One thing we can all agree on

It’s a question of whether we’re going to go forward into the future, or past to the back. -Dan Quayle

This is my last day writing before my spring break begins, and I’m hoping for some great weather as I prepare to head to the Oregon coast. Warm weather, clear skies… I can picture it now. In my dreams, it looks something like this.

It makes me think about global warming, the greenhouse effect, and whether this is really cause for concern or not.

On one hand, it’s definitely true that heating the planet up by even a few degrees will have catastrophic effects on our sea levels as the ice sheets over Antarctica and Greenland melt, displacing hundreds of millions of people and causing worldwide changes in the production of our food supply. For example, if the sea level rises by just 60 meters (that’s a partial melting of those ice sheets), all of the areas in red on the map will go from being (mostly habitable) land to being underwater.

Click to enlarge, of course, and note that huge sections of the U.S., India, China, England, and coastal Europe will be underwater. So that’s one hand.

But on the other hand, could changes to the atmosphere really be the cause of changes in temperature? In all honesty, it’s hard to answer that question just from looking at Earth.

Luckily, we have some other, nearby worlds to help shed light on our situation. Let’s take a look at the planets of the inner Solar System: Mercury, Venus, Earth, and Mars.

Mercury is the closest to the Sun, and gets awfully hot, topping out at about 450 Celsius (840 Fahrenheit) during the hottest part of the day & year. And as you might expect, of the four rocky planets we have to look at, Mars is the coldest. But if you look at the hottest part of Mars during the hottest times, it can get all the way up to about 27 Celsius (81 Fahrenheit), which isn’t too bad, considering how much farther away it is!

As you’d expect, Earth is cooler than Mercury but warmer than Mars. And I bet that you’d expect Venus — being closer than Earth but farther away than Mercury — would have temperatures in between Mercury’s and Earth’s.

Well. It turns out that Venus has an average temperature of about 460 Celsius (860 Fahrenheit), with no significant differences between day and night temperatures, even though a day on Venus lasts about 117 Earth days!

What gives? Venus is about twice as far from the Sun as Mercury, and therefore receives only about one-quarter of the energy from the Sun for each square mile of its surface as compared to Mercury.

So what gives? Why is Venus so hot? If you really want to know, all you have to do is look at Venus’ atmosphere. Take a look at the swirling clouds in the atmosphere, and you’ll immediately know something fishy is up.

Venus has an atmosphere that’s something like 93 times as thick as Earth’s, and over 96% of that atmosphere is carbon dioxide. That thick atmosphere lets lots of Solar Radiation (visible and ultraviolet light) in. When the planet tries to get rid of it — just like Earth does — it re-radiates that energy in the infrared. But carbon dioxide doesn’t let infrared light through very well, it reflects it back onto the planet’s surface.

This is how a greenhouse works, and that’s why it’s called the greenhouse effect!

This is a real effect, it really happens, and everyone agrees that this is the cause for why Venus is as hot as it is.

If everyone can agree on this, then the question isn’t whether putting carbon dioxide in our atmosphere is going to cause a greenhouse effect: it does. (And people contending otherwise, like this, are wrong, and now you know why.) The real question — the only one up for debate — is this: how much of an effect will adding carbon dioxide to our atmosphere cause?

Scientifically, it is no longer a question of whether global warming happens when you increase carbon dioxide in your atmosphere: it does. It’s now a question of how much carbon dioxide will lead to an unacceptable level of warming for Earth, and what — as the only species on the planet capable of doing something about it — we’re actually going to do.

Comments

  1. #1 Arvind Mishra
    March 19, 2010

    This is a perfect example of science popularization! A perfect article for a lay person on green house effect! Kudos!

  2. #2 Don EV
    March 19, 2010

    What I would like to know is why mars is not as hot as venus? I realize Mars is a longer distance from the sun but why isn’t it at least 300 to 400 degrees? Could it be the sulfuric acid clouds on Venus holding the heat in? Or could it be the atmosphere being more dense cause it? As with water vapor the acid clouds on Venus looks like it would be more of a green house gas than CO2? Here on Earth on a clear night it gets much colder than a cloudy one. Since there are always clouds on Venus, would this not be the same? Doesn’t the clouds block the suns heat from reflecting back into space?

  3. #3 liezl
    March 19, 2010

    We cannot stressed hard enough on the importance of doing our share in saving the environment.

    There are just too many vehicles around that produces carbon dioxide. Maybe we should go back to riding bicycle?

    There are just too many things that we can do like cleaning or replacing air filters on our aircon unit from time to time etc etc. The question is do we care enough to do the right thing?

    We should all really take action and do our share in our own little way. That is one thing we should all agree on.

