02 September 2008
Question Place 2
Have questions about science, particularly relating to oceanography, meteorology, glaciology, climate? Here's a place to put them. For that matter, about running as well. Maybe I'll answer them here, and maybe (as happened back when with Dave's note back in May) they'll prompt a post or three in their own right.
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13 comments:
It's easy to find daily updates of arctic sea ice *extent* from the Japan Aerospace Exploration Agency. Is there an online source for daily updates of arctic sea ice *area*? I want numerical data, not graphs or maps.
I don't know of one that prints out the area, but you can get daily, global grids of ice concentration at ftp://polar.ncep.noaa.gov/pub/cdas/ ... and the /archive subdirectory. The eng.YYYYMMDD files are on a 30' lat-long grid. The eng5min.YYYYMMDD are on a 5' lat-long grid. They get packed up month by month and put in the archive subdirectory.
They're in grib format (WMO standard) and a grib reading program is in pub/wgrib of the same site.
The 5' grids are the best to work with, but only go back to August 2004. There's a land mask change before that, and the grids are only 30' back to 1995. In 1995 they go to 60' resolution, and there are some changes in orientation as well. The oldest files are for 1978 (beginning of the satellite era). There are other land mask changes as well.
The plus side, particularly in the 8/2004-present, is that you've got the data on grid cell by grid cell basis and can make your own computations of area, extent, extent above a critical concentration limit (you'll see different things in summer with a 15% cutoff vs. 30, vs. 50), area and extent in geographic subdomains, etc.
What it the latest and greatest data on ocean heat content? I remember seeing somewhere that ARGO measured a drop in ocean temperatures - and saw a graph with a drop from 2003 to 2005. I've regularly seen skeptic comments about how the oceans have stopped heating, but haven't been able to to find any newer information to know whether that 2 year ocean cooling 'trend' has reversed since 2005, or grown to something a bit more significant.
Cool blog. I just began my freshman year of college, no classes in climate behind me, and am doing pre-calculus so please talk to me like I'm completely stupid. I did have just a few questions on global warming.
1) What exactly makes a gas a greenhouse gas? I've asked people how changing the amounts of gases which make up like a percent of the atmosphere can have such a small effect and they say its because the others are not greenhouse gases so they don't count
2) How is it possible to know what is causing the modern change? We've been through changes in the past so what makes it different today?
3) I understand that we dont know very much about cloud responses to warming, but I was told that we know how much precipitation changes. How is that possible? How exactly will precipitation change if it gets warmer?
Interesting questions Bart, thanks. Stupid you're not. You share with all of us the fact of ignorance, which you're doing the great thing of setting about curing. Since my areas of ignorance don't include your areas of question, I can help you some. Tables will, no doubt, be otherwise on other areas.
1) To be a greenhouse gas, it has to a) not absorb solar radiation (roughly anything from 4 microns and shorter, mostly down to 0.2 microns) and b) absorb earthly radiation (4 microns and longer, mostly down to say 100 microns).
Roughly speaking, any gas with only 1 or 2 atoms per molecule (Ar, N2, O2) is not a greenhouse gas. Any gas with 3 or more (H2O, CO2, N2O, CH4, O3, chlorofluorocarbons, ...) will be a greenhouse gas. To get hard core about why it works this way, you'll want to take quantum mechanics. But the fact that this was the case was observed long before quantum mechanics -- Tyndall published on it in 1861. (Also long before anyone was concerned about anthropogenic climate change.) See also my greenhouse misnomer note for some more details.
In the case of the atmosphere, which I mention a bit more about in introducing the atmoshpere, 99+% of the atmosphere is gases that are not greenhouse gases (N2, O2, Ar). All the interesting behavior of the atmosphere is due to the less than 1% that are greenhouse gases (H2O, CO2, ...).
This is a surprise, I agree. But it's also true that the cpu chip in your computer is almost nothing but silicon. If it were absolutely nothing but silicon, it wouldn't work. Adding very small traces (100 ppm, if I remember correctly) of other things is responsible for making your processor run. Energy transfer through the atmosphere is another of those cases where the small traces (water vapor averages about 2000 ppm through the atmosphere, I'll give more detail on that shortly, CO2 at 385 ppm (and rising), and others at even lower levels) control how the system at large works.
