by Judith Curry

“Arctic temperature anomalies in the 1930s were apparently as large as those in the 1990s and 2000s. There is still considerable discussion of the ultimate causes of the warm temperature anomalies that occurred in the Arctic in the 1920s and 1930s.” – IPCC AR5 Chapter 10

Readers of CE were introduced to the early 20th century Arctic warming in a post Mid 20th Century Global(?) Warming, with this figure from Polyakov et al 2003) shows a time series of surface land temperature in the Arctic from 1875-2000. The numbers above the abscissa show the number of stations used in the compilation. The graph shows a gradually rising surface temperature trend over the 105 years of the temperature record but the dominant feature is the nearly 2C (trough to ridge) warming from about 1920 to 1940, followed by a decline bottoming out in the mid-1960s. At that stage the temperature trends upward to almost the same level as the previous peak.

A subsequent post by Tony Brown entitled Historic variations in Arctic sea ice Part II: 1920-1950 provided anecdotal historical evidence of Arctic warmth during this period.

This early century Arctic warming has long fascinated me, and I have been collecting material for a new post on this topic.

Tamino

The particular impetus for posting on this topic at this time was a recent post by Tamino in response to my recent Senate testimony entitled (One of) the Problem(s) with Judith Curry. Tamino objects specifically to this text in my testimony:

Further, Arctic surface temperature anomalies in the 1930’s were as large as the recent temperature anomalies. Notwithstanding the simulations by climate models that reproduce the decline in Arctic sea ice, more convincing arguments regarding causes of sea ice variations requires understanding and ability to simulate sea ice variations in both hemispheres.

A key issue in understanding the recent decline in Arctic sea ice extent is to understand to what extent the decline is caused by anthropogenic warming versus natural climate variability.

Read Tamino’s post for his arguments. He concludes:

I think the IPCC goofed on this one — big-time — and if so, then Curry’s essential argument about Arctic sea ice is out the window.

I’ve studied the data. Not only does it fail to support the claim about 1930s Arctic temperatures, it actually contradicts that claim. By a wide margin. It ain’t even close.

What’s not hyperbole is how it looks to me: that Judith Curry cannot have studied the available data to draw that conclusion because the available data contradict it, that Judith Curry cannot have studied the supporting references because they don’t support it, and that if she believes it “because the IPCC report says so” then it’s obvious she’ll take the IPCC report’s word for what she wants to believe but not for what she doesn’t want to believe.

IPCC

Here is what the IPCC AR4 had to say about the early 20th century warming in the Arctic:

A slightly longer warm period [compared to the present], almost as warm as the present, was observed from the late 1920s to the early 1950s. Although data coverage was limited in the first half of the 20th century, the spatial pattern of the earlier warm period appears to have been different from that of the current warmth. In particular, the current warmth is partly linked to the Northern Annular Mode (NAM; see Section 3.6.4) and affects a broader region (Polyakov et al., 2003) (Chapter 3.2.2.4)

Here is what the IPCC AR5 has to say about the early 20th century warming in the Arctic in Chapter 10. The Executive Summary for Chapter 10 makes this statement:

It is likely that there has been an anthropogenic contribution to the very substantial Arctic warming over the past 50 years.

JC note: this is to be compared with ‘more than half’, ‘extremely likely’ for the overall attribution statement.

From the main text of Chapter 10 (JC bold):

Gillett et al. (2008b) detect anthropogenic influence on near-surface Arctic temperatures over land, with a consistent magnitude in simulations and observations. Wang et al. (2007) also find that observed Arctic warming is inconsistent with simulated internal variability. Both studies ascribe Arctic warmth in the 1930s and 1940s largely to internal variability. Shindell and Faluvegi (2009) infer a large contribution to both midcentury Arctic cooling and late century warming from aerosol forcing changes, with greenhouse gases the dominant driver of long-term warming, though they infer aerosol forcing changes from temperature changes using an inverse approach which may lead to some changes associated with internal variability being attributed to aerosol forcing. We therefore conclude that despite the uncertainties introduced by limited observational coverage, high internal variability, modelling uncertainties (Crook et al., 2011) and poorly understood local forcings, such as the effect of black carbon on snow, there is sufficiently strong evidence to conclude that it is likely that there has been an anthropogenic contribution to the very substantial warming in Arctic land surface temperatures over the past 50 years.

