I take issue with the central proposition of the paper, that “Key climate/earth system parameters that need to be restored to safe levels are:
ocean heat content
global surface temperature
|Geopiracy by ETC Group|
The climate science that I've seen referred to over the years on this topic suggests that ocean temperature rise is basically irreversible on human lifetimes. If we stop adding greenhouse gases and stop adding heat to the atmosphere, it may gradually cool back to where it was, but over centuries. In the meantime, warmer oceans means warmer climate and there's not much can be done to change it. Warmer oceans and climate also drive sea level rise.
I haven't seen research on how fast ocean acidification may be reversed, but I suspect it's similar if not slower.
I'm very happy to hear of research which contradicts me on either of these points, of course. But in the meantime, there is only one crucial parameter that we know for sure we can control: the excess greenhouse gases being added to the atmosphere every day, month and year.
(You could add that we can also stop destroying the biodiversity that gives ecosystems some stability and/or adaptability in the face of climate change. Indeed, biodiversity loss is a close second to climate change on the scale of major ecological threats to human civilisation. We will have to work to reverse this, too.)