In Part One we looked at a few basic numbers and how to compare “apples with oranges” – or the solar radiation in vs the earth’s longwave radiation going out.
And in Part One I said:
Energy radiated out from the climate system must balance the energy received from the sun. This is energy balance. If it’s not true then the earth will be heating up or cooling down.
Why hasn’t the Outgoing Longwave Radiation (OLR) increased?
In a discussion on another blog when I commented about CO2 actually creating a “radiative forcing” – shorthand for “it adds a certain amount of W/m^2 at the earth’s surface” – one commenter asked (paraphrasing because I can’t remember the exact words):
If that’s true – if CO2 creates extra energy at the earth’s surface – why has OLR not increased in 20 years?
This is a great question and inspired a mental note to add a post which includes this question.
Hopefully, most readers of this blog will know the answer. And understanding this answer is the key to understanding an important element of climate science.
Energy Balance and Imbalance
It isn’t some “divine” hand that commands that Energy in = Energy out.
Instead, if energy in > energy out, the system warms up.
And conversly, if energy in < energy out, the system cools down.
So if extra CO2 increases surface temperature… pause a second… backup, for new readers of this blog:
First, check out the CO2 series if it seems like some crazy idea that CO2 in the atmosphere can increase the amount of radiation at the earth’s surface. 10,000 physicists over 100 years are probably right, but depending on what and where you have been reading I can understand the challenge..
Second, we like to use weasel words like “all other things being equal” to deal with the fact that the climate is a massive mix of cause and effect. The only way that science can usually progress is to separate out one factor at a time and try and understand it..
So, if extra CO2 increases surface temperature – all other things being equal, why hasn’t energy out of the system increased?
Because the system will accumulate energy until energy balance is restored?
More or less correct. No, definitely correct – probably an axiom – and probably describes what we see.
Higher Surface Temperature – Same OLR - Does that make sense?
The question that the original commenter was asking was a very good one. He (or she) was trying to get something clear – if surface temperature has increased why hasn’t OLR increased?
Here’s a graphic which has caused much head scratching for non-physicists: (And I can understand why).
Upward Longwave Radiation, Numbers from Kiehl & Trenberth (1997)
For those new to the blog or to climate science concepts, “Longwave” means energy originally radiated from the earth’s surface (check out CO2 – An Insignificant Trace Gas – Part One for a little more on this).
Where’s the energy going? Everyone asks.
Some of it is being absorbed and re-radiated. Of this, some is re-radiated up. No real change there. And some is re-radiated down.
The downwards radiation, which we can measure – see Part Six – Visualization, is what increases the surface temperature.
Add some CO2 – and, all other things being equal, or weasel words to that effect, there will be more absorption of longwave radiation in the atmosphere, and more re-radiation back down to the surface – so clearly, less OLR.
In fact, that’s the explanation in a nutshell. If you add CO2, as an immediate effect less longwave radiation leaves the top of atmosphere (TOA). Therefore, more energy comes in than leaves, therefore, temperatures increase.
Eventually, energy balance is restored when higher temperatures at the surface finally mean that enough longwave radiation is leaving through the top of atmosphere.
If you are new to this, you might be saying “What?“
So, take a minute and read the post again. Or even – come back tomorrow and re-read it.
New concepts are hard to absorb inside five minutes.
Conclusion
This post has tried to look at energy balance from a couple of perspectives. Picture the whole climate system and think about energy in and energy out.
The idea is very illuminating.
The energy balance at TOA (top of atmosphere) is the “driver” for whether the earth heats or cools.
In the next post we will learn the annoying fact that we can’t measure the actual values accurately enough.. Which is also why even if there is an energy imbalance for an extended period, it is hard to measure.
Update – Part Three in the series on how the earth radiates energy from its atmosphere and what happens when the amount of “greenhouse” gas is increased. (And not, as promised, on measurement issues..)
Will you have a post on how the global OLR is measured? I can imagine the difficulty in measuring the global average OLR. Its accuracy is likely to be questionable, just as the average global temperatures are. The claimed changes in global average temperature is a very small percent of absolute (degrees K) temperature. With OLR, the accuracy capability of the global measuring method may be well outside the small percentage change, if any.
John Phillips:
Probably the next post will touch on that.
Love the site, deserves more traffic.
I wonder if I could ask you, Dr. Phillips, to help clarify something for me. I recently read Dr. Hansen’s book, in which he argues that we should look not to models to see what the earth will be like if we continue on the present path, but at the climate record, to other times when we had similar CO2 levels.
As you know, “peoplewhodontagreewithus-ists” make much of the fact that CO2 increases in response to rising temperatures. (They harp on the time lag, too, but that part doesn’t trouble me.)
What I am wondering is, how do we associate past warming with past CO2 levels, given the complex relationship between the two?
Excuse me if you’ve covered this (I looked, a bit). What Dr. Hansen said made a lot of sense, and I believe, given what we know about GHGs, that that level of CO2 has to have played a major role in driving temperatures, but I don’t know how to prove it to my own satisfaction and the satisfaction (if only!) of “skeptics.”
