This can cause confusion, and much worse. Sometimes people even use this confusion to promote their own goals, politically or otherwise. This is what has happened with the current climate change debate. An objective look and scientific data show that climate change is in fact happening, that, historically, a shifting climate rather than a stable climate is the norm on Earth, and there is a relationship between carbon in the atmosphere and global temperature – though there are also a great deal of complicating factors as well.
On the one hand, it would appear that it is very difficult to establish a historical trend of world temperatures – we have only been measuring and recording temperatures for around the past one hundred and fifty years – before that we supposedly know very little. And on the surface this is true, we do not know exactly what temperature existed before the current period. But this belies the fact that there are excellent proxy measurements that have been shown to be relatively accurate when calibrated to current temperatures. We have ice-core samples that demonstrate how much snow has fallen in particular years, which can give a rough gauge of temperatures (National Research Council), as well as tree rings which show how well trees grew in a particular year (NRC). This data, when taken collectively and studied with statistical and scientific eye, can give a relatively accurate approximation of temperature over the last two thousand years. This can be combined with historical records (good and bad harvests, for instance, or records of when lakes and rivers freeze and when they do not), to be extra sure that the calibration is accurate, which has been done by several scientists/historians (Edwards 129). But this is something that lay people have a hard time understanding, which might cause some to doubt their findings.