Greenhouse Earth

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Greenhouse Earth

Postby Graeme M on February 25th, 2020, 9:21 pm 

OK, this is a simple enough question but it's tackling something I am never quite clear on. People are always talking about surface temps rising due to CO2 emissions, GHGs causing the air to warm and the fabled "back radiation". Sure, I could go try to dig around the interwebs to get a better handle, but I wanted to ask whether my simple take on things is more or less right.

The earth is warmed by the sun, the warmed earth radiates to space. Without an atmosphere, the actual surface (dirt or water or trees or whatever) would be hot in the daytime and cool rapidly in the nighttime. However, we do have an atmosphere which consists of some gasses that are absorbers of the IR from the earth. This means that the atmosphere is warmed by the gas molecules being heated by both conduction and radiation, with radiation being the heavy lifter. All things being equal, the atmosphere on average is some temperature if measured say one metre above the ground. Add more GHG molecules and the atmosphere becomes warmer (that is, the time to radiate to space is longer and so the average temperature rises until equilibrium is reached - the heat is "trapped" in the atmosphere). The warmer air means that the earth radiates to space more slowly, thus the actual surface temperature rises over time as it radiates to space more slowly (takes longer to reach equilibrium). This latter effect is the equivalent of back radiation.

And that's about it. While regions may warm or cool at different rates as GHG concentrations rise (due to weather/climate patterns etc), as a general rule of thumb the matter in the atmosphere is warmed by the earth, and so the more matter the warmer the atmosphere. There is a broadly linear relationship between increasing cumulative GHG emissions over time and increasing atmospheric temperatures (measured at say one metre from the ground).

When folk talk about the "surface temperature", they are really meaning the temperature of the atmosphere close to the ground where we measure it. The actual average surface temperature (meaning the ground or the water) is not the same as the average near surface air temperature. The one we care about, in terms of climate change, is the air temperature.
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Re: Greenhouse Earth

Postby TheVat on February 26th, 2020, 11:11 am 

No time to address all this, but ocean temp changes are also quite important in climate change. And water and ice also play another role in amplifying warming through various positive feedback effects. And feedback can make things go nonlinear.
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Re: Greenhouse Earth

Postby TheVat on February 26th, 2020, 1:13 pm 

Here is a really good look at climate sensitivity, feedback and forcings, and so on, in remarkably clear language:

https://www.carbonbrief.org/explainer-h ... ensitivity

And, though I understand reluctance to get too far into the "interwebs," it is truly helpful to punch in some search terms and include "nonlinear effects" in your search. Carbon feedback, for example, can go nonlinear when permafrost melts or large tracts of forest burn. Methane feedback can go nonlinear when ancient bogs defrost - also methane hydrates from coastal areas liberated from sea ice. Ice itself has a nonlinear aspect when a critical melt temperature is reached and the albedo of large surface areas rapidly decreases.
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Re: Greenhouse Earth

Postby Graeme M on February 26th, 2020, 9:32 pm 

Hi Vat, yes, there are all sorts of wrinkles, but I am just trying to work out if my basic understanding is correct. At its simplest, the greenhouse effect is the sun warms the surface via SW and the surface warms the atmosphere via IR. Warming the atmosphere slows down radiation to space and makes the air warmer than space. More GHGs and the air warms more, meaning it takes longer for the earth's surface to radiate to space and raising the surface temperature.

Back radiation just means this slowing of the loss of heat (and consequently a warmer surface).

I have read that the atmosphere is transparent to incoming solar radiation, but given that some large proportion of incoming radiation is in wavelengths longer than 750nm, I'd have thought that the sun is also warming the atmosphere directly?
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Re: Greenhouse Earth

Postby TheVat on February 27th, 2020, 11:10 am 

Yes, oxygen and nitrogen are mostly transparent but some of the incoming solar radiation is longer wave and will cause C02 and CH4 and NOx molecules to increase their vibration. Those molecules capture both incoming IR and surface radiated IR. I forget the figures, but the visible spectrum is where most incoming radiation lies, something like 70%.
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Re: Greenhouse Earth

