Surface contribution to atmospheric warming

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Surface contribution to atmospheric warming

Postby Graeme M on November 25th, 2019, 4:39 pm 

Not sure if this should be here or maybe in the physics section. I hope you will forgive me if the question is vague or terminology incorrect, I am VERY fuzzy about the science in this question. I hope the question makes sense.

Greenhouse gasses don't just magically warm the atmosphere, they respond to IR from the heated earth. So while we may have more GHGs causing warming, presumably then the IR flux is larger than the capacity for say 280ppm of CO2 to absorb and reradiate. As GHGs accumulate there is sufficient IR emitting from earth's surface that more CO2 will absorb more.

But does the converse apply? That is, for a given concentration of GHGs does the atmosphere warm if the earth's surface radiates more IR? Or is the capacity exhausted at any given concentration once equilibrium is reached?
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Re: Surface contribution to atmospheric warming

Postby TheVat on November 25th, 2019, 5:30 pm 

https://scied.ucar.edu/longcontent/greenhouse-effect

I may have posted this page before. A fairly pellucid explanation. I will look tomorrow, if no one else does, for some better info on saturation effects, to address your second question.

Note that some energy absorbed by, say, CO2 is radiated out and some of that goes back to the earth and oceans and some to other adjacent molecules.
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Re: Surface contribution to atmospheric warming

Postby TheVat on November 25th, 2019, 5:36 pm 

Quick reminder to other members that this section of SCF does not deal with personal theories, and is just dealing with atmospheric physics and chemistry, so please confine replies to those areas.
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Re: Surface contribution to atmospheric warming

Postby Graeme M on November 25th, 2019, 6:48 pm 

Thanks, I think that description is more or less what I am getting at. IR from the heated earth radiates back to space; adding GHGs changes the rate of emission to space so that the atmosphere warms until emission to space is once again in balance with incoming radiation. The temperature of the atmosphere is a measure of how much outgoing IR is being absorbed and re-emitted (I think!), so that should be a function of both outgoing emission and GHG concentration. For a given rate of outgoing emission, more or less GHGs will change the temp of the atmosphere. But does the converse apply - that for a given concentration, does more or less IR emission change the temperature of the atmosphere. I'd have guessed yes, up to some point.

The reason I ask is that in the stuff I read, the focus is always on GHG concentrations. But GHGs respond to outgoing IR, so adding more outgoing IR must also have an effect.

I saw this abstract recently. It says, "Australian native vegetation holds more moisture that subsequently evaporates and recycles back as rainfall. It also reflects into space less shortwave solar radiation than broadacre crops and improved pastures, and this process keeps the surface temperature cooler and aids cloud formation. "

https://www.sciencedaily.com/releases/2 ... 180556.htm

That is saying that native vegetation has a lower albedo than broadacre crops (or bare earth, I suppose) which keeps surface temps lower. But that seems wrong. If a surface reflects less shortwave radiation, then the surface is absorbing more shortwave radiation and should surely heat up more. In which case, the atmosphere would warm more as increased IR from the warmed surface heats GHGs.

But then, that line of thought confuses me as it would seem to me that native vegetation would warm less than bare earth or broadacre crops. Bare earth/broadacre crops would seem to me more likely to heat up in the solar radiation and therefore emit more IR, warming the atmosphere more.

If that were so, then land use change that causes more IR radiation to space should also contribute to atmospheric warming. But as I rarely see that mentioned, perhaps I am wrong to think so.
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Re: Surface contribution to atmospheric warming

Postby TheVat on November 26th, 2019, 1:09 pm 

I think IR radiative effects have a lot to with not only albedo (certainly albedo's a more clearcut effect
where you have snowfields reflecting a lot of solar radiation promptly back to space) but also what is the best thermal sink. Humid air holds more heat than dry air - simply, more thermal mass. Dense vegetation, like a rainforest, designed to soak up lots of solar radiation (to power photosynthesis), and having considerable mass, will hold more heat. Which is why it is cooler than a grassland by day, and warmer than a grassland at night. The trees are absorbing the heat so the ground and the air under the canopy is cooler. At night, some of the thermal energy in the trees radiates into that below-canopy space, keeping away the night chill that a camper out on the prairie would be experiencing. Forests thus reduce extremes of temperature. In both locations, what radiates out to space, is lost. When you say

If that were so, then land use change that causes more IR radiation to space should also contribute to atmospheric warming.


