Constraining unforced variability

Pages 2k has a new Nature paper called Consistent multidecadal variability in global temperature reconstructions and simulations over the Common Era. What they do is present 2,000-year-long global mean temperature reconstructions using seven different statistical methods that draw from a global collection of temperature-sensitive palaeoclimate records which display synchronous multidecadal temperature fluctuations that are coherent with one another.

Credit: Pages 2k consortium.

The top panel of the figure on the left shows the temperature reconstruction for the last 2000 years, indicating a general cooling trend prior to 1850 and the subsequent warming that continues today. The lower panel shows the 30 to 200 year bandpass-filtered ensemble, showing the multi-decadal variability, which is consistent across all the statistical methods.

Credit: Pages 2k consortium

What I found interesting about the paper is that they then did a detection and attribution analysis to try and explain the pre-industrial multidecadal variability. The figure on the right shows that a substantial portion of the variability can be explained as being due to changes in external forcings, in particular volcanic aerosol forcing. In addition, the residual can be used to estimate the magnitude of the unforced/internal variability. This is consistent with the variability in pre-industrial control simulations, which suggests that models are probably not under-estimating unforced/internal variability.

The paper also considers 51-year running trends and they find that a majority of the ensemble members show the largest 51-year trend occurring in the twentieth century, indicating that the modern warming trend is outside the range of pre-industrial variability.

So, this looks like an interesting paper that illustrates that pre-industrial variability is a combination of forced (mostly volcanoes), and unforced variability that has a magnitude that is consistent with pre-industrial control simulations. It also demonstrates that the modern warming period is warming at a rate that is outside the range of pre-industrial variability. None of this is really a surprise, but it does seem to be a nice way to constrain unforced variability and illustrate that what we’re undergoing today is certainly unusual.

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8 Responses to Constraining unforced variability

  1. Steven Mosher says:

    Nice

  2. izen says:

    Given that a specific size of volcanic forcing will cause a specific pattern of cooling-warming that this paper attempts to better constrain;
    Does this constrain climate sensitivity at either end of the range ?

  3. izen,
    I’m not actually sure. It used a past millenium runs from 23 CMIP5 climate models. So, I guess one conclusion might be that the variability is consistent with the sensitivity in these models, but I’m not sure if that is a reasonable conclusion to draw.

  4. BBD says:

    The figure on the right shows that a substantial portion of the variability can be explained as being due to changes in external forcings, in particular volcanic aerosol forcing. In addition, the residual can be used to estimate the magnitude of the unforced/internal variability. This is consistent with the variability in pre-industrial control simulations, which suggests that models are probably not under-estimating unforced/internal variability.

    As you say, not exactly a surprise, but a useful addition to the body of evidence suggesting that climate is not self-propelling, nor subject to magick forcings as yet unknown to science (cue woo music).

  5. Chubbs says:

    Yes, all the warming is man-made. The only way to slow it down is to reduce the forcing increase.

  6. These are just my observations on long-term variability.

    Interesting that one of the best calibrated proxies, that of coral rings, is unfortunately not as useful as the other proxies for creating a hockey stick. The Pacific coral proxy data is almost perfectly calibrated to the modern-day instrumental ENSO variation but since the equatorial Pacific shows very little temperature trend, all one sees is the variability.

    In contrast, tree ring proxies are not very well calibrated against instrumental temperature data (since they are sensitive to droughts, floods, etc) but since they are off the equator, they are more sensitive to the AGW trends.

    For grins, I would like to see a hockey stick graph just based on coral proxies. This paper is close but I don’t see the plot.

    http://shadow.eas.gatech.edu/~kcobb/jones09.pdf

    “To date, proxy climate records derived from corals have mostly been used to reconstruct a specific feature of the tropical climate system, in particular ENSO and, to a lesser extent, decadal–interdecadal variability over the last several centuries (eg, Fleitmann et al., 2007). Here, after
    summarizing the basis for extracting climate information from corals, we focus on the broader objective of using coral records for large-scale climate reconstruction.”

    The Fleitmann paper appears to be a measure of coral uptake of tracer elements from land sources, so that would be more like a tree ring measure.

  7. JCH says:

    The more the reconstruction resembles Mann’s original hockey stick’s handle, the more likely ECS will be lower than 3.a bit ℃.

  8. frankclimate says:

    JCH: indeed, the result of the paper should be seen in the context of this former blogpost: https://andthentheresphysics.wordpress.com/2019/07/28/climate-sensitivity-and-decadal-temperature-variability/

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