A long time ago I started writing this article. I haven’t yet finished it.
I realized that trying to write it was difficult because the audience criticism was so diverse. Come to me you huddled masses.. This paper, so simple in concept, has become somehow the draw card for “everyone against AGW”. The reasons why are not clear, since the paper is nothing to do with that.
As I review the “critiques” around the blogosphere, I don’t find any consistent objection. That makes it very hard to write about.
So, the reason for posting a half-finished article is for readers to say what they don’t agree with and maybe – if there is a consistent message/question – I will finish the article, or maybe answer the questions here. If readers think that the ideas in the paper somehow violate the first or second law of thermodynamics, please see note 1 and comment in those referenced articles. Not here.
==== part written article ===
In 1997, J. T. Kiehl and Kevin Trenberth’s paper was published, Earth’s Annual Global Mean Energy Budget. (Referred to as KT97 for the rest of this article).
For some reason it has become a very unpopular paper, widely criticized, and apparently viewed as “the AGW paper”.
This is strange as it is a paper which says nothing about AGW, or even possible pre-feedback temperature changes from increases in the inappropriately-named “greenhouse” gases.
KT97 is a paper which attempts to quantify the global average numbers for energy fluxes at the surface and the top of atmosphere. And to quantify the uncertainty in these values.
Of course, many people criticizing the paper believe the values violates the first or second law of thermodynamics. I won’t comment in the main article on the basic thermodynamics laws – for this, check out the links in note 1.
In this article I will try and explain the paper a little. There are many updates from various researchers to the data in KT97, including Trenberth & Kiehl themselves (Trenberth, Fasullo and Kiehl 2009), with later and more accurate figures.
We are looking at this earlier paper because it has somehow become such a focus of attention.
Most people have seen the energy budget diagram as it appears in the IPCC TAR report (2001), but here it is reproduced for reference:
History and Utility
Many people have suggested that the KT97 energy budget is some “new invention of climate science”. And at the other end of the spectrum at least one commenter I read was angered by the fact that KT97 had somehow claimed this idea for themselves when many earlier attempts had been made long before KT97.
The paper states:
There is a long history of attempts to construct a global annual mean surface–atmosphere energy budget for the earth. The first such budget was provided by Dines (1917).
Compared with “imagining stuff”, reading a paper is occasionally helpful. KT97 is simply updating the field with the latest data and more analysis.
What is an energy budget?
It is an attempt to identify the relative and absolute values of all of the heat transfer components in the system under consideration. In the case of the earth’s energy budget, the main areas of interest are the surface and the “top of atmosphere”.
Why is this useful?
Well, it won’t tell you the likely temperature in Phoenix next month, whether it will rain more next year, or whether the sea level will change in 100 years.. but it helps us understand the relative importance of the different heat transfer mechanisms in the climate, and the areas and magnitude of uncertainty.
For example, the % of reflected solar radiation is now known to be quite close to 30%. That equates to around 103 W/m² of solar radiation (see note 2) that is not absorbed by the climate system. Compared with the emission of radiation from the earth’s climate system into space – 239 W/m² – this is significant. So we might ask – how much does this reflected % change? How much has it changed in the past? See The Earth’s Energy Budget – Part Four – Albedo.
In a similar way, the measurements of absorbed solar radiation and emitted thermal radiation into space are of great interest – do they balance? Is the climate system warming or cooling? How much uncertainty do we have about these measurements.
The subject of the earth’s energy budget tries to address these kind of questions and therefore it is a very useful analysis.
However, it is just one tiny piece of the jigsaw puzzle called climate.
It might surprise many people that KT97 also say:
Despite these important improvements in our understanding, a number of key terms in the energy budget remain uncertain, in particular, the net absorbed shortwave and longwave surface fluxes.
And in their conclusion:
The purpose of this paper is not so much to present definitive values, but to discuss how they were obtained and give some sense of the uncertainties and issues in determining the numbers.
It’s true. There are uncertainties and measurement difficulties. Amazing that they would actually say that. Probably didn’t think people would read the paper..
AGW – “Nil points”
What does this paper say about AGW?
What does it say about feedback from water vapor, ice melting and other mechanisms?
What does it say about the changes in surface temperature from doubling of CO2 prior to feedback?
Top of Atmosphere
Since satellites started measuring:
- incoming solar (shortware) radiation
- reflected solar radiation
- outgoing terrestrial (longwave) radiation
- it has become much easier to understand – and put boundaries around – the top of atmosphere (TOA) energy budget.
The main challenge is the instrument uncertainty. So KT97 consider the satellite measurements. The most accurate results available (at that time) were from five years of ERBE data (1985-1989).
From those results, the outgoing longwave radiation (OLR) from ERBE averaged 235 W/m² while the absorbed solar radiation averaged 238 W/m². Some dull discussion of error estimates from earlier various papers follows. The main result being that the error estimates are in the order of 5W/m², so it isn’t possible to pin down the satellite results any closer than that.
Based on these error estimates, we assume that the bulk of the bias in the ERBE imbalance is in the shortwave absorbed flux at the top of the atmosphere, since the retrieval of shortwave flux is more sensitive than the retrieval of longwave flux to the sampling and modeling of the diurnal cycle, surface and cloud inhomogeneities.
Therefore, we use the ERBE outgoing longwave flux of 235 W/m² to define the absorbed solar flux.
What are they saying? That – based on the measurements and error estimates – a useful working assumption is that the earth (over this time period) is in energy balance and so “pick the best number” to represent that. Reflected solar radiation is the hardest to measure accurately (because it can be reflected in any direction) so we assume that the OLR is the best value to work from.
