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Planetary energy imbalance?

Filed under: — gavin @ 3 May 2005


The recent paper in Science Express by Hansen et al (on which I am a co-author) has garnered quite a lot of press attention and has been described as the ‘smoking gun’ for anthropogenic climate change. We have discussed many of the relevant issues here before, but it may be useful to go over the arguments again here.

The key points of the paper are that: i) model simulations with 20th century forcings are able to match the surface air temperature record, ii) they also match the measured changes of ocean heat content over the last decade, iii) the implied planetary imbalance (the amount of excess energy the Earth is currently absorbing) which is roughly equal to the ocean heat uptake, is significant and growing, and iv) this implies both that there is significant heating “in the pipeline”, and that there is an important lag in the climate’s full response to changes in the forcing.

As we have discussed previously, looking in the ocean for climate change is a very good idea. Since the heat capacity of the land surface is so small compared to the ocean, any significant imbalance in the planetary radiation budget (the solar in minus the longwave out) must end up increasing the heat content in the ocean. This idea was explored by Levitus et al (long term observations of ocean heat content) and Barnett et al (modelling of such changes) in a couple of Science papers a few years ago. Since then, the models have got better (for instance, coupled models generally do not require ‘flux corrections’ to prevent excessive model drift any more), and the estimates of ocean heat changes, particularly over the last ten years have improved enourmously. The observational improvements come from incorporating satellite altimeter data (which mainly reflects changes in heat content) and, more recently, the ARGOS float network which is providing unparallelled coverage in sub-surface waters (particularly in the southern oceans). This implies that the estimates of ocean heat content changes over the last 10 years are the most accurate that we have had to date and thus provide a good target to compare against the models.

For their part, the model simulations that have been run for the IPCC AR4 have tried to simulate the climate of the last hundred or so years using only known and quantifiable forcings. Some of these forcings are well known and understood (such as the well-mixed greenhouse gases, or recent volcanic effects), while others have an uncertain magnitude (solar), and/or uncertain distributions in space and time (aerosols, tropospheric ozone etc.), or uncertain physics (land use change, aerosol indirect effects etc.). Given these uncertainties, modellers nevertheless make their best estimates consistent with observations of the actual forcing agents. The test for the modellers is whether they reproduce many of the elements of climate change over that period. Some tests are relatively easy to pass – for instance, we have discussed the model skill in response to the Mt. Pinatubo eruption in a number of threads.

The overall global surface temperature is also well modelled in this and other studies. While impressive, this may be due to an error in the forcings combined with compensating errors in the climate sensitivity (2.7 C for a doubling of CO2 in this model) or the mixing of heat into the deep ocean. Looking at the surface temperature and the ocean heat content changes together though allows us to pin down the total unrealised forcing (the net radiation imbalance) and demonstrate that the models are consistent with both the surface and ocean changes. It is still however conceivable that a different combination of the aerosol forcing (in particular (no pun intended!)) and climate sensitivity may give the same result, underlining the continuing need to improve the independent estimates of the forcings.

So how well does the model do? The figure shows the increase in heat content for 5 different simulations in the ensemble (same climate forcings, but with different weather) matched up against the observations. All lines show approximately the same trend, and the variability between the ensemble runs being greater than the difference with the observations (i.e. this is as good a match as could be expected). The interannual variability, predominantly related to ENSO processes, is different but that too is to be expected given the mainly chaotic nature of tropical Pacific variability, the short time period and the models’ known inadequacy in ENSO modelling. The slope of these lines is then related to the net heat imbalance of around 0.60+/-0.10W/m2 over 1993-2003, and which the models now suggest has grown to around 0.85+/-0.15 W/m2. The distribution of heat in the ocean in the different runs is quite large (figure 3 in the article) but clearly spans the variations in the observations, which is of course just one realisition of the actual climate.

What does this imply? Firstly, as surface temperatures and the ocean heat content are rising together, it almost certainly rules out intrinsic variability of the climate system as a major cause for the recent warming (since internal climate changes (ENSO, thermohaline variability, etc.) are related to transfers of heat around the system, atmospheric warming would only occur with energy from somewhere else (i.e. the ocean) which would then need to be cooling).

