|FACTOR 4: CHANGING ENVIRONMENTS|
Changing environments are a fact of life that are not easily avoided. As our population grows, cities and towns creep ever outward (Figure 16), vegetation matures, and land use patterns evolve, the environments we monitor change. These changes impact the climate record. The question is not whether these changes impact climate, but how, and to what magnitude and geographic scale.
How do we deal with environmental change? To some degree, knowledge gained from environmental changes is useful. For example, understanding that urban climates are generally warmer than surrounding suburban and rural areas is useful for energy planning and landscaping activities.
In many cases our thermometer records are from airports. Figure 17 is
an evening thermal picture (in the infrared, from ASTER) of Phoenix (at
about 10 p.m. local time) and note how the warm runways are the brightest
yellow area (warmest) in town. The official Phoenix climate observations
are made at the airport on these runways. Phoenix has grown into a major
metropolis with a very large and busy airport.
Regardless of data applications, in many cases, environmentally changing locations are unavoidable as measurement stations. Nonetheless, it is good practice, wherever feasible, to establish stations in environments expected to be as stable as possible over long periods of time unless special studies or research are the primary mission.
Let's look at several examples of the impact environmental change can have on climate data. Monthly temperatures departures from the multi-station NCDC-climate-zone average at Newark, New Jersey over a 17 month period (1991-92) indicate a discontinuity. The discontinuity is evident about 10-12 months into the record from this interval. Upon examination of the circumstances, it was found that:
1) The temperature sensor was within accuracy specifications, and,
Figure 18 depicts a similar situation that occurred at a COOP station out west. A gravel parking lot was paved to within a few feet of the station. This change resulted in an average monthly temperature increase at the site of over 2 degrees (F) within several months with greater impacts seen on days with sun and light winds. There was no mention of the pavement change in the metadata, and a data user would have no knowledge of this important station change.
Another example of non-compliant temperature sensor is exhibited in Figure 19. Here, the environmental change is vegetation encroachment. This situation impacts the radiation balance of the site, and additionally, may reduce the natural aspiration of the sensor. What is the net resultant impact on temperature? Only a sophisticated statistical evaluation of the data might yield an answer and without this evaluation, incorrect conclusions about climate variation and trends will result.
Since, in many cases, environmental change may not be fully anticipated or avoided, the best mitigating practice is to fully document environmental changes in the station vicinity in the station record (metadata). By practicing this principle religiously, we increase our chances of understanding the factors (both artificial and natural) driving changes and trends in the data.
Recommended Environmental-Change Related Actions
The following actions are recommended to minimize unwanted discontinuities in the climate record:
1) Correct non-compliant situations wherever possible. Keep vegetation in check. If non-compliance is severe and compromising data quality and remedial action is not an option, consider station relocation or closure of the station. Coordinate relocation/closure efforts with COOP modernization activities where possible. COOP modernization is now in the process of creating Regional Site Selection Teams that will be determining what existing sites will be modernized and where new stations need to be installed.
2) Ensure that the station metadata history contains reference to any situation that can bias measurements. Ensure compliance with the "ten principles of climate monitoring." Yes, it may be embarrassing to admit that station circumstances are not ideal, but it remains important to report the facts for the historical record.
3) Coordinate with your RCSPM and other climate services partners (NCDC,
RCCs, SCs) on any related issues.