Aerosol forcing and the PDO

There’s an interesting new paper by Smith et al. called Role of volcanic and anthropogenic aerosols in the recent global surface warming slowdown. A few months ago, I went to a seminar where one of the authors discussed this paper. I didn’t write about it at the time as there wasn’t a paper yet, but now that it’s out, I can do so.

There’s not really all that much to say, since Carbon Brief has covered it in some detail. The basic idea is that the supposed slowdown in surface warming is largely driven by a negative phase of the Pacific Decadal Oscillation (PDO), and partly by a recovery from Pinatubo. However, as illustrated in the figure below, at least some of the negative phase of the PDO is forced by aerosol emissions, mainly from China. Therefore, at least part of the supposed surface warming slowdown is externally forced.

Credit: Smith et al. (2016)

Credit: Smith et al. (2016)

Maybe the key consequence of the results in this paper is

The external factors that contributed to the GMST slowdown therefore appear to be diminishing or reversing. Barring a major volcanic eruption, and assuming continued aerosol mitigation especially in China, this suggests that GMST trends may increase rapidly over the coming few years.

Certainly seems true so far. We’ll have to wait and see if it continues. If we continue to increase our CO2 emissions, while also reducing aerosol emissions, it certainly won’t be all that surprising.

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10 Responses to Aerosol forcing and the PDO

  1. From the abstract: “However, here we show that coupled models from the Fifth Coupled Model Intercomparison Project robustly simulate a negative PDO in response to anthropogenic aerosols implying a potentially important role for external human influences.

    Totally not my field and I should probably read the paper fully, but when aerosol cooling is responsible for negative PDO values, it sounds like at least part of the “PDO” is not an oscillation within the climate system.

  2. Victor,
    Not my field either, but as I understand it, that’s at least partly the point.

  3. Christian says:

    If as i understood the Paper, the mechanism is easy, that aersols can weaken the adeluten-low, and therefore could force a negative PDO. I would say, that this (also true for vulcanic erruptions) is more a trigger for regime shifts in PDO.

    So, you get your “oszillation” (which is not time stable) and would explain why we find this in paleo reconstructions

  4. JCH says:

    There is a paper that makes funny statement about the PDO, so I will go find it and paste it below:

    By these statistical measures, the PDO should be characterized neither as decadal nor as an
    oscillation (but it is in the Pacific).

  5. Magma says:

    The basic hypothesis seems intuitively reasonable, that decadal-scale changes in regional aerosol production and optical density over the three main generating areas (SE Asia, eastern U.S. and Europe) could affect radiative and aerosol forcing and affect large-scale circulation patterns.

    The trick is in the details and those probably have to be left to specialists to thrash out. (Having accused more than my share of commenters of falling into the Dunning-Kruger trap, I’m going to respect my own limits.)

  6. BBD says:

    So perhaps Hansen was right, again. From Hansen et al. (2013) Climate forcing growth rates: doubling down on our Faustian bargain:

    The human-made aerosol climate forcing thus remains uncertain. IPCC (2007) concludes that aerosols are a negative (cooling) forcing, probably between -0.5 and -2.5 W m-2. Hansen et al (2011), based mainly on analysis of Earth’s energy imbalance, derive an aerosol forcing -1.6 ± 0.3 W m-2, consistent with an analysis of Murphy et al (2009) that suggests an aerosol forcing about -1.5 W m-2 (see discussion in Hansen et al (2011)). This large negative aerosol forcing reduces the net climate forcing of the past century by about half (IPCC 2007; figure 1 of Hansen et al 2011). Coincidentally, this leaves net climate forcing comparable to the CO2 forcing alone.

    Reduction of the net human-made climate forcing by aerosols has been described as a ‘Faustian bargain’ (Hansen and Lacis 1990, Hansen 2009), because the aerosols constitute deleterious particulate air pollution. Reduction of the net climate forcing by half will continue only if we allow air pollution to build up to greater and greater amounts. More likely, humanity will demand and achieve a reduction of particulate air pollution, whereupon, because the CO2 from fossil fuel burning remains in the surface climate system for millennia, the ‘devil’s payment’ will be extracted from humanity via increased global warming.

    So is the new data we present here good news or bad news, and how does it alter the ‘Faustian bargain’? At first glance there seems to be some good news. First, if our interpretation of the data is correct, the surge of fossil fuel emissions, especially from coal burning, along with the increasing atmospheric CO2 level is ‘fertilizing’ the biosphere, and thus limiting the growth of atmospheric CO2. Also, despite the absence of accurate global aerosol measurements, it seems that the aerosol cooling effect is probably increasing based on evidence of aerosol increases in the Far East and increasing ‘background’ stratospheric aerosols.

    Both effects work to limit global warming and thus help explain why the rate of global warming seems to be less this decade than it has been during the prior quarter century. This data interpretation also helps explain why multiple warnings that some carbon sinks are ‘drying up’ and could even become carbon sources, e.g., boreal forests infested by pine bark beetles (Kurz et al 2008) and the Amazon rain forest suffering from drought (Lewis et al 2011), have not produced an obvious impact on atmospheric CO2.

    * * *

    I also can’t help but wonder what implications Smith et al. might have for studies that estimate low sensitivity based on low aerosol negative forcing estimates.

  7. BBD,
    I guess the point is that even if the globally averaged aerosol forcing is smaller than we think, the distribution of the aerosols has a bigger impact on temperatures than the globally averaged value would suggest. This seems to be saying something similar to the recent Marvel et al. paper.

  8. BBD says:

    This seems to be saying something similar to the recent Marvel et al. paper.

    That’s my take, certainly. And if correct, Smith et al. is another step towards reconciling the lower aerosol negative forcing estimates with ‘mainstream’ sensitivity estimates.

  9. paulski0 says:

    Interesting. Been wondering about this possibility for a while.

    Also, the influence of negative PDO/IPO/ENSO on aerosol-cloud forcing. Meteorology is a factor in how aerosols influence cloud behaviour, so presumably global average aerosol forcing is modulated by climate variability.

    Not sure how robust this really is though. Looking at the supplementary information, they only have three different models producing the aerosol-only response and CanESM2 appears to show the strongest pattern. I was looking at the aerosol properties of this model’s CMIP5 runs a couple of years ago and found there is a large discontinuity in the carbonaceous aerosol emission inputs occurring at year 2000 such that the annual average burden suddenly drops by about 40%, also with a big change in annual cycle amplitude.

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