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Climate change and sensitivity: not all Watts are equal

Posted on 11 March 2014 by John Abraham

We hear a lot of talk these days about climate sensitivity. It is often considered the most important measure for predicting how much the Earth's temperature will increase as we emit heat-trapping gases like carbon dioxide.

But, the term sensitivity has to be used carefully because it can mean different things in different contexts. For instance, there is a long-term (equilibrium) sensitivity to doubling carbon dioxide which refers to the ultimate temperature reached by the planet if we were to double carbon dioxide. There are also shorter term (transient) sensitivities which relate to temperature changes as heat trapping gases increase at some specified rate.

Values for climate sensitivity in general can be obtained many ways. My favorite way is by looking at deep history. If we can measure how sensitive the climate was in the past, perhaps we can infer its sensitivity now. A second way is through the use of modern temperature records and recent greenhouse gas levels. A third way is through the use of climate models (computer programs that replicate the Earth climate system). Regardless of the method used, there is general agreement that if we were to double carbon dioxide, the Earth's surface temperature would eventually increase by 1.5–4.5°C (2.7–8.1°F). Obviously, if the Earth sensitivity is at the upper end of the range, we are in trouble.

Recently, there have been some studies which suggest that maybe the climate sensitivity is at the lower end of this range. Most of these studies have only used the second method to calculate sensitivity, a fact that will soon become important. In addition to real science studies, there have been policy organizations that have promoted these low-sensitivity results. But my question is, what does the science say? Fortunately, a paper just published in Nature Climate Change provides some guidance on this question. The study was completed by Dr. Drew Shindell from NASA, and what he found was exciting. It turns out, not all Watts are equal. Energy changes to the Earth system from changes of sun-reflecting particulates or from ozone have a different impact than energy changes from carbon dioxide.

The Earth has a greater sensitivity to particulates and ozone than to carbon. The reason for this seemingly strange behavior is that aerosols are largely located near industrialized areas in the Northern Hemisphere. This hemisphere also happens to contain much more land area than the south – and land regions are more sensitive to changes in energy, at least in the near term. In short, particulates and ozone impact more sensitive parts of the planet. Carbon dioxide, on the other hand, spreads out uniformly across the globe – it doesn't accumulate in one hemisphere or another.

However, let's not get too excited about particulates saving us from global warming. Dr. Shindell also showed that while in the short run, the cooling effect from particulates matters a lot, in the long run, it doesn't make much of a difference. The impact can be seen in this figure which shows the prior expectations of the climate (dashed line) alongside the revised prediction (solid). By 2050, there really is little difference.

 Temperature responses for uniform heating (dashed) and new non-uniform heating (solid).

Temperature responses for uniform heating (dashed) and new non-uniform heating (solid).

What does this have to do with climate sensitivity?

Click here to read the rest

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Comments 1 to 15:

  1. This seems to be the crucial part (from "below the fold"):


    " Multiple lines of evidence are now consistent showing that the climate sensitivity is very unlikely to be at the low end of the range. The consequences of climate change are thus likely to be towards the more damaging end of the estimates, unless we take action to quickly reduce our emissions."

    OT question: Has anyone else had trouble getting onto RealClimate recently? Are they shut down for maintanance, or under cyber attack?

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  2. What uniform heating? i am struggling with the units here. The y axis is fine. Uniform heating to me is a constant input of Watts (dQ/dt =k (J/S)) which into an adiabatic system would prosuce a linear rise. Is this uniform heating d2Q/dt2 =k? A few  ODEs might help me understand this a bit better - obviously the models are very complex but a simple state spaace model or equivalent simple ARMA proccess would help my small brain.

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  3. @wili

    have not been on RealClimate for a week or two. I have tried now without success; the site appears to be down. I'm sure they will be a prime target so hopefull it is just maintanence

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  4. Re. RealClimate: not sure what's going on but when I try to go to I immediately get a "Forbidden - You don't have permission to access / on this server."

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  5. I came here to ask the same question. Bummer - hope they're OK.

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  6. They've been down all day and it's been noted on Tamino's site as well....aurgh...

