Normally it is just a natural cycle, human temperament speeds this cycle.
The chart below shows the temperature and CO2 level change of the last 800,000 years. You can see a ~100,000 years cycle: a temperature spike (called interglacial) and the following ice age (called glacial). And you can see that we are in a warm period on macro level.
Below we have a closer look to the last 15,000 years. You can see a ~1000 year cycle of highs and lows and we are in a cold period on micro level.
Below is the last 2000 years. You can see the unusual spike at the last century.
The Earth is going to move into a micro-level warm period. The problem is that right now the warming is happening faster than usual and this is due to the sudden increase in CO2 levels among other factors. You can see the first graph to see how CO2 and temperature level changes in sync. The fear is that this kind of drastic change can trigger equally drastic reactions. It can get warmer than the usual peak or after that colder than the usual low and these can last longer or shorter, extreme meteorological events are also possible. As seen at the first graph we are in a macro-level warm period (interglacial period). Eventually we are going to enter the next glacial but we don't know if this changes would trigger that or not.