Irrigation and paint as reasons for a cooling bias

In previous posts on reasons why raw temperature data may show too little global warming I have examined improvements in the siting of stations, improvements in the protection of thermometers against the sun, and moves of urban stations to better locations, in particular to airports. This post will be about the influence of irrigation and watering, as well as improvements in the paints used for thermometer screens.

Irrigation and watering

Irrigation can decrease air temperature by up to 5 degrees and typically decreases the temperature by about 1°C (Cook et al., 2014). Because of irrigation more solar energy is used for evaporation and for transpiration by the plants, rather than for warming of the soil and air.

Over the last century we have seen a large 5 to 6 fold global increase in irrigation; see graph below.



The warming by the Urban Heat Island (UHI) is real. The reason we speak of a possible trend bias due to increases in the UHI is that an urban area has a higher probability of siting a weather station than rural areas. If only for the simple reason that that is where people live and want information on the weather.

The cooling due to increases in irrigation are also real. It seems to be a reasonable assumption that an irrigated area again has a higher probability of siting a weather station. People are more likely to live in irrigated areas and many weather stations are deployed to serve agriculture. While urbanization is a reason for stations to move to better locations, irrigation is no reason for a station to move away. On the contrary maybe even.

The author of the above dataset showing increases in irrigation, Stefan Siebert, writes: "Small irrigation areas are spread across almost all populated areas of the world." You can see this strong relation between irrigation and population on a large scale in the map below. It seems likely that this is also true on local scales.



Many stations are also in suburbs and these are likely watered more than they were in the past when water (energy) was more expensive or people even had to use hand pumps. In the same way as irrigation, watering could produce a cool bias due to more evaporation. Suburbs may thus be even cooler than the surrounding rural areas if there is no irrigation. Does anyone know of any literature about this?

I know of one station in Spain where the ground is watered to comply with WMO guidelines that weather stations should be installed on grass. The surrounding is dry and bare, but the station is lush and green. This could also cause a temperature trend bias under the reasonable assumption that this is a new idea. If anyone knows more about such stations, please let me know.

From whitewash to latex paint

Also the maintenance of the weather station can be important. Over the years better materials and paints may have been used for thermometer screens. If this makes the screens more white, they heat up less and they heat up the air flowing through the Louvres less. More regular cleaning and painting would have the same effect. It is possible that this has improved when climate change made weather services aware that high measurement accuracies are important. Unfortunately, it is also possible that good maintenance is nowadays seen as inefficient.

The mitigation skeptics somehow thought that the effect would go into the other direction. That the bad paints used in the past would be a cooling bias, rather than a warming bias. Something with infra-red albedo. Although most materials used have about the same infra-red albedo and the infra-red radiation fluxes are much smaller than the solar fluxes.

Anthony Watts started a paint experiment in his back garden in July 2007. The first picture below shows three Stevenson screens, a bare one, a screen with modern latex paint and one with whitewash, a chalk paint that quickly fades.



Already 5 months later in December 2007, the whitewash had deteriorated considerably; see below. This should lead to a warm bias for the whitewash screen, especially in summer.

Anthony Watts:

Compare the photo of the whitewash paint screen on 7/13/07 when it was new with one taken today on 12/27/07. No wonder the NWS dumped whitewash as the spec in the 70’s in favor of latex paint. Notice that the Latex painted shelter still looks good today while the Whitewashed shelter is already deteriorating.

In any event the statement of Patrick Michaels “Weather equipment is very high-maintenance. The standard temperature shelter is painted white. If the paint wears or discolors, the shelter absorbs more of the sun’s heat and the thermometer inside will read artificially high.” seems like a realistic statement in light of the photos above.

I have not seen any data from this experiment beyond a plot with one day of temperatures, which was a day one month after the start, showing no clear differences between the Stevenson screens. They were all up to 1°C warmer than the modern ventilated automatic weather station when the sun was shining. (That the most modern ventilated measurement had a cool bias was not emphasized in the article, as you can imagine.) Given that Anthony Watts maintains a stealth political blog against mitigation of climate change, I guess we can conclude that he probably did not like the results, that the old white wash screen was warmer and he did not want to publish that.

We may be able to make a rough estimate the size of the effect by looking at another experiment with a bad screen. In sunny Italy Giuseppina Lopardo and colleagues compared two old aged, yellowed and cracked screens of unventilated automatic weather stations that should have been replaced long ago with a good new screen. The picture to the right shows the screen after 3 years. They found a difference of 0.25°C after 3 years and 0.32°C after 5 years.

The main caveat is that the information on the whitewash comes from Anthony Watts. It may thus well misinformation that the American Weather Bureau used whitewash in the past. Lacquer paints are probably as old as 8000 years and I see no reason to use whitewash for a small and important weather screen. If anyone has a reliable source about paints used in the past, either inside or outside the USA, I would be very grateful.

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Related posts

Changes in screen design leading to temperature trend biases

Temperature bias from the village heat island

Temperature trend biases due to urbanization and siting quality changes

Climatologists have manipulated data to REDUCE global warming

Homogenisation of monthly and annual data from surface stations

References

Cook, B.I., S.P. Shukla, M.J. Puma, L.S. Nazarenko, 2014: Irrigation as an historical climate forcing. Climate Dynamics, 10.1007/s00382-014-2204-7.

Siebert, Stefan, Jippe Hoogeveen, Petra Döll, Jean-Marc Faurès, Sebastian Feick and Karen Frenken, 2006: The Digital Global Map of Irrigation Areas – Development and Validation of Map Version 4. Conference on International Agricultural Research for Development. Tropentag 2006, University of Bonn, October 11-13, 2006.