Postby Graeme M on February 27th, 2020, 4:42 pm 

That's a little different from what I'd thought though spectrum graphs I have seen suggest that yes, as much as 30% or more of incoming radiation is in the IR wavelengths. Which must mean that the sun directly heats the atmosphere to some proportion. Similarly with oxygen and nitrogen absorbing some radiation - the linked source suggests most of the UV is absorbed by oxygen. Does that also mean oxygen and ozone are warmed and radiate as well? Do we know the GHG forcing components for direct insolation, or are the forcing components scientists refer to inclusive of all heat sources (incoming insolation and IR from the surface)?

https://www.e-education.psu.edu/meteo300/node/683
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Re: Greenhouse Earth

Postby Graeme M on February 27th, 2020, 4:51 pm 

Looking at an energy budget description suggests that some 30% of incoming is reflected and 23% absorbed in the atmosphere on the way in, leaving just 47% of incoming to warm the surface. The 23% absorbed is interesting if it's true that something like 30-35% is absorbed on the way in according to spectrum graphs. Or is not all of the 30% or so of incoming IR absorbed? I suppose I'd need to know exactly how much incoming actually IS absorbed...

https://scied.ucar.edu/longcontent/energy-budget
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Re: Greenhouse Earth

Postby Graeme M on February 27th, 2020, 9:54 pm 

And while we are talking greenhouse effects, it occurs to me I don't actually know how a real greenhouse works. A quick Google tells me that very simply put, the sun warms the interior of a greenhouse and the glass acts as an insulator for the rate of loss of heat from the warmed interior. While the sun shines, the interior has to heat enough to overcome the thermal insulation effect in order to reach equilibrium. Nonetheless, the insulative properties are relatively poor and so the greenhouse will still lose heat quite quickly when the sun is not shining. The rate of heat loss and hence interior heating can be adjusted by using different materials for the transparent walls/roof and the interior flooring/structures. The similarity between the greenhouse and the atmosphere is that in effect, the warmed surface has to warm a further layer of matter enough in order for outgoing energy as IR to match incoming energy across the broader spectrum (UV to IR). More or less on track?
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Re: Greenhouse Earth

Postby bangstrom on February 27th, 2020, 10:34 pm 

Graeme M » February 27th, 2020, 3:51 pm wrote: The 23% absorbed is interesting if it's true that something like 30-35% is absorbed on the way in according to spectrum graphs. Or is not all of the 30% or so of incoming IR absorbed? I suppose I'd need to know exactly how much incoming actually IS absorbed...

https://scied.ucar.edu/longcontent/energy-budget


The amount of incoming radiation from the sun and the percentages of N2 and O2 are essentially constants. GHG’s can absorb IR both coming and going so the percentage of GHG’s in the atmosphere is critical factor determining temperature changes from the norm. The many percentages of ins and outs are of academic interest but the rise of GHG’s and their effect on climate are all we really need to know.

We need to understand that a rising percent of GHG’s in the atmosphere brings about a global rise in temperature which changes the equilibrium of the many physical and biological reactions on the surface of the Earth. Many of these changes risk a release of even more GHG’s into the atmosphere causing a run-away temperature rise that can’t be stopped or reversed until a new equilibrium is established at a higher temperature. The Earth will survive and possibly thrive but humanity may not.
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Re: Greenhouse Earth

Postby Graeme M on February 27th, 2020, 10:55 pm 

Bangstrom yes, but I am more just getting clear in my head the various bits of the whole. I have always read that the atmosphere is pretty much transparent to incoming insolation but now I am learning that's not quite true. So my interest in that is more academic - if GHGs absorb IR and the incoming spectrum contains a goodly percentage of IR then the sun warms the atmosphere directly. Not something I'd read before.
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Re: Greenhouse Earth

Postby bangstrom on February 27th, 2020, 11:16 pm 

You are right that GHG’s must work both ways and I don’t recall ever reading that either. The emphasis is on outgoing IR because visible light from the sun that is not reflected is absorbed and becomes heat. Heat energy from the Earth is radiated back into space in the IR spectrum so the Earth emits much more IR than it receives.
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Re: Greenhouse Earth

