Open Mind

Cold Comfort

July 8, 2008 · 12 Comments

In the last post, I looked at very hot weather for European stations (data from the European Climate Assessment & Dataset Project). I computed the degree-days, the sum of degrees over 90°F for days when the high temperature exceeds 90°F). But of course, global warming doesn’t just mean more hot weather, it means less cold weather as well. This time I’d like to look at both hot and cold — not the extremes, but the requirements for heating and cooling based on temperature.

This is usually estimated by the count of heating degree days. From Wikipedia:

the number of heating degrees in a day is defined as the difference between a reference value of 65°F (18°C) and the average outside temperature for that day. The value of 65°F is taken as a reference point because experience shows that if the outside temperature is this value then no heating or cooling is normally required. Occupants and equipment within a building usually add enough heat to bring the temperature up to a more comfortable level.

Suppose, for example, that the average temperature for a given day is 55°F. Since this value is ten degrees lower than the reference point of 65°F then one would say this is a ten degree-day. Obviously, the outside temperature is not always constant, so one needs a method to determine the average temperature. A simple way to do this is to compute the arithmetic mean of the high and low temperatures for the day. While not always correct, this is sufficiently accurate for most purposes and is done for practicality because these temperatures are always recorded by the weather bureau. Thus, in the previous example, if the high temperature were, say, 65°F and the low 45°F, then the average would still be 55°F for a ten degree-day.

Heating and cooling degree days can be added over periods of time to provide a rough estimate of seasonal heating and cooling requirements. In the course of a year, for example, the number of heating degree-days for New York City is around 5,000 whereas that for Barrow, Alaska is over 20,000. Thus, one can say that, for a given home of similar structure and insulation, four times the energy would be required to heat that home in Barrow than in New York

So I took daily mean temperature from ECA, and used it to compute heating degree days. I summed all the positive values for the year to compute heating degree days when the weather is hot, a number which is more commonly referred to as cooling degree days because it’s an estimate of how much cooling is required for comfort. I also summed all the negative values (actually, the absolute value of all the negative values) to compute what is often meant by heating degree days, an estimate of how much heating is required for comfort. As before, I did this for the longest single record in the ECA database (in this case, since we’re using daily mean temperature it’s Stockholm, Sweden), and computed an average for ECA stations with near-complete coverage of the 20th century (and into the 21st century).

Here’s how Stockholm has been feeling the heat (these are “cooling degree-days,” i.e., the amount of cooling needed for comfort because it’s too hot, the red line is a 30-year smooth to get the climate from the weather):

Frankly, Stockholm doesn’t require a lot of cooling because it just doesn’t get very hot there (compared to other inhabited locations). But the amount required for comfort has definitely increased in the last few decades; in fact it’s doubled since 1970.

Stockholm requires a lot of heating for comfort because it does get cold there, more so than most inhabited areas. As the climate has warmed, the amount of required heating has decreased:

The decrease has been substantial (and statistically significant), but Stockholm requires so much heating that it’s a much smaller fraction of the change in cooling needs.

Of course that’s just one station (albeit the longest record in the ECA data); here’s the average cooling required to “beat the heat” for ECA stations with near-complete coverage of the 20th century (and into the 21st):

Here’s the heating required to stave off the cold, averaged over the same stations:

What’s happened in Stockholm has, in general, happened in Europe. Warming climate has led to less heating required for comfort, more cooling.

Humans expend energy for both heating and cooling. I don’t really know how much energy is required to compensate for “too hot” compared to “too cold.” But if they’re the same, then the energy requirement will be proportional to the degree-days, whether heating or cooling. Under this assumption (which is only an approximation at best), we can compute the “sum of degree-days” by simply adding the heating-degree-days and the cooling-degree-days, to estimate how much energy is required for temperature comfort. Applying this to the average of ECA stations, we get:

From this we see one very real, palpable benefit from global warming (for Europe at least): less energy is required for temperature comfort than in the past, and we can fully expect even less to be required for the future.

