The original theory underlying the Rahn Curve assumed that as the size of government increased from zero, supplying law and order and some basic services, economic growth rates would increase until government reached some optimal size.
Well, it means that Stern is wrong in his discount rate, that the currently assumed social cost of carbon emissions is too high, that we should almost certainly be doing less about climate change than is currently planned and finally, that we should, again almost certainly, aim for more economic growth even at the expense of carbon emissions and climate change than is currently thought optimal.