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).
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.
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..)