You've got it a bit backwards. Air, compared to foliage or earth or water, doesn't hold all that much heat. More radiation to space (as on a cloudless night) is, to be tautological, more radiation to space. Overall, the system is losing more heat, and the air will, averaged across the diurnal cycle, have less heat from the ground and foliage to heat it. That said, more GHGs in that air certainly help it hold a bit more heat, and that tiny shift can have large effects on climate.
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Re: Surface contribution to atmospheric warming

Postby TheVat on November 26th, 2019, 1:27 pm 

Also, forgot to mention, that solar incoming radiation is mostly in the visible spectrum, so it's not really like we're under a pure heat lamp. We get some IR from the sun, but quite a bit of absorbed energy is visible light. So grasslands, on average paler than forests, absorb less from the visible spectrum and so are said to have a higher albedo. And so energy that would go into a forest system is lost outright by a grassland, making it on average cooler. As some climatologists have pointed out, converting Earth to a giant grazing range would have a cooling effect. Unfortunately, we would lose the carbon sequestering power of forestlands, and so this approach would backfire (as well as destroying the many ecological values of forests generally!) And we'd have other problems like rainfall declines, more extreme high and low temperatures inland, reduced daytime cloud cover over land, etc.
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Re: Surface contribution to atmospheric warming

Postby doogles on November 27th, 2019, 7:35 am 

I think that the link provided by TheVat is an excellent explanation of the principle of greenhouse gases.

Graeme M, the reference you provided about Australian native flora providing more water for rain and reflecting less infra-red than crop pastures was a little on the popular science side; there were no solid data or Materials and Methods that one could critically review. Apparently there was an intent to publish something in a peer-reviewed journal somewhere later, but no leads were given. Unfortunately, there was no substance that we could review critically.

We recently had a thread going on Climate Change in which I posted a number of references suggesting that greenhouse gases may just NOT be the main driving medium for increasing near surface temperatures. There were also some references to aerosols from native vegetation behaving both as minor blockers of IR reaching the surface and of being able to act as cloud condensation nuclei. More cloud, of course, means less solar radiation, including IR, reaching the surface. It's probably worth a look if you haven't seen that thread.

I also agree with TheVat that "Dense vegetation, like a rainforest, designed to soak up lots of solar radiation (to power photosynthesis), and having considerable mass, will hold more heat. Which is why it is cooler than a grassland by day, and warmer than a grassland at night. The trees are absorbing the heat so the ground and the air under the canopy is cooler. At night, some of the thermal energy in the trees radiates into that below-canopy space, keeping away the night chill that a camper out on the prairie would be experiencing. Forests thus reduce extremes of temperature."

On the subject of greenhouse gases being the great absorbers of IR, there appears to be a major school of thought that the absorption is linear, that is that heat retention increases proportionately with increasing concentrations of the greenhouse gases in the atmosphere. There is another minor school that suggests a doubling of carbon dioxide concentrations will have very little effect on near surface temperature increases.
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Re: Surface contribution to atmospheric warming

Postby TheVat on November 27th, 2019, 1:19 pm 

Given this thread is a pretty specific line of questions on surface radiation and heat gain/loss, I'm going to suggest we have a separate thread for alternative and/or personal theories on the topic of anthropogenic global warming. A place where the 3% (or whatever it currently is) of researchers who are looking at alternative causal factors in the current increase in surface temperatures can be discussed. To that end, Doogles, I might take your post's last section, starting with "the disappointing aspect...." and have that OP for a thread called something like "Dissenting Opinions on the IPCC Report" or similar. And you could cut/paste some of your other related material from other threads, if you would like to. This new thread would address what you refer to as "minor schools" of thought.

