2016 was only 0.02 degrees Celsius warmer than 1998 — a difference that is not statistically significant

See Scientists react to Earth’s warmest year: ‘We are heading into a new unknown’. Excerpts:

"Reaction: ‘This is no surprise’, but we have a lot to learn

As human-activities continue to put a pressure on the climate to warm, we will continue to see record global temperatures being periodically set under the current climate regime — especially when natural variability is acting to contribute a warming influence as well (as was the case in 2016). This is no surprise.
The question is not whether greenhouse gas emissions act to warm the climate, but rather, how large is their impact and in what manner is it manifest. It’s these latter questions, hand in hand with a better understanding of the complexities and intricacies of natural variability, that demand our scientific focus.
Observations from the last several decades indicate that we still have a lot to learn both in terms of our understanding of the observed history, as well as what changes can best be anticipated to lie ahead.
— Chip Knappenberger, assistant director, Center for the Study of Science, Cato Institute

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Reaction: Global surface temperature is a flawed metric and not ideal for monitoring global warming

Unfortunately, the surface temperature analysis contains several uncertainties and systematic biases when used to diagnose global warming. One of them is with respect to land minimum temperatures over land. Rather than measuring changes in heat content through depth in the atmosphere, even slight changes in vertical mixing of heat (even with no net heating) can produce warmer minimum temperatures.
Ocean heat content changes is, by far, the much better way to assess global warming. Ocean heat changes can be much more directly related to the radiative imbalance at the top of our atmosphere.
— Roger Pielke Sr., senior research scientist, University of Colorado-Boulder

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Reaction: Satellite-measured atmospheric temperatures show less-pronounced warming

The NOAA report refers to surface temperatures. However, the bulk atmospheric temperature is another important climate variable for people to know about. It too shows that 2016 was the warmest, edging out 1998 for first place. As in 1998, a strong El Niño was the main cause.
As the world cools now, the global atmospheric temperature change from February to December has been over one degree. 2017 will be cooler than 2016.
Overall, the atmospheric warming since 1979 has been about 0.25 degrees per decade, bolstered somewhat by two cold episodes caused by volcanic events in 1982 and 1991. Had these volcanoes not occurred, the upward trend would still be there, but smaller.
The atmospheric warming, while clearly present, is quite a bit less than anticipated from climate model projections which attempt to tell us how extra greenhouse gases affected the climate during the past 38 years.
— John Christy, professor of atmospheric science, University of Alabama in Huntsville

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2015 and 2016 were years with a very strong El Niño. Similar to the previous very strong El Niño in 1997-98, a spike in surface and atmospheric temperatures was observed.
According to surface temperature data sets, 2016 is the warmest year in the record (approximately the last 150 years). However, examination of the satellite data record of atmospheric temperatures, 2016 was only 0.02 degrees Celsius warmer than 1998 — a difference that is not statistically significant.
Understanding the difference in trends between the surface temperature data sets and the satellite data sets of atmospheric temperature remains an outstanding challenge for scientists.
What is the significance of the ‘warmest year’? Following the 1998 peak in warming, temperatures dropped substantially for the next several years. In 2016, we are seeing substantial drop in temperature anomalies following the warm peak in February. It will be another five years before we have perspective on this recent spike in temperatures.
— Judy Curry, president, Climate Forecast Applications Network"