Secondly, since the ocean warming is shown to be consistent with the land surface changes, this helps validate the surface temperature record, which is then unlikely to be purely an artifact of urban biases etc. Thirdly, since the current unrealised warming “in the pipeline” is related to the net imbalance, 0.85+/-0.15 W/m2 implies an further warming of around 0.5-0.7 C, regardless of future emission increases. This implications are similar to the conclusions discussed recently by Wigely and Meehl et al.. Many different models have now demonstrated that our understanding of current forcings, long-term observations of the land surface and ocean temperature changes and the canonical estimates of climate forcing are all consistent within the uncertainties. Thus since we are reasonably confident in what has happened in the recent past, projections of these same models under plausible future scenarios need to be considered seriously.


60 Responses to “Planetary energy imbalance?”

  1. 51
    Blair Dowden says:

    The IPCC 100-year temperature change model (IPCC AR4) discussed in the article appears to reproduce the 1940-1970 cooling period. I have not heard an explanation for this trend before, so I wonder if you can tell me what factors caused a cooling in a time of increasing greenhouse gas levels?

    [Response: The IPCC itself doesn't have any models, however the results from the this GISS model have been submitted to the archive run by IPCC so that they can be considered for the next assessment report (AR4).
    With respect to your question, the slight cooling trend seen in the results is a function of a combination of increasing aerosols, significant volcanic activity especially in the early 1960s and a plateau in both the greenhouse gas forcing and solar. Together that produces the 'blip' that you can see. Once internal variability is also taken into account the trend goes from near zero to negative over the different realisations. -gavin]

  2. 52
    John Dodds says:

    Re: #50 : “Response: Beacuse i) solar irradiance is not increasing appreciably, …”

    Where does the Solar Irradiance 0.3 W/m2 number in the IPCC/GISS 1750-2000 Forcings Bar Chart come from?

    The IPCC chapter on forcings (http://www.grida.no/climate/ipcc_tar/wg1/fig6-5.htm) did identify that for solar irradiance the 1750-2000 DATE is actually 1744-1996, these being the dates for the 11 year cycle bottoms. TDoes this mean that any forcing bar chart or table that uses the 0.3 value, is actually missing the 1.0 to 1.5 W/m2 that is attributable to the 11 year solar cycle value from 1996 to 2000 or 2003??. It was also very strange that I could not read anything close to 0.3 or 0.22 off the solar chart given by IPCC. My estimate was always over 1.5W/m2 perhaps meaning that the solar chart was an 11 year moving average and did not show yearly bottoms?? Another observation was that if you started in 1700 instead of 1744, then the solar irradiance change was an additional 1.5W/m2 larger, but CO2 was about the same. So, is the solar irradiance really ~ 3.0 or 4.0 W/m2 the difference from the values from 1364 to 1368 or do you have to include the 11 year cycle values also? or is it actually 0.3? or what?
    Just what numbers get used in the computer model- the real year over year changes or the ones you show in the forcings table?

    And with such a wide variation, just how meaningful is the choice of DATES for the Forcing charts & curves? It certainly biases the conclusions or inferences taken from the forcing curves

    [Response: The only thing that really matters is the long term change because the 11-year cycle is heavily damped by the ocean thermal inertia. You are also confusing a change in the solar irradiance i.e. 0.1% of 1365 W/m2 = 1.3W/m2 over a solar cycle to what is on the forcings chart which is the net forcing once you allow for the geometry and albedo effects - i.e. divide by 4 and multiply by 0.7 = 0.24 W/m2.
    The model runs cited above only used data from 1880 on, so the difference between 1700 and 1750 is not relevant. I think 1750 was chosen for that graph because the anthropogenic effects are close to zero, however, solar and volcanic forcing were varying significantly at that time. Those variations are small compared to the subsequent 20th century forcings, but if you were particularly interested in the 18th and 19th C, you would need to be careful about what start dates you used. -gavin]

  3. 53
    John Dodds says:

    Re #50 Response (part ii): Beacuse… ii) the bigger effect is the reduction in outgoing long wave radiation by increasing GHG concentrations. -gavin]

    Is the “reduction in outgoing long wave radiation…” represented by the GHG Forcing of 1.5 W/m2 in the 1750-2000 Forcing Bar chart? If so then just HOW does all this energy get created? How is the 1.5W/m2 FIVE times larger than the 0.3 W/m2 from the solar input? It is the solar input energy change (the ONLY source of energy change from equilibrium to the earth ) that causes the greenhouse effect in the first place. All airborne gases are totally incapable of creating energy by themselves. Doesn’t the LAW of. conservation of energy apply to Forcing charts? .
    Could it be that the Forcing bar chart is an incomplete and very misleading representation of the total energy change process? Not only is the forcings chart missing all the energy associated with water vapor feedbacks attributable to GHGs, aerosols etc , and the energy change in the ocean identified above, BUT is it also missing the negative forcing associated with the energy that the GHGs etc emit to space?

