Last month we had a look at the Bigger, better, new hockey stick from a study by Marcott et al. Since then Skeptical Science has a post on the phoney skeptical/denialist critique of the study. We now have a new study, Continental-scale temperature variability during the past two millennia, by 78 scientists from 24 nations analyzing climate data from tree rings, pollen, cave formations, ice cores, lake and ocean sediments, and historical records from around the world published in the journal Nature Geoscience. The study found:
There were no globally synchronous multi-decadal warm or cold intervals that define a worldwide Medieval Warm Period or Little Ice Age, but all reconstructions show generally cold conditions between ad 1580 and 1880, punctuated in some regions by warm decades during the eighteenth century.
The real interest was in the regional variations on a continental scale. Temperatures were identified for the Arctic, North America, South America, Europe, Asia, Australasia and the Antarctic. There was insufficient data for Africa.
This is from the summary at Science Daily (thanks to John D for sending me the link):
The evolution of temperature across all the continents was noticeably more similar within the hemispheres than between the Northern and Southern Hemisphere. “Distinctive periods, such as the Medieval Warm Period or the Little Ice Age stand out, but do not show a globally uniform pattern,” says professor Heinz Wanner.
By around 1500 AD temperatures did indeed fall below the long-term mean everywhere. However, in the Arctic, Europe and Asia this temperature drop occurred several decades earlier than in North America and the Southern Hemisphere. These new findings will certainly stimulate vibrant discussions within the research community, Wanner believes.
Long-term cooling trend reversed
The most consistent feature across the regions over the last 2000 years was a long-term cooling trend, which was likely caused by a combination of factors such as an overall increase in volcanic activity, a decrease in solar irradiance, changes in land cover, and slow changes in earth’s orbit. This cooling only came to an end toward the end of the 19th century.
The warming during the last century has reversed this long-term cooling, the study found. It remained cold only in Antarctica. An analysis of the average temperatures over 30-year periods indicates that interval from 1971-2000 was probably warmer than any other 30-year period in the last 1400 years.
Cooler 30-year periods between the years 830 and 1910 AD were particularly pronounced during weak solar activity and strong tropical volcanic eruptions. Both phenomena often occurred simultaneously and led to a drop in the average temperature during five distinct 30- to 90-year intervals between 1251 and 1820.
In most cases, they tell us, “the data used were highly resolved, attesting to short-term variations over decades or less, rather than smoothing over centuries.”
Of course, the study was identifying atmospheric temperature, which accounts for only 2.3% of the earth’s energy balance. Here is an image from Skeptical Science showing where the energy from global warming goes:
With around 93% of the heat going into the ocean we should take an interest in what’s happening there. Nutticelli et al did a study last year updating the story as represented in this image:
The ocean has an average depth of some 3700 metres but I gather that by measuring to 2000 metres most of the expected heat from radiative forcing can be accounted for. Yet another study, this one by Balmaseda, Källén and Trenberth found that 30% of the warming has occurred below 700 meters, which is unusually high. They think this is because of changed wind patterns associated with the Pacific Decadal Oscillation (PDO) which is only temporary. The PDO changes on inter-decadal time scale, usually about 20 to 30 years. The PDO is linked with changes in the El Niño–Southern Oscillation (ENSO), which is thought to be involved in changes of the energy balance between upper and lower ocean layers, and between ocean and atmosphere. There is more going on than PDO and ENSO, however, so there is plenty of scope to improve our understanding of the mechanisms at at work.
Skeptical Science has published two graphs which they call The Escalator. I’ve frozen them here so you can compare. This is how skeptics view global warming:
This is how realists view global warming:
But really, both are correct. The second simply represents the longer trend. In recent times more heat has been distributed lower into the oceans, which must happen to reach a new equilibrium. Guess whose turn it will be next?
The first thing to bear in mind is that a trend line is a model. A warming trend is not a theory of how climate changes. If a complex, non-linear system fails to follow a trend, look at the model to see whether it represents the theory sufficiently well.
In a nutshell, the theory says greenhouse gases act like a blanket, trapping heat near the surface. This creates a radiation imbalance at the top of the atmosphere. The earth system warms to return this balance by increasing the heat escaping from the top of the atmosphere so that energy out equals energy in. This is a slow process, taking centuries, because the ocean has to warm sufficiently to support a hotter atmosphere. The scientific confidence in this aspect of climatology is extremely high. A simple trend line is sufficient to measure this process.
But on decadal time scales, the trend-line model fails. Most of the heat trapped in the earth system goes into the oceans. The top 700m of ocean increased in heat content from 3 x 1022 Joules in 1997 to 10 x 1022 Joules in 2010, in a highly non-linear manner, due to mixing rates between the surface and deep ocean. The atmosphere holds as much heat as the top 3m of ocean, about 0.4% of the heat content above. Why on earth then, with highly non-linear processes in the ocean, would we expect a gradual warming trend in the atmosphere?
If anything the non-linear step changes accord more closely with human experience. In geological time you get something like this:
All that happened with a fluctuation of CO2 concentration between 180 and 300 ppm. With 450 to 550 ppm we bring into play a range of sea level change of 200 metres from the Last Glacial Maximum to an ice-free planet. That’s roughly 20 metres on average for each degree Celsius. That relationship is starkly represented by this graph from Archer 2006 (see Roger Jones again):