. Sinds de industriële revolutie is de zeespiegel versneld gestegen in vergelijking met de paar duizend jaar daarvoor.

 

Blijkbaar is de zeespiegel over lange tijdsschalen erg gevoelig voor de gemiddelde temperatuur op aarde. Zo was 125.000 jaar geleden de zeespiegel gemiddeld ongeveer 6 meter hoger dan nu, terwijl de gemiddelde temperatuur maar één of twee graden hoger was. Niet echt geruststellend.

 

IPCC

In zijn karakterisering van het IPCC klimaatpanel heeft Le Pair het blijkbaar over het handjevol mensen dat nodig is om de organisatie draaiende te houden. Maar het eigenlijke IPCC proces is veel belangrijker uit wetenschappelijk oogpunt: Daar dragen duizenden wetenschappers uit de hele wereld op vrijwillige basis aan bij. Ze zijn geen IPCC werknemer, maar dragen als wetenschapper bij aan het proces, namelijk het maken van een samenvatting en synthese van de relevante, recente onderzoeksresultaten. De IPCC rapporten zijn een afspiegeling van de huidige wetenschappelijke inzichten op het gebied van klimaatverandering. Je moet met betere argumenten komen om die cumulatief opgedane kennis zomaar eventjes terzijde te schuiven.

 

Update: Wordt vervolgd hier.


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