Most paleoclimate records suggest that during the last interglacial (so-called Eemian period, a mere 125 thousand years back) the
planet was substantially warmer than present (close to 2C in a global sense) despite the fact that GHG concentrations were within the historical range of variability of glacial-interglacial cycles (180-280 ppm). This fact has been used incessantly to make the claim that “CO2 was not important during previous warm periods”. Statements like these have been used to indirectly justifying ignoring the theory of anthropogenic climate change.
So, what gives? Can we explain why the planet was warmer 125 kya ago despite the fact that radiative forcing from GHG would have been substantially lower than today? Is the last interglacial a decent analog to present day climate? Let’s take a look at the evidence.
1. Continental plates were not significantly different from today, so those are largely ruled out.
2. Factors associated with aerosols and volcanic activity have very low confidence and are not suggested to have contributed.
3. We have fairly strong confidence and knowledge of orbital parameters and can state that phasing of the three Milankovitch moments were quite different during the last IG than present.
Let’s examine #3 in more detail. Results suggest that eccentricity (the shape of the orbit) was less circular (more eccentric) during the last IG (see the green line below) and the seasonality of inso
lation (through precession and obliquity) was significantly higher (red line). This allowed for summer insolation at high-latitudes (65N, black line) to be significantly greater than today leading to significant melt and negative mass balance of the cryosphere, invoking a positive ice-albedo feedback to drive warmer global temperatures. It is important to note here that these changes are not simply due to the amount of energy incident to the planet increasing or decreasing, but instead the means through which it is distributed latitudinally and seasonally (and invoking feedback processes).
Orbital parameters alone suggest that the globe should be going through a slow cooldown over the last 6kya in the absence of other forcing mechanisms. Why? Today perihelion (when the Earth is closest to the sun, hence largest insolation potential) occurs during in January, and vice-versa for aphelion. This effectively muddles the seasonality of insolation at northern latitudes (not as much insolation during summer, more in winter). As ice-sheets and land-ice is most likely to form in the NH due to the abundance of high-latitude landmass, the present parameters are not conducive for the retreat of cryospheric content in the NH.Flip back to 10kya when perihelion occurred during NH summer and you would find a time where conditions were more amenable for melt.
This brings us back to the present climate where we know that orbital parameters are not “stellar” for warming. Furthermore, orbital projections are not suggestive of conditions conducive to warming in the pipeline (see the right side of the graph below) over the next 1000yrs. So while GHG forcing may not have been THAT important 125kya ago; it just might be today.
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