  4. #4 David Marjanović
    March 19, 2010

    The atmosphere of Mars is very, very, very thin, so there’s a lot less CO2 above each square meter.

    The sulfuric acid aerosols block heat from coming in. That’s where the slight cooling on Earth in the 1970s and 80s came from.

  5. #5 Rod
    March 19, 2010

    You are preaching to the choir

  6. #6 Don EV
    March 19, 2010

    David, I understand mars atmosphere is thin, but isn’t the reason for venus being so hot isn’t the CO2 atmosphere but the density of the atmosphere?”93 times as thick as Earth’s”
    It looks like the density of the atmosphere alone would hold the heat in wouldn’t it? This is why the questions, Mars and venus’s atmosphere are very simular with the exception of there density.

  7. #7 Don EV
    March 19, 2010

    I do not claim CO2 does not have a green house effect but, I do not agree that the CO2 on Venus is why it is so hot.
    I would think the clouds and density of the atmosphere on Venus would be more of a green house effect?

  8. #8 Douglas Watts
    March 20, 2010

    There has never been a question about the specific role and effect of CO2 in the warming of the atmosphere. This is like arguing whether U-235 has a role in nuclear fission. Unf., humans are not willing to take responsibility for our own actions. It’s our weak spot.

  9. #9 Andrew G.
    March 20, 2010

    Don EV @7,

    What matters is not the proportion of CO2 but the density; not the overall density but the density of CO2 specifically.

    Imagine shining an infrared beam through a container filled with nothing but CO2. The amount that would be absorbed would depend (for a fixed container size) only on the density of CO2 present – the more molecules of CO2 present near the path of a photon, the more likely it is to get absorbed.

    Furthermore, putting something else in the container as well which is transparent to infrared would make little difference to the result – it’s not the overall density that counts, but the density of the non-transparent substance.

    So Venus’ atmosphere has a huge density of CO2 and thus a huge “greenhouse”* effect, whereas Mars’ atmosphere has a very low density of CO2 and thus a low “greenhouse” effect.

    In the case of Earth’s atmosphere, by releasing more CO2 we are increasing the density of atmospheric CO2 accordingly, and thus increasing the amount of radiated heat which is absorbed in the atmosphere and retained rather than escaping into space.

    (* – real greenhouses don’t actually work by the “greenhouse effect”; they primarily block heat loss by convection.)

  10. #10 MadScientist
    March 20, 2010

    Greenhouses keep things warm by quenching convection – if you don’t allow the air in the greenhouse to mix with the cold air outside, the greenhouse stays warmer. CO2 does not quench convection; it absorbs mid infrared energy (thermal infrared) in a number of broad range of frequencies and re-radiates that energy more or less evenly in all directions – so a large fraction of energy which would otherwise go straight into space gets trapped on earth. The bulk of the sun’s energy is in the visible region to the ultraviolet and also a fair bit into the near infrared; that energy can pass through the atmosphere fairly easily and is absorbed at the surface, which warms the surface. (Everyone is probably familiar with how hot asphalt roads get.) The earth radiates energy primarily in the thermal infrared – and much of that energy is easily trapped by water vapor and CO2.

  11. #11 Alex Besogonov
    March 20, 2010

    Good article, but it’s misleading.

    CO2 does NOT trap heat in Venus’ atmosphere. In fact, ALL absorbed Sun’s energy is re-radiated.

    What CO2 does, is increasing the temperature at which all incoming heat is re-radiated.

  12. #12 Kevin
    March 20, 2010

    This article proves the maxim that a little knowledge is a dangerous thing. Here is an analogy:

    The ocean is full of water, and aircraft carriers can float in this water. That atmosphere has water vapor in it, therefore if we increase the water vapor in the air (say, by making more tea on our stoves), then we might see aircraft carriers floating in the sky.

    CO2 is a trace gas that contributes almost nothing to the “greenhouse effect”; hence, small increases in a trace gas still make it a trace gas.

  13. #13 sixthlight
    March 20, 2010

    Nice article, but the person who made that map of the world with a 60 metre sea rise managed to eliminate my entire country. Does that mean that they think we’re going to be entirely flooded or that we already are?

  14. #14 IBY
    March 21, 2010

    @kevin
    It is true that water vapor is the biggest contributor, but CO2 is the second largest contributor. Even at a concentration of 300ppm, without it, the Earth would be quiet uncomfortable to live in. And note, a few degrees of change can throw all the ecosystems out of balance. And note, both are trace gases on Earth. And believe it or not, little things do make great differences. For example, the ozone in the ozone layer is around 2 to 8 parts per billion, but it is enough to block most of the short UV ray radiation. So as you can see, trace gases are extremely important.