If that were the only effect of global warming, then it’d be great! We could feel free to “turn up the A/C” and cool off during summer, knowing that we’d easily recoup that energy during winter due to less need for heating. Alas, that’s far from the only impact of global warming. There are nasty things like mass extinctions, ecosystem collapse, sea level rise, more powerful and chaotic storm activity, changes in the hydrological cycle (drought and flood, deserts turning to swamps and swamps to deserts), and the simple fact that life on earth has adapted to conditions of the recent past, and any change in the environment puts stress on the ecosphere. Given the litany of unpleasant effects, the reduction of heating requirements in a warmer world is … cold comfort.

Categories: Global Warming

12 responses so far ↓

  • S2 // July 8, 2008 at 9:46 pm

    I don’t know about Europe as a whole, but in Scotland air-con is conspicuous by it’s absence. We don’t spend money on cooling.

    Maybe that’s just because we’re Scots, though. :)

    Joking aside, I think it is pretty rare in Europe - which is one of the reasons why the 2003 heat wave killed so many people (including one of my relatives).

    [Response: You have my sympathy; may he (she) rest in eternal peace.]

  • Joseph // July 8, 2008 at 10:16 pm

    Is there any merit to the idea that GW means there will be warmer summers and colder winters?

    I plotted difference between Jul-Aug and Feb-Mar temps from a Sea surface NH dataset, and the trend looks flat to me. So just based on that, I’d say ‘no’.

  • John Mashey // July 8, 2008 at 10:34 pm

    Of course, as I’m sure tamino knows, it’s more complicated than that, since:

    a) The cost of heating and cooling depends on relative cost of heating fuel and electricity in a specific area, and then on building design.

    b) But also, heating and cooling are not symmetric, especially in dense urban areas subject to UHI.

    When it’s cold and people are heating buildings, heat escapes, keeps ambient temperature higher, thus lessening need for heat. Even in Barrow Alaska, the UHI effect added 2.2C over nearby rural areas. See:

    Put another way,when it’s cold, people inc cities help each other out by heating, and of course, inside buildings, running lights, computers, refrigerators, ovens, etc generates waste heat as well.

    But, when it’s hot, everything works the wrong way. Air conditioners exhaust hot air, raising the ambient temperature, which makes air conditioners work harder. Neighbors are actually making it harder.

    I haven’t seen a quantification that puts together these effects with degree days, but there’s not doubt that the effects are real. In either winter or summer, if I go down the hill into Palo Alto, it gets warmer, and during the summer, walking along a (not really dense) street, one can hear the hum of the ranks of air conditioners, and it gets really hot.

  • georgedarroch // July 9, 2008 at 1:27 am

    It also requires about twice as much energy to cool a volume of air by one degree as it does to heat it, IIRC.

  • John Mashey // July 9, 2008 at 2:03 am

    George: have you a reference for that? I looked but didn’t find anything before I had to run.

    Of course, note the fundamental difference again: one can consume fuel to heat a volume, but when one runs a normal air conditioner, one is really cooling one volume and moving the heat elsewhere, while using electricity.

  • Timothy Chase // July 9, 2008 at 2:11 am

    Tamino wrote:

    I don’t really know how much energy is required to compensate for “too hot” compared to “too cold.” But if they’re the same, then the energy requirement will be proportional to the degree-days, whether heating or cooling. Under this assumption (which is only an approximation at best), we can compute the “sum of degree-days” by simply adding the heating-degree-days and the cooling-degree-days, to estimate how much energy is required for temperature comfort.

    Raypierre touched on this a little while ago over at RC back on JUne 15th:

    Put it all together with the energy efficiency of the air conditioner itself and air conditioning comes in at a whopping 2.19 times less efficient than heating. for a given amount of temperature difference between house and environment. That means that so far as carbon emissions go, heating a house to 70 degrees when the outside temperature is 40 degrees is like cooling the same house to 70 degrees when the outside temperature is 83.7 degrees.

