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What does "1.5 °C above pre-industrial levels" mean?

The dignified representatives of the world's countries who gathered in Copenhagen enjoy many childish games. But one of the favorite ones was a pissing contest: who can restrict the rise of the global mean temperatures more toughly?



Does Earth on August, 17th, 1773 plus 1.5 °C equal Eta Carinae? Al Gore probably "thinks" it does. Click to zoom in a little bit.

Some primitive tribes from some small Pacific islands - such as Tuvalu and Kiribati - want to improve the previous 2 °C limit proposed by officials from richer countries. They demand that the globe's temperature will never jump more than 1.5 °C above the pre-industrial level: Google News.

You can see that the difference between the wild tribes who have just jumped down from a tree on one side and the carbon diplomatic representatives of the rich countries on the other side is a factor of 3/4.

But what the hell does the bound mean? There have been many pre-industrial temperatures because the natural factors were always changing - and are still changing - the global mean temperature by anything between fractions of a Celsius degree and 10 Celsius degrees. Depending on the point where you start, you get very different answers for the "pre-industrial temperatures".

Their pissing game - which is supposed to eat trillions of dollars - is based on the thoroughly unscientific, inherently creationist assumption that there was only one temperature before the man built his first engines. But the temperature was changing at all time scales. Even if the humans have contributed something to the warming - which is conceivable, theoretically justifiable, but supported by no empirical evidence - they have surely changed nothing qualitative about the existence and character of natural climate variability. Look at these reconstructions, possibly the best ones we have today:



Moberg et al. (up), Loehle (down).

For example, according to the Loehle reconstruction, the "temperature deviation" in 1700 was around -0.6 °C while it was close to 0 °C around 1770. Well, the year 900 was also pre-industrial but the temperature anomaly was close to +0.6 °C at that time. Even when you look at the most recent millenium, the temperatures in different years differed by more than 1 °C.




Clearly, the term "pre-industrial temperature" is meant to make this temperature reading "canonical" or "holy" so that we should keep ourselves in the vicinity of their temperature. What the "pre-industrial temperature" actually means is an arbitrarily chosen and mostly unknown temperature at a random year during the feudal era.

Because they believe that the temperature has increased by 0.8 °C since the pre-industrial times, they must mean a year between 1700 and 1800 as their pre-industrial day of creation. But which one?



Temperatures above the "pre-industrial era" in the last 450,000 years. Note that even the limit of 2.0 °C was probably breached 8,135 years ago and 128,357 years ago, among other periods.

Even if those people agree what temperature we're going to be subtracting, it's still impossible to agree on the temperature difference with the impressive accuracy of 0.1 °C. Different teams that publish their global mean temperature data have very different ideas about the trend.

For example, UAH MSU satellite measurements end up with a 0.13 °C per decade during the last 30 years when GISS claimed something like 0.2 °C per decade which is 50% higher. So if our Tuvalu leaders allow us an additional 0.7 °C of warming and we extrapolate the trends from the last 30 years (which almost certainly understates the warming), we will have 54 years left according to UAH MSU but only 35 years according to GISS.

If the 2 °C figure is adopted, we are allowed 1.2 °C of extra warming which is 92 years according to UAH MSU and 60 years according to GISS. So our remaining time before the U.N.-sanctioned catastrophe is something in between 35 and 92 years but it may also be negative or, more likely, millions of years.

And what should exactly happen when this "catastrophically hot" average brightness temperature of the Earth around -14 °C is reached? Should the people stop driving or breathing? Should the people of the world commit suicide? I assure you that this won't stop temperature from changing.

Even after the bloody crackpots exterminate billions of cows or people or prevent them from driving, the global mean temperature will keep on jumping up or down by something like 0.2 °C per decade, 0.8 °C per century, 2 °C per millenium, 8 °C per ten or more millenia (and cooling into another ice age is likely to be the next step on this timeframe).

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reader JB said...

I suggest that we use a period that no one can argue is pre-industrial as a baseline period. The years 1100-1200 should work well. Or to be completely certain maybe a block of time between 200 and 600 BC.