Due to Thanksgiving in the USA, I'm a bit short of time right now, but I will get back to this.
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Re: Surface contribution to atmospheric warming

Postby Graeme M on November 27th, 2019, 8:02 pm 

Thanks for these comments. Just a minor recap, the argument as I understand it is that incoming solar radiation is primarily in the shortwave bands and does little to heat the atmosphere (though see Etmanin et al from I think 2016 or so who have noted greater absorption by CH4 in those bands than has previously been understood). What heats the atmosphere is IR from the heated earth. Heat radiated to space ends up in equilibrium with incoming solar radiation; the presence of greenhouse gasses slows the transmission to space and raises the atmosphere's temperature until equilibrium is reached. The usual best guess for equilibrium sensitivity is about 3C for a doubling of CO2 from 280ppm.

My main interest in this question is the extent to which IR from the heated earth is the primary driver of atmospheric warming. Hence my question more generally around the extent to which GHGs are already saturated in terms of IR absorption and how much scope there is for further heating if the outgoing IR increases given a stable concentration of CO2. Specifically, it is the extent to which different kinds of vegetation etc affect outgoing IR.

The Vat suggests that forests are better heated by incoming solar radiation than are grasslands which would suggest that forests radiate more IR to space after dark than do grasslands (when I say radiate to space, I just mean radiate away from the surface - in the absence of GHGs that radiation would simply escape to space, with GHGs that radiation is "trapped" to some extent).

Still, I would have thought that anything that causes more IR from the surface must contribute to warming of the atmosphere (for it is only outward bound IR that causes the GHGs to warm). Replacing natural vegetation cover with say bare earth would seem to be a likely example. Replacing forest with grasslands also, except that it seems grasslands are better reflectors than absorbers of incoming solar radiation.

Thanks for the tip re the climate change thread.
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Re: Surface contribution to atmospheric warming

Postby Graeme M on November 28th, 2019, 6:08 am 

I tracked down the paper referred to in my link. It argues that an increase in albedo from land clearing contributes to local climate warming and drying, including amplification during El Nino events. I am still not sure why that would result from increased albedo if reflected radiation is more to the ultra-violet end of the spectrum given that shouldn't really warm the atmosphere. But maybe there is SOME atmospheric warming from shortwave radiation? Still seems odd to me. Intuitively it seems to me that a uniform lighter surface heats more than a darker varied surface. perhaps while albedo is higher, so too is the IR from heating of the surface? Or I am just not getting my head around it at all.

https://eprints.usq.edu.au/7419/
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Re: Surface contribution to atmospheric warming

Postby TheVat on November 28th, 2019, 11:27 am 

Generally, its longwave emissions that are absorbed by gases like NO, CH4, CO2, water vapor. I hate to just dump a citation and run but holiday obligations mount. And the Schmidt paper is a good one.

https://scholar.google.com/scholar_look ... 0JD014287&

Given the complex interaction, it's possible that albedo increase can raise near-surface temps during the day (where you have heat from the sun, PLUS longwave emissions from bare ground, as would be notably seen in desert areas) while at the same time leading to overall net cooling because water vapor levels are low and there is a cold nighttime where there is less heat stored in thermal mass and then more has already radiated to the air during the day.
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Re: Surface contribution to atmospheric warming

Postby TheVat on November 28th, 2019, 11:36 am 

The Vat suggests that forests are better heated by incoming solar radiation than are grasslands which would suggest that forests radiate more IR to space after dark than do grasslands (when I say radiate to space, I just mean radiate away from the surface - in the absence of GHGs that radiation would simply escape to space, with GHGs that radiation is "trapped" to some extent).


Sorry, I must have worded my comment poorly, as that is not quite what I was saying. Forests convert more incoming radiation to photosynthetic activity and "tree swapping" i.e. trees radiating at each other and at other heat sinks below the canopy, so they would tend to radiate LESS longwave radiation to space or the airspace above. Again, this is so complex, and dependent on moisture levels and foliage and cloud cover, that I would want to say this is something I'd like to research more.
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