World's Hottest Year…Probably

World's Hottest Year…Probably

2014 was a warm year, according to NASA and NOAA measurements of Earth's average global temperature, quite likely the warmest in the past 135 years, since we started making measurements.

The 2014 average global temperature was 1.24°F warmer than the 20th century average of 57.0°F - plus or minus 0.16°F, according to NOAA. This means that Earth's average temperature was 1.08°F to 1.40°F warmer than normal (and we can say that with 95% confidence).

Headlines last week read that 2014 was the warmest ever, however, we can't say for sure that 2014 was the warmest year. Why is that? It's because of uncertainty. Then again, we do know quite a bit about how certain we are that 2014 was the hottest year, and we have uncertainty to thank for that too.

Uncertainty - the casual, nonscientific definition of that word means unpredictable and unreliable. Something that's uncertain is doubtful or unknown. I'm uncertain what I'm going to eat for breakfast tomorrow, for example, because I haven't really given that much thought.

But, in science, uncertainty means something different. Uncertainty tells us how certain we are. Almost nothing is absolutely certain in Earth science. When uncertainty is reported by a scientific study, the researchers are letting us know how sure they are of the results. Researchers use math to calculate scientific uncertainty.

As a simple example, let's say Molly took her temperature 10 times. Sometimes the thermometer read 101.5°F and sometimes it read 101.4°F. The highest it read was 101.8°F. Because the measurements were not exactly the same each time, we know that there is some uncertainty.

This uncertainty could be for several reasons. Perhaps Molly's temperature is actually changing. If so, then the timing of the measurements would make a difference. If Molly had taken measurements at a different time then they might be slightly different. The same is true for the Earth -  measurements taken at slightly different times might be different. Perhaps Molly's thermometer is a little imprecise. This could also be the case for our measurements of Earth's temperature too, even if those are taken with a very high quality thermometer.

We can calculate to find that Molly's average temperature is 101.52°F. We can also calculate the confidence interval and find that 95% of the time, Molly's temperature is between 101.517°F and 101.523°F.

Molly clearly has a fever since this range of numbers is quite a bit higher than 98.6°F, which is typical body temperature. No math calculations were necessary to know that Molly is ill. Someone should get her chicken soup.

Like Molly, Earth is warmer than usual. All of the warmest years on record happened recently. Nine of the ten warmest years in the 135-year period of record have occurred in the 21st Century. All of the coolest years happened decades ago. Three years nearly tie for having the highest heat - 2014, 2010, and 2005. While 2014 is the highest, uncertainty means that the range in likely temperatures overlaps. What we can say is that, of the three hottest years, 2014 is the most likely to take home the gold as our world's hottest (until next year, perhaps).

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