Below are depicted 3 temperature trends extending out to 2100. Using NCDC global temperature data, the three trends have a 2100 temperature spread of approximately 2.0°C. The green trend line represents the long-term trend, and it's end point almost perfectly matches the IPCC prediction of what temperatures will be, based on a doubling of CO2 levels from 280 ppm, the pre-industrial level (note: doubling of CO2 causes a 1.1°C increase and any further CO2 increase has almost zero impact on temperatures).
The blue trend represents the cooling trend that developed in 2001. The red trend represents the "hot" trend that climate alarmists are concerned about since its appearance in the mid-1970's. Note that the red trend is above the IPCC prediction for the CO2 doubling mark, but it is well below the highest temperatures reached in the Medieval Warming Period.
What about those purple stars? Those are several of the climate model predictions that the IPCC and others state are likely. How could any of the historical trend lines possibly make gigantic shifts up to those purple stars? Well, it's called "positive" feedback. When temperatures reach the black/green star temperature, that's when the climate models' positive feedback supposedly does its magic and boosts temperatures to the purple stars. Unfortunately, when temperatures reached the Medieval gold/red star in the past, that old magical "positive" feedback moment did not occur, nor did it occur at other past times when temperatures were even higher.
Why not? Here's why not. And for good reason, positive feedback is unsustainable in nature unless we are talking about nuclear reactions, but that's not what the natural climate is about. Global temperatures may increase, but they won't do so in either an accelerated or runaway mode as global warming alarmists and climate modellers speculate.