by Judith Curry

It has been claimed that the early-2000s global warming slowdown or hiatus, characterized by a reduced rate of global surface warming, has been overstated, lacks sound scientific basis, or is unsupported by observations. The evidence presented here contradicts these claims. – Fyfe et al.

A very important new paper has been published in Nature: Making sense of the early 2000’s warming slowdown [link to full paper]. Unfortunately the paper itself is behind paywall. The list of coauthors on this paper is particularly significant:

John C. Fyfe, Gerald A. Meehl, Matthew H. England, Michael E. Mann, Benjamin D. Santer, Gregory M. Flato, Ed Hawkins, Nathan P. Gillett, Shang-Ping Xie, Yu Kosaka and Neil C. Swart

Ed Hawkins’ blog post

Co author Ed Hawkins has a blog post on the paper, that includes the 3 figures from the paper. Excerpts:

The new Fyfe et al. paper is mainly in response to Karl et al. and Lewandowsky et al., who made the following statements in their abstracts:

“These results do not support the notion of a ‘slowdown’ in the increase of global surface temperature” – Karl et al., 2015, Science

“there is no evidence that identifies the recent period as unique or particularly unusual” – Lewandowsky et al., 2016, BAMS

Firstly, climate scientists agree that global warming has not ‘stopped’ – global surface temperatures and ocean heat content have continued to increase, sea levels are still rising, and the planet is retaining ~0.5 days of the sun’s incoming energy per year.

I think there is also broad agreement that climate scientists have probably not chosen the right words (e.g. ‘hiatus’) to describe the temporary slowdown, especially when talking to the media and the public.

However, there has very clearly been a change in the rate of global surface warming. There are clear fluctuations in the rate of global temperature change in the past. We also expect similar fluctuations in future – global temperatures will not increase smoothly or linearly.

Just focusing on the observations, the most recent observed 15-year trends are all positive, but lower than most previous similar trends in the past few decades. This is a clear demonstration that the rate of change has slowed since its peak.

The absolute value of the trend is not really relevant for such an assessment – it is far more instructive to examine how global temperatures have changed relative to our expectations, as represented by the CMIP5 simulations, for example.

Observations should fall outside the simulated spread sporadically because of internal variability – we do not expect the observations to always match the ensemble mean. However, the recent observations are all continuously outside the ±1σ spread of the simulations for a lengthy period, which is obviously unusual. It is also not just global temperatures that have been unusual – the tropical Pacific sea surface temperatures & winds have also behaved well outside the simulated range.

These analyses all suggest that the early-2000s were indeed ‘particularly unusual’ – so we strongly dispute Lewandowsky et al.’s statement quoted above.

Reality has deviated from our expectations – it is perfectly normal (& indeed essential) to try and understand this difference. Oddly, Lewandowsky et al. seem to disagree, suggesting that trying to explain this event “departs from long-standing practice“, which I think is utterly bizarre and simply wrong.

Note that there are important issues with the radiative forcings used in CMIP5 (particularly solar & volcanic), which do not necessarily match the real world, especially after 2005. I hope that at least some CMIP5 models will be rerun with the updated CMIP6 forcings to determine the size of this effect. In addition, when an ‘apples-to-apples’ comparison is performed, the consistency between observations and simulations is much improved. This type of research has been valuable and is ongoing.

Finally, the issue of natural variability merits further discussion. The 1972-2001 period shows higher ratios (more warming per unit forcing) than the other periods. This period also corresponds to when the Pacific Decadal Oscillation was in its positive phase, suggesting that these variations in the Pacific have caused a large part of the difference between models and observations.

Overall, there is compelling evidence that there has been a temporary slowdown in observed global surface warming, especially when examined relative to our expectations, which can be explained by a combination of factors. Research into the nature and causes of this event has triggered improved understanding of observational biases, radiative forcing and internal variability. This has led to more widespread recognition that modulation by internal variability is large enough to produce a significantly reduced rate of surface temperature increase for a decade or even more — particularly if internal variability is augmented by the externally driven cooling caused by a succession of volcanic eruptions.

The legacy of this new understanding will certainly outlive the recent warming slowdown.

Nature News

Nature News has an article by Jeff Tollefsen entitled Global warming hiatus debate flares up again. The article is notable for quotations from several climate scientists:

Susan Solomon: Susan Solomon, a climatologist at the Massachusetts Institute of Technology in Cambridge, says that Fyfe’s framework helps to put twenty-first-century trends into perspective, and clearly indicates that the rate of warming slowed down at a time when greenhouse-gas emissions were rising dramatically. “It’s important to explain that,” Solomon says. “As scientists, we are curious about every bump and wiggle in that curve.”

Tom Karl: For his part, Karl acknowledges that it is important to investigate how short-term effects might impact decadal trends, but says that these short term trends do not necessarily elucidate the long-term effects of rising greenhouse-gas concentrations in the atmosphere. “What gets obfuscated is the goal of uncovering the warming due to persistent greenhouse forcing [by human emissions],” Karl says. “It is simply not possible to gain insight on that underlying trend from short, segmented 10- to 20-year periods.”

Gavin Schmidt: Gavin Schmidt, director of NASA’s Goddard Institute for Space Studies in New York, is tired of the entire discussion, which he says comes down to definitions and academic bickering. There is no evidence for a change in the long-term warming trend, he says, and there are always a host of reasons why a short-term trend might diverge — and why the climate models might not capture that divergence. “A little bit of turf-protecting and self-promotion I think is the most parsimonious explanation,” Schmidt says. “Not that there’s anything wrong with that.”

JC reflections

Well, regular readers of Climate Etc. will not be surprised by the results of this paper:

The significance of this paper is1) the list of authors (many if not most whom are IPCC authors) and 2) the lucid explanation and context that is provided for the overall debate on this topic. The ‘hiatus’, ‘pause’ or whatever has been a dominant topic in both the scientific and the public debate on climate change since about 2012.

I don’t disagree with anything in this paper or in Ed’s blog post. A few specific comments:

I agree that the preferred term is ‘slowdown’; this gets us away from details of the trends and disagreement among different datasets. I also agree that the main significance is the discrepancy between climate model simulations.

I am a little surprised that they chose 1972 as the demarcation between the large hiatus during mid century and the late 20th century warming period. I would have chosen 1976, associated with the shift to the warm PDO/IPO.

I think the issue of multi-decadal variability is more complicated than is portrayed by the paper, which focuses on PDO/IPO.

The topic that didn’t receive sufficient attention in the paper IMO was solar, particularly the possibility of indirect solar effects.

Overall, the authors are to be congratulated for a very well done and important paper, that given the author list arguably redefines the consensus on this topic.

As for Gavin’s comment, it is totally bizarre. Gavin’s recent papers on the topic claim to explain the slowdown by forced variability. He is wrong; multi-decadal ocean oscillations play an important role. Calling this definitions and academic bickering is beyond bizarre.

JC message to Lewandowski et al.: (snip)