Lots of climate researchers and climate change activists have been discombobulated by the fact that global average temperature increases have been considerably slower during the first years of this century than most climate models projected. There have been scores of studies that have tried to explain away this inconvenient fact. One of the more heralded studies was published by researchers associated with the National Oceanic and Atmospheric Administration (NOAA) in June, 2015. That study eliminated the hiatus by controversially adjusting ocean temperature data derived from robot buoys to match earlier data temperature data taken by ocean-going ships.

Now a group of climate researchers in Nature Climate Change have published an article, "Making sense of the early-2000s warming," that argues the hiatus is real and not well understood. Interestingly, it includes as co-authors some of the more prominent climate researchers who have challenged the notion of the "pause." For example, last June, Pennsylvania State University climatologist Michael Mann crowed:

Mann is now a co-author on the new study that pulls that stake out:

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.

Has Mann become climate change "denier" now? Hardly.

About the new study, Nature News reports:

The latest salvo in an ongoing row over global-warming trends claims that warming has indeed slowed down this century. An apparent slowing in the rise of global temperatures at the beginning of the twenty-first century, which is not explained by climate models, was referred to as a "hiatus" or a "pause" when first observed several years ago. Climate-change sceptics have used this as evidence that global warming has stopped. But in June last year, a study in Science claimed that the hiatus was just an artefact which vanishes when biases in temperature data are corrected. Now a prominent group of researchers is countering that claim, arguing in Nature Climate Change that even after correcting these biases the slowdown was real. "There is this mismatch between what the climate models are producing and what the observations are showing," says lead author John Fyfe, a climate modeller at the Canadian Centre for Climate Modelling and Analysis in Victoria, British Columbia. "We can't ignore it." Fyfe uses the term "slowdown" rather than "hiatus" and stresses that it does not in any way undermine global-warming theory.

A graph comparing climate model projections, a.k.a., Coupled Model Intercomparison Project Phase 5 (CIMP5) with satellite data from Remote Sensing Systems and the University of Alabama in Huntsville is most illuminating. See below.

Overlapping trend in the temperature of the lower troposphere (TLT), spatially averaged over the near-global (82.5°N, 70°S) coverage of two satellite-based datasets; model results are from 41 simulations of historical climate change performed with 28 CMIP-5 models, with RCP8.5 extensions from 2005. Peaks in the running 15-year trends centred around 2000 reflect recovery from the Pinatubo eruption in 1991.

Another co-author, Ed Hawkins, who is at the National Centre for Atmospheric Science at the University of Reading, notes:

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.

Indeed. But if the rate of temperature increase continues to remain low, at what point do the models and projections of catastrophic warming get called into question by mainstream researchers?