    [Response:I think you are fairly well confused here. You appear to think that there should be some relation between the solar radiation variation, and the GHG radiative forcing. There isn't. The GHG forcing can be positive while the solar var could be negative. What you need to do is throw away whatever concept of radiative forcing you are using, and use the one which the chart was prepared with, ie http://www.grida.no/climate/ipcc_tar/wg1/214.htm - William]

    If the forcings chart is missing all these components then how can it be used to conclude that GHGs are bigger than solar in driving the global climate change? It only looks at part of the picture.

    I go back to the original #50 question: Why isn’t it that the SUN (solar input energy) is driving the climate, and since it has been increasing,

    [Response: Because the recent changes in solar output have been small, and don't fit the temperature record at all well http://www.grida.no/climate/ipcc_tar/wg1/244.htm#61111 - William]

    then why should we not expect the earth air and water to increase in temperature until a new equilibrium is reached. AND the corollary, when the solar input decreases would we not expect the temerature to go down as it did in 1999-2000, and 1940-70 and 1400 thru 1700 and all the other times (in spite of the increasing CO2). After all this is what happens every single day. CO2 in the air is just another gas It has a slightly bigger heat CAPACITY (most of which is not used), than the O2 and N2 that it would REPLACE in the air, BUT the total energy dictated by the Solar input will just be redistributed slightly more into CO2 & H20 & less into O2 and N2, but they all go up if there is a net increase in energy. The energy content of the air (and water) will still go up and down with the solar input. The sun is causing global warming.

    Please note that the conclusion above in no way casts doubt on the fact that the temperature hockey stick exists, that the air is warming, that the ocean is warming, that the ice & glaciers are melting, that Global Warming exists, or that the model estimates for temperature are not valid (assuming that you agree with the future estimate for the solar input). The conclusion is just that the SUN causes it, and the CO2 concentration is basically irrelevant, and along with water vapor, self regulating. Man can not control the sun.

  4. 54
    John Dodds says:

    William: Re #53: Response:I think you are fairly well confused here. You appear to think that there should be some relation between the solar radiation variation, and the GHG radiative forcing. There isn’t…”

    Yes I agree I am VERY confused about what a forcing is. I have read all of Ch 6 in IPCC. It doesn’t make sense. It makes so little damn sense that I do not believe ANY conclusion that comes from a forcing chart. AND that is the only place I have seen any evidence that CO2, as opposed to the sun, is the cause of global warming.

    [Response:When you read a large scientific document tht has undergone extensive peer review and survived very hostile scrutiny, and you discover that you don't understand it, then there are two possibilities: (a) its wrong; (b) you are wrong. Naturally, possibility (b) is not so pleasant to contemplate so you're inclined to go for (a), but an awful lot of other people have read it and do understand it and do agree with it. So I think you need to brush up your understanding of the science - William]

    Re there being NO relationship between a solar radiation variation and a GHG forcing. This may be true of a Forcing, but it is NOT true of reality.
    It is the solar radiation that causes the greenhouse effect. If there were no change in the solar radiation then there would be no change in the temperature/energy content of the air. If you add CO2, under conditions of no change in solar, then the CO2 (with its higher heat capacity) steals some of the energy from the other air molecules. THERE IS NO change to the air temp. YOU CANNOT CREATE ENERGY (temperature) OUT OF NOTHING.

    “[Response: Because the recent changes in solar output have been small, and don't fit the temperature record at all well "
    This observation is NOT relevant.
    It happens EVERY DAY. The solar increases from dawn to noon. The air temp increases from dawn to mid afternoon (due to earth response time lags) even tho the solar has been reducing since noon.

