Global Climate Report - Annual 2017
Calculating the Probability of Rankings for 2017

« Global Climate Report - Annual 2017

Calculating the Probability of Rankings for 2017

Evaluating the temperature of the entire planet has an inherent level of uncertainty. The reported global value is not an exact measurement; instead it is the central value within some range of possible values. The size of this range depends on the method used to evaluate the global temperature anomaly, the number and placement of the stations used in the analysis, and so on. Because of this, NCEI provides values that describe the range of this uncertainty, or simply "range", of each month's, season's or year's global temperature anomaly. These values are provided as plus/minus values. For example, the 2017 temperature anomaly was reported as "0.84°C above the 20th century average, ±0.15°C.". This may be written in shorthand as "+0.84°C ±0.15°C". Scientists, statisticians and mathematicians have several terms for this concept, such as "precision", "margin of error" or "confidence interval".

The plus/minus numbers, which are presented in the data tables of the monthly and annual Global State of the Climate reports, indicate the range of uncertainty (or "range") of the reported global temperature anomaly. For example, a reported global value of +0.84°C ±0.15°C indicates that the most likely value is 0.84°C warmer than the long-term average, but, conservatively, one can be confident that it falls somewhere between 0.69°C and 0.99°C above the long-term average. More technically, it is 95% likely that the value falls within this range. The chance of the actual value being at or beyond the range on the warm side is 2.5% (one in forty chance). Likewise, the chance of the actual value being at or beyond the cool end of the range is 2.5% (one in forty chance).

Using a Monte Carlo approach (Arguez et al, 2013), NCEI considered the known uncertainty of the global land and ocean annual temperature in the 2017 annual ranking. Taking into account the uncertainty and assuming all years (1880–2017) in the time series are independent, the chance of 2017 being

  • Warmest year on record: 10.2%
  • One of the five warmest years: 94.6%
  • One of the 10 warmest years: 99.7%
  • One of the 20 warmest years: 100.0%
  • Warmer than the 20th century average: 100.0%
  • Warmer than the 1981-2010 average: 100.0%

NCEI follows these conventions to categorize the confidence associated with assertions made with respect to ranks used in the report:

> 99%"almost certain"
90% - 99%"very likely"
66.7% - 90%"likely"
50% - 66.7%"more likely than not"
33.3% - 50%"more unlikely than likely"
10% - 33.3%"unlikely"
1% - 10%"very unlikely"
< 1%"almost certainly not"


Arguez, A., T.R. Karl, M.F. Squires, and R.S. Vose, 2013: Uncertainty in annual rankings from NOAA’s global temperature time series. Geophysical Research Letters, 40, 5965–5969, doi:10.1002/2013GL057999.