Widespread global use of thermometers didn't exist before the early 19th century, so how do we get estimates of global temperatures before that? We use proxies. For example, we can estimate past annual temperatures from tree rings. NOAA has a good explanation here. Clearly, the more proxies we have, the more certain we are about temperatures pre-thermometers.
Planet Earth is warmer than it has been for at least 2,000 years, according to a study that took its temperature from 692 different “natural thermometers” on every continent and ocean on the planet.
In the most comprehensive assessment of how the climate has changed over the period to date, researchers looked at a host of sources of historic information, including tree rings, ice cores, lake and sea sediments, corals, mineral deposits and written records.
What they found confirmed the famous “hockey stick” graph, showing an undulating, but broadly flat, line followed by a sharp uptick that begins at around 1900.
The only plausible explanation for this sudden change is fossil fuel emissions, which have increased the level of carbon dioxide in the atmosphere from about 280 parts per million in the 19th century to more than 400 today.
The warming effect was predicted by the Nobel Prize-winning Swedish scientist Svante Arrhenius in 1895.
[Read more here]
The "Hockey Stick" chart of global temperatures was first devised by Mann, Bradley and Hughes (MBH) in 1999. The denialists had an immediate bulgy, and Mann's life was threatened. Makes you wonder doesn't it--why should ordinary folk care about some academic study on global temperatures, unless of course, they were in the pay of those whose profits and revenues were threatened by steps to reduce global warming?
This new study confirms the general shape of the MBH99 study. One of the authors of the study describes how much work they had to do to get this reconstruction here.
The chart below shows the original MBH Hockey Stick with an overlay of this latest estimate. The blue line shows a smoothed version of the original MBH99 calculation. The green line shows a 30 year moving average of the latest estimate. The red line is a smoothed version of measured temperatures. The pale blue shading shows the error margin of the original MBH99 estimates. Note that the error term is less likely at the edges and more likely towards the centre. Note also that the error term of the new estimate should be appreciably lower because more time series were used in its calculation. Also, the red line only goes up to 2013. Since then temperatures have risen another 0.3 degrees.
|The original northern hemisphere hockey stick graph of Mann, Bradley & Hughes 1999, smoothed curve shown in blue with its uncertainty range in light blue, overlaid with green dots showing the 30-year global average of the PAGES 2k Consortium 2013 reconstruction. The red curve shows measured global mean temperature, according to HadCRUT4 data from 1850 to 2013. Source: Wikipedia|
Climate change: Nearly 700 'natural thermometers' demonstrate the world is warmer than its ever been
The Hockey Stick is Alive; long live the Hockey Stick
The Hockey Stick Graph