[...] climate is in overall equilibrium, the energy radiated out will match the incoming energy. See The Earth’s Energy Budget – Part Two and also Part One might be of [...]
Robert:
Your comment got trapped in the spam queue because it breached a point of etiquette. I resurrected it with a minor edit. I’ll take a look at your question shortly, but I’m not “Dr. Phillips”..
Robert:
Thanks for kind comment.
As to your question – it seems like a massively complex problem – which you also allude to. Climate effects are inter-related and non-linear.
There’s also the argument “CO2 levels have been much higher in times of similar temperatures, so there is nothing to worry about..” – I haven’t dug into that to find out even how strong the evidence is – because many other conditions would have been different.
We have had times of higher overall temperatures that have been more moderated than present day distributions – lower temperatures in the tropics and higher temperatures at the poles than present. But circumstances were much different – continents in different locations and who knows what else.
The Ghosts of Climates Past are a “gift that keeps on giving” for both sides of the discussion.
I do plan to cover some aspects of the more recent past – like the last million years.
Forgive my ignorance; but would out going radiation not be less until an equilibrium was reached?
Alf:
You are right. I thought that’s what I said.
Increase CO2, less outgoing radiation.. finally a new equilibrium at higher temperatures.
I might have labored the point so much that the key point was obscured..
Robert,
I’m just another reader of the blog like you, and have no PhD.
Considering the fluency and authority with which you review the science, you can understand my confusion. Thanks for your response.
Hi — great site.
I’m still missing something, though. If you add more CO2, then more energy will be absorbed and re-radiated in all directions — including up. Is there some latency between the absorption and re-radiation? If not, I still can’t see how the TOA radiation doesn’t increase “all things being equal”
Tim W.
Tim:
Of course, there’s no latency, (except of interest to people who love quantum mechanics) between absorption and re-radiation.
Well, let’s suppose a “layer” of atmosphere absorbs 1W/m^2 of energy. (And let’s suppose it’s a new change, someone poured in some CO2 to that layer, or water vapor, depending on your favorite greenhouse gas)
Now that energy increases the temperature of that layer. So the layer radiates energy out.
Lets simplify and say 1 W/m^2 is immediately radiated out-
0.5 W/m^2 upwards and 0.5 W/m^2 downwards
So at TOA we have lost 0.5 W/m^2, which is now heating up the surface (redirected)
But instantaneously there won’t be any temperature increase. Why not?
Because of the specific heat capacity of the oceans, the atmosphere and so on. Like turning the heat on the stove, the water takes time to heat up.
More energy comes into the system than out so the whole system has to heat up – where the heat ends up depends completely on the characteristics of that system, a very complex problem.
But simplifying it down we end up with an increased temperature at the surface. Eventually the temperature increase at the surface is high enough that the “lost” 0.5W/m^2 is now made up for and energy equilibrium is now re-established.
“All things being equal” of course.
Science,
I am a bit dense so bear with me. If the earth is at equilibrium and radiating 265 W/m^2 at the TOA and we add a bit of CO2, the added CO2 absorbs energy (say 1 W/m^2). Now that 1 W/m^2 is re-emmitted with 0,5 going down (as a temperature increase) and the other 0.5 going up. Why isn’t the emission then 265.5 W/m^2 at the TOA?
ps. never mind! The lightbulb just went on.
[...] Another post to check out – The Earth’s Energy Budget – Part Two [...]
[...] 21, 2010 by scienceofdoom In the previous article in this series, The Earth’s Energy Budget – Part Two we looked at outgoing longwave radiation (OLR) and energy imbalance. At the end of the article I [...]
Wouldn’t the main reason you don’t see OLR increase is that, from space, you are (mostly) seeing the radiation associated with the “top” of the troposphere?
Since that temperature isn’t expected to change (much), you wouldn’t expect the OLR to increase….
Carrick:
This post really tries to cover the basic idea of energy balance.
Once we move deeper into the subject it all gets more complicated. Like “how does the equilibrium get restored”?
There’s more in Part Three – with the concept of where the atmosphere radiates from.
[...] by energy leaving from the top of the atmosphere – otherwise temperature will increase (see The Earth’s Energy Budget – Part Two). And if one “layer” of the atmosphere totally absorbs it will still radiate energy to [...]
[...] Update – new post The Earth’s Energy Budget – Part Two [...]
[...] Part Two – which explained energy balance a little more [...]
[...] In long term equilibrium energy in = energy out. However, we want to know what happens if something disturbs the system. For example, if increased CO2 reduces OLR then heat will be added to the climate system until eventually OLR rises to match the old value – but with a higher temperature in the climate. The same is the case with any other forcing. (See The Earth’s Energy Budget – Part Two). [...]
[...] The most important point to understand is that the atmosphere and surface are heated by the sun via radiation, and they cool to space via radiation. While all of the components of the climate are inter-related, the fundamental consideration is that if cooling to space reduces then the climate will heat up (assuming constant solar radiation). Which part of the climate, at what speed, in what order? These are all important questions but first understand that if the climate system radiates less energy to space then the climate system will heat up. See The Earth’s Energy Budget – Part Two. [...]