Limiting global temperature increase to x above "Pre-industrial Levels"
August 20, 2021 4:21 PM   Subscribe

WHAT IS PRE-INDUSTRIAL LEVEL? What is the actual target? I get that these are averages and not just a single flat number, but we have an idea, right? Why is it so hard to say "1.5 degrees over X TEMPERATURE" ( (or even 1.5 above (x1 thru x2))? What is the base we're comparing to, or even better... WHAT IS THE END GOAL GIVEN THE VARIOUS TARGETS of .5 - 4 degrees in the models?

When looking for a simple number (or range of numbers) I get explanations of why it's hard to get what that even means "(how far is preindustrial)" "how much is geologic variability" "We tend to mean 1850-1900" etc...

But nobody is giving a temperature!

I see things like this :
"Averaged as a whole, the global land and ocean surface temperature for March 2020 was 1.16°C (2.09°F) above the 20th century average of 12.7°C (54.9°F) and the second highest in the 141-year record. Only March 2016 was warmer at 1.31°C (2.36°F)."

Thsi is great, ok, so I see March was 1.16 above 12.7 so 13.86 or so. Great, I have an idea what that means. But that's above the 20th century average.

So why is it so damn hard to find what the actual numbers are, because when I add 2-4 degrees c above the average, it turns out to be a HECK of a lot warmer than it sounds (for those in Farenheit land).

SOrry for the editorial, but can someone give me an idea of what these temperatures are compared to and what a general estimate of what they would mean (I get these are global average, not max, not minimum, so like 54.9 is obviously a lot cooler than the hot summer temps I think of, but it's much warmer than the subzero temps I'm also used to.

But when I put in say, 4 degrees c to F I see almost 39.2 degrees. in Fahrenheit, that's like 2/3 of the 20th century average! That sounds insane. But people here "pfft, 4 degrees" and just think it's nothing. (Admittedly that's the extreme).

Just - can someone give some basic simple numbers (again - I get there's ranges and leeways and averages) so I know even if I get a number it's not "ha! we've hit X" just wanting an idea of what it was because according to some things I"M ready we're already near 1.5 of 20th century, how is that not much different from preindustrial?
posted by symbioid to Science & Nature (6 answers total) 2 users marked this as a favorite
Best answer: Well, for one thing, the Fahrenheit and Celsius temperature scales have different zero points, so if you enter "4 degrees c to F" into a converter it will probably assume you want to know what the absolute temperature 4 degrees Celsius is in Fahrenheit, rather than a difference of 4 degrees Celsius. Every change of 1 degree C is a change of 1.8 degrees F, but the absolute temperature 1 degree C is the absolute temperature 33.8 degrees F, because 0 C is 32 F.

An increase in temperature of 4 C is 7.2 F.
posted by biogeo at 4:35 PM on August 20, 2021 [7 favorites]

Best answer: One reason you may be having trouble finding the answer to your question is that it's actually harder to define and measure an absolute average global temperature than it is is to define and measure an average global "temperature anomaly." For a little bit of explanation on that point, this page at Basically, if you ask the question, "What is the average temperature of Earth right now?" that is actually very hard to answer, because you have to further ask whether you mean land surface temperature or oceanic temperature or what have you, and how to account for these differences. But if you ask "How much hotter is the Earth right now than it was 100 years ago?", you can actually give a much better answer by just looking at the overall agreement in how much each local area's records have changed. I think this is partly why climate scientists always use and report temperature anomaly instead of absolute temperature: a change of temperature is much more consistently observed and well-defined than any given value for the exact temperature would be.
posted by biogeo at 4:59 PM on August 20, 2021 [11 favorites]

Response by poster: Thanks for the info. I hadn't thought about change vs absolute re: your first answer, makes sense.
I have so many questions that just... boggle my mind more now. But at least you gave me a starting point. Thanks!

And if anyone has any further contributions I would gladly accept to help inform my understanding (or lack thereof LOL).
posted by symbioid at 6:20 PM on August 20, 2021

Best answer: It's sets of data from multiple sites showing higher average temperatures for those sites (the trend) and the "pre-industrial extrapolation" uses tree growth and winter ice deposits to proxy for the temperature in that area. From those things the scientists use data to infer trends about those location average temperatures. The sets of data are at, should include the specific locations for which they created average pre-industrial temperatures.

The key part for use now is "multiple sites" so reading for your/one location is not going to give you predictive power over how advanced to human-caused temperature rise is.
posted by k3ninho at 9:52 AM on August 21, 2021

Best answer: If you want to understand the science of climate change generally, I strongly recommend Spencer Weart’s The Discovery of Global Warming (free online) and Richard Alley’s The Two Mile Time Machine
posted by sindark at 1:48 PM on August 21, 2021 [1 favorite]

Best answer: I googled and found things like this page that explains why it’s hard to define pre-industrial temperatures - mainly because the global average temperature varied a lot over hundreds and thousands of years.

One of the pages that links to describes the pros and cons of choosing different periods for the baseline.
posted by fabius at 4:26 AM on August 22, 2021 [1 favorite]

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