By Steve Goddard
The classic cure for hyperventilation is to put a paper bag over your head, which increases your CO2 levels and reduces the amount of Oxygen in your bloodstream. Global warmers have been hyperventilating over CO2 on Venus, ever since Carl Sagan made popular the idea of a runaway greenhouse effect. That was when he wasn’t warning about nuclear winter.
Sagan said that marijuana helped him write some of his books.
I bought off on the “runaway greenhouse” idea on Venus for several decades (without smoking pot) and only very recently have come to understand that the theory is beyond absurd. I explain below.
The first problem is that the surface of Venus receives no direct sunshine. The Venusian atmosphere is full of dense, high clouds “30–40 km thick with bases at 30–35 km altitude.” The way a greenhouse effect works is by shortwave radiation warming the ground, and greenhouse gases impeding the return of long wave radiation to space. Since there is very little sunshine reaching below 30km on Venus, it does not warm the surface much. This is further evidenced by the fact that there is almost no difference in temperature on Venus between day and night. It is just as hot during their very long (1400 hours) nights, so the 485C temperatures can not be due to solar heating and a resultant greenhouse effect. The days on Venus are dim and the nights are pitch black.
The next problem is that the albedo of Venus is very high, due to the 100% cloud cover. At least 65% of the sunshine received by Venus is immediately reflected back into space. Even the upper atmosphere doesn’t receive a lot of sunshine. The top of Venus’ atmosphere receives 1.9 times as much solar radiation as earth, but the albedo is more than double earth’s – so the net effect is that Venus’ upper atmosphere receives a lower TSI than earth.
The third problem is that Venus has almost no water vapor in the atmosphere. The concentration of water vapor is about one thousand times greater on earth.
Composition of Venus Atmosphere
0.965 CO2
0.035 N2
0.00015 SO2
0.00007 AR
0.00002 H2O
Water vapor is a much more important greenhouse gas than CO2, because it absorbs a wider spectrum of infrared light – as can be seen in the image below.
http://www.globalwarmingart.com/images/7/7c/Atmospheric_Transmission.png
The effects of increasing CO2 decay logarithmically. Each doubling of CO2 increases temperatures by 2-3C. So if earth went from .04% CO2 to 100% CO2, it would raise temperatures by less than 25-36C.
Even worse, if earth’s atmosphere had almost no water (like Venus) temperatures would be much colder – like the Arctic. The excess CO2 does not begin to compensate for the lack of H2O. Water vapour accounts for 70-95% of the greenhouse effect on earth. The whole basis of the CAGW argument is that H2O feedback will overwhelm the system, yet Venus has essentially no H2O to feed back. CAGW proponents are talking out of both sides of their mouth.
So why is Venus hot? Because it has an extremely high atmospheric pressure. The atmospheric pressure on Venus is 92X greater than earth. Temperatures in Earth’s atmosphere warm over 80C going from 20 kPa (altitude 15km) to 100 kPa (sea level.) That is why mountains are much colder than the deserts which lie at their base.
The atmospheric pressure on Venus is greater than 9,000 kPa. At those pressures, we would expect Venus to be very hot. Much, much hotter than Death Valley.
http://en.wikipedia.org/wiki/File:Emagram.GIF
Wikipedia typifies the illogical “runaway greenhouse” argument with this statement.
Without the greenhouse effect caused by the carbon dioxide in the atmosphere, the temperature at the surface of Venus would be quite similar to that on Earth.
No it wouldn’t. 9000 kPa atmospheric pressure would occur on earth at an altitude many miles below sea level. No such place exists, but if it did – it would be extremely hot, like Venus. A back of the envelope estimate – temperatures on earth increase by about 80C going from 20 to 100 kPa, so at 9,000 kPa we would expect temperatures to be in the ballpark of :
20C + ln(9000/(100-20)) *80C = 400C
This is very close to what we see on Venus. The high temperatures there can be almost completely explained by atmospheric pressure – not composition. If 90% of the CO2 in Venus atmosphere was replaced by Nitrogen, it would change temperatures there by only a few tens of degrees.
How did such bad science become “common knowledge?” The greenhouse effect can not be the cause of the high temperatures on Venus. “Group Think” at it’s worst, and I am embarrassed to admit that I blindly accepted it for decades.
Blame CO2 first – ask questions later.
