Uncharted Territory

January 18, 2010

Musings of the Hemispheres

Filed under: Global climate trends, Global warming, Science — Tim Joslin @ 12:58 pm

I should really try to finish one blog post before I start another on a similar topic. My last (published) post noted that the North Atlantic Oscillation (NAO), an atmospheric phenomenon, is not a climate driver, rather it’s a measure of the state of the climate – incidentally, I’m pleased to discover this morning that Philip Eden at Weatheronline.co.uk holds similar views. In a post that may or may not ever appear, I was going to note similar thoughts about the so-called Atlantic Multidecadal Oscillation or AMO.

It’s not my understanding of the oceanic circulation that great surges of current drive the climate. Rather the oceanic circulation is itself driven by changes in heat distribution at the surface. OK, there may be timelags and of course there’s the El Nino, but that’s about it. If the El Nino is driven by ocean currents (which I believe it is), these are, crucially, east-west, not north-south. The planet loses heat partly because heat moves (in water and air) from the tropics towards the poles where it is more easily radiated away (or used to melt ice). I suggest, therefore, that changes in oceanic circulation are primarily caused by changes in the absorption of heat at the surface. For example, if the planet is warming, you’d expect a general strengthening of oceanic circulation.

What’s piqued my renewed interest in this topic is James Hansen’s release of temperature data for 2009, available over at Realclimate. In particular, Hansen includes separate temperature graphs for the southern and northern hemispheres. I reproduce these here for convenience (the diagrams in my previous post were from Wikipedia, btw):

Fig 1: Annual temperatures

Fig 2: Running mean temperatures

Looking at the right hand graphs, comparing temperature changes in the hemispheres, we see that sometimes the northern hemisphere warms quicker than the southern hemisphere, whereas at other times the reverse is true. What would we expect, though? Well, there’s a lot more water in the southern hemisphere and a lot less land. We’d therefore expect the south to warm (and cool) slower than than the north (and, in the long-term, catch up when temperatures stabilise at a different level). And, indeed, this is what appears to happen most of the time: since the mid 1970s, the northern hemisphere has warmed much faster than the south; on the other hand, the cooling (clearest in Fig 1) caused by the Mt Pinatubo eruption in 1991 was most evident in the northern hemisphere.

But – there’s always a “but” – for significant periods of time (I consider the weather can affect annual data, but not decadal trends) – for example, from around 1950 to the mid 1970s – the southern hemisphere has actually warmed when the northern hemisphere has cooled. This requires explanation.

There are only two possibilities: either the one I’ve already dismissed, that large amounts of heat are, by some unexplained causal mechanism, transferred between the hemispheres, or, that there are factors causing the hemispheres to gain different amounts of heat at different times. Specifically, for several decades from around 1950, the southern hemisphere must have either gained heat, whilst the northern hemisphere lost it, or, more probably, two countervailing factors were involved: one causing a general warming and the other cooling, but disproportionately of the northern hemisphere.

We know that increased levels of greenhouse gases are tending to warm the planet. The inescapable conclusion is that another factor tends to cool the northern hemisphere more than the southern hemisphere. The argument that from the 1940s through to the 1970s this was “global dimming“, caused by sulphur dioxide and other pollution is highly persuasive. Most of this pollution is emitted in the northern hemisphere and doesn’t stay in the atmosphere long enough to spread evenly.

What’s happening now, though?

Well, what strikes me in Hansen’s graphs is the levelling off of warming over the last few years. There’s not yet really enough data to reach any sort of conclusion, but Hansen notes 2009 was the second warmest year on record. In fact, though, his data (Fig 1), suggest it was the warmest year in the southern hemisphere and around the 7th warmest in the north.

Given the rapid industrial development of China and India, it seems justifiable to hold a working hypothesis that we face renewed global dimming.

You would expect a layer (or layers) of reflective particles in the atmosphere to reflect a greater proportion of light from the sun in the winter than in the summer, so another way to test the hypothesis would be to examine seasonal rates of warming over the past century or so. The trouble is, seasonal temperatures are very much affected by poleward heat transport and weather patterns themselves dependent on whether the planet (or hemisphere) is warming or cooling, but nevertheless I’d expect average temperatures in continental interiors (with stable seasonal weather patterns and especially anticyclonic conditions in both winter and summer) at high latitudes to fall more in winter than in summer during periods when global dimming increases, i.e. when the rate of warming of the planet as a whole slows down.