    Wired Magazine’s Incoherent Truths
    June 15, 2008, Raypierre

    … but note: this makes certain assumptions regarding the technology being employed — and he points out that we can do better.

  • Zeke Hausfather // July 9, 2008 at 5:01 am

    If you heat your house using natural gas, it will be considerably less CO2-intensive per joule than cooling your house using electricity, assuming that you live in a country with a grid primarily dependent on fossil fuel generation.

    For a country like France, where the majority of the generation is nuclear, the outcome is less clear-cut.

  • Bob Tisdale // July 9, 2008 at 10:35 am

    Returning to my days of heating, ventilating and air conditioning (HVAC) systems for non-residential buildings: Unadjusted degree days are a poor source for determining heating and cooling energy use. Degree days are based on a base temperature of 65 deg F and assume that cooling is required above that outdoor air temperature and heating below. Degree days that have not been adjusted fail to consider an occupied building’s break even temperature, which is the outdoor air temperature at which the internal heat gain of a building (from lighting, equipment, occupants, solar, etc.) equals the heat loss. Back in the late 70s and early 80s, break even temperatures for many non-residential buildings (hospitals, large office buildings, etc.) were as low as 40 deg F. I’ve lost track since then.

    Degree days also fail to consider economizer cycles on HVAC systems. When the environmental controls determine that outdoor air is more economical to cool than recirculated air, outdoor air (above the volume required to meet ventilation codes) is used. When outdoor air temperatures are between that economizer switchover temperature (somewhere between 65 and 75 deg F depending on the enthalpy of the return air and outdoor air sources and the type of economizer controls employed) and 55 deg F, outdoor air supplements mechanical cooling and the costs for cooling the air are reduced. (Note: 55 deg F is used as an example of the controlled supply air temperature for HVAC systems. It varies, too, through design and via controls.) Between 55 deg F and the break-even temperature of the exterior zones, mechanical cooling should not be required. Since interior zones have no external heat losses (they’re surrounded by exterior zones), occupied interior zones require cooling year round.


  • Henk Lankamp // July 9, 2008 at 11:54 pm

    S2 is right. Just a few homes in Europe have airco, although many offices and shops do. So you can’t just sum the two different degree days, at least not for Europe.
    But the figures are clear: on average summers are getting warmer and winters becoming milder.
    An extreme hot summer like 2003 is a combination of causes: persistent southern winds (air originating from Africa), low precipitation in the southern parts of Europe in springtime (so little evaporation), and global warming.

  • Eli Rabett // July 10, 2008 at 4:51 pm

    Having worked and visited in Europe since the early 1970s, my impression is that A/C is much more common. A place where this is clearest to me is in labs. When I first went there, there were essentially no air conditioned labs, but increasingly in the 1980s and 1990s more and more time was lost to days in which the equipment could not work because of the heat. Some places I was at went to swamp A/C (get a bunch of car radiators with fans and pump the water from the house cooling water lines through it) and other kluges, but increasingly now places are getting retrofitted for HVAC systems.

  • chriscolose // July 11, 2008 at 4:20 am

    hey, don’t we love McIntyre’s graphs?

  • dhogaza // July 11, 2008 at 5:59 am

    The man has descended into outright silliness. Posts like this, his recent attempt to discredit a handful of well-known papers in Nature by doing a count of words he doesn’t like, his other lame attempt at “humor” are nothing but pandering to the fan club.

    Maybe he understands that he’s got no traction where it counts, in the political world outside the likes of the increasingly irrelevant Sen. Inhofe.

    His blog should provide fodder for any well-schooled Congresscritter should McIntyre be called for testimony again. McCarthyism sucked, but the likes of him and his gang (Nixon, and especially Roy Cohn) would riddle his poor carcass …

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