The earth has been warming since the end of the last ice age and will continue to do so until,the next ice age begins.
It must have been all those cave men buring whatever was available to stay warm that caused the ice age to end.
Reluctantly, climate scientist admit average temperature in the 1930's were warmer than they are today. As best as can be determined scientifically, the 1300s were warmer than today. Must have been all those bodies they burned who died from the plague. Recent studies have also shown that warming stopped several years ago.
The funniest aspect of this whole canard is that one of the scientist who started the whole global warming insanity was a leading advocate of the new ice age just a few years before he discovered he could make more money by touting warming as the problem.
Why don't people believe any of this? It may be true that the earth's average temperature has increased a bit, but
who said that it should not change over the course of time. As you said, it has increased and decreased.
People need to believe conspiracy theories. And that sells.