IPCC Precipitation Map

IPCC Precipitation Map

Originally uploaded by heinemanfleck.

By what seems like a series of coincidences (or not?), I’ve been reading and writing and trying to write lately about maps – the information in them, the things cartographers leave out of them, their implications for understanding things about the world around us, and also the choices we make about what we don’t need to understand. So I’m launching a new category today on Inkstain: maps.

Today’s entry is Figure 6 from the IPCC report released yesterday. Given the frequent use of the phrase “global warming” as shorthand for this complex thing we all talk about, much of the attention ends up focused on temperature. This is reasonable, given that globally averaged temperature is an area of relatively greater understanding. But we don’t live globally. We live “here”, wherever “here” might be. And central to our life “here” is how much precipitation falls from the sky. Figure 6 in the new report attempts to convey in a visually understandable way what is known and not known about this centrally important issue.

The map’s color choices are obvious: brownish means dry, blueish means wet. (As an aside, I was at a talk yesterday where a guy used the opposite color scheme on a climate map – the red end of the rainbow for wet anomalies, blues and greens for dries – very confusing.) But beyond wet and dry, there’s a second kind of information that we also need to know, which is the confidence of the forecast. Places with no confidence are just left white. Places with higher confidence get a little stippled overlay.

I found this map very useful yesterday as I was writing about the IPCC report.


  1. what bodes ill in that graph is all the white covering most of North America in the JJA (right) panel. It’s white because the models that went into the ensemble disagreed roughly evenly about the sign change. So what’s a poor policymaker type to think? The precip picture for the west isn’t just winter/spring SWE in the mountains, obviously.

  2. For my purposes (water supply in NM), DJF is the critical time period. But if I was an arid land rancher/farmer who depends on cattle that eat grass, I’d be left clueless by this. What I like about the map is the efficient way it shares that cluelessness.

    On the CPC’s long lead forecast maps, they used to leave areas with no sign in the forecast white, with a big “CL” to indicate “climatology”. I also used to kid them that it really stood for “clueless”. Now they use “EC” (equal chances) instead.

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