Catastrophic climate change is hardly something new for the human race. The list of climate catastrophes that did in civilizations, cultures, and, very nearly, the whole human race is long but also rather poorly known and understood. Modern humans appeared in a particularly catastrophic geological time, the Pleistocene, in which giant ice sheets repeatedly covered big chunks of land and drove us from much of Europe (or exterminated those unlucky enough to be caught). The genetic evidence makes it clear that our population went through repeated bottlenecks when our numbers were reduced to a few thousands.
The advent of agriculture and food storage, in the form of crops and herds, allowed our numbers to multiply rapidly, and may have made us less vulnerable to certain vagaries of weather but even more vulnerable to longer term climate shifts. Many a cultural or civilizational collapse can be traced to a widespread drought, including the collapse of the First Kingdom in Egypt and the massive collapse of most of Bronze Age civilization around 1177 BC.
Before agriculture, humans can be considered to be essentially innocent bystanders to the processes of climate change, but agriculture gave us the ability to modify climate on the local and regional scale, which we frequently did by means of deforestation, overgrazing, and other destructive practices. Affecting climate on the global scale probably* had to wait until the industrial revolution and widespread use of fossil fuels started increasing the amount of CO2 in the atmosphere.
Since then atmospheric CO2 has increased by 45%, and temperature has been increasing. CO2 continues to increase and the rate of increase also seems to be increasing, despite mostly unserious efforts to control it. Temperature would continue to increase for a long time even if fossil fuel burning ceased tomorrow, which it clearly will not. The supposed treaty goal of holding temperature increase to 1.5 C is quite likely completely unrealistic, so it is clear that we will have to adapt to substantial climate change. So how bad are the effects likely to be?
I will focus on the obvious, since many uncertainties exist. Temperature by itself may not be the biggest threat. A recent study found more than twenty times as many deaths due to cold as to heat. Indirect effects are more frightening. The last time temperatures were a couple of C above the preindustrial average was shortly after the last ice age, and those were pretty good times for humans - the dawn of agriculture and all that. On the negative side, sea levels were several meters higher, which would be catastrophic for low islands, coastal cities, and places like Bangladesh and Florida. Even more worrisome is the treat that some of the world's agricultural breadbaskets, including much of the US, might become permanently drought stricken.
I strongly support action to limit CO2 emissions, but even in the most optimistic scenarios, much of these effects will happen, so we had better start preparing for them.
*Though some have argued that methane produced by rice farming and other agriculture started affecting climate as much as 8000 years ago.