    Associated question: Just what does the CO2 box in a forcing chart represent, (since apparently solar irradiance is different from a solar forcing (per response 52)? Is it the change in energy/m2 for the CHANGE in CO2 eg from 1750's 290ppm to 378ppm, or is it the TOTAL CO2 - all 378ppm or what? & Just where is the energy forcing part that comes from the natural change in solar irradiance acting on the natural 290ppm (75%)of CO2, vs the part that comes from the change in irradiance acting on the change in CO2, ie the anthropogenic CO2 (25%)?- are they both in the CO2 box? & if so how can you conclude that anthropogenic CO2 is causing global warming? or is this another item missing in a Forcing chart? I am totally confused about what a forcing chart represents and can be used for, AND especially confused about trying to draw ANY conclusions from it. The IPCC Ch 6 is of no help.

    [Response: Forcings are just a metric we use to be able to compare different changes to the climate. It happens to be that the forcings (which can be calculated very easily) actually are able to tell how a model will eventually respond to different forcings without you having to do each individual experiment. The forcings we are talking about in the IPCC graph, or as shown in the Planetary Imbalance post are essentially the global, annual mean energy flux difference at the tropopause when you instantly change the composition or the solar input and then recalculate the radiation. All the forcings are defined as differences from, say 1850, to the present. Thus it is only the change from 280 to 380 ppm for CO2 that is counted, and only the change in irradiance. Because it is a global mean number, you need to adjust the change in solar constant for the geometry and the albedo as I explained above.
    No one is disputing that almost all the energy comes from the sun, but we are looking at CHANGES in the energy fluxes, not the absolute numbers. -gavin]

  5. 55
    Blair Dowden says:

    Gavin, thanks for your response to #51. Increasing aerosols is interesting and probably significant, although I am sure there are large uncertainties in both the amount produced in that timeframe, and what their effect is. The recent “global dimming” issue illustrates that the effect of aerosols is still not well understood.

    I thought volcanic effects lasted for only a few years, and would not greatly affect a 30 year trend.

    I am interested in what is meant by “a plateau in both the greenhouse gas forcing and solar.” Greenhouse gases were increasing in that period; are you saying there was a saturation effect?

    The 1940-1970 cooling does not appear that significant on the graph on this page, but does in other graphs I have seen, particularly the one on page 4 (“Observed Arctic Temperature, 1900 to present”) in the summary of the recent Arctic Climate Impact Assessment. I notice that a similar dip around 1960 is present in both graphs, presumably a short-term volcanic effect.

  6. 56
    Heiner Grupe says:

    Re: #3

    As figured out, climate science has a communication problem.
    But not only NGO’s and “enviro-hysterics” exaggerate.
    Why is HAnsen himself talking about “the proof” and “the smoking gun”.
    Such terms are not helpfull at the state of debate, we achived!

  7. 57
    John Dodds says:

    OK OK You all have convinced me. A forcing is a man made device that is used for estimating responses in the GISSWorld computer model. It has nothing to do with reality. Forcings do NOT comply with conservation of Energy, because the solar forcing (the SOLE significant source of energy to the Earth ecosystem) is smaller than most of the other forcings. You can NOT draw conclusions from forcings because the forcings charts are NOT cause and effect charts and because they are incomplete. eg no WV feedback, no ocean energy content, no longwave out numbers. etc etc

    SO where does the conclusion that GHGs are causing global warming come from? GHGs are totally incapable of generating energy. They can only absorb the solar-in energy. If you put a container of CO2 in the sun, it will come to the equilibrium temperature of the surroundings. – CO2 is not magical, it cannot generate energy to get to a higher temperature than the surroundings.

    The #50 high school physics concept that the SUN, the sole source of energy, is the cause of warming is STILL VALID. The solar irradiance has increased since 1700. The earth model, due to lags, is not yet there – it is warming. The air adjusts daily to the new equilibrium, The ocean with a bigger mass & slower response time is still warming. The land is VERY slow, probably hasn’t budged. The solar irradiance “energy in” exceeds the “energy out”. Hence the world is warming up to come to equilibrium. Which according to the GISSWorld model is in a few hundred years and several degrees away – but IF AND ONLY IF, the solar irradiance stabilizes.

    The fact that the recent changes in solar energy are small (#50, response i) is irrelevant. Every day, the changes in solar energy after noon are negative, and yet due to earthly lags, the air temp continues to get warmer for a few hours. Maybe we are rolling over into cooling as the solar gets smaller?? For Response ii) Yes, the longwave is smaller, that is because there is a net energy imbalance, because the earth is heating up and absorbing energy to try to reach the equilibrium IMPOSED by the changes in solar irradiance in.

