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Paper: Today’s Climate Still Impacted By Medieval Warm Period, Other Climate Surprises

pacific ocean sunAt Die kalte Sonne site here, Prof. Fritz Vahrenholt and Frank Bosse published an analysis of Gebbie et al 2019. What follows is the translation in English.


Climate surprises

A paper very worth reading from the USA from January 2019 in Science (Geoffrey Gebbie of the Woods Hole Oceanographic Institution/Peter Huybers of Harvard University, hereinafter GH19) is titled “The Little Ice Age and 20th-century deep Pacific cooling”.

It shows fascinating science.

The authors evaluated temperature measurements made in the deep sea by the famous expedition of the “HMS Challenger” in the 1870s.

The ship sailed the Atlantic and Pacific, and probably provided the first data on the oceans down to depths of over 2000 meters.

The recalibration of the old data alone is a work of art! What the paper found: The Pacific down in depths has cooled from 1870 to today, the Atlantic not.

With a model of the global waters down to such depths, the authors got to the bottom of the cause and concluded: The circulation of the deep sea means that the Pacific depths today are still influenced by the Medieval warm period (MWP, about 950-1250 AD) and the transition to the Little Ice Age (about 1500-1800 AD).

The warmed up water from 1,000 years ago needs that long a time until it arrives at depths of 3,000 meters in the Pacific!

This implies two things: The MWP was a globally large-scale event, as we have also demonstrated in the MWP project (not represented in climate models in this way and so it is an “unknown factor” to the IPCC) and it is still at work today.

The GH19 paper is evidence.

The temperature development in the Pacific from the paper:

pacific temp depths

Fig. 3: The temperature development in the Pacific to depths of 5,500 meters. The MWP at the surface warmed the water until 1300 AD and the subsequent cooling of the “Little Ice Age” (LIA) later had a cooling effect on the Pacific Ocean. After 1750, the ocean was still “warmed up” and cooling continues even until today. Source: Supplements Fig. S5 from GH19.

Today’s climate still impacted by the Medieval Warm Period

This is a wonderful example that our climate was NOT in equilibrium around 1750 like all climate models assume. We are still feeling the effects of MWP today.

If we now assume an equilibrium in the ocean around 1750 for the determination of the sensitivity “ECS“, i.e., the long-term (several centuries) temperature increase with a doubling of the CO2 concentration.

And if we trace the warming back to anthropogenic impulses to this day, then we indeed neglect that there was still residual heat (completely unknown for models as already mentioned) in the 1750s.

CO2 effect on ocean warming overstated

The growth in the total heat content of the oceans to this day is, therefore, smaller than models assume and this leads to a lower sensitivity to anthropogenic [human-caused] effects.

Nic Lewis also stated this in this commentary on the work by noting a significant reduction in the size of climate sensitivity from the findings of GH19, even if one follows the IPCC guidelines in the following calculations.

IPCC report needs fundamental revising

Should the findings of Gebbie and Huybers be confirmed, then the IPCC report needs to be fundamentally revised, especially for long-term temperature forecasts.

But this also applies to the warming to be expected by 2100, as reflected in the TCR (transient climate response) estimates when the CO2 concentration doubles.

Reader F.B. posted a comment on Judith Curry’s blog for discussion at the beginning of January 2019. In it, he tried to derive the natural variability 1950-2016 from observations and NOT from models.

Taking into account all assumptions made by the IPCC, what ultimately results in a climate sensitivity (TCR) of about 1.3 °C/doubling the carbon dioxide content in our atmosphere.

If one adds the known influences of volcanoes, the Pacific El Nino/La Nina cycles (ENSO for short) and the solar influences (also assumed by the IPCC to be exactly the same) and takes into account the internal variability since 1950, one can only reconstruct the annual temperature trend only with this magnitude:

global temps since 1950

Fig. 4: Reconstructed global temperature since 1950 (dark blue) and the real observed (green).

The agreement is astounding and it was not created by the excessive fudging and tuning of many parameters, as is done in many climate models. Even the order in which the parameters are “tuned” plays an important role in the result.

A sensitivity of only 1.3°C

The good reconstruction results if a TCR of 1.3 is used, unlike the models for the IPCC’s 5th Assessment Report, which do it by using 1.85. The TCR of 1.3 is the most important parameter for the results.

That is 30% less warming than models see. This is also the result of Lewis/Curry (2018), which we reported here.

An additional result of this IPCC-conforming approach is that (almost) all longer-term warming, in the example shown since 1950, is assumed to be caused by “anthropogenic forcing”, essentially CO2.

Longer-term studies also show this: if a similar approach is chosen and the time spans from 1870 onwards are investigated, then it inevitably follows that all tendential warming comes from anthropogenic drives, in this case since 1870.

All such methods contain this requirement.

Any long-term natural warming source is excluded by assuming our climate was in “equilibrium” in the 1750s, i.e., that it was not driven by long-term drivers and that only (short-term in the sense of decades) internal variability, volcanoes and ENSO had an impact on the climate, with the exception of even more long-term changes caused by the Earth’s orbit (ice ages, interglacial ice ages).

Climate sensitivity “likely to be lower”

Against the background of the paper by Gebbie and Huybers presented above, it is however highly questionable that influences acting on longer timescales can be neglected from the outset by the IPCC approach.

In plain language, this means that the above-mentioned estimate of sensitivity is rather an upper limit and, considering the ocean warmth was not in climate equilibrium in the 1750s, it is likely to be lower.

Focusing on observations leads to surprises

We keep hearing, again and again, that science can hardly find anything new on anthropogenic climate change. “The science is settled.” But don’t fall for it!

GH19 is a shining example of true science when it comes to climate. We look forward to more news and observations.

If you don’t concentrate on models, but rather work on empirics, then we are always in for a surprise!

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