In scientific language, the word "significant" has a very specific meaning. It means that the probability of an observation being random rather than being a result of the specific experimental conditions is less than 5%--or, as we scientists express it, the P value is less than 0.05. (P<0.05). Scientists use many weasel words (it reflects our acknowledgement that we can be wrong), but "significant" is not one of them.
Yet it is being expanded beyond the capability of science in the situation. We cannot conduct two worlds, a control world without anthropogenic input and an observation world with it, and make observations for both at the depth possible with modern meteorology.
Except that is not the context. Significant was not used to describe the likihood, but it was used as a magnitude, so its meaning is unclear.