Doug McNeall tweeted a link to this somewhat enigmatic essay yesterday. It’s by Wally Broecker, the so-called “Father of Global Warming”, an epithet which he earned by virtue of his fairly accurate prediction of global temperature rise (predicted to be mainly due to rising CO2 emissions) in 1975.

It’s pretty obvious that Wally Broecker does not nowadays care much for the accolade, if he ever did:

It’s also very obvious, from the main subject of his essay, that he considers his prediction of global warming for the 40 odd years hence from 1975 to have been correct – but for the wrong reasons.

Given that those reasons were:

1. A cessation of the natural cooling influence of the Dansgaard cycle
2. The commencement of dominant CO2 GHG warming

We are left to ponder the possibility that Wally Broecker is not quite as certain as he once was that GHG warming is mainly responsible for the rapid temperature rise after the late 1970s.

I can’t imagine that this revelation, this scientific epiphany, has come to Broecker very suddenly. He’s probably known for years that his scientific reasoning re. natural variability at least, was faulty. But he has waited until now, when establishment climate science is going through a particularly rough period, to write this rather enigmatic essay which hints that all might not be quite as it seems with respect to natural climate variability and hence, by logical deduction, the attribution of modern climate change mainly to anthropogenic GHG warming.

Broecker recently became aware of the fact that Dansgaard’s Arctic oxygen isotope record – which he used as a proxy for global temperature in 1975 – was in fact far more closely related to the phase and amplitude of the North Atlantic Oscillation. This in itself is rather interesting because the NAO, during winter, governs the severity of northern European winters especially. Take a look at the standardized winter NAO index since 1950:

We can see the deep trough which gave us the very severe winters of the 1960s and early 70s, the partial recovery to 1975/76, the dip into the late 70s/early 80s, followed by the pronounced rise to the mid 90s, coincident with the rapid phase of global warming. Thereafter, the winter NAO index began to decline again, resulting in the severe winter of 2009/10 and the exceptionally cold December of 2010. Remarkably, after 2010, it did a very sharp about turn and went very positive in 2014/15/16, which explains the extremely wet, very warm winters we experienced during those years. So, although NAO is not a very accurate proxy for global temperature, there is a link there somewhere, and it is probably via a teleconnection to ENSO.

And speaking of ENSO, Broecker, identifying 1976-77 as a significant turning point in global climate, indicates that at the time (coincident with an El Nino event) the equatorial Pacific underwent a marked change whereby upwelling weakened considerably, resulting in significant warming of the California current plus other effects in South America and elsewhere. Broecker also notes that that the PDO underwent a sharp phase change from “unusually large negative” to positive at the time. All of these events signify what is known as the Pacific Ocean Climate Shift of 1976/77. It is obvious that a major and abrupt shift in global atmospheric/oceanic circulation took place at that time, the physics of which still eludes scientists today.

Broecker speculates that “perhaps in the new circulation regimen [initiated during 1976/77], the ocean absorbed less of the heat generated by the atmosphere’s growing CO₂ content”. If so, then naturally we would expect global mean surface temperature to rise considerably as heat stayed in the upper layers of the Pacific during this period of decreased upwelling. It is equally plausible of course that incident solar UV (perhaps boosted by clearer tropical skies) also warmed the ocean surface considerably more during this period – energy which was not mixed into deeper layers, thus leading to accelerated surface warming. Whatever the case, natural variability almost certainly has played a part in the rapid warming post 1976. Broecker speculates that the same mechanism, in reverse, may be responsible for the 21st century pause:

Which, if we are brutally honest, is a version of the ‘ocean ate my warming’ explanation for the Pause. But I do wonder if it is more than that, if Wally Broecker is saying not that the Pacific is responsible for moderating and enhancing global warming due only to humans, but that it is responsible of itself for moderating natural global warming via the absorption and release of incident short wave solar radiation, hence bringing into question the attribution of all global warming post 1950 to CO2 emissions. I could be misinterpreting his essay of course, but it is rather enigmatic. It’s also worth noting that Broecker based his calculations in 1975 on a transient climate response of 2.4°C, which is considerably higher than many current estimates, so the amount of warming which he predicted due mainly to GHGs, may need to be augmented by unaccounted for natural forcings (internal/external).

Finally, a rash of new science papers recently released tends to confirm the increasing recognition by scientists of the role played by natural variability in climate change.