The phrase "huge inertia" means that something is resistant to change or movement. It refers to a situation where an object or person requires a lot of force or effort to start or stop. Full definition
The problem is, our instinct to wait and see is at odds with another characteristic of climate change, which is that it's a problem with huge inertia. (dotearth.blogs.nytimes.com)
Some of the earlier estimates seemed to imply a «headroom to 1.5» of less than a decade of current emissions — clearly unachievable given the long timespans and huge inertia. (scientificamerican.com)