Little to no proof. In fact, based on the evidence, the opposite is true - countries are using more resources than ever before. New technologies have barely made a difference. See here: World Energy Outlook.
US government observations agree there’s been no difference (Source):
See also here, where the IEA states emissions will rise exponetinally regardless of efforts to switch.
Great question! Simplest answer: thermometers. Simple instruments such as thermometers and barometers have been used for centuries. Governments began to collect data from these instruments beginning in the early 1700s. (There are early data sets, but these focused on local or route specific locations rather than globally. For example, shipping companies collected ocean temperatures during the 1600s along specific routes to report conditions to insurance companies.).
The old-school instruments were placed in locations all around the world (locations ranged from trees, church steeples and clocks, tall poles, cliff faces, to just stuck in the ground). Governments collected the temperatures typically for military, farming, and shipping purposes.
The U.S. Weather Bureau, established in 1735, was sporadically managed by a few individual states (rather than the Federal Government). The bureau collected local information - not global.
In 1814, the U.S. Federal Government established the U.S.’s first nation wide weather service. Army doctors and ‘war’ hospitals were instructed to keep diaries of local weather. But, again, this was not a global system.
In 1870, President Ulysses Grant established the U.S. National Weather Service (NWS):
The beginning of the National Weather Service we know today started on February 9th, 1870, when President Ulysses S. Grant signed a joint resolution of Congress authorizing the Secretary of War to establish a national weather service. This resolution required the Secretary of War:
“to provide for taking meteorological observations at the military stations in the interior of the continent and at other points in the States and Territories…and for giving notice on the northern (Great) Lakes and on the seacoast by magnetic telegraph and marine signals, of the approach and force of storms”
After much thought and consideration, it was decided that this agency would be placed under the Secretary of War because military discipline would probably secure the greatest promptness, regularity, and accuracy in the required observations. Via NOAA
The NWS worked internationally. It collected data from its own instruments, and also from data shared by other countries, such as Denmark, France, India, and the U.K.
The NWS’s information was collected over time, and digitized into big data sets. These sets are used today!
The chart below shows temperature data over 1,000 years. (NOTE: This chart is from wikipedia entry “Temperature record of the past 1,000 years." I do not endorse this chart. I’m posting for illustrative purposes to help answer anon’s question about records from 1880).
Note the black line (far right). It shows collected instrument data from 1850 to 2004. Data prior to 1850 is collected by climate proxies.
Finally, if you’re interested, you can read about the weather data sets collected in the 1850s. This paper, Uncertainty estimates in regional and global observed temperature changes: a new dataset from 1850, covers the history of that data, as well as issues with using it in modern climate models.
Hope that helps!