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 in a new commentary in Nature Climate Change by Fyfe et al. contradicts these claims.

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. Figure 1 shows rolling 15-, 30- and 50-year trends computed for different surface and satellite global temperature datasets. 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.

This simple comparison shows the cited statement from Karl et al. to be erroneous, but is this period ‘particularly unusual’, to use Lewandowsky et al.’s words?

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.

Figure 1 also compares the observed trends with the CMIP5 simulations (grey shading). Note that CMIP5 also shows a recent drop in the expected rate of change for 15-year trends – the earlier peak is because of accelerated trends starting just after the 1991 eruption of Mt. Pinatubo, e.g. 1992-2006.

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. Figure 2 shows the ratio of the change in temperature and the change in anthropogenic radiative forcing for three periods. 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. As further evidence, model simulations which produce a fluctuation in their Pacific variability similar to that observed (either by chance or by design – see Fig. 1e) also better reproduce the observed fluctuations in global temperatures.

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.

Point of clarification: Although I am a co-author of the Fyfe et al. paper, I disagree with the sentence at the start of the ‘Claims and counterclaims’ section of the paper. My views are represented here.