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Volcanoes may be responsible for most of the global surface warming slowdown

Posted on 3 December 2014 by dana1981

A new study has found that when particulates from small volcanic eruptions are properly accounted for, volcanoes may be responsible for much of the slowdown in global surface warming over the past 15 years.

Sulfur aerosol particulates pumped into the atmosphere from volcanic eruptions cause short-term cooling by blocking sunlight. Until recently, climate scientists thought that only large volcanic eruptions had a significant impact on global temperatures. There haven’t been any big eruptions since Mount Pinatubo in 1991. However, studies published over the past few years have found that even moderate volcanic eruptions can pump significant amounts of aerosol particulates into the atmosphere.

Virtually all research into the climate influence of volcanic aerosols has used satellite measurements of particulates in the upper atmosphere (the stratosphere). These satellite measurements only monitor the volcanic aerosol at heights of 15 km and above. The new paper by David Ridley and colleagues studied the amount of volcanic aerosols in portions of the stratosphere that lie below 15 km.

To do this, the researchers combined data from satellites, ground-based instruments in the AERONET program, and from instruments on weather balloons. The study was co-authored by 17 climate scientists, including some leading experts in aerosol research.

By combining all of these measurements, the scientists found that there is also a significant amount of volcanic aerosol in portions of the stratosphere below 15 km They concluded that for recent eruptions, between 30 and 70% of the overall amount of volcanic aerosol in the stratosphere has come from below 15 km. Since the year 2000, the study estimates that volcanoes have had a cooling influence on global surface temperatures. The likely range of this volcanic cooling influence lies between 0.05 and 0.12°C.

As the authors of the paper note, this cooling influence is not taken into account in the climate model simulations incorporated into the latest IPCC report,

The climate model simulations evaluated in the IPCC fifth assessment report [Stocker et al., 2013] generally assumed zero stratospheric aerosol after about 2000, and hence neglect any cooling effect of recent volcanoes

Although the global surface temperature data have been within the range of model simulations, they’ve been towards the lower end of those model runs over the past 10–15 years.

IPCC AR5 Figure 1.4. Solid lines and squares represent measured average global surface temperature changes by NASA (blue), NOAA (yellow), and the UK Hadley Centre (green). The colored shading shows the projected range of surface warming in the IPCC First Assessment Report (FAR; yellow), Second (SAR; green), Third (TAR; blue), and Fourth (AR4; red).

IPCC AR5 Figure 1.4. Solid lines and squares represent measured average global surface temperature changes by NASA (blue), NOAA (yellow), and the UK Hadley Centre (green). The colored shading shows the projected range of surface warming in the IPCC First Assessment Report (FAR; yellow), Second (SAR; green), Third (TAR; blue), and Fourth (AR4; red).

The measured surface warming has been about 0.13°C less than the average of model simulations since 2000. The estimated volcanic cooling from this new paper (0.05–0.12°C), not included in those climate models, could account for most of that discrepancy.


Combine this with the approximately 0.06°C surface cooling due to more heat being stored in the deep oceans, and the slowdown is both fully accounted for and temporary.

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Comments 1 to 14:

  1. Maybe someone can help me understand the part about sulfur aerosol particulates blocking sunlight. Are these particulates highly reflective in any of the more abundant solar wavelenths? 

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  2. If there is a volcanism related 'slowdown' over the period of 1999-2014 , that requires those years to have been unusually volcanically active compared the previous 30, which included Pinutabo and St Helens.

    Is that demonstrated in the paper? It doesn't give enough detail in the abstract.

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  3. Tristan... Try the second paper that Dana links to.

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  4. Ahh, thanks Rob :)

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  5. MThompson: iirc, aerosols have a cooling effect because they scatter light. 

    Scattering by atmospheric molecules explains some of the blueness of the sky. Scattering is stronger when the wavelength is shorter, and blue light has a shorter wavelength so it's scattered more strongly. Instead of passing through the sky above our heads, more of it is scattered down and we see it.

    Visible light has a shorter wavelength than infrared light, so it is much more strongly scattered by aerosols. This means that they bounce back more light from the Sun than they do from the Earth, so they have a net cooling effect.

    I think this is a reasonable summary of what's going on. There's more detail to it as well: in reality it's related to the size parameter rather than the wavelength alone, aerosols have other effects (like helping to form bigger particles), while black carbon and soot helps to absorb more light so has a warming effect. There is also the 'phase function', which determines the direction of scattering... but all of this is accounted for and the result is that volcanic aerosols cool the surface.

