Climate reconstructions are not real time temperature data. They are estimates based on climate proxies. They are not accurate. They may have margins of error of +/- 2 degrees C. or more.
We know from anecdotes that some centuries in the past 100 years were unusually cool, and others were unusually warm.
We know from geologists that Canada was covered by a thick ice glacier about 20,000 years ago, which had melted by 10,000 years ago, with no help from CO2 emissions.
We know from physicists that our planet is not in thermodynamic equilibrium -- the climate is always changing.
The predictions of future climate doom ignore the falling temperatures, as CO2 levels rose, from 1940 to 1975 -- originally reported as almost -0.5 degrees C. global cooling by NCAR in 1975 ... later "adjusted" to no global cooling at all by NASA-GISS, gradually over many decades, so few people would notice.
The measured warming from 1910 to 1940, although based on not very accurate measurements, happened with almost no CO2 increase = natural global warming. But all natural causes of warming and cooling in the past are ignored by the IPCC, which described natural causes of climate change as "noise" in 1995.
The greatest fraud is 65 consecutive years of predictions of a coming global warming crisis, first specified in the 1979 Charney Report, of warming at a rate much faster than has been experienced since then. By "since then", I mean the period since 1979 with reasonably accurate global average temperature compilations made using data collected by NASA weather satellites in the troposphere -- a stable environment where the greenhouse effect occurs.
The weather satellites capture about 95% of Earth's surface temperatures, and require guessing for the 5% of the surface area over both poles. Alternative land surface temperature compilations require at least 4x more guessing for missing data (called "infilling", but guessing is more accurate). And that is the percentage of guessing done today -- there was far more guessing one century ago, as you can clearly see at the link above.
Prior to World War II, there were too few weather stations on land. Prior to 1920, the few weather stations were mainly in the US, Europe and the West coast of Australia.
Ocean surface temperatures were "measured" haphazardly with buckets and thermometers, almost entirely in Northern Hemisphere shipping lanes. There were very few ocean surface temperature measurement in the Southern Hemisphere, which is mainly oceans.
The bottom line is no temperature data before 1979 are accurate enough for science ... other than the broad statement that Canada is warmer now than it was 20,000 years ago, when covered with a thick ice glacier.
There are many hundreds of climate reconstruction studies that DID NOT FIND a flat temperature trend for several thousand years before the 1900s, as falsely claimed by the IPCC.
There are three land weather stations in Central England that have measurements since the 1600s. They reflect about +3 degrees C. warming in Central England since the cold 1690s, during the coldest decade of the Maunder Minimum low solar energy period. There are no other measurements worth quoting before satellite data starting in 1979.
Another 50 to 100 years of global warming similar to the 1975 to 2020 warming, would be even better. Especially when you know the prior warming period mainly affected colder, high latitude areas of the Northern Hemisphere, mainly during the six coldest months of the year, and mainly at night (TMIN). Think of warmer winter nights in Siberia. And let's hope for more of that !