Sorry, it's easy to forget that many people have never encountered these facts before. Climatologists continuously rewrite history which is why it's so confusing. In 1999, NASA climatologist James Hansen was wondering why the USA hadn't got any warmer in the 20th century:

https://web.archive.org/web/20090404150607/http://www.giss.n...

This is a very interesting historical article which NASA has long since deleted from their website. Look at the two graphs, in particular the graph of US temperatures on the left. The text says this:

How can the absence of clear climate change in the United States be reconciled with continued reports of record global temperature? Part of the "answer" is that U.S. climate has been following a different course than global climate, at least so far. Figure 1 compares the temperature history in the U.S. and the world for the past 120 years. The U.S. has warmed during the past century, but the warming hardly exceeds year-to-year variability. Indeed, in the U.S. the warmest decade was the 1930s and the warmest year was 1934.

OK, so in 1999 the historical record said there was no clear climate change in the USA and the hottest year was 1934. Temperature declined from +1.5F (anomaly) to about -0.2F in 1970. Now look at the modern NASA temperature graph:

https://data.giss.nasa.gov/tmp/gistemp/CUSTOM_GRAPHS/87aa1e2...

If you look at the 1900-2000 period in both graphs, the story is very different. Now 1999 is hotter than 1934. In the old graph, the period between 1980 and 2000 doesn't do much. In the modern graph you see rapid warming.

So what's going on? As you can see, by the year 2000 climatologists were getting seriously bothered by the US data. It can't really be called global warming if it isn't global. Also, the US temperature network had a lot of problems but it is by far the most comprehensive for 20th century data. Much of the world doesn't have any data at all for much of the 20th century! Theory and observed data didn't match, so they went looking for reasons to change the data. If you look hard and long enough you can find such reasons, and they had plenty of time. The weather stations were never intended for climate monitoring so their siting and procedures weren't good enough for that. More and more adjustments started being made until the picture you see today: they cooled the dustbowl years and warmed the most recent history. Fast forward 20 years and they're so comfortable with changing historical data that some temperature records rewrite the entire history of every thermometer reading, every month.

This isn't really kosher, obviously, hence the construction of the new Climate Reference Network. Unlike the historical network this one is built for climatologists, with very careful siting away from heat sources. It shows no warming. So to recap: in 1999, the weather stations show no warming in the 20th century. A few years later a new network opens, and it shows no warming either. The appearance of warming comes from big piles of FORTRAN that reprocess the old data. Although this is about the US data, the global thermometer network has even bigger problems with siting, rewritten histories etc.

Hopefully that's enough to go on, if you feel like researching the topic further!