I've seen that graph posted before. Looks to me like global cooling will be the bigger problem longterm.
The data for that chart comes from N.O.A.A. so it is based on good scientific data.
There are things we can conclude from the data, and things we cannot.
1) Global temperatures have been higher than now
2) Global temperatures cycle on a period of ~120,000 years
3) Global temperatures are very likely to drop again
4) Humanity has not caused the current heat cycle
5) Atmospheric C02 complies with 1,2,3 above
6) Atmospheric C02 LAGs Global temperature variation
7) Global temperatures drop after C02 reaches ~300ppm
8) Global temperatures rise after C02 drops to ~200ppm
Now 7 and 8 do not prove causal effect, but they do suggest that global warming/cooling, if caused by atmospheric C02, is driven to reverse itself, not increase itself.
You're absolute correct...I think. Look, the long-term mega-trend clearly shows that the Earth is cooling when compared to its beginnings some 4.5 billion years ago. Geologic time starts at the Hadean Period (or Eon) for God's sake. The name was given for a reason and I don't think it was because this was a really cool time in Earth's history.
There's another issue: radioactive decay from parent substances have been providing a whole bunch of heat to Earth since it has formed and been added to by meteoric strikes. As the parent isotopes turn into less radio active (and less heat producing) daughter isotopes, there will be less and less heat generated from these chemical processes the further we go into the future. The only thing that may save us from the cooling -- and perhaps part of God's plan? -- is that the Earth may be moving closer to the sun, countering the cooling effects. All of this is speculation though as to what God is doing...shoot for all I know, the opening pages in the Book of Genisis may all be literally true.