Like most natural mechanisms, the global climate system has a certain amount of built-in give. But push past that point, and forcing factors, or environmental processes that affect climate, take over. This could create feedbacks that will alter climates for decades or centuries to come.
One nightmare scenario begins when global climate change melts arctic ice too quickly. As the resultant freshwater spreads across the North Atlantic Ocean, it shuts down a looping global current vital to global climate called the thermohaline circulation (THC). The THC runs off a blend of heat and density, and its motion helps transport heat around the world. For example, Atlantic surface waters warm up near Florida and flow northeast toward Europe, which partly explains why London has a temperate maritime climate even though it shares the same latitude with Calgary, Canada and Kiev, Ukraine.
Research suggests the THC has shut down in the past, likely due to massive freshwater dumps that occur during waning ice ages. Whether such a shutdown will occur because of climate change remains unclear, but the bulk of data says the THC will more likely experience a slowdown [source: Hausfather].
In the unlikely worst case, however, the effects of a mini-ice age combined with other climate change stresses could be nothing short of seismic.