Of course, we could also put a bit more effort into simply measuring the strength of sunlight at the top of the atmosphere (to account for variations in solar output) and at ground level in cloudless conditions (or controlling for cloud cover).

If the analysis that the climate is being affected by renewed global dimming is correct, it’s really bad news. What it implies is that, when the presently industrialising nations reduce their sulphur emissions (and assuming other countries don’t repeat the exercise), we could be in for another period of rapid warming (several tenths of a degree C per decade on average in the northern hemisphere), similar to that over the last quarter of the 20th century.

14/6/10: Minor correction, AMO stands for Atlantic Multidecadal Oscillation, not “Meridional”.



  1. Hi my friend Stephen brought your post to my attention. It’s an interesting area of climate. I thought it might be useful to you to consider something I checked on the GISS website. If you compare the period 1975-1988 (when the NH temperatures “undershot” the SH temperatures on the first graph (b)) with the previous 30 yrs, it’s notable that the divergence between the hemispheres appears to be largely explained by the ocean SSTA’s rather than the land, which doesn’t appear particularly supportive of the idea of aerosols playing a major role.

    Comment by Chris — January 18, 2010 @ 2:13 pm

    • Well I’m rubbish, I misread the graph when i came up with 1975-1988……
      So here’s the same again but for 1966-1988 vs 1936-1965

      Comment by Chris — January 18, 2010 @ 2:23 pm

  2. Hmm, hours of fun to be had with NASA’s database!

    I want to compare the period 1950-74 when the SH was warming faster than the NH with other periods. Seems best to me to go for an earlier baseline, e.g. 1915-40 when the planet as a whole was cooler, but the NH was warming faster than the SH. If I do this, I can attribute the NH cooling to land rather than sea:

    Annual mean temps 1950-74 relative to 1915-40

    So, even though it was on average warmer during the putative global dimming period, 1950 to 1974, than the baseline 1915 to 1940 (OK, OK, I should have done 1939 for an exact quarter-century), certain land areas were in fact cooler.

    All very speculative, I know. But.

    I’m slightly surprised to see coldspots in SH land masses too. I’m starting to wonder if a point I nearly put in my original post could in fact be the case: perhaps the average surface temperature of land is directly affected more by global dimming than is that of the oceans. To think out loud, for the purposes of determining surface temperature, the ocean surface holds much more heat than land areas, so the rate at which it can be radiated away (reduced by GHGs) may be more important in determining the average temperature than on land where the rate at which sunlight warms the land may be critical. Further, perhaps the anomalies arise because we measure surface temperature. Perhaps the atmosphere over land still gains heat even during global dimming episodes – it’s just the surface that is colder.

    Another possible factor is that warmer seas can provide moisture which cools the land surface by allowing surface heat to be lost (to higher in the atmosphere, at least initially) through evaporation (and melting for that matter) and the albedo to be increased if it falls as snow.

    That’s enough speculation for now!

    Whatever the precise physics I have to say a period of renewed global dimming – if we are entering into one – would be politically difficult, to say the least. Especially if we don’t really understand what’s going on.

    Comment by Tim Joslin — January 18, 2010 @ 5:55 pm

  3. Just to let you know my last link was STILL wrong, as it defaulted back to Dec rather than annual when i changed the years…..
    Anyway, glad to see you’ve been having some fun with the database too 🙂

    Comment by Chris — January 19, 2010 @ 10:47 am

    • And here’s the final (hopefully?) link:

      Comment by Chris — January 19, 2010 @ 10:47 am

      • Chris, Your result is a bit of a mixed picture, but clearly shows NH as colder than the SH. The trouble is, I disagree with the dates you’ve chosen. We don’t know if the baseline of Hansen’s graph is correct (I actually thought of adjusting it), so can only definitely attribute global dimming to the period when the NH is cooling relative to the SH. We simply don’t know when the NH has been coolER than the SH – this is not a meaningful question. I chose 1950-74 as I can see that over that period the NH was definitely coolING relative to the SH. Your 1966-88, though, includes some of the global dimming period and some of the correction period with the hemisphere mean temperature lines crossing at the start and end of the period.

        I’m not sure how much we can read into these graphs. Maybe all mine supports is the idea in my previous post that coolING affects seasonal weather patterns.

