Paleoclimate: The End of the Holocene

marcott_fig_2

Figure 2 Changes in incoming solar radiation as a function of latitude in December, January and annual average, due to the astronomical Milankovitch cycles (known as orbital forcing). Source: Marcott et al., 2013.

In the bottom panel we see the sunlight averaged over the year, as it depends on time and latitude. It declined strongly in the mid to high latitudes over the Holocene, but increased slightly in the tropics. In the Marcott reconstruction the global temperature curve is dominated primarily by the large temperature changes in northern latitudes (30-90 °N). For this, the middle panel is particularly relevant: the summer maximum of the incoming radiation. That reduces massively during the Holocene – by more than 30 watts per square meter. (For comparison: the anthropogenic carbon dioxide in the atmosphere produces a radiative forcing of about 2 watts per square meter – albeit globally and throughout the year.) The climate system is particularly sensitive to this summer insolation, because it is amplified by the snow- and ice-albedo feedback. That is why in the Milanković theory summer insolation is the determining factor for the ice age cycles – the strong radiation maximum at the beginning of the Holocene is the reason why the ice masses of the last Ice Age disappeared.

However a puzzle remains: climate models don’t seem to get this cooling trend over the last 5,000 years. Maybe they are underestimating the feedbacks that amplify the northern orbital forcing shown in Fig. 2. Or maybe the proxy data do not properly represent the annual mean temperature but have a summer bias – as Fig. 2 shows, it is in summer that the solar radiation has declined so strongly since the mid-Holocene. As Gavin has just explained very nicely: a model-data mismatch is an opportunity to learn something new, but it takes work to find out what it is.

Comparison with the PAGES 2k reconstruction

The data used by Marcott et al. are different from those of the PAGES 2k project (which used land data only) mainly in that they come to 80% from deep-sea sediments. Sediments reach further back in time (far further than just through the Holocene – but that’s another story). Unlike tree-ring data, which are mainly suitable for the last two thousand years and rarely reach further. However, the sediment data have poorer time resolution and do not extend right up to the present, because the surface of the sediment is disturbed when the sediment core is taken. The methods of temperature reconstruction are very different from those used with the land data. For example, in sediment data the concentration of oxygen isotopes or the ratio of magnesium to calcium in the calcite shells of microscopic plankton are used, both of which show a good correlation with the water temperature. Thus each sediment core can be individually calibrated to obtain a temperature time series for each location.

Overall, the new Marcott reconstruction is largely independent of, and nicely complementary to, the PAGES 2k reconstruction: ocean instead of land, completely different methodology. Therefore, a comparison between the two is interesting:

Marcott_PAGES2k

Figure 3 The last two thousand years from Figure 1, in comparison to the PAGES 2k reconstruction (green), which was recently described here in detail. Graph: Klaus Bitterman.

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