2) is the 'attribution problem'. It is, ultimately, quite difficult. Take a look on google scholar (http://scholar.google.com/) for some examples of how people are trying to attack it in the science. (Scholar is better for getting a feel for the science than a web search is. Harder reading often.)
For the intuitive level understanding, there are a couple of things to look at. One is (disregarding how it has happened) levels of atmospheric CO2, CH4, and some other greenhouse gases are observed to have risen over the last century or so. The effect expected, from conservation of energy considerations, of that increase is a warming. This is rather akin to expecting a fire to burn higher/hotter if you throw gasoline on it. It could be that the amount and rate are just such to blow out the fire. But the odds of that are slim.
A different intuitive point is the speed and magnitude of the warming over the last few decades. Unlike some of the earlier warming/cooling periods, which were either slow (due to changes in the earth's orbit -- ice age cycles) or faster but not so large, the last 30 years show no large effect from volcanoes, solar cycles, etc., and are both large (for climate) and fast (for climate). Changes in the radiation budget of the atmosphere can fill both requirements, and we see that there have indeed been large changes (over 30% in the CO2 concentration) in the greenhouse gas levels.
Now that's intuitive level stuff. A 'sniff test' kind of thing. Getting to the hard core business involves some serious data and analysis methods.
3) Maybe you can track down and let me know about the sources which are putting confidence about future changes in precipitation? My understanding is that the confidence about the future precip is similar to that about cloud changes, and for the same reasons.
For recent changes in precipitation, we have reason for confidence because of satellite programs like TRMM (Tropical Rainfall Measurement Mission). They provide us with substantial (though not always global, TRMM is tropical) information about precipitation amounts and changes with changing conditions over the last several to many years. My understanding (this being an area I haven't followed much, so take with a grain of salt; better yet, check out your library and the science journals) is that the global mean pattern observed so far is for more precip with greater warming. This makes intuitive sense, but intuition is not infallible. For the future, I understand the expectation is for a general increase in precip, but for some regions like the US southwest and Great Plains, to dry out. Again, though, not my area and I haven't studied it much so it could easily be the other way round.
bart,
Robert Grumbine has addressed your questions well, and I'll throw in a few extra comments
Gases absorb and emit infrared light if they vibrate at the same frequency of the light, and if its vibration has a dipole moment that effects the electric field (maybe Dr. Grumbine can think of a good analogy for this for simplification). For the diatomic gases (O2, N2) this dipole moment does not take place, and they do not absorb or emit infrared radiation. Thus under Robert Grumbine's criteria 1b, they are not greenhouse gases (GHG's). Don't worry if that sounds all fancy, just know that only GHG molecules (and clouds) absorb and emit infrared radiation. ~99% of the atmosphere does not.
The other molecules do matter in the sense that collisions increase the absorption by the GHG molecules, so it does matter that the other molecules are there...but it's only the GHG molecules that are doing the radiating, and it's their presence which keeps the planet from becoming an iceball. There are in fact some exotic cases on other planets where diatomic gases such as H2 and N2 can become greenhouse gases, but not on Earth
I don't really agree with him that GHG's can't absorb solar radiation, but this is bound to lead to technical details. Assume that it is correct for now. The broad brush explanation of the greenhouse effect is that visible and near-infrared sunlight comes in and goes through the atmosphere just fine (okay, some gets reflected away by clouds, but then it's as if it were never there), while far-infrared radiation *emitted by the earth* is absorbed very strongly by the atmosphere.
So this is precisely why changing the concentrations of those gases which exist in relatively small quantities matters. We're also changing oxygen because you need that for the combustion of fossil fuels, but there's so much of it, and what we're doing is negligible, so no one really cares.