=============================
UPDATE: Lubos Motl has written an essay and analysis that broadly agrees with this post. See it here
Steve Goddard,
Sorry if my cite about Venus cooling wasn’t the right one, here are a couple that explain it though.
http://www.science-frontiers.com/sf014/sf014p03.htm
“The second law of thermodynamics is a general principle, which places constraints upon the direction of heat transfer. To maintain the high surface temperature of Venus there should be no net flow of heat through the atmosphere. However, when the Pioneer Venus probes looked at the amount of radiant energy passing through the atmosphere, each one found more energy being radiated up from the lower atmosphere than enters it as sunlight. And, if this were not enough, the night probe site was shown to be about 2K warmer than it was at the day probe site. The Russian probes, Vega 1 and 2, also “recorded a pronounced upward radiation flux.” These findings simply show that Venus’ surface is hot and still cooling.”
http://www.holoscience.com/news.php?article=9aqt6cz5
“Probably the most damning for the greenhouse theory is the data from all of the US and Russian probes showing that the thermal gradient of the atmosphere is from base to cloud tops, i.e. the heat source is at the bottom, not outside. In short, the claims that have been made publicly notwithstanding, the planet is not in thermal equilibrium. According to the probe data, the emitted surface infrared flux is 40 times more than enters as sunlight. About 2 percent of the heat at the surface can be attributed to solar input. Overall, Venus emits 15 percent more energy than it receives from the sun, implying a heat output 10,000 times greater than Earth’s. Although Venus rotates 243 times more slowly than Earth (58-day dark period), nightside temperatures are slightly higher than on the day side–contradicting the notion that the sun is the heat source. Rigorous mathematical modeling by the thermodynamicist George R. Talbott showed that given an incandescent state 3,500 years ago as the recent-origin theory proposes, the cooling curve over that period yields a temperature today exactly as observed.”
KK
“Nick, Venus emits more energy than it receives in SW.”
You need to provide some evidence for that. I’ve pointed out the huge disparity between IR leaving the surface (>12000 W/m2), and that leaving TOA. It’s actually nearly 2 orders of magnitude – Venus only absorbs and reemits as IR about 164 W/m2, because of its high albedo. If you really want to claim Venus emits that much more than it receives, there should be much evidence. If not, then something is absorbing the surface IR.
Willis at 7:04. Your comments about the emissivity of snow is exactly what I also have found and yes it has been determined by experiment (I read the articles). Thus I strongly suspect that the data is correct however creates an apparent paradox, maybe you can explain it.
If one looks at the Nimbus 3 plots of thermal infrared emission from earth to space over the antarctic the plot shows an equivalent black body temperature of 180K in the atmospheric window. Now there is no place in the antarctic that is as cold as 180K, even the high plateau is at about 220k. There is also no place in the atmosphere that is as cold as 180K, so how can the emission be so low? The only explanation I can come up with is that Nimbus is looking at a surface significantly hotter than 180K – (somewhere between 220K and 250K depending on which portion of the Antarctic it is looking at) but a surface with a low emissivity. For a 220K surface to look like a 180K black body it would need to have an emissivity of around 0.45. One cannot explain the situation by claiming that the black body curves overlayed over the Nimbus data are simply wrong because there is to much internal consistency. For example, the equivalent black body temp at the CO2 line is 215K which is consistent with the temperatures over the meditteranean and over the Sahara and these match the tropopause temperature (in that lies another very interesting paradox but thats another subject) which I would expect. Further, the emission at the CO2 line is higher than the emission from the atmopsheric window yet the surafce is warmer than the tropopause (we know by the way that the CO2 emissivity is 1 since it is a deep column of gas) . Then again the black body temperatures for both the meditteranean and the Sahara in the atmospheric window pretty well match surface temperatures which is again what one would expect. I could go on – the Nimbus data with overlays is internally consistent and matches reality on several points so it also would seem to be correct. Yet the emissivity of snow and the Nimbus data seem to be in conflict, how come?
This is far from a trivial point because if the emissivity were low in the thermal infrared then covering the surface with soot would raise both emissivities. However when one calculates the energy gain from absorbed sunlight versus the energy loss from thermal emission to space above the arctic circle the latter dominates considerably. If one raises the emissivity for both visible and thermal infra red energy it turns out the increase in energy loss is greater than the increase in energy gain and the net impact is additional cooling not warming. Hence negative not positive feedback. Your experiment about throwing wood ash on snow and observing what happens is not relevant unless it is done in the Arctic or Antarctic because at more benign latitudes the solar energy is so much higher. Please respond, I would be very interested in your thoughts on this issue.
