Systematically, for numerous issues, those advocating for action to mitigate our headlong rush over the Catastrophic Climate Chaos Cliff understate the case. The scientific community, notably the International Panel for Climate Change (IPCC), is ‘conservative’ — taking a very measured (scientific, one might say) approach with use solely of already published peer-reviewed literature which makes the IPCC reports dated by years the moment they’re released. Advocates for action on environmental issues often overestimate the costs of action in part because it so difficult to assess the dynamic nature of technological, policy, and cultural response to mandates to reduce pollution. And, supporters for action typically understate the benefits from action due to, in part, the difficult analysis to assess systems-of-systems benefits. There are very understandable (even defendable) reasons for this, but (even though we can find the examples that break this rule) remember a systemic truth:
Yesterday, 
Oil Change International provided an example of this fundamental truth with the (powerful) graphic to the right (full size table here). What a powerful set of images to get across the point that developed countries are subsidizing fossil fuels more than they are financing climate change action in the developing world. Wow. A powerful image that will leave an impression.
There is a problem. This significantly understates the extent of subsidization of fossil fuels for a very simple and very significant reason: externalities are not included in the equation. Increased cancer rates, polluted rivers, acid rain, greenhouse gas emissions, etc, are not included, they are ‘external’ to the graphic just as they are ‘externalities’ to the financial contracts to buy energy. That such costs are ‘externalities’ is one of the major reasons why we are failing to restructure our energy systems to a low-carbon future — advocates of action understate the case for action when they leave such externalities external to the discussion.
Note: See here for a discussion, using the social cost of carbon and the U.S. burning of coal, of how Oil Change International’s graphic/analysis understates the case.