If the absorbed solar radiation and the OLR had been, say, 25 W/m² apart then the error estimates couldn’t have bridged this gap. And the choices would have been:
- the first law of thermodynamics was wrong (150 years of work proven wrong)
- the earth was cooling (warming) – depending on the sign of the imbalance
- a mystery source of heating/cooling hadn’t been detected
- one or both of the satellites was plain wrong (or the error estimates had major mistakes)
So all the paper is explaining about the TOA results is that the measurement results don’t justify concluding that the earth is out of energy balance and therefore they pick the best number to represent the TOA fluxes. That’s it. This shouldn’t be very controversial.
And also note that during this time period the ocean heat content (OHC) didn’t record any significant increase, so an assumption of energy balance during this period is reasonable.
And, as with any review paper, KT97 also include the results from previous studies, explaining where they agree and where they differ and possible/probable reasons for the differences.
In their later update of their paper (2009) they use the results of a climate model for the TOA imbalance. This comes to 0.9 W/m². In the context of the uncertainties they discuss this is not so significant. It is simply a matter of whether the TOA fluxes balance or not. This is something that is fundamentally unknown over a given 5-year or decadal time period.
As an exercise for the interested student, if you review KT97 with the working assumption that the TOA fluxes are out of balance by 1W/m², what changes of note take place to the various values in the 1997 paper?
This is the more challenging energy balance. At TOA we have satellites measuring the radiation quite comprehensively – and we have only radiation as the heat transfer mechanism for incoming and outgoing energy.
At the surface the measurement systems are less complete. Why is that?
Firstly, we have movement of heat from the surface via latent heat and sensible heat – as well as radiation.
Secondly, satellites can only measure only a small fraction of the upward emitted surface radiation and none of the downward radiation at the surface.
Surface Fluxes – Radiation
To calculate the surface radiation, upward and downward, we need to rely on theory, on models.
You mean made up stuff that no one has checked?
Well, that’s what you might think if you read a lot of blogs that have KT97 on their hit list. It’s easy to make claims.
In fact, if we want to know on a global annual average basis what the upward and downward longwave fluxes are, and if we want to know the solar (shortwave) fluxes that reach the surface (vs absorbed in the atmosphere), we need to rely on models. This is simply because we don’t have 1,000′s of high quality radiation-measuring stations.
Instead we do have a small network of high-quality monitoring stations for measuring downward radiation – the BSRN (baseline surface radiation network) was established by the World Climate Research Programme (WCRP) in the early 1990′s. See The Amazing Case of “Back Radiation”.
The important point is that, for the surface values of downward solar and downward longwave radiation we can check the results of theory against measurements in the places where measurements are available. This tells us whether models are accurate or not.
To calculate the values of surface fluxes with the resolution to calculate the global annual average we need to rely on models. For many people, their instinctive response is that obviously this is not accurate. Instinctive responses are not science, though.
Digression – Many Types of Models
There are many different types of models. For example, if we want to know the value of the DLR (downward longwave radiation) at the surface on Nov 1st, 2210 we need to be sure that some important parameters are well-known for this date. We would need to know the temperature of the atmosphere as a function of height through the atmosphere – and also the concentration of CO2, water vapor, methane – and so on. We would need to predict all of these values successfully for Nov 1st, 2210.
The burden of proof is quite high for this “prediction”.
However, if we want to know the average value of DLR for 2009 we need to have a record of these parameters at lots of locations and times and we can do a proven calculation for DLR at these locations and times.
An Analogy – It isn’t much different from calculating how long the water will take to boil on the stove – we need to know how much water, the initial temperature of the water, the atmospheric temperature and what level you turned the heat to. If we want to predict this value for the future we will need to know what these values will be in the future. But to calculate the past is easy – if we already have a record of these parameters.
See Theory and Experiment – Atmospheric Radiation for examples of verifying theory against experiment.
End of Digression
And if we want to know the upward fluxes we need to know the reflected portion.
The Earth’s Energy Budget – Part One – a few climate basics.
The Earth’s Energy Budget – Part Two – the important concept of energy balance at top of atmosphere.
Earth’s Annual Global Mean Energy Budget, Kiehl & Trenberth, Bulletin of the American Meteorological Society (1997) – free paper
Earth’s Global Energy Budget, Trenberth, Fasullo & Kiehl, Bulletin of the American Meteorological Society (2009) – free paper
Note 1 – The First Law of Thermodynamics is about the conservation of energy. Many people believe that because the temperature is higher at the surface than the top of atmosphere this somehow violates this first law. Check out Do Trenberth and Kiehl understand the First Law of Thermodynamics? as well as the follow-on articles.
The Second Law of Thermodynamics is about entropy increasing, due to heat flowing from hotter to colder. Many have created an imaginary law which apparently stops energy from radiation from a colder body being absorbed by a hotter body. Check out these articles:
Note 2 – When comparing solar radiation with radiation emitted by the climate system there is a “comparison issue” that has to be taken into account. Solar radiation is “captured” by an area of πr² (the area of a disc) because the solar radiation comes from a point source a long way away. But terrestrial radiation is emitted over the whole surface of the earth, an area of 4πr². So if we are talking about W/m² either we need to multiply terrestrial radiation by a factor of 4 to equate the two, or divide solar radiation by a factor of 4 to equate the two. The latter is conventionally chosen.
More about this in The Earth’s Energy Budget – Part One