Rich Werne thanks for the public email address I will attempt to use it.
jcrabb
The atmosphere doesn’t “create heat.” Heat is continuously introduced from outside the atmospheric system – i.e. the Sun.
Julian Braggins
Excellent links! Thanks much for the information.
stevengoddard says: May 6, 2010 at 2:12 pm
“@Hans Erren
I made the point that it is the pressure, not the concentration of CO2 that is important. You seem to be agreeing with me, without realizing it.”
Steve,
Sure I do realise it’s the adiabatic heating downward from the venusian cloudtops (the effective photosphere) that make the surface of Venus hot, but as you can read in Bullock and Greenspoon, even without clouds the effective photosphere would be sitting far above the solid surface due to the complete CO2 saturation of the infrared spectrum.
The effective photosphere height for Venus is at 20 mBar pressure whereas on earth it is at 500 mbar. And that difference is due to the effect of CO2 infrared saturation.
http://pds-atmospheres.nmsu.edu/education_and_outreach/encyclopedia/atmospheric_parameters.htm
Hans Erren
I understand what you are saying. My point is that the reason CO2 saturates the infrared spectrum on Venus is because of the high pressure. Without the high pressure, it wouldn’t happen.
CRS, Dr.P.H.
Good point. Despite the apparent disagreements about details, I don’t see anyone arguing that the greenhouse effect can be responsible for the high temperatures on Venus.
stevengoddard says:
May 8, 2010 at 5:09 am
“The atmosphere doesn’t “create heat.” Heat is continuously introduced from outside the atmospheric system – i.e. the Sun.”
So where is the sun, in your little hypothesis?
What is Governing the Temperature of Earth?
The surface of Planet Earth is heated by the incident shortwave electromagnetic radiation from the sun. In order to balance the temperature of the earth, most of this incident radiation energy is, contrary to the assumption made in the AGW hypothesis, removed from the surface by convective cooling by a flowing atmosphere and by evaporation (removal of latent heat) from the surfaces of the seas, lakes and moist soil; and by transpiration of plants. Consequently, the lower atmosphere, is heated by convective heat transfer from the surface of Earth; by release of latent heat during condensation of water vapour (formation of clouds) and; to a small extent, by thermalisation of excited GHG molecules. The so heated air parcels increase in volume (reduce their density) and rise towards higher altitudes, while their temperature reduce along the lapse rate function. The rising warm air is replaced by cooler air, thus maintaining a continuous convective churning/stirring of the atmosphere that we call wind.
In the upper part of the troposphere and in the stratosphere, IR energy can more efficiently be emitted towards space to achieve the required radiation balance of Earth’s. That is, the estimated effective radiation balance temperature for the earth, 18°C, is established in the upper portion of troposphere and in the stratosphere and not at the surface of the earth, as anticipated by IPCC and the AGW proponents.
The bottom portion of the troposphere and thereby also the surface of the earth, exhibit an average temperature of +15°C. This is approximately 33 K higher than what can be estimated based on the radiative balance with space. This temperature increase is the direct result of the presence of an atmosphere, with its given mass, within the gravitational field of the earth (see contributions by Jelbring and Thieme below). The atmosphere (the air), with its mass, is attracted towards the surface of the earth by the gravitational field. Most of this atmospheric mass will thus reside close the surface of the earth where it is compressed to a higher pressure (on average 1.013 bar) and obtains thus a higher temperature.
Correspondingly, the atmosphere becomes thinner (less compression and thus lower pressure) and cooler, the higher the altitude is. This pressure and temperature distribution is governed by a restricted thermodynamic equilibrium, so-called adiabatic condition. Such a condition prevails when the size of the air parcels is large in comparison to the length scales for heat conduction and diffusion during the time interval pertaining to the state change. Adiabatic changes in temperature thus occur due to changes in pressure of a gas while not adding or removing any heat from or to the surrounding. This entails that the total energy content in an air parcel with a given mass remains constant when it moves vertically, that is, the sum of potential and thermal energy remains constant, independent of the altitude.
