I've always been intrigued by the Doomsday clock a device created by the Bulletin of the Atomic Scientists to represent the likelyhood of mankind bringing about its own destruction. It's currently at 6 minutes to midnight, and has ranged between 17 minutes (as the cold war wound down in 1991) and 2 minutes (the testing of the hydrogen bomb, and mounting global tensions in the early 50s). While mostly focussed on nuclear doom and political armageddon, they also take into account climate change and biosecurity. As an object it's easy to understand and provides a powerful image of our ticking towards our end. As a generator of news and public interest it's also powerful. It combines the views of concerned experts into a press-worthy event and allows a way for opinion to drive coverage.
Climate change will never provide a route to our immediate extinction, only a set of hardened circumstances for our older selves and our descendants, so metaphores of catastrophy don't match its effects. Nevertheless, some way could be found to deliver a punchy message about its impacts or likelyhood that can be modified to as changes in politics science or technology, drive us towards a solution or a worsening of our situation. Picking an actual impact would be problematical, as we're only just becomming able to predict these, and the science surrounding sea ice retreat, ocean acidification or species extinction would change the severity of the index and so, perhaps, remove its power as a public point of debate.
Perhaps something as simple as a thermostat would work, with the dial swinging in and out of our comfort zone, or the controls for an oven burning a pizza to a crisp.
(Thermostat from here)