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  6. Can someone explain in a quick summary why the big swings in projected ranges from FAR to SAR, and then TAR?  Also, I am assuming that the 2 deg on the y-axis is the 2 degree value we always hear discussed as the point we want to avoid exceeding.  Is there a delay effect such that even if we stopped emissions completely today that we might still reach the 2 degree point?

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  7. 1) "I am assuming that the 2 deg on the y-axis is the 2 degree value we always hear discussed as the point we want to avoid exceeding"

    No. The chart set its "0 degree" mark at the average for the last half of the 20th century, specifically 1961 - 1990. Whereas the 2 degree C limit dicussed in treaties is from a "0 degree" mark from the 1800s, iirc.


    2) "Is there a delay effect such that even if we stopped emissions completely today that we might still reach the 2 degree point?"

    Yes...well, probably. But in any case, we are not going to 100% stop emissions this year. So it's a moot point. Continued emissions at anything like todays levels for the next few years makes it pretty much impossible to stay below 2 degrees, as I understand it.

    If you have a few moments, look at this from one of the top climatologists in the UK: Kevin Anderson 'Rhetoric to Reality'

    If you have more than a few minutes, try this:

    Real clothes for the Emperor: Facing the challenges of climate change

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  8. MarkR @5, you have the essentials correct except that the blueness of the sky (and redness of sunsets) is due to Rayleigh Scattering, something that requires particles sizes about a tenth or less of the wavelength of light, it 38 to 75 billionth of a meter.  Sulfate aerosols typically have sizes from 100 billionth to 5 millionth of a meter, and hence are scattered primarilly by Mie Scattering, which does not discriminate by wavelength.  Because aerosol sizes have a size distribution, a small proportion of aerosols are small enough for Rayleigh scattering and will contribute to reder sunsets.   Further, the wavelength of thermal radiation from the Earth is sufficiently large that scattering by aerosols is restricted to Rayleigh scattering, and is hence less efficient as you note.  For a mathematical treatment, these lecture notes may be useful.

    In addition to the scattering effect (the aerosol direct effect), aerosols form condensation nuclei for clouds, resulting in the aerosol indirect effect.  The more condensation nuclei, the more water droplets in a given cloud, and the smaller the droplet size.  That in turn results in a higher albedo in the cloud.  This is seen in the phenomon of ship tracks, where clouds are thicker and more reflective over the paths of ships (as seen below):

    For reference, here is one of the early papers on the effects of aerosols.

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  9. rkrolph @6, it is an often neglected subtlety of climate change that the duration it takes to reach the full equilibrium response to a change in forcing is approximately equal to the duration required for the CO2 partial pressures of the atmosphere and ocean to equalize.  That means, in the 100 plus years it will take to from current temperatues to the approximately 2 C Equilibrium Climate Response to 400 ppmv of CO2, the CO2 concentration of the atmosphere would have reduced to 340 ppmv.  That means with the immediate cessation of all emissions, we are very unlikely to reach the 2 C limit.  This only applies with the cessation of all emissions.  Even continuing emissions at 10% of the current rate will sustain CO2 levels at a near constant level making it near 50/50 that we will exceed the 2 C limit at some time in the next century.

    Unfortunately the IPCC tends to focus only on events in this century, and consider another 150 ppmv of CO2 in the atmosphere to be the limit compatible with having a 50/50 chance of avoiding the 2 C limit as of 2080-2100.  As a matter of practical policy that is probably correct, but it does commit us to future sequestrtion of CO2 from the atmosphere.

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  10. Tom @8: there is a hidden wavelength dependence in Mie scattering. In the lecture notes you give, through the size parameter. This is labelled as 'x' in Eq. 9.36, and is (for a spherical particle) the diameter divided by the wavelength.

    The scattering efficiency has a wavelength-squared depndence, even in the Mie regime. The phase function is also dependent on size parameter, with larger particles showing more forward scattering. And the scattering cross section also increases with particle size, assuming no major changes in the scattering efficiency.

    So I'm pretty sure that aerosol scattering still has some wavelength dependence. It's weaker than Rayleigh scattering, but it is there. 

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  11. I meant to say circumference divided by diameter...

    Anyway, Tom's comments provide more detail as to why volcanic aerosols tend to have a cooling effect.

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  12. MarkR @11, perhaps circumference divided by wavelength?  But other than that, yes you are correct.

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  13. MarkR@11

    "circumference divided by diameter"

    I'm afraid that's just Pi in the sky, Mark.


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  14. So this would mean that some of the "missing heat" never actually made it to earth? 

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