The (vertical) circulation of the air is a continuous process that is driven by the dynamics of the planetary atmosphere. At high altitude (high potential energy), a parcel of air of given mass has a large volume and low temperature (low thermal energy). When brought to lower altitudes (low potential energy and high thermal energy) it becomes compressed and thus heated. Again, the temperature of the air parcel will be governed the lapse rate function, which is a consequence of the prevailing thermodynamic equilibrium under adiabatic conditions.
The mathematical description of this mechanism has been called the adiabatic model of atmospheric temperature (see the paper by Chilingar, Khilyuk and Sorokhtin below) and can closely (within 0.1%) describe the long-term average temperature distribution as function of altitude in the troposphere. The normal atmospheric pressure (1.013 bar) near the surface of the earth will bring the air temperature to around +15°C, or delta-T_Earth=33K higher than the temperature determined from the radiation balance between incident (short-wave) radiation from the sun and outgoing (long-wave) radiation from the upper part of the atmosphere towards space.
The cyclic heat input from the sun (over day and night; seasonal variations at different latitudes, etc.) provides the driving forces behind the continuously circulating atmosphere and its vertical transport of heat from the surface of the earth to higher altitudes, where heat balance can be established by long-wave radiation towards space. Well proven, basic physics considerations (radiation balance and adiabatic compression) therefore directly explain the observed temperature of Earth’s atmosphere, without the need to revert to obscure and unverified greenhouse effects from greenhouse gases.
The Hothouse Venus:
Planet Venus, with its CO2 rich atmosphere (96.5% CO2) has a very high surface temperature, in the order of 462°C (735 K). This high surface temperature is often claimed to be the result of a runaway greenhouse effect due to the high CO2 concentration, evaporation of the surface water and subsequent rise of the levels of other GHGs. This, of course, sounds plausible in the ears of the AGW proponents, who want to scare the inhabitants of Planet Earth that similar dramatic temperature increases (beyond the tipping point) may develop also here, if we do not take appropriate measures to curb the emission of GHGs.
There exists, however, a physically well-founded, explanation for the high surface temperature on planet Venus, namely the adiabatic model of atmospheric temperature (see paper by Chilingar, Khilyuk and Sorokhtin below), mentioned above. Not only is the concentration of CO2 much higher at Venus than on Earth, but also the atmospheric pressure at the surface of Venus is much higher, approximately 90 bar. While the gravitational effects on the mass of the (dynamic) planetary atmosphere heats the bottom of the atmosphere and the surface of Earth by delta-T_Earth=33K above the effective temperature expected from radiation balance alone, the same effect on Venus heats its surface by delta-T_Venus=507K above its effective radiation balance temperature, which is 45°C (228 K). The adiabatic model of atmospheric temperature also well describes the temperature of the Venusian atmosphere as function of altitude, within 1%. Again, there is no need to revert to an (unverified) runaway greenhouse effect to explain the high surface temperature of Venus. Plain, well established, physics suffices. Consequently, the (runaway) greenhouse effect does not exist.
Suggested Reading:
• H.R. Jelbring. The Greenhouse Effect as a Function of Atmospheric Mass, Energy & Environment, Volume 14, Number 2 – 3, May 2003, pp. 351-356, http://ruby.fgcu.edu/courses/twimberley/EnviroPhilo/FunctionOfMass.pdf
• Hans Jelbring, Politics and the Greenhouse Effect, http://www.tech-know.eu/NISubmission/pdf/Politics_and_the_Greenhouse_Effect.pdf
• Heinz Thieme, The Thermodynamic Atmosphere Effect – explained stepwise, http://freenet-homepage.de/klima/atmoseffect.htm
• L. F. Khilyuk, G.V. Chilingar, On global forces of nature driving the Earth’s climate. Are humans involved? Environmental Geology, Volume 50, Number 6 / August, 2006, pp. 899-910, http://www.springerlink.com/content/t341350850360302/ (behind pay-wall)
• G.V. Chilingar, L.F. Khilyuk, O. G. Sorokhtin. Cooling of Atmosphere Due to CO2 Emission. Energy Sources, Part A: Recovery, Utilization, and Environmental Effects, Volume 30, Issue 1, January 2008, pages 1 9
http://www.mitosyfraudes.org/Calen9/Chillingar_Atm_Cooling_due_to_CO2.pdf
carrot eater
The sun warms the atmosphere, causing it to convect. This is not a “little hypothesis.” It is basic physics.
