IPCC Climate predictions change in AR5

Shortly after I left town for a 2 week trip to the East Coast, the IPCC released their latest assessment on climate change, it’s Fifth Assessment is nicknamed AR5.  Today I pulled up some of the report from the web.    It was full of surprises.

The press did not surprise.  Gloom and doom has been everywhere these past two weeks.   Lost is all the hoopla is a significant change in the way the IPCC makes predictions.  They have become more circumspect.    Gone are absolute short term predictions like this one from AR4 made in 2007:

For the next two decades a warming of about 0.2°C per decade is projected for a range of SRES emissions scenarios. Even if the concentrations of all GHGs and aerosols had been kept constant at year 2000 levels, a further warming of about 0.1°C per decade would be expected. Afterwards, temperature projections increasingly depend on specific emissions scenarios. {3.2}

AR4 included the following charts to help explain their short term temperature predictions:

https://www.ipcc.ch/publications_and_data/ar4/syr/en/fig/figurespm-5-l.png

Now look at what AR5 says:

It is more likely than not that the mean global mean surface air temperature for the period 2016–2035 will be more than 1°C above the mean for 1850–1900, and very unlikely that it will be more than 1.5°C above the 1850–1900 mean (medium confidence).

A quick peek at the East Anglia University  temperature data set will allow us to interpret what 1 degree C since 1900 really means:

The world average from 1850 to 1900 was about 0.7 degree C cooler than the year 2000 baseline used in AR4.   So the new report is saying that the world has about a 65% chance (more likely than not) of being about 0.3 degrees C warmer on average between 2016 and 2035 than it was in the year 2000.  They also say that temperature is very unlikely to be as high as 0.8 degrees C higher.

Compare that statement  to the chart from AR4.  The IPCC predicted  0.8 degree C above 2000 as a most likely case in the year 2035.    The AR5 report lowers their estimate and changes the way it is calculated.  AR5 deals with average temperatures over a period of time while AR4 made much more specific and higher predictions.  AR4 effectively ignored natural climate variation.  AR5 does not repeat the mistake.

AR5 includes two significant temperature prediction caveats.   They acknowledge that natural climate variation makes specific temperature predictions difficult in the short run and they included a statement on volcanic activity:

This projection is valid for the four RCP scenarios and assumes there will be no major volcanic eruptions or secular changes in total solar irradiance before 2035.

IF the earth experiences a significant  (Mt. Pinatubo equivalent) volcanic eruption, then IPCC projections will likely be wrong according to the IPCC.   How likely are Pinatubo equivalent eruptions?  Volcanic eruptions are measured using the Volcanic Explosivity Index (VEI).   Mt. Pinatubo was a 6.   A chart courtesy of Wikipedia describes VEI and offers their best guess for each classification.  Here it is:

https://climateswag.files.wordpress.com/2014/04/6ac18-veitable.jpg

This chart says both VEI 5 (Mt St. Helens) and VEI 6 (Mt. Pinatubo) eruptions happen at rates that are less than one every 100 years.   What are these guys smoking?    The Wall Street Journal published a chart that shows each VEI 5 or higher event in the last 200 years.

Between April of 1815 and August of 1991, the Earth produced 19 VEI 5 or greater eruptions.   Mt. Tambora,  got things started in April of 1815.   This category 7 event was really special.  5,000 feet of this Indonesian volcano disappeared in a single event.   There was so much crap in the air that 1816 was named the year without a summer.  It was followed by 14 category 5 events, and  4 category 6 events culminating with Mt. Pinatubo in 1991.

Recent history is going to give us the best data.  Recent history says the IPCC is ….well…wrong.   We probably will have a significant eruption before 2035.   I suspect wild ass guessing at the UN.  Mother Nature gave us 4 category 6 events in the 108 year period from August of 1883 (Krakatoa) to June of 1991 (Pinatubo).

I am glad AR5 noted the exclusion of volcanic activity and made some references to natural climate variation.  These inclusions make the work so much better. The admission really was necessary.  AR4 short term temperature predictions were so flawed that they had to do something to explain why they were so wrong.

AR4 was full of bad wild ass guessing….and at least  AR5 acknowledges that they might be wrong.  The IPCC admits that there is a one in three chance that they are being too aggressive simply because normal climate variation makes specific predictions difficult.  A VEI 6 Volcanic eruption will totally mess with their predictions.

Lets assume VEI 6 eruptions happen every 40 years or so.   A VEI 6 volcanic eruption between now and 2035 carries a 50% probability.  We must then reduce the probability that the IPCC predictions are correct by that 50% since they admit they assumed a zero probability.

I’m just glad to see the IPCC adjusting to the real world…at least a little bit.

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