Climate change, in particular the warming, has resulted in two things from the perspective of beetles biology. The first is those overwintering conditions. Typically, when we get good long stretches of below -35o, -40o, for several days or weeks at a time, that is really effective at killing those beetles during the winter. We typically see high mortality rates in those cold conditions.
However, we haven't been getting those types of cold winters. As a result, those beetles are relaxed from that factor, and we end up with larger populations come summer.
The other factor is that the warmer climate is putting on a lot of moisture stress. It's a lot drier. Those trees have reduced capacity to defend themselves against the beetle when they're water-stressed. That further contributes to the susceptibility of the forests.