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  7. StBarnabas: In this context, uniform heating means uniform between the northern and southern hemisphere, and says nothing about time variation of the heating. Non-uniform heating involves the same total global heating, but more of it occuring in the southern hemisphere where it has less impact on temperatures.

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  8. Realclimate was down for me last night. It's back online today.

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  9. Kevin C @7

    many thanks. That makes total sense.

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  10. Just a quick addition to John's summary; Shindell's paper also noted an interesting thing, which is that any reductions in aerosols in the future would also have a warming effect, but while the RCPs do model aerosol trajectories, most of the attempts to minimise potential TCR or ECP do not (another function of the cherry picking involved, I guess).

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  11. RealClimate is up and kicking...

    Recent article by gavin contains an apparent typo: "It never rains but it pause" (my emphasais) but gavin explained it to be an intentional pun. Can you guess the pun? Hint: think about rhotic british-like pronounciation (well represented by gavin himself) and what's happening in UK right now....

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  12. Wili wrote in 1:

    OT question: Has anyone else had trouble getting onto RealClimate recently? Are they shut down for maintanance, or under cyber attack?

    Earlier this week I had trouble but was able to get through shortly after that.  My assumption was a DDoS attack by someone incompetant.  If it were down due to maintanence I would expect it to remain down for a bit rather than be available only half a minute or so later.  So DDoS seemed a safer bet.

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  13. BaerbelW wrote in 4:

    Re. RealClimate: not sure what's going on but when I try to go to I immediately get a "Forbidden - You don't have permission to access / on this server."

    That sounds like a shutdown in response to attack, possibly due to an intrusion, similar to the Climategate upload.  The details are more likely to be discussed in a less public forum.

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  14. Kevin C writes in 7:

    StBarnabas: In this context, uniform heating means uniform between the northern and southern hemisphere,... Non-uniform heating involves the same total global heating, but more of it occuring in the southern hemisphere where it has less impact on temperatures.

    Personally, I would still expect the northern hemisphere to warm more quickly than the south, both for the 20th century and for the period of modern global warming, from 1975 to present. There is more land than water in the northern hemisphere, and given that land has less thermal inertia, it has been able to warm more quickly. Then with the atmosphere as the intermediary, I would expect the land to drag the northern oceans along with it.

    In any case, at least for the two hemispheres as a whole, the northern hemisphere warmed more quickly than the southern but was was more sensative to aerosols mid-century, with some cooling from 1940 to 1980, whereas there was only a single year of statistically significant cooling in the southern hemisphere.

    Tamino states:

    The cooling effect of man-made sulfates also helps explain the hemispheric asymmetry in temperature history. Most industrial activity is in the northern hemisphere, so most of the anthropogenic sulfate cooling should be there too. The northern hemisphere has warmed faster than the southern because there’s more land in the north than the south, and land has far less thermal inertia than ocean. But if sulfates are mostly in the northern hemisphere, that means that there should have been a stronger mid-century cooling effect in the north than in the south — and that’s exactly what we observe:

    Anthropogenic Global Cooling
    August 23, 2010

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  15. It should be noted that the same discussion about which hemisphere is experiencing more ocean warming and the role of aerosols in this is taking place in the comment section of Global warming not slowing - it's speeding up beginning with comment 23 by Hyperactive Hydrologist:

    Is ocean heat content increasing faster in the southern hemisphere? If so would this add weight to the idea that the northern hemisphere is experiencing increased dampening due to aerosols?

    and continuing as far as 27-28 by Tom Curtis where he states regarding a graph by Bob Tisdale:

    ... the SH is gaining heat 13 times faster than the NH per unit area, which would definitely seem to support Hyperactive Hydrologist's suggestion..   I suspect, however, that there are other major factors involved....   Tisdales graph only plots heat gain since mid-2005.  That means he only plots it over a period in which the SOI has shown a distinct, and very strong trend towards record high levels (ie, from record high El Ninos to record low La Ninas). The tongue of water that is modulated by ENSO lies, primarilly, just south of the equator. Large changes in ENSO, therefore, may well have significantly different effects in either hemisphere, so that may be another major factor in the difference.

    Regardless, it is an interesting question.

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