    So, if there is one lesson to be learned in all of this is that small changes can cause enormous unforeseen consequences.

  15. #15 rick king
    March 21, 2010

    A year or 2 ago I saw an article about NASA scientists noting that they saw more green plantlife on earth as of late compared to the 80′s/90′s. They said the increase in CO2 has led to more plantlife which appears to be soaking it in.

  16. #16 Sam
    March 21, 2010

    Unfortunately, judging by the comments the only thing wrong about this post is the title.

  17. #17 Dave R
    March 21, 2010

    Even more unfortunately, it isn’t just commenters on blogs who are dumb enough to reject the greenhouse effect, it also happens in scientific journals: http://scienceblogs.com/deltoid/2009/03/gerlich_and_tscheuschner_oh_my.php

  18. #18 Randolph Lopez
    March 22, 2010

    Great post Ethan! Concerning the global warming denialists, well… I regret sharing the same planet with you.

  19. #19 Adam Kamp
    March 22, 2010

    Kevin’s post is kinda dumb, but there’s one important aspect in which he’s potentially right: while no one doubts that carbon dioxide causes global warming, there also needs to be a showing that humans are capable of adding enough carbon dioxide to the atmosphere to actually cause a significant change.

    Without that showing (which I understand and believe has been made elsewhere), this post is sort of posing, and answering, a question that no one is actually interested in.

  20. #20 Ethan Siegel
    March 22, 2010

    Adam,

    It was 11 months ago, but I’ve already done that here:

    http://scienceblogs.com/startswithabang/2009/04/how_could_we_affect_the_earth.php

  21. #21 rob
    March 22, 2010

    wow. that link to a person that contends otherwise has some pretty, um, unique theories.

  22. #22 Vicki
    March 22, 2010

    A minor question: if I’m reading that map right, the area underwater would include not just the entire Hudson Valley– which I can believe, given that the elevation of that river at Troy, about 200 km inland, is 4 feet (1.2 meters) above sea level–but a corridor clear through to the Niagara or St. Lawrence. Is that really to be expected, or did someone just look at the elevation of Lake Champlain and extrapolate?

    As I said, this is a minor question: with that much loss, much of it densely populated areas, the Adirondacks aren’t going to be on a lot of people’s radar.

    Did you draw this map, or should I be asking someone else this question?

  23. #23 Sili
    March 22, 2010

    What CO2 does, is increasing the temperature at which all incoming heat is re-radiated.

    And how, pray tell, is the temperature increased if more heat has not captured by Venus compared to what it would have captured, had it not had CO2 in its atmosphere?

    Noöne is claiming that Venus isn’t in equilibrium, but that equilibrium has been pushed to a higher temperature due to the extra CO2.

  24. #24 Adam Kamp
    March 22, 2010

    Thanks for the pointer, Ethan! (I only started reading the blog in August or September or so.) That post really does help some things, but there’s still a premise there that prevents it from being entirely convincing: to wit, that the tiny amount of CO2 currently in our atmosphere has an outsized importance on why our planet is the temperature it is. This may well be true (and I’m inclined to believe it is, for no other reason than that the people who seem to think most scientifically believe this is the case), but since I can’t really grok the science myself, I have trouble believing it in my heart of hearts on say-so, you know?

  25. #25 Dave R
    March 23, 2010

    Adam Kemp @ 24:

    the tiny amount of CO2 currently in our atmosphere

    This article gives a good explanation…
    http://moregrumbinescience.blogspot.com/2009/11/how-co2-matters.html

  26. #26 David Holland
    March 23, 2010

    Adam @ 24,
    If your don’t want to read Dave R’s link here is the part that most directly answers your question:

    “The three major gases are nitrogen (N2), oxygen (as O2), and Argon (Ar), which comprise well over 99% of the atmosphere, and none of which absorb energy emitted by the earth. All greenhouse gases are trace gases, water vapor (H2O) included.”

    As far as the greenhouse effect is concerned nitrogen, oxygen and argon might as well not exist. They are transparent to infrared radiation.

  27. #27 Adam Kamp
    March 23, 2010

    Thank you very much! I’m definitely in the camp of “global warming skeptic–not denier, just cautious doubter–who’s slowly coming around,” and this helps quite a bit.

  28. #28 mansour daftarian
    March 26, 2010

    Global warming is real and the cosequences are alarming and irreverseable.

    The scientific evidences which prove the global warming is the consequence of our energy situation are now finally , after two decades, realised and the findings remain unassailable.

  29. #29 Doug
    July 9, 2010

    A nice, but rather one-sided discussion. What’s missing is the effect the biosphere has on regulating the atmosphere, but that’s better left to ecologists.