Postby Graeme M on February 28th, 2020, 5:17 am 

Ahh.. just noticed my link above to the spectrum of incoming solar radiation also notes the absorption bands and I should have noted the wavelengths too. Very little longwave IR in the incoming insolation, it's mostly near IR and SW IR. So yes, as far as CO2 and CH4 are concerned, the atmosphere is pretty transparent to incoming radiation. Though according to that graphic water vapour is doing a fair bit of absorbing around 1.0 to 1.5 nm. So yes, the atmosphere is largely transparent to incoming insolation. Conduction must also play a part in warming the atmosphere, but in terms of GHG warming I assume that is somewhat irrelevant (that is, I am assuming all atmospheric gasses are warmed by conduction so CO2 at such a small concentration has a very small part to play in conductive warming).
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Re: Greenhouse Earth

Postby Graeme M on February 28th, 2020, 5:22 am 

Found this on Quora and I think it's interesting - it suggests that LW IR delivers 8.5% of the warming forces.

"The atmosphere loses energy to space as 59% of the 71% of the total solar flux thermalised. That is, at equilibrium the incident radiation absorbed from incident sunlight is emitted in the form of infrared by the surface and the atmosphere. This is 71% of the incident solar flux. It can be seen that the infrared losses to space are largely from the atmosphere at 83% of the total losses to space.

This energy that is emitted back to space from the atmosphere became atmospheric energy largely by latent heat transfer from the surface at (25/59x100)~42%. Secondly direct absorption of incident solar radiation heats the atmosphere at (23/59x100)~39%. Joint third and fourth at (5/59x100)~8.5% is convection and long wave heat transfer from the surface.

Except that latent heat transfer is coupled with convection as ‘moist convection,’ as convection carries the water vapour up to the condensing levels.

Hence the atmosphere is heated as follows;

51% of its energy from moist convection.

39% of its energy from direct absorption of incoming solar radiation

8.5% by long wave heat transfer from the surface to all absorbing species.

(I am aware that the rounded figures fall just short of 100%, but that is the breakdown.)"

Image
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Re: Greenhouse Earth

Postby Graeme M on February 28th, 2020, 5:26 am 

I like this as it seems to fit with a question I was wondering - if the atmosphere is an average of 14C and the main warming force were GHGs, then doubling CO2 would deliver more than 3C extra (median ECS estimate). But if LW IR to GHGs delivers just 8.5% forcing, that would make far more sense. Given we are told that the atmosphere delivers a surface temp 33C more than otherwise, is it dumb to take 8.5% of that as the component from LW IR from the surface? That gives us 2.75C so a doubling of CO2 should deliver something close to that, which is not far from the median estimates for climate ECS?
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Re: Greenhouse Earth

Postby bangstrom on February 28th, 2020, 7:10 pm 

Graeme M » February 28th, 2020, 4:26 am wrote: That gives us 2.75C so a doubling of CO2 should deliver something close to that, which is not far from the median estimates for climate ECS?

All atmospheric changes involve multiple feedback loops both positive and negative so the changes are not one to one. A doubling of the CO2 level does not imply a doubling of the effect. An increase in GHG’s raises air temperatures which melts more polar ice decreasing the Earth’s albedo allowing the oceans and land to absorb more energy from the sun. Greater surface temperatures increase the decomposition of soil organic matter and methane hydrates adding more CO2 and methane to the air. Much of the Earth’s methane is in the solid form of methane hydrates which are only stable in permafrost or cold polar waters. Rising ocean temperatures also decrease the ability of the oceans to absorb CO2. A small change in CO2 levels can have as oversized effect.

Rising temperatures and CO2 levels also promote plant growth which has a negative effect on the CO2 levels but the effect of plants is limited. More plants mean more plant material that either must decompose or burn. Dead plants return every bit of the carbon they stored back to the atmosphere and decomposition consumes exactly as much oxygen as the plant contributed to the atmosphere while it was alive.
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