Increasing Heat Island Effect’s Influence on Urban Temperature Records Introduces Bias in Climate Studies

When it comes to human-caused climate change, urban warming is a big player.

Sourced through Scoop.it from: www.cato.org

>”Perhaps no other climatic variable receives more attention in the debate over CO2-induced global warming than temperature. Its forecast change over time in response to rising atmospheric CO2 concentrations is the typical measure by which climate models are compared. It is also the standard by which the climate model projections tend to be judged; right or wrong, the correctness of global warming theory is most often adjudicated by comparing model projections of temperature against real-world measurements. And in such comparisons, it is critical to have a proper baseline of good data; but that is easier acknowledged than accomplished, as multiple problems and potential inaccuracies have been identified in even the best of temperature data sets.

One particular issue in this regard is the urban heat island effect, a phenomenon by which urban structures artificially warm background air temperatures above what they normally would be in a non-urbanized environment. The urban influence on a given station’s temperature record can be quite profound. In large cities, for example, urban-induced heating can be as great as Tokyo’s 10°C, making it all the more difficult to detect and discern a CO2-induced global warming signal in the temperature record, especially since the putative warming of non-urbanized areas of the planet over the past century is believed to be less than 1°C.  Yet, because nearly all long-term temperature records have been obtained from sensors initially located in towns and cities that have experienced significant growth over the past century, it is extremely important that urbanization-induced warming – which can be a full order of magnitude greater than the background trend being sought – be removed from the original temperature records when attempting to accurately assess the true warming (or cooling!) of the natural non-urban environment. A new study by Founda et al. (2015) suggests this may not be so simple or straightforward a task.

Working with temperature records in and around the metropolitan area of Athens, Greece, Founda et al. set out to examine the interdecadal variability of the urban heat island (UHI) effect, since “few studies focus on the temporal variability of UHI intensity over long periods.” Yet, as they note, “knowledge of the temporal variability and trends of UHI intensity is very important in climate change studies, since [the] urban effect has an additive effect on long term air temperature trends.”

[…]

Such findings as these are of significant relevance in climate change studies, for they clearly indicate the UHI influence on a temperature record is not static. It changes over time and is likely inducing an ever-increasing warming bias on the temperature record, a bias that will only increase as the world’s population continues to urbanize in the years and decades ahead. Consequently, unless researchers routinely identify and remove this growing UHI influence from the various temperature data bases used in global change studies, there will likely be a progressive overestimation of the influence of the radiative effects of rising CO2 on the temperature record. “<

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LA’s Urban Heat Island Effect Alters Weather

Over the last 60 years urban areas of Southern California have lost significant amounts of fog due to the heat created by paved roads and buildings.

Source: www.scpr.org

>” A new study reports that coastal fog in Southern California is on the decline, especially in heavily urbanized areas.

In particular, Los Angeles saw a 63 percent decrease over the last 60 years.

You can blame the heat island effect created by city streets and buildings, said the study’s author Park Williams of Columbia University’s Lamont-Doherty Earth Observatory in New York.

Fog may be a nuisance for drivers, but according to Williams, it also plays a crucial role in hydrating many costal ecosystems.

These include mountains with coastal forests and hillsides covered in chaparral, which easily burns when conditions are too dry.

“They all receive water directly from fog and benefit from the shading of these clouds,” Williams said.

In fact, he noted that in some parts of Southern California, fog may provide plants with almost as much water as rain does. Williams says this loss of coastal fog could impact the regional environment.

Fog typically forms when the air is cool enough for clouds to condense close to ground level. This often happens at night and in the early morning.

However, Williams said this process is being upset by all the concrete in urban areas, which absorbs heat in the day and slowly releases it over night, raising temperatures.

“When you increase the temperature of the surface of the Earth, then you essentially need to go higher up into the atmosphere before [it] is cool enough to promote condensation,” Williams explained.

The end result is that as cities heat up, clouds rise and fog disappears.

Data for the study came from the detailed logs of the 24 coastal airports between Santa Barbara and San Diego.

“Of course airports have been collecting really good data on clouds because the presence of clouds and their hight in the atmosphere really affects air travel,” he said.

Many of these logs had hourly updates on cloud height, some dating back to the 1940s.

Using this information, Williams and his colleagues determined that the greatest loss of fog occurred in Ontario where there was a nearly 90% decrease over the last 60 years.

Other airports such as LAX, Burbank’s Bob Hope, Long Beach Airport and John Wayne Airport in Orange County also saw a considerable decrease in the average amount of fog.

However, less urban areas like Santa Barbara and the undeveloped the Channel Islands remained quite misty.

Williams says this trend is concerning because man-made climate change is expected to heat things up even more in the future.

Coastal fog can help cool an area down but as cities continue to bake, they will gather and emit even more heat, driving away even more fog.

“That can then feedback until the cloud layer is eaten away entirely in the daytime,” he said.

Soon, Williams hopes to explore how much water fog provides Southern California in general to see whether the continued loss of these low clouds could dry out the region even more.

His current paper appears in the journal Geophysical Research Letters.”<

 

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