Arctic temperature anomalies in the 1930s were apparently as large as those in the 1990s and 2000s. There is still considerable discussion of the ultimate causes of the warm temperature anomalies that occurred in the Arctic in the 1920s and 1930s (Ahlmann, 1948; Veryard, 1963; Hegerl et al., 2007a; Hegerl et al., 2007b). The early 20th century warm period, while reflected in the hemispheric average air temperature record (Brohan et al., 2006), did not appear consistently in the mid-latitudes nor on the Pacific side of the Arctic (Johannessen et al., 2004; Wood and Overland, 2010). Polyakov et al. (2003) argued that the Arctic air temperature records reflected a natural cycle of about 50–80 years. However, many authors (Bengtsson et al., 2004; Grant et al., 2009; Wood and Overland, 2010; Brönnimann et al., 2012) instead link the 1930s temperatures to internal variability in the North Atlantic atmospheric and ocean circulation as a single episode that was sustained by ocean and sea ice processes in the Arctic and north Atlantic. The Arctic wide temperature increases in the last decade contrast with the episodic regional increases in the early 20th century, suggesting that it is unlikely that recent increases are due to the same primary climate process as the early 20th century.

Turning to model based attribution studies, Min et al. (2008b) compared the seasonal evolution of Arctic sea ice extent from observations with those simulated by multiple GCMs for 1953–2006. Comparing changes in both the amplitude and shape of the annual cycle of the sea ice extent reduces the chance of spurious detection due to coincidental agreement between the response to anthropogenic forcing and other factors, such as slow internal variability. They found that human influence on the sea ice extent changes has been robustly detected since the early 1990s. The anthropogenic signal is also detectable for individual months from May to December, suggesting that human influence, strongest in late summer, now also extends into colder seasons. Kay et al. (2011b), Jahn et al. (2012) and Schweiger et al. (2011) used the climate model (CCSM4) to investigate the influence of anthropogenic forcing on late 20th century and early 21st century Arctic sea ice extent and volume trends. On all timescales examined (2–50+ years), the most extreme negative extent trends observed in the late 20th century cannot be explained by modeled internal variability alone. Comparing trends from the CCSM4 ensemble to observed trends suggests that internal variability could account for approximately half of the observed 1979–2005 September Arctic sea ice extent loss.

JC note: In my essay for Climate Dialogue on the Arctic sea ice decline, I argued that the decline was 50% (+/- 20%) anthropogenic. Mine was the ‘skeptic’ position as opposed to the spiral of death position; looks like the skeptic position is mainstream IPCC!

So, who are the Arctic-knowledgable authors that contributed to AR5 Chapter 10? Names that I spotted on the lead author list are: Igor Mokhov, James Overland.

For context, the statements in my Senate testimony were specifically with regards to statements made by the IPCC AR5 and how they represented a ‘pullback’ relative to the IPCC AR4. The did not represent my own independent assessment. However, I don’t disagree with anything the IPCC AR5 wrote on this topic.

Arctic surface temperature record

Back to the topic that has Tamino in a tizzy: he says the recent temperatures are much warmer than the temperatures circa 1930, and that the IPCC is wrong (and I am wrong because I quoted the IPCC). In the literature, there are a number of different time series plots of Arctic surface temperatures:

(full manuscripts for all but Yamanouchi are available online at the links)

Update: new references added

A 2009 study of warming in Greenland by Jason Box et al found that ” The annual whole ice sheet 1919–32 warming trend is 33% greater in magnitude than the 1994–2007 warming.” Link

Some of the plots show the recent temperatures to be comparable to the earlier temperatures; others show current temperatures to be much warmer. The discrepancies to occur owing to the spatial variability of the trends. In particular there is a strong latitudinal trend with the warmest temperature anomalies circa 1930 occurring at latitudes higher than 70N (Bekrayev et al., Yamanouchi) and also in the Atlantic sector (Overland and Wood).

So back to the IPCC AR5 statement:

“Arctic temperature anomalies in the 1930s were apparently as large as those in the 1990s and 2000s”

Well, average temperatures above 70 N during the 1990’s were lower than in the 1930’s. The most recent Arctic temperatures that are published (say for 2005-2010) are higher. The IPCC’s statement is not incorrect, and the citation of the IPCC statement in my Senate testimony has been defended.

JC conclusion

This issue further highlights the importance of doing a better job on the climatology of Arctic surface temperatures; not just in the early part of the 20th century, but there are substantial discrepancies in various climatologies even of the last decade. Two recent posts addressed this issue: