Heat is on to cut LA’s temperature
Climate change is mostly responsible for hotter cities. But other forces are at work and they can be influenced. reports.
Globally, 2016 was the warmest year on record. In Los Angeles, temperature records were shattered last (US) summer during scorching heat waves that saw highs of 37.8 degrees Celsius for five days straight.
If you think the city is too hot, you’ve got company at City Hall. Los Angeles Mayor Eric Garcetti agrees, and he wants to do something about it.
As part of a sweeping plan to help LA live within its environmental means, Garcetti has pledged to reduce the average temperature in the metropolis by 3 degrees Fahrenheit over the next 20 years.
It’s a noble goal. But how do you turn down the thermostat of an entire city in a warming world? And in a place as vast, sprawling and heterogeneous as Los Angeles, how do you measure success?
These questions have never been more relevant. LA’s heat problem is expected to worsen over the coming decades.
Climate models suggest that by 2050, the temperature in downtown L.A. will exceed 35C some 22 days a year. In 1990, only six days were that warm. The San Fernando Valley is expected to see 92 days of this extreme heat per year, compared with 54 in 1990.
Climate change is primarily responsible for the warming trend, but it’s not the only force at work.
Angelenos are also contending with an additional layer of misery caused by what’s known as the ‘‘urban heat island effect’’.
It means that cities – with their asphalt streets, dark roofs, sparse vegetation and car-clogged roads – are almost always a few degrees warmer than the more rural areas that surround them.
The mayor’s plan to cool the region won’t compensate for all the effects of climate change.
‘‘We can’t geoengineer the atmosphere,’’ said Matt Petersen, chief sustainability officer for the office of the mayor.
But Petersen believes the city’s residents can do something about the way the city traps heat. By counteracting this heat island effect, he hopes to reduce the amount of warming LA will experience in the future.
‘‘What we are trying to do is create a research collective to help us reach our target,’’ Petersen said. ‘‘It’s a huge challenge.’’
‘‘There is all this variation across the city,’’ said George BanWeiss, a USC environmental engineer and veteran of the Lawrence Berkeley National Laboratory’s Heat Island Group .
The built environment is mostly responsible for the problem. More than half of city surfaces are covered by dark pavements and dark roofs. Traditional asphalt absorbs up to 90 per cent of the sun’s radiation. As the asphalt gets hotter, it warms the air around it, adding to the overall heat. Even after the sun goes down, that accumulated heat lingers for hours and continues to transfer warmth to the night air.
One way to combat this heat sink is to replace the city’s streets and sidewalks with hi-tech materials – called ‘‘cool pavements’’ – that reflect more sunlight and stay cooler during the day and at night.
In the summer of 2015, the city’s Bureau of Street Surfaces tested one of these cool pavements at a carpark in Encino. The new surface was approximately 11 degrees Fahrenheit cooler than regular pavement in the midafternoon.
Scientists and policymakers are also investigating ‘‘cool roofs’’ and their potential to reduce the overall temperature of the city. Studies have found that in Los Angeles, widespread deployment of cool roofs could reduce the city’s temperature by as much as 2 degrees Fahrenheit.
But it’s unlikely that a single strategy will be the most effective option for all neighbourhoods.
‘‘The heat island effect is a regional phenomenon, and the way you choose your mitigation strategy could vary block to block,’’ Ban-Weiss said.
If an area has no tree cover but lots of cool roofs, adding more cool roofs won’t be as useful as planting trees. On the other hand, if an area has lots of trees, adding reflective pavements won’t reduce temperatures because the sidewalks don’t get much sunlight anyway.
Also, some regions of the city require more cooling than others.
To address the hyper-local nature of the heat island effect, Ban-Weiss and his graduate students are modelling microclimates of areas as small as a few blocks. They started with a neighbourhood in El Monte, a city that is relatively warm compared to its surroundings.
After painstakingly building a computer model that included each tree and building, the researchers were able to analyse the effects of various heat mitigation strategies, comparing how it would feel if streets had more reflective surfaces, if every grassy yard were shaded by trees, and if every roof were covered in grass.
They found that cool roofs and green roofs had little effect on the thermal comfort of a person walking down the street, and that putting more trees in unshaded areas was the most effective cooling strategy. However, in areas that were already shady, the most significant effect came from cool pavements. – Los Angeles Times