I just saw this article about a new climate-change index [LINK]. I'm not sure whether I agree with how they are defining the index, but I do totally support the existence of these kinds of indices. If one (or a few) can gain popularity, I think they will be a great way to communicate the degree of climate change the Earth is experiencing. One pitfall of these indices is that you want to measure the climate change, and not things that are not part of the changing climate. For example, this index includes atmospheric CO2 concentration, but this is the forcing on the system, not the response of the system. So imagine we stop CO2 emissions, then the CO2 levels in the atmosphere will stay about the same or decrease, but that doesn't mean the climate won't be changing still (cf. Soloman et al. 2009). On the other hand, arctic sea-ice extent, which is also included in this new index, has potentially large natural variability. I think this comes out when we consider the rapid sea-ice melt season of 2007, which was largely due to a high pressure system sitting in one place for a while (e.g., Zhang et al. 2008). Depending on how they implement the index then, the arctic sea-ice term might be diminishing the overall climate change "factor" because the short-term trend is for sea-ice recovery, but the long-term trend remains and shows decreasing summer sea-ice extent. I'm sure they thought about these issues when designing the index, but I have a feeling we're going to see a bunch of these indices come and go over the next few years. Eventually a few will get picked up and become standard.