Hottest year on record, statistically speaking II

Last portfolio entry, I looked at the statistics showing that the odds of the earth’s warming trend since 1970 is random are infinitesimally small.  Today I’m going to look at the other side of that coin.

Climate scientists – and scientists in general – are dealing with mounds of complex data gathered from across the world in different ways.  For that reason, when they make a declaration that, for example, 2014 was the warmest year on record, or that “warming of the climate system is unequivocal,” they also like to quantify the confidence they have that the findings are true.

In the case of the announcement that 2014 is the warmest year on record, NOAA gave that a 48% probability and NASA gave it a 38% probability (source pdf).  On the face of it, this seems pretty low.  How can NOAA and NASA state 2014 is the warmest year with less than a 50% probability?

There are several ways this uncertainty is created.  First, there are multiple teams of scientists measuring temperatures around the world, and each may be taking readings from different weather stations or in slightly different ways.

Second, the historical record goes back to 1880, and in that time weather stations may have moved and procedures for measuring temperature may have changed.  Today’s scientists don’t have the scientists of 1900 to talk to – they have to rely on records left behind.

Gavin Schmidt, director of NASA’s Goddard Institute for Space Studies, said that any one year’s average temperature reading has an uncertainty of plus or minus 0.05 degrees C or 0.09 degrees F.

Then consider that the differences in average annual temperature we are talking about between 2014 and the next couple of hottest year contenders, 2005 and 2010, are tiny.  According to Dr. James Hansen, the former director of Goddard Institute for Space Studies:

The two agencies use slightly different methods, so they have different readings for the difference between 2014 and the previous warmest year, 2010, with N.O.A.A. putting it at 0.07 degrees Fahrenheit (0.04 degrees Celsius), while NASA got 0.036 degrees (0.02 Celsius) — which this analysis says is well “within uncertainty of measurement.” (source)

Because there are so many sources of uncertainty, it turns out that a 48% or 38% certainty that 2014 is the hottest year on record is actually pretty high.  The next most likely year is 2010, at 18%. This means that 2014 is a little more than 2.5 times more likely than 2010 to be the warmest on record.  That’s how scientists can say pretty definitely that 2014 is the hottest.

However, the 48%/38% certainty that 2014 is the hottest year on record did not escape the notice of climate skeptics – who really need to be called climate deniers because they spend their careers trying to discredit any science showing the earth is warming.

The Daily Mail published an article by David Rose, who held up the certainty figures for ridicule and acted as if NASA and NOAA had been trying to hide them, even though they were discussed publicly at the presentation of findings to the media.

Here the skeptics seem to be pouncing on an artifact of how science works which the public doesn’t understand, and playing it up in an effort to discredit the science.

While it might be only 48%/38% certain that 2014 is the warmest year ever, it is not uncertain that 2014 was among the warmest.  In fact, even with all the sources of uncertainty discussed above, scientists are 90.4% certain that 2014 is among the five warmest years, and 99.2% certain it is among the 10 warmest.

There’s also the matter of the warming trend shown in the graph from NOAA.  Whether or not 2014 sets a specific record – and indications are that it did – it is still a matter of direct observation that every year since 1978 has been warmer than average, and that temperatures are on a clear trend of increasing.

Taking scientific probabilities of certainty out of context and presenting them without any explanation of where they came from is a fundamental misrepresentation of the science.  But that is what the professional climate skeptics do, which is why the public is still so confused about climate change.

Further reading
Freedman, Andrew. Climate scientists rebuff skeptics’ arguments against 2014 ‘warmest year’ claim. Mashable. January 20, 2015.
Revkin, Andrew.  How ‘Warmest Ever’ Headlines and Debates Can Obscure What Matters About Climate Change. The New York Times. January 21, 2015.

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