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Friday, 2 October, 2009

How we know global warming is happening, Part 2

In our last post, we determined whether global warming was still happening by adding up all the heat content of the climate system.  Murphy 2009 performed this analysis and found that that planet has been accumulating heat up to 2003. Unfortunately their data ends there as the ocean data they used from Domingues 2008 stops at the end of 2003. So how do we find out what's happened from 2003 until now? Unfortunately, there is no time series (that I know of) of the planet's total heat content up to present time. However, we do have the next best thing.

Global hydrographic variability patterns during 2003–2008 (Schuckmann 2009) analyses ocean temperature measurements by the Argo network, constructing a map of ocean heat content down to 2000 metres (H/T to Chris for bringing it to my attention). This is significantly deeper than other recent papers that focus on upper ocean heat, only going down to 700 metres. They constructed the following time series of global ocean heat:


Figure 1: Time series of global mean heat storage (0–2000 m), measured in 108 Jm-2.

Globally, the oceans have still been steadily accumulating heat right to the end of 2008. Combined with the results of Murphy 2009 who finds the planet accumulating heat right to 2003, we now see a picture of unbroken global warming. Over the last 5 years, the oceans have been absorbing heat at a rate of 0.77 ± 0.11 Wm−2.

So what is our planet's total energy imbalance? Indulge me as I perform some rough back-of-a-napkin calculations. Murphy 2009 found that about 5.6% of the planet's energy imbalance went into the land and atmosphere. In other words, 94.4% of global warming goes into the oceans. So if the ocean is absorbing 0.77 ± 0.11 Wm−2, this puts the total energy imbalance at around 0.82 ± 0.12 Wm−2. This is a slight underestimate as Murphy 2009 included ocean heat down to 3000m (remember this is back-of-a-napkin stuff).

How does this value compare to other estimates of energy imbalance? Hansen 2005, using ocean heat data, calculated the planet's energy imbalance around 2003 to be 0.85 Wm−2. Trenberth 2009 examined satellite measurements of incoming and outgoing radiation for the March 2000 to May 2004 period and found the planet accumulating energy at a rate of 0.9 ± 0.15 Wm−2.

All these estimates are consistent with each other. Most importantly, all find a statistically significant positive energy imbalance. The empirical data has spoken. Cancel the global cooling party. Global warming is still happening.

Posted by John Cook at 22:27 PM

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Comments

Comments 1 to 13:

  1. Is the 0.77Wm-2 supposed to be averaged over the whole surface of the Earth? The graph seems to be referring to oceans on their own.

    The graph covers 6 years which equals to 6*365.25*24*60*60 seconds = 189345600 seconds(A)

    Eyeballing the graph it looks as though the six year period has seen an ~1.5*10^8 Jm-2 increase.(B)

    Dividing B by A gives us ~0.79W-m2(pretty close to the 0.77) storage in the oceans, but the oceans are only ~70% of the Earth's surface. This gives us 0.56Wm-2 averaged over the surface of the Earth.

    I wonder if we're comparing apples to oranges.

    Cheers, :)
  2. There is a potted history of the(apparent) ocean cooling found in 2007 here:
    http://earthobservatory.nasa.gov/Features/OceanCooling/

    It seems there was bad data from the XBT floats and theArgo floats, one giving consistently high results the other consistently low. Cross checking with ERBES data ( which are not consistent with CERES) and correcting the float data gave a very different picture of ocean heat content.
    Also there is extensive info on Argo float position and coverage etc plus data at:
    http://wo.jcommops.org/cgi-bin/WebObjects/Argo.woa/wa/
  3. Shawnhet, I think your analysis applies to an imaginary circumstance where only the oceans absorb heat under the influence of positive radiative imbalance. But additional heat is absorbed by the land, atmosphere, and in melting ice. According to Hansen et al (2005), this amounts to around 0.05 W.m-2 globally averaged. So the globally-averaged heat uptake is of the order of 0.6 W.m-2 during 2003-2008 (according to the analysis of Schuckmann, 2009).

    If we’re comparing this value with the value estimated by Hansen et al (2005) for example (0.85 W.m-2 in 2003), we should probably consider the fact that the sun was radiating energy at around 1 W.m-2 more (top of solar cycle) than in 2008/9 (smack bottom of solar cycle). If we consider the effect on absorbed solar radiation at the surface, this amounts to a (temporary!) reduction near 0.175 W.m-2 (1 x 0.25 spherical geometry x 0.7 albedo) now, compared to 2003. So we should probably be comparing 0.6 W.m-2 to something quite a bit less than 0.85 W.m-2 (something around 0.75 W.m-2 perhaps).