Siebert, S., Kummu, M., Porkka, M., Döll, P., Ramankutty, N., and Scanlon, B.R., 2015: A global data set of the extent of irrigated land from 1900 to 2005. Hydrology and Earth System Sciences, 19, pp. 1521-1545, doi: 10.5194/hess-19-1521-2015.

See also: Zhou, D., D. Li, G. Sun, L. Zhang, Y. Liu, and L. Hao (2016), Contrasting effects of urbanization and agriculture on surface temperature in eastern China, J. Geophys. Res. Atmos., 121, doi: 10.1002/2016JD025359.

Temperature trend biases due to urbanization and siting quality changes

The temperature in urban areas can be several degrees higher than their surrounding due to the Urban Heat Island (UHI). The additional heat stress is an important medical problem and studied by bio-meteorologists. Many urban geographers study the UHI and ways to reduce the heat stress. Their work suggests that the UHI is due to a reduction in evaporation from bare soil and vegetation in city centers. The solar energy that is not used for evaporation goes into warming of the air. In case of high-rise buildings there are, in addition, more surfaces and thus more storage of heat in the buildings during the day, which is released during the night. High-rise buildings also reduce radiative cooling (infrared) at night because the surface sees a smaller part of the cold sky. Recent work suggests that cities also influence convection (often visible as cumulus (towering) clouds).

To study changes in the temperature, a constant UHI bias is no problem. The problem is an increase in urbanization. For some city stations this can be clearly seen in a comparison with nearby rural stations. A clear example is the temperature at the station in Tokyo, where the temperature since 1920 rises faster than in surrounding stations.



Scientists like to make a strong case, thus before they confidently state that the global temperature is increasing, they have naturally studied the influence of urbanization in detail. An early example is Joseph Kincer of the US Weather Bureau (HT @GuyCallendar) who studied the influence of growing cities in 1933.

While urbanization can be clearly seen for some stations, the effect on the global mean temperature is small. The Fourth Assessment Report from the IPCC, states the following.

Studies that have looked at hemispheric and global scales conclude that any urban-related trend is an order of magnitude smaller than decadal and longer time-scale trends evident in the series (e.g., Jones et al., 1990; Peterson et al., 1999). This result could partly be attributed to the omission from the gridded data set of a small number of sites (<1%) with clear urban-related warming trends. ... Accordingly, this assessment adds the same level of urban warming uncertainty as in the TAR: 0.006°C per decade since 1900 for land, and 0.002°C per decade since 1900 for blended land with ocean, as ocean UHI is zero.

Next to the removal of urban stations, the influence of urbanization is reduced by statistical removal of non-climatic changes (homogenization). The most overlooked aspect may, however, be that urban stations do not often stay at the same location, but rather are relocated when the surrounding is seen to be no longer suited or the meteorological offices simply cannot pay the rent any more or the offices are relocated to airports to help with air traffic safety.

Thus urbanization does not only lead to an gradual increase in temperature, but also to downward jumps. Such a non-climatic change often looks like an (irregular) sawtooth. This can lead to artificial trends in both directions; see sketch below. In the end, what counts is how strong the UHI was in the beginning and how strong it is now.



The first post of this series was about a new study that showed that even villages have a small "urban heat island". For a village in Sweden (Haparanda) and Germany (Geisenheim) the study found that the current location of the weather station is about 0.5°C (1°F) colder than the village center. For cities you would expect a larger effect.

Around the Second World War many city stations were moved to airports, which largely takes the stations out of the urban heat island. Comparing the temperature trend of stations that are currently at airports with the non-airport stations, a number of people have found that this effect is about 0.1°C for the airport stations, which would suggest that it is not important for the entire dataset.

This 0.1°C sounds rather small to me. If we have urban heat islands of multiple degrees and people worry about small increases in the urban heat island effect, then taking a station (mostly) out of the heat island should lead to a strong cooling. Furthermore, cities are often in valleys and coasts and the later build airports thus often are at a higher and thus cooler location.

A preliminary study by citizen scientist Caerbannog suggests that airport relocations can explain a considerable part of the adjustments. These calculations need to be performed more carefully and we need to understand why the apparently small difference for airport stations translates to a considerable effect for the global mean. A more detailed scientific study on relocations to airports is unfortunately still missing.

Also the period where the bias increases in GHCNv3 corresponds to the period around the second world war where many stations were relocated to airports, see figure below. Finally, also that the temperature trend bias in the raw GHCNv3 data is larger than the bias in the Berkeley Earth dataset suggests that airport relocations could be important. Airport stations are overrepresented in GHCNv3, which contains a quite large fraction of airport stations.



With some colleagues we have started the Parallel Observations Science Team (POST) in the International Surface Temperature Initiative. There are some people interested in using parallel measurements (simultaneous measurements at cities and airports) to study the influence of these relocations. There seems to be more data than one may think. We are, however, still looking for a leading author (hint).

If the non-climatic change due to airport relocations is different (likely larger) than the change implemented in GHCNv3, that would give us an estimate of how well homogenization methods can reduce trend biases. Williams, Menne, and Thorne (2012) showed that homogenization can reduce trend errors, that they improve trend estimates, but also that part of the bias remains.



In the 19th century and earlier, thermometers were expensive scientific instruments and meteorological observations were made by educated people, apothecaries, teachers, clergymen, and so on. These people lived in the city. Many stations have subsequently been moved to better and colder locations. Whether urbanization produces a cold or a warm bias is thus an empirical and historical question. The evidence seems to show that on average the effect is small. It would be valuable when the effect of urbanization and relocations would be studies together. That may lead to an understanding of this paradox.

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Related posts

Changes in screen design leading to temperature trend biases

Temperature bias from the village heat island

Climatologists have manipulated data to REDUCE global warming

Homogenisation of monthly and annual data from surface stations