In which I continue an occasional series (2014, 2015) on whether particular years are the warmest on record.

A question I have been asked many time since about March 2020 was whether 2020 was going to be the warmest year on record and then, once the new year was rung in, whether 2020 was the warmest year on record? In a sense, the answer – the correct answer I would aver – hasn’t changed since March: it was maybe then, and it is maybe now. Some years are too close to call and 2020 was one such.

While 2020 was still underway, that indeterminate answer was found to be acceptable. The year was not over and the remainder of months yet to be experienced impressed upon the asker the unavoidable existence of some small doubt. In late November, with ten months in hand, 2020 looked set to land somewhere amongst the three warmest years. Although some interpreted this as a euphemistic admission that 2020 was merely the third warmest year, others rightly apprehended it to mean that the final answer was not yet decided.

November and December often hold surprises for the careful watcher of global temperature and 2020 had surprises indeed, furnishing first a November whose warmth was almost without equal in our short records and then a December that showed the first cold bite of La Niña. In short, 2020 kept us guessing to the very end.

In January, the situation stood thus. The nominal rankings – those that take the central estimate as precise and true – of 2020 were: 1st (GISTEMP), 2nd (NOAAGlobalTemp), 2nd (HadCRUT5), 2nd (ERA5), 2nd (Berkeley Earth), 2nd (HadCRUT4), 3rd (JRA-55). A data set produced by JMA from in situ measurements also had it in first place. But we know that global temperatures are not precise and though, by January, we had data for every month of 2020, we did not have data everywhere. Data gaps in space have an analogous effect to temporal gaps impeding our ability to state with ultimate precision what the temperature of the year is and how it sits in relation to its near-neighbours.

The spread between data sets is one indication of this and includes in it the effects of other factors deleterious to our ability to provide for the curious to their exact satisfaction. No measurement is perfect and the margins of error attendant on each one, though diluted by averaging across so many thousands of independent recordings, still serve to perturb the resulting global average from its true value. What that “true” value is is also a matter of small debate. Each centre that exerts itself in calculating a global mean temperature, provides as a result – intended or otherwise – an answer to a problem that differs in important matters to that of its peers. Whether one uses sea-surface temperatures or air temperatures measured just above the marine surface, affects the final answer, as do myriad decisions that must be made practically in the construction of a data set. Even if we could measure the individual data points precisely, our failure of definition, which stems perhaps from a more fundamental inability to specify precisely what global temperature is, would still lead to a range of answers. Though as for that, the divergence between these estimates in recent years is very small.

Relying simply on the spread of answers available from these global centres, we cannot say with certainty whether 2020 was the first, second or third warmest. The data sets, albeit alike in their claim to authority, disagree. However, none contradicts the statement that 2020 was one of the three warmest years. There is an advantage, I would argue, to a statement that cannot be refuted by available evidence, and that its advantages exceed those of a statement, which though having the attraction of boldness and apparent certitude, is nonetheless susceptible to counter examples.

This is not to say, that the causes for the differences between data sets should not be explored. I believe strongly that they should. By better understanding the reasons for them, we might succeed in bringing our ideas into sharper focus. To this worthy aim, I append two caveats. First, we should not seek simply to reduce disagreement; we should seek out and understand the causes for these disagreements and thereby enact improvements across the full range of data sets. Second, that there may be necessary limits on how far it is practical, if not possible, to pursue this aim, with improvements being harder to find, the closer the agreement becomes.

When considering how to rank the year, we might also consider the estimates of uncertainty provided with some of these data sets. GISTEMP, NOAAGlobalTemp, HadCRUT4 and HadCRUT5 and Berkeley Earth routinely generate such information and it figures in some of the statements made by these groups. GISTEMP, which finds 2020 ahead of 2016 by a margin smaller than the estimated uncertainty, noted that 2020 was the joint warmest year on record. Copernicus, relying on their ERA5 reanalysis, likewise noted that 2020 was joint warmest on record though they do not routinely publish an estimated uncertainty. Berkeley Earth note that despite its nominal ranking of second place, 2020 was not so different from 2016 that their positions might not in reality be exchanged. This they summarised by saying that “2020 and 2016 could also be regarded as effectively tied“.

Later on though, Berkeley Earth say “As a result, 2020 is nominally the second warmest year to have been directly observed, though the three warmest years are all tightly clustered together relative to the uncertainty in these measurements.” Indeed, 2020 sits almost exactly half way between 2016 and 2019 in their analysis. 2020 is in the deliciously poised position of being effectively tied with the nominally warmest and third warmest years. Although it is very unlikely that 2019 was warmer than 2016, 2020 could flip places with either one if the years are jiggled round within their uncertainty bounds. Therefore, it is not quite correct to say that 2016, 2020 and 2019 are statistically indistinguishable from each other as 2016 and 2019 can be told apart. A similar calculation made using the HadCRUT5 ensemble finds that while 2020 is most likely second warmest on record, there is a fair chance of it being first or third warmest. The precise probabilities of these outcomes are themselves uncertain and vary from data set to data set.

By now, it ought to be clear that any attempt to talk specifically about rankings of a year like 2020 are doomed to confusion. If one person says something that sounds definitive, it will seem to contradict what another person has said with equal conviction and the unpicking this necessitates* distracts from the essential message, which is this: it’s no surprise that 2020 was an exceptionally warm year, warmer for sure than 168 years in the global temperature record and very likely much longer than that. The decade on decade rise in temperature is unmistakable. The decade 2011-2020 was clearly warmer than 2001-2010, which was warmer than 1991-2000, which was warmer than 1981-1990 and indeed any decade prior to that. Global temperatures have clearly increased and, to borrow the words of the IPCC, “most of the observed increase in global average temperatures since the mid-20th century is very likely due to the observed increase in anthropogenic greenhouse gas concentrations”. We see physically consistent effects in other indicators like ocean heat, sea level, glacier and ice-sheet mass balance, humidity and many others.

* For a perfect example see this Guardian article which pedals back from its headline from the very first sentence.