Thinking globally

Isaac Held has a recent post called addicted to global mean temperature, that I wanted to highlight. It’s quite relevant to the energy balance models that I’ve dicussed before. You should probably just read Isaac Held’s post, but I thought I would highlight a few things that I found quite illuminating.

One point that the post makes relates to Cowtan & Way (2014) and discusses the implications of undersampling of the temperatures in some regions of the globe. If you try to infill in the undersampled region, you might find that this increases the global temperatures and, consequently, increases estimates of the transient climate response. However, this doesn’t mean that the response to CO2 has increased everywhere, it’s really telling us that

the response to CO2 has a different pattern than what we had thought, not that the response to CO2 is everywhere larger than previously estimated.

I will add, however, that it would seem that improving the sampling would then make it easier to do comparisons with models that do not suffer from this kind of undersampling.

What I found particularly useful was the explanation for why we might expect the feedback response to be non-linear, or – more correctly – not globally constant as we warm.

In models, the effective strength of the radiative restoring is stronger for perturbations in tropical temperatures than for perturbations in high latitude temperatures. In addition, temperature responses are less polar amplified in the initial as compared to the final stages of the approach to a new equilibrium with elevated CO2.

However, as pointed out above, this really means that if we consider the initial period only, we would mostly be underestimating the response in the polar regions.

I’ll just add one more thing that I found interesting and that relates to what I’ve been stressing recently; climate change is essentially irreversible on human timescales. Even if we were to entirely halt emissions, the decay in atmospheric CO2 would largely balance the rise in temperatures, resulting in global temperatures remaining fixed. However, even this is subtler than I had realised

As another example, consider the accumulated emission perspective on long-term climate change after emissions cease, in which slow carbon uptake over centuries compensates approximately for the slow equilibration of the climate to the evolving CO2 levels. The southern ocean plays a leading role for both carbon and heat uptake. And from a global perspective these are competing to change the same global mean temperature. But CO2 is well mixed in the atmosphere on time scales longer than a year or two, so any uptake of carbon affects both hemispheres with roughly equal radiative forcing. But uptake of heat in the Southern Oceans affects the southern more strongly than the northern hemisphere. This distinction can get lost when discussing this accumulated emission perspective.

If I understand this properly (and I may not) what I think it is pointing out is that the northern hemispheres – with more land – equilibrates more rapidly than the Southern Hemisphere. Therefore, what will probably happen is that the NH will rapidly warm towards a higher temperature than the SH, which will maintain an energy imbalance, with most of the energy going into the oceans. The global temperature may be largely fixed, but that doesn’t mean there will be no regional variability, or that there won’t be regional warming/cooling (as the NH and SH equilibrate). At least, I think that is right.

Anyway, those are a few things that I found interesting. You should probably read Isaac Held’s post, as there is more than just this and I may not have quite captured all the subtleties. I think it is quite easy to simply think in terms of globally averaged quantities, while forgetting that there are significant regional variations (both in time and space) and that these can influence what we might infer if we consider globally average quantities only.

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14 Responses to Thinking globally

  1. entropicman says:

    “the northern hemispheres – with more land – equilibrates more rapidly than the Southern Hemisphere. Therefore, what will probably happen is that the NH will rapidly warm towards a higher temperature than the SH, which will maintain an energy imbalance, with most of the energy going into the oceans. ”

    You can see this in the GISS data . The NH maintains a higher mean temperature and a faster rate of warming than the SH. This is particularly noticeable after 1990.

  2. EM,
    Yes, I had realised that there was a difference now, but I hadn’t put two and two together to realise that this would also influence how we would respond if we were to suddenly stop emissions. Seems obvious now, though 🙂

  3. entropicman says:

    Perhaps you are suffering from seepage. 🙂

    Reality is regrettably a lot more complicated than one number.

    Focusing on the global surface temperature makes the message simpler for politicians and the public, which is why the policy discussions, the IPCC (and the deniers) use it.

    From a total energy perspective we have land, ocean, ice, troposphere, stratosphere, THC, ENSO and god knows what else all being measured and changing at different rates. From a scientific viewpoint rate of energy accumulation is probably more useful.

  4. EM,

    From a scientific viewpoint rate of energy accumulation is probably more useful.

    Yes, I agree. That is indeed a more fundamental way to view this.

  5. entropicman says:

    But try explaining that at BH

  6. I have 🙂 (as have you, I realise)

  7. Michael 2 says:

    I suffer from “see page” every day!

