The same way to deal with depression about anything. Depression is an illness that renders one incapable of helping themselves or others and debilitates people from contributing to the solution to the problems which contributed to the depression. Depression should be treated as an illness to be cured before anything else.

As far as being not being depressed by events beyond one's control, it requires effort since even being aware and concerned by such problems of this magnitude is highly unnatural and highly unintuitive. It is necessary to consciously come to peace with the world and humanity as it is. The universe is chaotic and we have as much control over the collective will of billions of completely ignorant and afraid people as we do over natural disasters just with less ability to predict when issues will occur. Although it is the case that we have a good grasp of the environmental issue, we have absolutely no grasp of how get plutocrats not to behave like plutocrats having no care for anything or anyone other than increasing their personal fortunes meaninglessly or to get the majority of people to understand the degree to which it is a problem that we continue to allow this. Being upset that humans are failing to achieve stable societies just as we have always failed for the entire 10,000 years we have been trying to achieve them is having unrealistic and unfair expectations of our species. Everyone is trying their best and no one knows what they're doing really. It may be scarier to consider how much less agency people have than they believe they have initially, but it does allow one to have the comfort of more consistently helpful expectations of oneself and others.