Julian Braggins says:
May 8, 2010 at 12:31 am
I followed that link. Nearly at the bottom I read much to my amazement:
On the Venusian surface, nitrogen molecules are converted to carbon monoxide molecules by a catalytic surface nuclear reaction in the presence of red-hot iron. The brilliant French chemist, Louis Kervran, when investigating carbon monoxide poisoning of welders, discovered this surprising nuclear transformation.
I had never heard of this character, even when I was actively following cold fusion (which still hasn’t gone away!) I couldn’t finish the rest of the article, but did spend a little time reading about all the marvelous biological transmutations that somehow have escaped my notice. Chemtrails are more believable….
Sense and Science wrote May 8, 2010 at 7:20 am …
Very good post, thank you for taking the time to write it.
Thank you “Sense and Science, and Steve for explaining why temperature changes with altitude. Sorry I was so slow in picking it up. There’s a detail I’m not sure I get, though.
At some point there is a balance between solar heating and radiation out. On the moon, that would be the surface, since there is no atmosphere to make things complex. I would imagine that on Venus it would be the altitude where the atmosphere is essentially opaque to IR. You start there and can calculate temperatures up and down from there. Change all the atmosphere on Venus to Argon (or N2 or any other IR transparent gas), and the altitude would be the surface, so you would see a vastly cooler surface. It would appear that there really is a greenhouse effect after all, and that sets the starting point for the temperature model.
Is this right, or am I missing something?
stevengoddard says:
May 8, 2010 at 6:05 am
CRS, Dr.P.H.
Good point. Despite the apparent disagreements about details, I don’t see anyone arguing that the greenhouse effect can be responsible for the high temperatures on Venus.
——-
REPLY: Thanks, Steve! I don’t know the origin of that urban legend (Earth is at risk of a “Venus GH Effect,” perhaps Karl Sagan as noted above, but I do recall reading about it in the early 1970’s.
It surprises me that serious scientists are still hyperventilating over that scenario for Earth! Comparing two planets to one another in that fashion is just pure, simple folly.
BTW, I once considered writing a sci-fi novel about the terra-forming of Venus! You’d want to use green sulfur-bacteria like C. Chlorobium limicola forma thiosulfatophilum, and you’d have to add water to the planet. Several decent-sized comets would do it.
Great post by Sense and Science. I especially recommend the reference
Heinz Thieme, The Thermodynamic Atmosphere Effect – explained stepwise, http://freenet-homepage.de/klima/atmoseffect.htm
All basic physics and no mention of CO2.
It should be re-posted as a headliner.
Lon Hocker
May 8 2010 11.08 am.
All you are missing is that the phenomenon you describe is nothing like a greenhouse.
It’s just the natural consequence of gravity and total atmospheric density just as Steve says. Those two features are what sets both pressure and temperature at the surface.
Composition dictates density and thus temperature but the same density can arise from an infinite number of compositional variations. To change the surface temperature one must have a significant change in total atmosphere density. More human CO2 could never do that for the Earth.
Density describes the total molecular weight (mass) for the planetary atmosphere however it is composed. Each molecule present whether it is in the form of a gas or solid particulates has it’s own mass and that mass dictates the amount of energy it is capable of absorbing from incoming photons before re radiating.
CO2 molecules have more mass than Nitrogen molecules and so carry more heat energy but what matters for setting the base surface temperature is the total density of the atmosphere under the influence of gravity. A pure Nitrogen atmosphere will have a very low density and so will be much cooler than a pure CO2 atmosphere.
On Venus there are many sulphuric acid clouds. They add greatly to total atmospheric density and so contribute greatly to the temperature of Venus.
The rate at which energy is shifted away from the surface by convection and other processes is a seperate issue.
A unique feature of water is that the phase changes from water to ice and water to vapour and back again are so powerful in shifting energy away that they seem capable of neutralising even changes in temperature that would otherwise arise from changes in the density of the air above the oceans.
For that reason I have proposed elswhere that we should regard Earth’s oceans as an integral part of the atmosphere for energy processing purposes with the air above being powerless to change the equilibrium set by the ocean surfaces combined with solar energy input.
