Mid Pliocene Warm Period

Is the warm period in the Mid Pliocene analogous to today’s warming climate? If so can we use it to infer future temperature rise? This is what Wikipedia says:

The mid Pliocene warm period is considered a potential analog of future climate. The intensity of the sunlight reaching the earth, the global geography, and carbon dioxide concentrations were similar to present. Furthermore, many mid-Pliocene species are extant helping paleotemperature proxies calibration. Model simulations of mid-Pliocene climate produce warmer conditions at middle and high latitudes, as much as 10–20 °C warmer than today above 70°N. They also indicate little temperature variation in the tropics. Model-based biomes are generally consistent with Pliocene paleobotanical data indicating a northward shift of the Tundra and Taiga and an expansion of Savanna and warm-temperate forest in Africa and Australia.

Carbon dioxide

Carbon dioxide concentration during the mid Pliocene has been estimated at around 400 ppmv from 13C/12C ratio in organic marine matter[12] and stomatal density of fossilized leaves,[13] decreasing carbon dioxide levels during late Pliocene may have contributed substantially to global cooling and the onset of northern hemisphere glaciation.[10]

Interestingly though the subject is only mentioned once in the IPCC AR5 report concerning Sea Levels (5.6.1), which is surprising if the above statements are true.   Here is their Benthic Foram proxy data on which the warm period is based.

As you can see what they define as the warm period is centred  at 3.2 million years ago. So was the sunlight intensity really the same as today? I have used the Laskar astronomic ephemeris to calculate seasonal insolation at 65N back to 5 million years ago. This is what I get.

Daily insolation at 65N for different months over 6 millions.

There is an exact match to current orbital conditions 2.8 million years ago but not 3.2 million years ago. The Mid Pliocene ‘warm’ period corresponds to an era of high eccentricity more like that during the Eemian which was warmer than the holocene despite slightly lower CO2 levels.

Therefore I do not think the claim that the Mid Pliocene warm period tells us very about future warming  is valid. I also think it is hard to justify that it was really that much warmer than earlier times either. Here is my plot of the LRO4 stack.

LRO4 Benthic Foram stack plotted in forward time.

I doubt whether you could claim a clear Mid Pliocene warm signal here. This may be why it doesn’t feature in AR5.

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6 Responses to Mid Pliocene Warm Period

  1. Kevin O'Neill says:

    Why 65N? Don’t you need to make an argument why that’s THE important latitude or that it is representative of the planet as a whole?

    There’s also this:

    “In summary, the integrated solar insolation and the annual mean insolation are dominated by obliquity cycle, whereas the mean daily insolation is dominated by precession cycle except for the winter insolation in high latitudes which has abundant obliquity cycles. Therefore, it is the integrated solar insolation or annual mean insolation rather than monthly mean insolation that takes the dominant role in controlling climate change ”

    Astronomically modulated late Pliocene equatorial Pacific climate transition and Northern Hemisphere ice sheet expansion

    • Clive Best says:

      Only because I had it already calculated beforehand for a different reason. The point is that orbital parameters for the Mid Pliocene ‘Warm’ period (if it really existed) with CO2~400ppm had a different distribution of solar energy than today.

      The paper you quote from is not directly about this period but rather the slow cooling from 5m y ago and YES CO2 acted as a positive feedback to cooling in this case.

  2. paulski0 says:

    I suspect “intensity of sunlight” refers to change in the Sun’s total output rather than to insolation distribution on Earth. Over hundreds of millions of years change in Solar luminosity is significant for climate but not so much over a few million years.

    The fact that model simulations show relative high levels of warmth in the Arctic but little change at the Tropics should make it clear that insolation distribution is considered to be quite different at that time.

    • Clive Best says:

      Actually the annual sunshine can change with eccentricity. It is a small effect – 1-3% Eccentricity follows a 100,000y cycle modulated by a 420k cycle. However, I think the distribution of insolation with latitude makes a difference.

  3. Kevin O'Neill says:

    paulskio writes: “The fact that model simulations show relative high levels of warmth in the Arctic but little change at the Tropics should make it clear that insolation distribution is considered to be quite different at that time.”

    If you mean quite different in the arctic, I’m not confident this is really true. Transport from lower latitudes to higher latitudes is quite significant in today’s arctic and likely was then as well. So any global increase in insolation will affect the arctic through transport. It doesn’t require direct increase of insolation at higher latitudes – though direct increases will obviously have an effect as well.

  4. Ron Clutz says:

    Clive, someone who knows the paleo field needs to look at James Hansen’s new paper.

    http://www.earth-syst-dynam.net/8/577/2017/

    He claims it is warmer now than anytime in the Holocene and well into Eemian temps.

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