    I had written rather a lot comparing northern hemisphere / southern hemisphere oscillation as it compares somewhat weakly to a tuned circuit in electronics. You can measure a lot of current (heat) or a lot of voltage (temperature) but they are out of phase; you won’t see both at the same time.

  8. Catalin C says:

    I think a simple point related to the physics of AGW is that the same amount of heat distributed perfectly uniform will radiate into space the least amount of energy at equilibrium, and as such will generate the largest average temperature for the equilibrium. IMHO better ocean mixing is to a large extent the 2nd cause of the much warmer conditions during Pliocene (after of course CO2, but CO2 was about the same as today and much smaller than what will be seen by the end of this century).
    Getting back to our days, one reason why we see with some (certainly imperfect) methods lower values for ECS is the (temporary) increase in such discrepancies in rates of warming, which are to be expected given the constant (but unequally-distributed) increase in radiative imbalance.

  9. izen says:

    I had read the Issac Held post – additced to global temperature when it was posted a day or two ago. I think I got the same sense from it that ATTP describes/ That combining the local temperature records can be useful for simple models, but obscures the local variation in the forcing effects. Climate sensitivity is actually a local phenomena with variations betwee latitudes and hemispheres. A subtlety that is lost in the global surface temperature metric.

    I was interested to see what was in the follow-up post –
    http://www.gfdl.noaa.gov/blog/isaac-held/2015/05/13/59-how-not-to-evaluate-climate-models/

    Which sounds as if it expands on the point, but have been unable to access the Isaac Held site all day.
    Has anyone else had the same problem?
    Has ATTP’s recommendation pushed so much traffic that it has overwhelmed the NOAA server?!


  10. Climate sensitivity is actually a local phenomena with variations betwee latitudes and hemispheres. A subtlety that is lost in the global surface temperature metric.

    Yet when one considers how much that ENSO, or a significant volcano, has on the global temperature record, it is no longer a local metric. A strong ENSO event will have repercussions that will propagate globally.

    Yes, Isaac Held’s blog is out of commission today as I checked earlier.

  11. izen says:

    @-WHT
    “Yet when one considers how much that ENSO, or a significant volcano, has on the global temperature record, it is no longer a local metric. A strong ENSO event will have repercussions that will propagate globally.”

    But it erases the ‘fingerprint’ of distribution that can enable attribution.

    Does the ‘hiatus’ in GISS contain in the regional data an indication of a change in the forcings or feedbacks? Or is there the ‘known’ pattern of an internal acronymic process (PDO, ENSO) evident in the distribution of change?
    (rhetorical questions, but if anyone has good answers..!)

  12. Certainly, but if the shown graphs at least compensated for the known ENSO variation, one wouldn’t need to act so naive about where most of the variability comes from.

    It’s kind of like having a Dolby noise reduction system and not using it because you prefer to hear the hissing and scratches.

  13. John Hartz says:

    Perhaps we ought to pay more attention to global warming’s evil twin…

    Sea creatures are set to shrink as the world’s oceans become more acidic. That is the startling warning given by an international group of biologists who have charted the likely impact of rising carbon dioxide levels on marine life.

    The group reveals that not only are hundreds of marine species likely to be wiped out as more and more carbon dioxide is dissolved in the Earth’s oceans but also that creatures that do survive – in particular those with shells, such as clams, oysters and snails – will be left puny and shrunken as a result.

    “We have already seen this effect in commercial oyster beds in the US, where marine farmers have had to stop growing young oysters in sea water because their shells could no longer form properly in our increasingly acidic seas. Instead they have to grow them in tanks where water acidity can be controlled,” said marine biologist Professor Jason Hall-Spencer, of Plymouth University.

    “And as the oceans get even more acidic, the problem of species shrinkage – known as the Lilliput effect – will become more and more common. It is a clear warning of the extreme dangers we are facing as carbon emissions continue to rise around the planet.”

    Shellfish species shrinking as rising carbon emissions hit marine life by Robert McKie, The Observer, May 16, 2015

  14. Michael 2 says:

    WHT says “It’s kind of like having a Dolby noise reduction system and not using it because you prefer to hear the hissing and scratches.”

    Indeed. I dislike the background “breathing” as the background noise is elevated then suppressed noticeably. A better device was the Phase Linear “autocorrelator”. Why oh why did I get rid of my Phase Linear 4000 pre-amp. It is a bit like the Dolby but works in frequency bands so you don’t get the breathing effect.

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