Global Temperature Uncertainty
What is a range of uncertainty?
Evaluating the temperature of the entire planet has an inherent level of uncertainty. Because of this, NCDC 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, a month's temperature anomaly may be reported as "0.54°C above the 20th Century average, plus or minus 0.08°C." This may be written in shorthand as "+0.54°C +/- 0.08°C." Scientists, statisticians and mathematicians have several terms for this concept, such as "precision", "margin of error" or "confidence interval".
Understanding & Interpreting Uncertainty Ranges
What do the "plus or minus" numbers mean?
The plus/minus numbers indicate the range of uncertainty (or "range") of the reported global temperature anomaly. The reported global value is not an exact measurement; instead it is the central — and most likely — value within a range of possible values, according to the method used to evaluate the global temperature anomaly. The plus/minus numbers indicate the scope of this range. For example, a reported global value of +0.50C +/- 0.08°C indicates that the most likely value is 0.50°C warmer than the long-term average, but may fall between 0.42°C and 0.58°C above the long-term average. More technically, it is 95% likely that the value falls within this range. The chance of actual value being beyond the range on the warm side is 2.5% (one in forty chance). Likewise, the chance of the actual value being beyond the cool end of the range is 2.5% (one in forty chance).
What is the most likely actual value?
According to the methods used, the most likely value is the calculated central value, the one that is reported before the plus/minus range. Values nearer to this central value are more likely than those farther from it.
What causes this plus or minus range?
The dominant reason for this range is the number and scope of temperature observations reported during the period (the month, season or year in question). The global temperature value is a composite of many thousands of temperatures recorded around the world. If there are many areas with missing or few temperature observations can increase the uncertainty associated with the global temperature anomaly calculation. Click here for more information on how NCDC compiles its monthly observations.
Which is more likely, plus or minus?
They are equally likely.
Why do the plus/minus values change over time?
Many temperature observations are recovered months or even years after they are recorded. When these are added to the record, they increase the precision ("reduce the uncertainty") associated with the estimated temperature.
If there is less-than-perfect certainty, how are scientists confident in global temperature values?
As more and more data builds a long-term series, there is less and less influence of single "outliers" on the overall trend, making the long-term trend even more certain than the individual points along it. Put another way, the chance of any single month's value being an "outlier" (outside its range of uncertainty) is about 5%, or one-in-twenty. The chance of two consecutive months' values being outliers is about 0.25%, or one-in-four-hundred. The chance of three consecutive months being outliers is 0.0125%, or one-in-eight-thousand. The chance of a long series of temperatures all occurring near outside the range is incredibily small. Over the long term, these individual "outliers" are few, and balance out because they are evenly distributed (a roughly equal number of points above and below).
What impact does this have on ranks?
NCDC uses the calculated central (most likely) value to establish the monthly rank ("fourth warmest", etc.). This provides a consistent approach from month to month.
- Smith, T. M., and R. W. Reynolds (2005), A global merged land air and sea surface temperature reconstruction based on historical observations (1880-1997), J. Climate, 18, 2021-2036.
- Smith, T. M., et al. (2008), Improvements to NOAA's Historical Merged Land-Ocean Surface Temperature Analysis (1880-2006), J. Climate, 21, 2283-2293.
- Global Temperature Anomalies: More Information