Of Hawks and Handsaws

Guest Post by Willis Eschenbach

“I am but mad north-north-west: when the wind is southerly I know a hawk from a handsaw”

William Shakespeare, Hamlet, Act II, Scene ii

Following on from my look at the USHCN temperature dataset, I have gone north (if not north-north-west) and looked at the NORDKLIM dataset. This dataset covers Norway, Sweden, Finland, Denmark, and Iceland.  Of the seventy-five stations in the dataset, fifty of them have records covering the period from 1900 to 1999. Figure 1 shows the average of those selected temperatures for that period.

Figure 1. Average of the 50 long-term stations in the NORDKLIM dataset. The warmest year in the data is 1934. Photo is of Tromso, Norway, 70° North Latitude.

As before, I wanted to look at the changes in different months, to see when during the year the warming occurred. Figure 2 shows the decadal changes in the temperature for each month.

Figure 2. Decade-by-decade changes in the temperature of the Nordic countries. Photo is of Tromsoe, Norway, 70° North Latitude

As you can see, the changes are similar to those in the US. The summer temperatures have not changed. Winter temperatures (January to March) have warmed. One difference is that the winter warming is larger in the NORDKLIM temperatures.

The more I look at these datasets, the more I think that we are looking at the Urban Heat Island (UHI) effect. This is the change in the recorded temperature due to increasing development around the recording station. Increasing houses, buildings, industry, parking lots, and roads all increase the recorded temperature at nearby stations. The NORDKLIM notes say:

Especially one should notice that stations represent local conditions, which may have been effected e.g. by urbanisation

This effect is known to be greater in winter than in summer. In a study done in Barrow, Alaska, for example, there is a 4.5°C difference in the UHI effect between January and July. The winter to summer difference in the UHI in Fairbanks, Alaska is estimated to be 1.2°C.

In addition to the physical development (buildings, roads, etc.), another reason for this UHI can be seen in the photos used to illustrate the graphs. This is the direct usage of energy in the cities. For example, estimates of the energy usage for the New York City area are on the order of 5 * 10^18 joules annually. This gives a local forcing of ~ 20 W/m2.

How large an effect is this? Well, to get this amount of forcing from increasing CO2, instead of merely doubling, it would have to increase by more than forty times …

The colder the city is on average, the more effect that this will have. A building kept at 70°F (20°C) will have little effect on temperature if the local temperature is only slightly below that. If the temperature is below freezing, on the other hand, this will be a much larger effect.

In addition, the colder the weather, the more energy is put into heating the buildings. This also increases the winter UHI. As a result, we would expect the effect we have seen, that the recorded change in winter temperatures is greater in the NORDKLIM dataset than in the USHCN dataset.

My conclusion? At least part of the warming in the US and the NORDKLIM datasets is the result of UHI distortion of the records. An unknown but likely significant amount of this UHI heating is due to direct energy consumption in the cities.

And knowing how much of the temperature change is from UHI is harder than telling a hawk from a handsaw.