Climate scientists cannot explain why early 20th century warming occurred, so why should anyone trust them on more recent warming?


A careful look at the early 20th century global warming, which is almost as large as the warming since 1950. Until we can explain the early 20th century warming, I have little confidence IPCC and NCA4 attribution statements regarding the cause of the recent warming.

This is an issue that has long interested me. Peter Webster wrote a previous post Mid 20th Century Global(?) Warming, which focused on the warm bump that culminated in the 1940’s. My interest in this period was reignited while working on my report Sea Level and Climate Change. Then, the recent paper by Zanna et al. discussed in Ocean Heat Content Surprises further made the wheels turn.

In response to the Ocean Heat Content thread, David Appell posted a link to this paper on Twitter:

The early 20th century warming: Anomalies, causes and consequences

Gabi Hegerl, Stefan Bronniman, Andrew Shurer, Tim Cowan

Abstract: “The most pronounced warming in the historical global climate record prior to the recent warming occurred over the first half of the 20th century and is known as the Early Twentieth Century Warming (ETCW). Understanding this period and the subsequent slowdown of warming is key to disentangling the relationship between decadal variability and the response to human influences in the present and future climate. This review discusses the observed changes during the ETCW and hypotheses for the underlying causes and mechanisms. Attribution studies estimate that about a half (40–54%; p > .8) of the global warming from 1901 to 1950 was forced by a combination of increasing greenhouse gases and natural forcing, offset to some extent by aerosols. Natural variability also made a large contribution, particularly to regional anomalies like the Arctic warming in the 1920s and 1930s. The ETCW period also encompassed exceptional events, several of which are touched upon: Indian monsoon failures during the turn of the century, the “Dust Bowl” droughts and extreme heat waves in North America in the 1930s, the World War II period drought in Australia between 1937 and 1945; and the European droughts and heat waves of the late 1940s and early 1950s. Understanding the mechanisms involved in these events, and their links to large scale forcing is an important test for our understanding of modern climate change and for predicting impacts of future change.”

HELLOOOOOO!

This paper ‘shocked’ me for several reasons. First, I can’t imagine how I missed this paper when it was first published in Oct 2017 – apparently it received no publicity (oops now I remember, this was when i messed up my neck/shoulder/hand). Second, every time in the context of an attribution argument that I say ‘but the early 20th century global warming (not to mention the mid century cooling) and all those heat waves and droughts,’ I (along with my argument) am dismissed. I will paraphrase something I recall Gavin Schmidt saying: “We understand the late 20th century warming and have good forcing data, so no point to paying attention to the early warming where the data is far inferior.” And last but not least, the AMO and PDO are explicitly considered in Hegerl et al.’s attribution argument.

The Hegerl et al paper is actually pretty good (as far as it goes). Lets take a closer look at their analysis and argument, then I will take it a bit further.

Hegerl et al. provides a summary of forcing from CO2, volcanoes and solar (Figure 4, below). In 1910, the atmospheric CO2 concentration has been estimated to be 300.1 ppm; in 1950 it was 311.3 ppm; and in 2018 it is 408 ppm. So, the warming during the period 1910-1945 was associated with a CO2 increase of 10 ppm, whereas a comparable amount of warming during the period 1950 to 2018 was associated with a 97 ppm increase in atmospheric CO2concentration – almost an order of magnitude greater CO2 increase for a comparable amount of global ocean warming. Back when CO2 concentrations were lower, each molecule had a greater radiative impact – but not THAT much.
Read more: https://judithcurry.com/2019/01/23/e...lobal-warming/