> It would be challenging to fairly compare data from the last 10 or so years to a similar timeframe 100 years ago.
Why? A thermometer works exactly the same it did 100 years ago. For such a simple measurement like temperature I can't think of many reasons why measurements from the 30s could be significantly different (as in more than fractions).
I also guess that if the data was considered unreliable for any reason it would not be used.
I don't have any particular reason to believe the data is unreliable, I'm inclined to believe temperates are actually rising, but it's fun to try to come up with reasons why the rise might be artificial:
- Most cities grow and develop over time and the parts of cities with more buildings and especially with more cement will radiate more heat. If an observation station was built somewhere less developed then city growth might artificially push up temperatures around the station. This effect might be strongest somewhere like Phoenix or LA.
- This one seems unlikely (well, I'm sure it happens but it's very likely to have already been corrected for) but aggregation methods might have changed over time. I could imagine in the 30s someone might check the thermometer every hour and record the current temperature in a logbook. Modern thermometers are almost certainly automated. If the "highest temperature" is measured by recording the temperature each minute and taking the max then you're going to get a higher number than the manual method.
- site selection might have changed: observation posts probably used to be buildings which needed to be large enough to fit equipment and the people to operate them. City real-estate is expensive and meteorology used to be a cost-center so these buildings were probably placed at the edge of town. I'm sure modern stations are much smaller, they don't need a full-time staff to operate them, so they can be placed closer to the (hotter) centers of cities. Alternatively: I'm sure many weather stations are now situated at airports, an option which was not available in the 30s, and airports are particularly pavement-heavy (hotter) parts of cities.
> Most cities grow and develop over time and the parts of cities with more buildings and especially with more cement will radiate more heat. If an observation station was built somewhere less developed then city growth might artificially push up temperatures around the station. This effect might be strongest somewhere like Phoenix or LA.
The temperature record - at least, what's used for generating long-term trends - is homogenized to remove discontinuities (step-wise or linear) associated with this sort of drift or other significant changes. Regardless, this effect likely wouldn't matter for the stats the website illustrates.
> This one seems unlikely (well, I'm sure it happens but it's very likely to have already been corrected for) but aggregation standards might have changed over time. I could imagine in the 30s someone might check the thermometer every hour and record the current temperature in a logbook. Modern thermometers are almost certainly automated. If the "highest temperature" is measured by recording the temperature each minute and taking the max then you're going to get a higher number than the manual method.
When manual reading of the temperatures was the norm, you'd have a mercury and an alcohol thermometer to record the daily max and min, respectively. For a mercury thermometer, it would have a stopper that rises and records the maximum level the liquid reaches. A human needs to invert the thermometer to reset it. Same for an alcohol thermometer, albeit in reverse (records the minimum). So granularity of observation doesn't matter - the daily max, historically, is truly the maximum temperature taken since the last time a human reset the stopper.
> site selection might have changed: observation posts probably used to be buildings which needed to be large enough to fit equipment and the people to operate them. City real-estate is expensive and meteorology used to be a cost-center so these buildings were probably placed at the edge of town. I'm sure modern stations are much smaller, they don't need a full-time staff to operate them, so they can be placed closer to the (hotter) centers of cities. Alternatively: I'm sure many weather stations are now situated at airports, an option which was not available in the 30s, and airports are particularly pavement-heavy (hotter) parts of cities.
Also accounted for by homogenization. In most cases when siting changes, you consider that site a new station.
While you're here, do you happen know of a good book / other resource to read if I want to learn more about how we decided climate change was real and anthropogenic? I take it for granted; I've never actually looked at any data.
The single, best general audience easy read on the basics of why we know the planet is warming due to human greenhouse gas emissions is Kerry Emanuel's ["What we know about climate change"](https://www.amazon.com/What-About-Climate-Change-Press/dp/02...).
I guess you missed the main point of lack of data in addition to possible quality issues. Certainly I would assume we collect many more air temperature readings now then we did in the 1800’s even into the 1900’s. I can’t imagine there are nearly as much temperature data from 100 years ago compared to how much data is being collected in the last 30. You didn’t answer the real question, of course a thermometer works the same, but does the collection methods work the same? Did we have satellite data 100 years ago? Obviously not. We are collecting a much wider variety of data from a variety of sensors and it has built up especially in the last 50 years. I haven’t seen the data I’m only asking the question. What does the data look like, I’d love to compare just a slice from one city. Someone post the data from that data set that compares 100 years ago reading to ones collected in 20 year intervals. My guess is that there is progressively more and more data as you move forward. If I’m wrong show me the data.
Yes, more or less, they're the same. It's simple enough to restrict measurements to just surface observations made with a thermometer of some sort if you want a wholly apples-to-apples, long-term temperature record. You don't have to add in satellite data (satellites don't directly retrieve surface air temperature so the direct comparison is fraught) although there do exist techniques which leverage it to augment the lack of spatial coverage of surface observations, especially over oceans.
Quality issues have been studied ad nauseum over the past thirty years. Nearly half a dozen independent efforts have all tackled this problem. Probably the most notable recent one is the Berkeley Earth Land/Ocean Temperature Record (Rohde and Hausfather, 2020 -https://essd.copernicus.org/articles/12/3469/2020/essd-12-34...).
Why? A thermometer works exactly the same it did 100 years ago. For such a simple measurement like temperature I can't think of many reasons why measurements from the 30s could be significantly different (as in more than fractions).
I also guess that if the data was considered unreliable for any reason it would not be used.