Tuesday, July 7, 2015

Urban Traffic Hotspots of Greenhouse Gas Concentrations

Cities, traffic, and CO2: A multidecadal assessment of trends, drivers, and scaling relationships (6 page pdf, Conor K. Gately, Lucy R. Hutyra, and Ian Sue Wing, Proceedings of the National Academy of Sciences of the USA (PNAS), Apr. 9, 2015)

Also discussed here: Maps pinpoint where cars pollute the most (Barbara Moran-Boston University, Futurity, Apr. 15, 2015)

Today we review research into the contributions that vehicles make to greenhouse gas emissions over the last 33 years on urban roads in the USA where 87% of the counties have low population densities (or are sprawled towns and cities). Results indicate that top-down estimates of national or regional GHG emissions may be as much as 50% in error unless these emissions are estimated at the scale of roads and streets (approx 1 km resolution) as opposed to estimating them from population only. The authors suggest that the contributions from on-road emissions will likely increase as population and road densities increase in the future, suggesting that carbon policy makers focus on per capita emissions (which in turn would be linked to low or high driving rates) to capture the greater emissions from urban hotspots.

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Key Quotes:

“Over the past 40 years, the global urban population rose from 1.51 to 3.91 billion people, and it is expected to reach 6.3 billion people by 2050 ..CO2 emissions from transportation comprised 23% of global fossil fuel carbon emissions in 2010 ..with over 40% of those emissions produced by road travel in urban areas”

“The United States, with 5% of the world’s population and 30% of the world’s automobiles, emits 45% of global transportation CO2 emissions…Nationally, the on-road sector represented 28% of total fossil fuel CO2 emissions in 2012 and is responsible for almost half of the growth in total US emissions since 1990..urban areas are responsible for 80% of on-road emissions growth since 1980 and for 63% of total 2012 emissions”

“before the peak in 2004, urban per capita emissions rose 19% from 1980 levels, whereas average urban population density rose 30% over the same time period. This discrepancy suggests that the future trajectory of per capita on-road emissions may not be as strongly coupled with trends in urban densification as previously believed.”

“only 46 counties (comprising 13% of the US population) had a population density greater than 1,000 persons per square kilometer. For the remaining 87% of the US population that lives in lower density counties, our results indicate that the ongoing urban growth in these counties is likely to produce substantial increases in local on-road emissions, because the reductions in per capita emissions at lower levels of urban density will not be sufficient to stabilize the total emissions growth.”

“Cars are the hardest part of the emissions equation to quantify…They are moving all the time at different speeds, and there are different cars on the road at different times of day.”

“DARTE [Database of Road Transportation Emissions ] complements NASA’s Orbiting Carbon Observatory 2, which is collecting global data on atmospheric carbon dioxide…We need good bottom-up data to match what we’re measuring looking down from space. That’s what we need to really advance greenhouse gas polices.”

“it may make more sense for policy makers to focus on reducing local per capita emissions, rather than total emissions, because most growing urban areas should expect total on-road emissions to continue to rise with the population over the next decades.”

“Uncertainty in the magnitude of on-road emissions at the national level is estimated to be on the order of 3–5% for developed countries…but at subnational or state scales, existing inventories disagree by as much as 40%... and at city scales, uncertainty can be as large as 50–100%”

“Traffic sensors are widely considered to measure total vehicle volumes with 95–99% accuracy” “Our results highlight the importance of cities as sources of CO2 and the need for improved fine-scale inventories for monitoring and reporting of emissions.”

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