by Judith Curry

Two new papers were published last week of relevance to the hiatus.

For background, see my invited presentation to the American Physical Society on this topic: Causes and implications of the pause.

First, lets take a look at the ‘spin’ surrounding this issue and these papers:

The titles pretty much speak for the articles: The inference is that hiatus has now been explained; and that it should end soon with a warming ‘burst.’

And now for the spin-free zone. Lets take a look at these two papers and see what we can actually infer and and learn from them.

Quantifying the likelihood of a continued hiatus in global warming

C. D. Roberts, M. D. Palmer, D. McNeall & M. Collins

Abstract. Since the end of the twentieth century, global mean surface temperature has not risen as rapidly as predicted by global climate models (GCMs). This discrepancy has become known as the global warming ‘hiatus’ and a variety of mechanisms have been proposed to explain the observed slowdown in warming. Focusing on internally generated variability, we use pre-industrial control simulations from an observationally constrained ensemble of GCMs and a statistical approach to evaluate the expected frequency and characteristics of variability-driven hiatus periods and their likelihood of future continuation. Given an expected forced warming trend of ~0.2 K per decade, our constrained ensemble of GCMs implies that the probability of a variability-driven 10-year hiatus is ~10%, but less than 1% for a 20-year hiatus. Although the absolute probability of a 20-year hiatus is small, the probability that an existing 15-year hiatus will continue another five years is much higher (up to 25%). Therefore, given the recognized contribution of internal climate variability to the reduced rate of global warming during the past 15 years, we should not be surprised if the current hiatus continues until the end of the decade. Following the termination of a variability-driven hiatus, we also show that there is an increased likelihood of accelerated global warming associated with release of heat from the sub-surface ocean and a reversal of the phase of decadal variability in the Pacific Ocean.

Published in Nature [link].

The paper is further explained by

Co-author Doub McNeall’s blog post University of Exeter press release

JC comment: I think this paper is a useful contribution, and I have no concerns/questions about the methodology they used. The interesting result is that once you have a 15 year hiatus (something that is already pretty unlikely), then according to climate models, the probability of it continuing another 5 years is 25%.

Atlantic and Pacific multidecadal oscillations and Northern Hemisphere temperatures

Byron A. Steinman, Michael E. Mann, Sonya K. Miller

Abstract. The recent slowdown in global warming has brought into question the reliability of climate model projections of future temperature change and has led to a vigorous debate over whether this slowdown is the result of naturally occurring, internal variability or forcing external to Earth’s climate system. To address these issues, we applied a semi-empirical approach that combines climate observations and model simulations to estimate Atlantic- and Pacific-based internal multidecadal variability (termed “AMO” and “PMO,” respectively). Using this method, the AMO and PMO are found to explain a large proportion of internal variability in Northern Hemisphere mean temperatures. Competition between a modest positive peak in the AMO and a substantially negative-trending PMO are seen to produce a slowdown or “false pause” in warming of the past decade.

Published in Science [link].

The paper is explained by:

JC question to Michael Mann: How can the pause be both ‘false’ and caused by something?

Bob Tisdale critiques this paper in two posts at WUWT:

Punchline: The Steinman et al. (2015) analyses do not explain the slowdown and stoppage of surface warming in the Southern Hemisphere…and, as noted in the earlier post, they had to redefine multidecadal temperature variability to try to explain it in the Northern Hemisphere.

JC comment: I agree with Bob Tisdale’s comments on this paper. I do not regard the Steinman et al. paper to be a useful contribution owing to methodological problems in ‘reinventing’ the AMO and PDO indices; this issue was discussed in these previous posts:

Interestingly, they create a new index the NMO, which is the average of their AMO and PDO; sort of like a ‘poor man’s stadium wave.

JC reflections

I found the Palmer et al. paper to be interesting, in that given a 15 year hiatus, that the models predict a 25% chance of 5 more years. My take on how the pause will play out is summarized in the stadium wave post: I expect that the natural variability will contribute to a continuation of the hiatus into the 2030’s, with solar and volcanoes being a wild card.

With regards to the Steinman and Mann paper, they state in the Penn State press release:

Using a wide variety of climate simulations, the researchers found that the AMO and PMO are not significantly correlated; they are not part of the global “stadium wave” oscillation, as some researchers had claimed.

Their arguments are based on the assumption that model simulations provide ‘correct’ estimates of both the forced signal and the correct level of intrinsic multidecadal variability. The Kravtsov et al. paper shows that this assumption is not justified.

Brilliant to use climate models that don’t correctly simulate internal variability, particularly the PDO, to claim that the AMO and PDO aren’t correlated and not part of a stadium wave oscillation. /sarc The stadium wave paper showed that it is the lag that gives rise to maximum correlation between the AMO and PDO. Steinman and Mann conclude that the linear detrending is likely to cause artificially high observed (lagged) correlations between the AMO and PDO in the stadium wave.

So where does all this leave us? Internal variability only partly explains the pause – how big that part is remains somewhat unclear, but there is growing evidence that it is the major cause of the pause. The critical issues are how long will the pause last, inability of climate models to simulate a pause beyond 20 yrs with increasing CO2, and what will happen once the internal variability flips to a warming situation.

Is there a looming ‘warming spurt’ at the ‘end’ of the pause? Maybe, but it depends on what is going on with the sun. Many solar scientists expect a coming solar cooling. How increasing CO2, solar cooling and internal variability will play out in terms of influencing the global climate over the next several decades is of central importance to our understanding of the climate system, which still has substantial uncertainties (no matter what the IPCC, EPA etc say).

Bottom line: The pause in global warming is NOT finally explained.