    The energy balance is VERY SIMPLE. Why are you complicating it with forcings and extraneous ecosystem model details to get the warming conclusion?
    The GISSWorld model by itself is a real wonder (seriously – no sarcasm intended) given all the things it models etc. It is very useful to predict what the temerature increase will be and when it may occur, IF none of the assumptions change. AND the only one that matters is the solar insolation (ie solar irradiance modified by orbital etc factors). If it decreases or is flat, like it was from 1300 to 1700, or 1999 to 2000, and 1940-70 or like it does from noon to dawn everyday at a specific location, then the earth will cool – WITH the appropriate lag time factors. The amount of CO2, in spite of its greater heat capacity, is irrelevant since it is small compared to the total earth mass. The CO2 will absorb or release as much energy as is required to come into balance with the solar in energy and its immediate surroundings, and probably within a few minutes if not hours.

    WHY are you complicating this? The sun causes the earth warming and cooling, not man. – It is the ONLY (significant) energy source. CO2 & GHGs can NOT CAUSE temperature changes. They can only react to solar imposed forcings, to come to a new equilibrium. Using Kyoto to change the level of CO2 will have absolutely ZERO effect on global temperatures. (It will however cost a fortune!) If there is less CO2 then there will be more water vapor (feedback!) UNTIL we reach the equilibrium temperature imposed by the solar insolation. If there is more CO2 then there will be less N2 & O2 and water vapor at the SAME equilibrium temperature imposed by the sun.

    The Hansen paper cited above, (with Gavin as a co-author) is JUST PLAIN WRONG is assigning a warming cause to GHGs.

    [Response: Sigh... Almost every line here contains an error. I've done my best to try and explain what the forcings charts show, why we think greenhouse gases have an impact on surface temperatures, why we think solar forcing is a smaller term, but there are limits. The situation is not 'very simple' - if it were, we'd have all moved on to more interesting questions already. -gavin]

  8. 58
    Steve Bloom says:

    John, the good old greenhouse analogy may be useful to consider: GHGs are called GHGs because they act in a manner analogous to the glass in a greenhouse. In a greenhouse receiving sunshine, the air temperature inside is distinctly higher than ambient due to the glass being less transparent to outgoing radiation than incoming radiation (different wavelengths). The important point relative to your misunderstanding is the the air in the greenhouse is *not* heated as a result of the glass being heated and then emitting that heat to the inside air. If childhood memory serves, greenhouse glass will feel cold to the touch on a cold sunny day even while the air inside the greenhouse remains quite a bit warmer than ambient.

    Another analogy to consider in Venus v. Earth. If one were to suddenly replace the current Venusian atmosphere with one similar to Earth’s, Venus would be way cooler. Clearly there’s something about the atmospheres that makes a much larger difference that what could be directly attributed to insolation.

    [Response:The GH analogy is not a good one. See http://www.wmconnolley.org.uk/sci/wood_rw.1909.html or http://www.ems.psu.edu/~fraser/Bad/BadGreenhouse.html - William]

  9. 59
    Nick J says:

    I was wondering if you could explain in more detail what an “energy imbalance” was. For instance, you mentioned in your “Planetary Energy Imbalance?” post that the energy imbalance has grown to around “0.85+/-0.15 W/m2.” Does this mean that, on average, every square meter of the earth is experiencing a net change of +0.85 and -0.15 joules every second? And how do energy imbalance numbers relate to forcings numbers?

    [Response: You are half right. The numbers imply that over every square metre of the Earth, an extra 0.85 Watts is being absorbed - more is coming in, than leaving. The +/- 0.15 is the error bar in the estimate taken from the different 'realizations' of the model simulation. Therefore, some simulations had 1 W/m2 imblance, some only 0.6 W/m2. The forcings are a measure of the imbalance that would occur if the atmopshere was changed instantaneously. If the climate was in equilibrium with the changes, the imblance would be zero. Therefore, the actual imblance is a measure of the un-realised forcing - what still needs to be dealt with before equilibrium would again be reached. -gavin]

  10. 60

    [...] Real Climate agrees with this viewpoint as they report in a May 3 2005 weblog Planetary energy imbalance? This weblog from Real Climate states that their  key conclusions with respect to this issue are [...]


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