Evidence for global warming has taken a really long time to gather. If you measure temperatures in a place like the Arctic once a year, it takes a lot of years to spot a trend that things are getting warmer. Also there are lots little fluctuations that can hid bigger trends. Also, not all scientists are convinced by the global warming evidence that’s been found so far, and are trying to come up with different theories to explain it. And even scientists who are convinced that global warming is happening have different ideas about why its happening and what we should do about it.
Science is like that – you gather evidence then try to interpret it in a way that makes sense, but evidence on a planet sized scale takes time.
In addition to Katherine, humans have only been around for a very short time compared to the life of the Earth and have been taking measurements for an even shorter time. It takes many measurements to give an accurate picture and depending on the time scale it can give you very different results.
It has taken a long time to develop technology that can give us insights into the past and the development of the earth to make sure we have a more accurate view of what has been happening!