Being pessimistic or optimistic doesn't preclude measuring things. Many people are pessimistic about climate change precisely because we're measuring the amount

Being pessimistic or optimistic doesn't preclude measuring things. Many people are pessimistic about climate change precisely because we're measuring the amount of CO2 being released and because we're measuring changes in temperature, weather, etc. over time. Same with believing and disbelieving - sure, some people believe in religious-type stuff that has an absence of evidence, but there are lots of other things to believe or disbelieve based on evidence. I believe that Covid vaccines help to prevent serious illness precisely because there is a tremendous amount of evidence to that end.