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stevewepay | 10 years ago

How exactly do they do a fair comparison between 1880 and today? I'm assuming the temperature readings today are very accurate, but how accurate were the measuring devices going back in history, and their standards for taking temperature? Most of the thermometers I have in my house are +- 2F, which would eliminate the differences entirely.

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cryptoz|10 years ago

There are significant temperature records that go back much farther than 1880 even. Many navies and armies kept records from all over the world, so we actually have lots more data than people think. There are also bodies of science that deal with 'aftcasting', which is making a weather forecast for a date in the past. These techniques have been refined to the point where we can make 1-day aftcasts for ~1850 that are comparable to modern 2-3 day weather forecasts in terms of accuracy, even with the limited data available (things like barometric pressure readings from ships).

I'm not sure about the technology used at the time, but the basic answer to your question that that they had a lot more thermometers than you expect, distributed globally too. And as we're discussing global averages, the noise about +-2F in your house isn't really the same problem.

Edit: Here's a neat 100-year reanalysis paper if you're interested. Abstract link here, full pdf available on the page: http://journals.ametsoc.org/doi/abs/10.1175/BAMS-87-2-175. "Feasibility of a 100-Year Reanalysis Using Only Surface Pressure Data"

rnovak|10 years ago

To be honest, and this is my opinion only, but 2-3 day forcasts, at least in my area, are horribly inaccurate. In my state, it's a common joke that being a meteorologist is the only job you can have where you can be wrong 95% of the time, and still keep your job.

Seriously, our forecasts are nearly always wrong. So.....saying we can make as accurate of forecasts for 200 years ago...that's not really saying much.

This is after moving to two cities in the same state, more than 100 miles apart, the forecasts still never get better.

guscost|10 years ago

Prior to the satellite record historical temperature data is much less reliable, since each source is in a local area and may have different heat island effects, calibration, or quality of instruments. Additionally not as many stations were operating in 1880, which means more uncertainty about the data we do have.

Folks have tried to get around this by finding proxies for temperature (measurements that are theorized to track temperature, like the width of tree rings for example), and then calibrating that data by comparing it with the intervals for which we have reliable data (since 1970 for satellite data, or earlier for whatever thermometer data might be suitable). The long-term trends are identified and argued based on these proxies.

So to answer your question, they can calibrate a proxy using the satellite record, see what it suggests about 1880, and combine that with the available temperature data, but that's about it.

pgrote|10 years ago

I do not know. I am ignorant of the means in which NASA gathers temperature from the middle of the ocean and presume it is satellite. If that is the case, how was the temperature in the middle of the ocean ascertained in 1880? I would presume they base it on datapoints collected using thermometers by those at sea.

bjwbell|10 years ago

In 1880 via ship measurements. They'd take out a bucket of sea water and measure the temperature. They still use buoys as part of the sea surface temperature measurement.