2) On your next question, remember that there is a *detection* and an *attribution* problem. The first deals with the fact that climate has changed or is changing, etc...while the second involves lots of detective work to see what is causing it. Don't fool yourself by confusing the two and saying that just because climate has changed before, it must be natural now (do arsonists get off on the line that 'fires always happened?').
In fact, the past record shows us that almost every climate change in the past involved changing greenhouse gases in some way or another. So there's no contradiction between past changes and now, but rather the past is very strong evidence that the late 20th century change is unique in some ways and required human-induced forcing. See The Scientific Basis for Anthropogenic Climate Change.
3) My answer to your question 3 may differ from Robert Grumbines. There is probably much less uncertainty on the global mean precipitation response even for models which handle cloud feedbacks differently. A lot of the uncertanties in cloud feedbacks come from uncertainty in the albedo effect of low clouds. That is, how much more (or less) incoming sunlight is reflected away by changing cloud cover as climate warms. The jury is still out on that one.
However, the scattering of visible radiation has little impact on the tropospheric energy budget. This balance is that somehow or another, the increase in infrared radiative cooling of the troposphere must be balanced by the increase in latent heat release (precipitation). So it is the feedbacks that affect tropospheric energy *absorption* that affect the tropospheric energy budget, so the temperature dependent effects of precipitation on visible reflection is small.
The globally-averaged precipitation response to warming is rather small (only a few percent per degree), but what matters is that precipitation gradients will change, and you may get more intense rather than more frequent precipitaton events. The regional changes matter quite a bit, and there's still a lot of question marks at these levels.
And I'll try to be shorter next time :-/
Chris, you have read some of my notes, haven't you? :-) I'm not concerned about someone taking a while to say something useful. Much better a long, good answer, than a short bad one.
You're right, of course, that greenhouse gases can absorb solar. Ozone is the prime example, absorbing both solar (ultraviolet) and earthly (9.6 microns, right near the peak of earth surface emissions). Still, the balance of their absorption is much towards the terrestrial rather than solar.
I've never seen or thought of a nice intuitive way to explain why polyatomics have absorption lines in the terrestrial IR. Still thinking about it and keeping my eyes open; and if you see one, let me know.
Thanks for the word about precipitation. When I first (and only time) looked in to it much, a decade or so ago, even the direction of change was an open question between models.
I really should update some things, or mention them, or something. I'm almost never 'Dr. Grumbine'. I only use Robert in formal situations or in filling out forms (which carried over to blog forms). Mostly, I'm Bob. 'Dr. Grumbine' is any of several people, including an OB-GYN who lives not far from me and whose name shows up often in the Web of Science. (Fortunately he's Francis. But there are some R. and Robert Grumbines out there too.)
Bart: Between the two of us (so far) did we give you some useful answers and leads? If not, what's the problem?
Thanks guys,
Just one more. Are greenhouse gases the only requirement you need for a greenhouse effect? If so, should there no be a straightforward relation between how warm natural CO2 makes the planet, and we can use that to see how much a change in CO2 warms the planet? But isn't water vapor the most important greenhouse gas and so CO2 will only have a small effect?
The way I see it is that a lot of radiation from the surface is going to the atmosphere out to space, but a lot of it is absorbed close to the ground, and clouds and water vapor are very prevalent. So water and clouds would re-direct a lot of the energy back down, and more CO2 could only do a little bit.
Just one more. Are greenhouse gases the only requirement you need for a greenhouse effect?
Yes.
If so, should there no be a straightforward relation between how warm natural CO2 makes the planet, and we can use that to see how much a change in CO2 warms the planet?
No. Just because an effect is due to only 2-3 sources does not mean that it can be handled straightforwardly. Even if there were only 1, it can be quite difficult to get accurate answers.
But isn't water vapor the most important greenhouse gas and so CO2 will only have a small effect?
The conclusion follows only if water vapor is overwhelmingly the most important greenhouse gas, and if we didn't care about 'small' changes to the greenhouse effect.