    So the heat uptake data determined by Schuckmann 2009 is of a similar order to that determined by Hansen et al 5 years ago. It is somewhat lower than the value determined by Trenberth (2009); however there must be significant uncertainty in these values. The main point is that current data is increasingly at odds with the notion (see blogosphere ad nauseum!) that the oceans are cooling. They’re not. The evidence supports the rather straightforward expectation that a positive radiative imbalance results in absorption of heat and global warming.
  4. We should also bear in mind that each component of the system responds to change differently, according to its' physical characteristics.
    The oceans represent an enormous buffer as they absorb and emit heat more slowly than the land or atmosphere.
    It seems to me that we should not expect an immediate correlation between atmospheric cooling and ocean cooling. There will be a time delay.
    Presently SST data suggests a diminution or even cessation in the warming trend dating from around 2003 which should eventually be reflected in the data for ocean temps.
    I'm not aware of any direct comparison available for both sets of data...Chris..any suggestions?
  5. BTW..how much of the increase in ocean heat content is due to vulcanism?
  6. Thanks for that Chris,

    I concur that the warming for that period seems to be more like 0.6Wm-2 rather than the 0.82 number mentioned above by John Cook.

    I just wanted to make sure I was reading it right.

    Cheers, :)
  7. shawnhet
    why do you average the heat absorbed by the ocean over the whole earth?
  8. Riccardo, my point was that some of numbers quoted above refer to the Earth as a whole(Hansen & Trenberth) while Schuckmann is talking about the oceans only. That was not particularily clear above.

    Cheers, :)
  9. Thank you for this great post. I will be updating my site with much of this information. :)
  10. Mizimi -- the known number of volcanos isn't changing, there's no unusual warming at the bottom of the ocean in volcanic areas, the earthquake maps show no change, so likely none. Same for volcanos above the ocean, these happen but not in an increasing number so they're part of the background noise, not part of the increase.
  11. The ocean heat content graph still has some amount of noise in it. I wonder if the total global heat content would be a more monotonic line, since the planet is steadily taking up more heat over time.
  12. Nice article John, and the new website design is terrific. Thanks so much (and you should advertise, or if you did I missed it).

    All: see IPCC WG I, Section 5.2.2.3,Implications for Earth's Heat Balance, p.8 of the PDF, an especially Figure 5.4, which has a great chart that shows the heat takeup of various Earth elements, with error bars. Of course, there is newer data,but this is a great, an publicly-accessible summary, an that one chart eliminates a lot of silliness.

    Volcanoes aren't there, because undersea volcanoes are essentially irrelevant: they just don't produce enough heat to be interesting. Of course, big surface volcanoes cane relevant, but only when (like Pinataubo), they send enough sulfate aerosols high enough to cool the Earth.

    "Undersea volcanoes" are what I'd call M2 memes propagated by the usual blogs to confuse the unwwary. I.e., to see that they are wrong, you need to know just a little bit of climate science.

    (John: maybe it's worth augmenting your volcanoe discussion with a bit about undersea volcanoes, if you don't want to give that it's own item.
    Response: New website design? You haven't been here in a while, have you, John?

    Are you sure that link to the IPCC report is correct? If you have links to data on undersea volcanoes, please do post them - would be much appreciated.
  13. I really don't understand what this heat content thing proves. The fact of the matter is that the AGW argument has ben propagated by the correlation between CO2 and temperature, not heat content (so are you now changing the definition?)

    Also, you would expect the heat content is a lagging indicator of energy induced. So even if the current leveling off of temperature reflects a reduction in the cause of global warming, you wouldn't pick it up in the heat content until you got to levels where it created a cooling environment.
    Response: This "heat content thing" proves that the planet is in energy imbalance. That the planet is accumulating heat. And as we've seen elsewhere, the imbalance is caused by an enhanced greenhouse effect in turn caused by increasing CO2 (and to a lesser effect, CH4).

    Our climate system is governed by a relatively simple principle: when the planet is in energy imbalance, global temperatures change. Eg - when you add heat to the system, it warms. How much the global temperature changes is determined by climate sensitivity.

    Of course, even while the planet is accumulating heat, it's possible for the surface temperatures to show short term cooling trends due to the exchange of heat between ocean and atmosphere. For example, over the last few years, we've transitioned from El Nino conditions to La Nina conditions which has had a slight cooling effect on surface temperatures.

    This internal variation is like a noisy signal imposed upon the long term warming signal. But some have interpreted several years of cooling as a sign that global warming has stopped and we've now entered a long term cooling period. By noting that the planet is still in energy imbalance, we are reminded that global warming is still happening.

    Re heat content lagging energy induced, satellite measurements of energy in and out give us a relatively instantaneous picture of the planet's energy imbalance - they also find more energy coming in than escaping back out to space.

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