One can call the warming effect of gravity and density a greenhouse effect but it is not what is normally meant by the term and that is what Steve is gently pointing out (Ithink).
stevengoddard says:
May 7, 2010 at 10:38 pm
“…Gas pressure on the other hand is caused by movement of molecules.”
But the pressure at the surface equals the weight of the gas above, halve the temperature ,the surface pressure remains the same, liquefy it or freeze it, and the pressure at the surface still remains the same. It’s only when the gas is confined in a vessel that pressure increases with temperature, heat the atmosphere of a planet and it expands.
CRS, Dr.P.H. says:
May 8, 2010 at 11:48 am
It surprises me that serious scientists are still hyperventilating over that scenario for Earth! [Venus type runaway GH]
Are they? Name one.
Supercritical CO2 is neither a liquid nor a gas. It’s a separate state of matter. It can become a gas without vaporization and a liquid without condensation. It has chemical properties unique to that state.
An atmosphere composed of a supercritical fluid must surely be unique in our solar system and may have little relation to any other atmosphere.
Steven Wilde
All you are missing is that the phenomenon you describe is nothing like a greenhouse.
You didn’t address the question I had. Let me repeat it:
“At some point there is a balance between solar heating and radiation out. On the moon, that would be the surface, since there is no atmosphere to make things complex. I would imagine that on Venus it would be the altitude where the atmosphere is essentially opaque to IR. You start there and can calculate temperatures up and down from there. Change all the atmosphere on Venus to Argon (or N2 or any other IR transparent gas), and the altitude would be the surface, so you would see a vastly cooler surface. It would appear that there really is a greenhouse effect after all, and that sets the starting point for the temperature model.”
It would appear, since the energy balance isn’t at the surface on Venus, but way up in the atmosphere, that something is holding the heat in. This is usually called the greenhouse effect. If you can find a flaw in this argument, please let me know.
Thanks
Steve Goddard in post “I bought off on the “runaway greenhouse” idea on Venus for several decades (without smoking pot) and only very recently have come to understand that the theory is beyond absurd.”
Sounds to me like you were happy to accept what is an established theory, only to reject it when it became ideologically inconvenient.
“But the pressure at the surface equals the weight of the gas above, halve the temperature ,the surface pressure remains the same, liquefy it or freeze it, and the pressure at the surface still remains the same. It’s only when the gas is confined in a vessel that pressure increases with temperature, heat the atmosphere of a planet and it expands.”
How would you propose to halve the temperature without altering gravity and density ?
If the gas were to liquify or freeze how would you prevent it from dropping out of the atmosphere thereby reducing density, temperature and pressure ?
The atmosphere is to some extent confined in a vessel, namely gravity. Weaken the gravity and the enclosure grows to reduce both pressure and temperature. Strengthen the gravity and the enclosure shrinks to increase both pressure and temperature.
One has to assume a constant energy flow from outside however.
If the external energy source changes within ‘reasonable’ parameters then expansion or contraction of the atmosphere will result but temperature and pressure at the surface remains the same.
If the external energy source ceases altogether the constituents of the atmosphere will change state and fall to the ground reducing density, pressure and temperature.
If the external energy source becomes far more active then molecules will be stripped away to space reducing density, pressure and temperature.
Stephen Wilde, the pressure at the surface will remain the same whatever state the matter above it is in, only by changing gravity or the amount of matter above the surface will the pressure change.
Andrew W says:
May 8, 2010 at 12:52 pm
CRS, Dr.P.H. says:
May 8, 2010 at 11:48 am
It surprises me that serious scientists are still hyperventilating over that scenario for Earth! [Venus type runaway GH]
Are they? Name one.
——
REPLY:
http://www.sjsu.edu/faculty/watkins/GWvenus.htm
“The planet Venus is often cited as an example of a runaway greenhouse effect and used to alarm people about the effect of increasing levels of CO2 in the Earth’s atmosphere. This material is to put matters in perspective.”
and:
“Ian Pearson, the UK minister for science and innovation, said: “Understanding the influencing factors of global warming on Venus could help us in mitigating the threat here on Earth.”
http://news.bbc.co.uk/2/hi/7117303.stm
(Pearson has a Ph.D in Industrial and Business Studies, University of Warwick, MA)
—–
You don’t have to look very hard to see that AGW proponents still try to draw a direct (and scary) comparison between the climates of Venus and future climate of Earth.