The greenhouse effect is 33 C (I haven't detailed how this is so, the simplest climate model being too simple to answer this). Large climate changes, like ice age cycles, are associated with changes of about 10% in the greenhouse effect. Doubling CO2 (and the leverage it likely exerts on water vapor), is expected to be something like a 10% increase in greenhouse effect. We have to care about rather modest changes to the total greenhouse effect.
The way I see it is that a lot of radiation from the surface is going to the atmosphere out to space, but a lot of it is absorbed close to the ground, and clouds and water vapor are very prevalent. So water and clouds would re-direct a lot of the energy back down, and more CO2 could only do a little bit.
The question isn't whether it's a 'little bit' with respect to the radiation, but whether it's small compared to what would have significant effects on people, sea level cities, food supplies, ... The latter is quite a bit more sensitive than the former.
I'll be taking up at length just what fraction of the greenhouse effect water vapor is (many lies and much laziness out there on the topic), how much of a climate change is 'large', how much of the greenhouse effect is subject to human modification, and such like.
First, though, I'll be taking up the question of why rare gases (H2O, CO2, O3, ...) are important for the structure (temperature included) of the atmosphere.
bart,
in addition to greenhouse gases you also need a lapse rate (i.e., a temperature drop with altitude)...but that is something you get on any planet with an atmosphere. Again, there are extraterrestrial cases where you can get a greenhouse effect from scaterring as opposed to absorption, and that would work in an isothermal atmosphere. But yes, if you have greenhouse gases in the atmosphere, you'll get some kind of effect from that.
You cannot linearize over the whole greenhouse effect as I think you're hinting at. Removing all the CO2 in the atmosphere would cause a temperature drop of about 7 C (before feedbacks) while doubling it produces a change of a little over 1 degree (before feedbacks).
I think I'm going to do a post as well on water vapor, since it keeps coming up. RealClimate has a discussion here however, which is a good read. For more concerning the greenhouse effect, you need to read this one
By the way, the greenhouse effect does not actually work primarily by re-directing more infrared energy downward. You also need to consider what is happening in the upper layers of the atmosphere, and infrared is coming from all layers, not just the surface. And generally any extra IR energy downward will come through higher temperatures rather than the direct effects of increased emission by CO2. This may become clearer if you read the second link above.
Sorry for posting more in an outdated place but it's the only related thread. I have been looking into this more, and I think I have a good basic understanding of the greenhouse effect. I am still confused on a couple of points that I need clarification.
1) I read somewhere that carbon dioxide has a very low emissivity. Isn't the emissiviy which governs its greenhouse "Strength." I don't remember the number but it was something like 0.2, but I don't get this concept?
2) I read elsewhere ( I can only research what I read, I don't really have the ability to check much of this for myself) that models assume a constant lapse rate. Chris said the lapse rate is required for the greenhouse effect, but from everything I look at people only catgorize in "Dry" or "Moist" cases, but doesn't it vary everywhere over the globe?
Sorry if these are silly questions-- I'm working on it.
Bart, you can stop worrying about whether the questions are bad. a) they're not and b) you can trust me to tell you if they are. And if option b happened, which doesn't seem at all likely, I've been there myself. After a D'Oh moment, we move on.
Please do track down your source on the emissivity of carbon dioxide. What you remember, which is likely exactly what it said, just doesn't make sense. By Kirchoff's Law (for radiation, not circuits), a consequence of the Law of Conservation of Energy, gases are exactly as good emitters as they are absorbers, and at exactly the same wavelengths. Any statement of 'poor emitter' has to be made in comparison to something else, and under a particular circumstance. Ozone, for instance, is a 'poor' greenhouse gas in that it doesn't absorb in much of the wavelength band that the earth emits at. It does, however, absorb strongly in a zone that nothing else absorbs much, and that is near the peak surface emission from the earth. So it becomes a 'good' greenhouse gas, in that consideration. Or at least an important greenhouse gas.
The only 0.2 that comes to mind in connection to CO2 as a greenhouse gas is that it is about 20%, by count, of the molecules of greenhouse gases in the atmosphere. Water vapor is most of the remaining 80%.
The lapse rate, I'll take up in a new post. Again, please let me know the source for the statement about the models.
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