There Is a Number for That

I harbor the unproven thesis that for any phenomenon, no matter how complex, there is someone who can quote a single summarizing number purporting to represent the topic in question.  That can be very handy, or very confusing, and we don’t always have the time to figure out which is which.

I also harbor the more-proven thesis that without a good sense for the meaning of the numbers punctuating our information-age discussions, our dialogue – much less any analysis – is going to be content-free at best, and badly wrong at worst. Where possible, I think we owe it to ourselves to get that sense, and not rely solely on experts to tell us whether something is really small, or really big, or just about right.

What brought all this up was a couple of recent conversations about climate change. Before we talk about rising sea levels; neurotic and violent weather cycles; insect, crop, and animal habitat destruction; depleted sea oxygen, or any other feature attributed to rising global temperatures, we have the underyling phenomena of rising temperatures themselves.  Rising temperatures are at the origin of other climate-related problems, but ironically temperatures may be the least discussed, and most poorly-understood, of all climate-change phenomena.

It’s really not surprising.  We can figure out what a 20-foot rise in sea levels would mean, or three years without rain would mean, or what 30 hurricanes per year instead of 15 hurricanes would mean.  But if I say that the “rate of global warming” is roughly 0.05 F/year (0.03 C/year),  it’s not very clear how we can relate to that.   Is 0.05 F/year a lot?   What does that number really mean?

Global warming rates aren’t the simplest things to grasp.  Compared with concrete and visual pheonema warming rates are dry and abstract.  And unless we’re already thinking about the problem we probably don’t have good comparisons handy.  We know 80F (about 27 C) is getting hot, we know that 32 F (0 C) is freezing, and we know that 0.05 F (0.03 C) is something we could never detect on our own.   In fact, even if we added up a decade’s worth of 0.05 F/year changes, we couldn’t detect it.  If that weren’t enough, the rates are also very aggregated numbers – they are a time trend over many years; they give an average temperature over an entire year; and they finally average over many points on the surface of the Earth.

I don’t blame people for thinking it’s not worth the trouble, but I wanted see if I could align the rates with something that I understood a little better..To that end, I put together a scale of concrete examples for comparison – namely looking at the annual average temperature of some cities here in the US, along with a few key temperature metrics – in particular the number of months experiencing frost, and the number of months in which cooling equipment will be used.

Before looking at that, what precisely is this rate number, anyway?  All of the numbers quoted here are the 50-year trending of land-based warming – the global and US rates are essentially identical, at 0.05 F/year (0.03 C/year) and 0.049 F/year – as reported by the NOAA National Centers for Environmental Information site.  (Note: the NOAA sometimes uses mixed units, they often report  degree F/decade, but sometimes report degree C/decade).  The US warming rates over the last 50 years look like this:




There are significant flucutations in the average temperatues, but also a clear trend.  (Unsurprisingly, the fluctuations on a global scale are less.) The NOAA site is excellent, but I didn’t think they made it particularly easy to estimate the error associated with the temperature trend – however did find a page sugggesting it’s about 20%.

To help cast this trend into concrete terms, I put together several charts with frames of reference for the temperature trending.  Each chart gives a number line of annual average temps for selected US ciries. Applying warming rates of the last 50 years,  each city would reach temperatures in 50 or 80 years equivalent to warmer cities now, and those are shown. It’s a concrete but approximate metric, offering a sense of how 2 to 5 F differences in annual average temperature translate to practical measures.



Chart 1: 50- and 80-year temperature-warming horizons.

The left-hand side of Chart 1 aligns the annual average temperature of US cities with the more practical metrics of number of cooling and frost months.  There’s a correlation, albiet a rough one, between these metrics. The annual average temperature doesn’t completely describe climate, but it does give a general picture.  The annual averages can be deceptive, because a small difference describes quite different climates. – Detroit and Atlanta have different climates and numbers of hot/cold months, but differ only by 13 F (7 C) in annual average temperature.

The right-hand side of the chart compares current temperatures to future state,  using the same city-based number line.   Detroit will achieve temperatures up to those currently seen in Columbus;  Nashville will achieve temperatures up to those currently in LA.    Such statements are guidance rather than prediction, intended to give a sense of what several degrees F in average temperature can mean in terms of climates we currently know.

In the same spirit, Charts 2 and 3 are similar to Chart 1, and look at shifts in the number of frost and cooling months  It can be seen that the difference induced by 50 to 80 years of continual warming often aligns with a month more cooling weather, and a month less frost weather.

This scaling exercise isn’t a true climate predictions – for one thing, the meaning of a degree’s change in annual average temperature in the current climate won’t be identical that that in a warmer future climate, and as I mentioned the association between average temperatures and frost/cooling months isn’t precise even for current climates.  Its purpose is to give concrete and reasonable handles on an abstract but commonly-quoted number.

That said, as a way assigning conceptual signficance to a warming rate, I personally found the exercise helpful – scaling makes the rate number concrete, and as a result there is a (not particularly pretty) picture.  Extended over less than a lifetime, the warming rates of the last 50 years effectively could “move everyone south” by a distance of roughly 100 miles, and cost many of us a month of winter and add a month of summer.   It’s easy to see that those kinds of differences are more than sufficient to alter habitats, crop cycles, and weather patterns.  And when the time scale to correct and reverse current trends is on the order of decades, it’s also easy to see why climate scientists almost universally recommend urgent policy action.


Chart 2: Additional cooling months on the 80-year horizon.



Chart 3: Reduction in frost months on the 80-year horizon.

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