  30. #30 WoW Accounts
    July 22, 2010

    CO2 does NOT trap heat in Venus’ atmosphere. In fact, ALL absorbed Sun’s energy is re-radiated.

    What CO2 does, is increasing the temperature at which all incoming heat is re-radiated.

  31. #31 galvanized tub
    August 3, 2010

    . . . is that we are not worth of being called the most intelligent species of the planet!!!

  32. #32 dance beginners
    August 21, 2010

    Earth but farther away than Mercury — would have temperatures in between Mercury’s and Earth’s.

  33. #33 buy body magic
    August 31, 2010

    Atsushi Yes the php tags are missing as this forum seems to be stripping them. What are you using for image upload?

  34. #34 sudoku puzzles to print
    September 11, 2010

    I would have asked for Final Fantasy 7 for my family’s computer but we were that one Mac family that you knew back in the 90′s before the iPod made it cool to own one.

  35. #35 Candence
    September 13, 2010

    what i dont get it.!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

  36. #36 Lab Software
    September 19, 2010

    Rising sea level are a real problem, I must admit though that I am enjoying a strangely warm summer though in the UK. Its a shame we cant get a balance between good and bad!

  37. #37 SEO Lincolnshire
    October 1, 2010

    Whoah 460 degrees on venus – im moving there, nice and warm!!

  38. #38 Video Production London
    October 5, 2010

    Im still not convinced that the data regarding mercury having a lower temperature than venus can be correct…. its further from the sun!

  39. #39 Kathy
    November 11, 2010

    Doesn’t co2 make up only about 2% of the earths atmosphere? Certainly nowhere near the 96% co2 that the atmosphere of Venus has.

  40. #40 dave neill
    November 17, 2010

    This post really does help some things, but there’s still a premise there that prevents it from being entirely convincing: to wit, that the tiny amount of CO2 currently in our atmosphere has an outsize importance on why our planet is the temperature it is. This may well be true (and I’m inclined to believe it is, for no other reason than that the people who seem to think most scientifically believe this is the case), but since I can’t really grok the science myself, I have trouble believing it in my heart of hearts on say-so, you know?

    Without it, the Earth would be quiet uncomfortable to live in. And note, a few degrees of change can throw all the ecosystems out of balance. And note, both are trace gases on Earth. And believe it or not, little things do make great differences. computer science shows for example, the ozone in the ozone layer is around 2 to 8 parts per billion, but it is enough to block most of the short UV ray radiation. So as you can see, trace gases are extremely important.

  41. #41 vigrx
    November 22, 2010

    Yeah! We humans are the only living on earth that can save it from global warming or just even survive its effects if the fight is already lost.

  42. #42 maopo
    January 20, 2011

    Well, you can believe whatever you like cant you!
    Some say the temperatures are going up, others that their going down, co2 levels are good, or their bad, neither side really looks closely at the arguements of the opposing faction, and its all very heated and emotional. this is not how science is supposed to be. http://www.itunes.com/download

  43. #43 maih
    April 28, 2011

    i hate the way people litter ti makes me feel like they don’t care about their country

  44. #44 anisa anis
    July 11, 2011

    wat can we do now!!!

  45. #45 Wow
    July 11, 2011

    “neither side really looks closely at the arguements of the opposing faction”

    Only half true.

    Look here:
    http://www.skepticalscience.com/argument.php

    Also take a look at the “arguments” on the side opposing the IPCC. Note how many of them are incompatible. Note how they DO NOT argue amongst themselves over it.

    It’s because they’re not FOR their argument, they’re AGAINST the IPCC.

  46. #46 Wow
    July 11, 2011

    “Could it be the sulfuric acid clouds on Venus holding the heat in?”

    The clouds reflect more light, not trap the heat.

    “Or could it be the atmosphere being more dense cause it?”

    If it were the density alone, then the atmosphere would cool because it’s radiating and when cooling, reducing the volume. You have to remember that the equation is:

    PV=nRT

    But those claiming that the pressure of Venus’ atmosphere is causing the warming forget that there’s a “V” there. If the pressure increases, volume can decrease and the temperature remain unchanged.

    “isn’t the CO2 atmosphere but the density of the atmosphere?”93 times as thick as Earth’s”"

    But that would mean the temperature would be 93 times that of earth, surely. And that would be 300×93 Kelvin = 28,000K. I.e. hotter than the sun.

    “CO2 is a trace gas that contributes almost nothing to the “greenhouse effect”;”

    Cyanide is a trace element that will kill you if ingested.

    Judge not me by my size, as Yoda would say.