        That said, I’m having great difficulty in getting my head round the idea that the oceans can warm at the same time as the land is cooling. Maybe the water vapour feedback is stronger over the seas than over land and outweighs global dimming; maybe it’s simply that sulphur particles are removed more effectively over the seas; maybe it’s something to do with cloud formation – perhaps there’s no shortage of nuclei over the seas, but there normally is over land…

        Btw, the Mt Pinatubo anomaly:

        Mt Pinatubo temperature anomaly (1992-3 vs whole of 1990s)

        appears to have a quite different signature to that for global dimming. Maybe the difference is that volcanoes inject their dimming particles much higher in the atmosphere. Or maybe the Pinatubo result is too distorted by a couple of years’ weather. (I did El Chichon – 1983-4 vs the 1980s – as well, but that appears to have been affected by an El Nino).

        Tricky stuff. I’m going to the Royal Society’s geo-engineering session later today. I’ll be very interested to hear whether they seriously propose meddling with the climate by inducing global dimming.

        Comment by Tim Joslin — January 19, 2010 @ 11:28 am

  4. FYI. I went to a talk on aerosols yesterday by Jonannes Quaas ( http://www.mpimet.mpg.de/~quaas.johannes ). It was a really excellent talk.
    His talk was about observing / evaluating model results of aerosol effects from sattelite data.

    There’s three effects
    (direct (clear sky effect -scattering); first indirect effect (more cloud droplet nucleation – more clouds); second indirect effect (more cloud. nuclearion – smaller droplets in clouds, less rain, longer longevity of clouds)

    The direct effect is the largest, followed by the 1st indirect effect.

    Overall, he provided convincing evidence from two methodologies that the total effect on radiative forcing from aerosols is about 1.15+/-0.4 Wm-2

    I also asked him about trends from aerosols; most of the published work shows a decline since mid 80s, and not much sign (yet) of the explosive growth in China.

    Here are the references for the trends:

    Mishchenko et al., Sci 2007:

    Li et al., Ann Geo 2009:

    Richter et al., Nature 2005:

    Hope this is useful.

    Comment by Stephen Stretton — January 19, 2010 @ 6:27 pm

  5. I simply do not know the answer to the question. I will ask the author.

    Comment by Stephen Stretton — January 21, 2010 @ 5:16 pm

  6. […] might also be pertinent to point out that 1995 was quite a warm year (see the graphs in my post when the 2009 data appeared a while […]

    Pingback by Harrabin’s Hamfisted Interview « Uncharted Territory — February 17, 2010 @ 6:01 pm

  7. […] – could affect the temperature of the entire Northern Hemisphere (NH). [See previous post Musings of the Hemispheres – there may be similar processes in the SH, but I'm not going to discuss those just […]

    Pingback by Spin Snow, not Sea Ice: the AMO is Real! « Uncharted Territory — February 23, 2010 @ 1:55 pm

  8. ((FYI)
    >> Reply from author of presentation.

    Dear Stephen,
    Since we base the estimate on scaled climate model simulations, the black carbon effect is partially included, to the extent to which the models simulate it. I believe none is looking at the effect of snow-albedo decrease, but the warming effect by absorption in the atmosphere is included, as well as scattering effects and interactions with clouds.
    I don’t think there is a sound basis for estimating such a very large positive forcing by black carbon (1 Wm-2). But of course this is still ongoing research, and within the AEROCOM initiative, there might soon be new assessments on this.

    Comment by Stephen Stretton — February 24, 2010 @ 5:19 pm

  9. […] playing field, since the eruption of Pinatubo in 1991 cooled the whole planet for a few years (see the graphs from James Hansen that I posted in 2010), making 2011 more freakish, since there hasn’t been a recent eruption. But, in the CET […]

    Pingback by How unusual was the cool UK summer of 2011? « Uncharted Territory — September 3, 2011 @ 7:05 pm

  10. […] (as measured by the mean surface temperature) appears to have slowed over the past decade or so. I suggested a while back that, in view of the rapid industrialisation of China in particular, perhaps renewed global dimming […]

    Pingback by The Earth is a Fridge « Uncharted Territory — September 7, 2011 @ 9:33 am

  11. […] I noted in a comment on a previous post that El Chichon was followed by a strong El Nino. As can be seen from the graph I gave at the time, […]

    Pingback by Interestingly, Volcanoes can Trigger El Ninos « Uncharted Territory — September 23, 2011 @ 3:04 pm

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