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Road transportation emerges as key driver of warming: NASA analysis

For decades, climatologists have studied the gases and particles that have potential to alter Earth's climate. They have discovered and described certain airborne chemicals that can trap incoming sunlight and warm the climate, while others cool the planet by blocking the Sun's rays.

Now a new study led by Nadine Unger of NASA's Goddard Institute for Space Studies (GISS) in New York City offers a more intuitive way to understand what's changing the Earth's climate. Rather than analyzing impacts by chemical species, scientists have analyzed the climate impacts by different economic sectors.

Each part of the economy, such as ground transportation or agriculture, emits a unique portfolio of gases and aerosols that affect the climate in different ways and on different timescales.

Motor vehicles give off only minimal amounts of sulfates and nitrates, both pollutants that cool climate, though they produce significant amounts of pollutants that warm climate such as carbon dioxide, black carbon, and ozone.
Credit: NASA's Langley Research Center
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"We wanted to provide the information in a way that would be more helpful for policy makers," Unger said. "This approach will make it easier to identify sectors for which emission reductions will be most beneficial for climate and those which may produce unintended consequences."

In a paper published online on Feb. 3 by the Proceedings of the National Academy of Sciences, Unger and colleagues described how they used a climate model to estimate the impact of 13 sectors of the economy from 2000 to 2100. They based their calculations on real-world inventories of emissions collected by scientists around the world, and they assumed that those emissions would stay relatively constant in the future.

Snapshots of the Future

In their analysis, motor vehicles emerged as the greatest contributor to atmospheric warming now and in the near term. Cars, buses, and trucks release pollutants and greenhouse gases that promote warming, while emitting few aerosols that counteract it.

The on-road transportation sector releases significant amounts of carbon dioxide, black carbon, and ozone—all substances that cause warming. In contrast, the industrial sector releases many of the same gases, but it also tends to emit sulfates and other aerosols that cause cooling by reflecting light and altering clouds.
Credit: NASA GISS/Unger
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The researchers found that the burning of household biofuels -- primarily wood and animal dung for home heating and cooking -- contribute the second most warming. And raising livestock, particularly methane-producing cattle, contribute the third most.

On the other end of the spectrum, the industrial sector releases such a high proportion of sulfates and other cooling aerosols that it actually contributes a significant amount of cooling to the system. And biomass burning -- which occurs mainly as a result of tropical forest fires, deforestation, savannah and shrub fires -- emits large amounts of organic carbon particles that block solar radiation.

The new analysis offers policy makers and the public a far more detailed and comprehensive understanding of how to mitigate climate change most effectively, Unger and colleagues assert.
"Targeting on-road transportation is a win-win-win," she said. "It's good for the climate in the short term and long term, and it's good for our health."

Due to the health problems caused by aerosols, many developed countries have been reducing aerosol emissions by industry. But such efforts are also eliminating some of the cooling effect of such pollution, eliminating a form of inadvertent geoengineering that has likely counteracted global warming in recent decades.

"Warming should accelerate as we continue to remove the aerosols," said Unger. "We have no choice but to remove the aerosol particulate pollution to protect human and ecosystem health. That means we'll need to work even harder to reduce greenhouse gases and warming pollutants."

Unger's model finds that in 2020 (left), transportation, household biofuels and animal husbandry will have the greatest warming impact on the climate, while the shipping, biomass burning, and industrial sectors will have a cooling impact. By 2100 (right), the model finds that the power and industrial sector will become strongly warming as carbon dioxide accumulates.
Credit: NASA GISS/Unger
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By the year 2100, Unger's projections suggest that the impact of the various sectors will change significantly. By 2050, electric power generation overtakes road transportation as the biggest promoter of warming. The industrial sector likewise jumps from the smallest contribution in 2020 to the third largest by 2100.

"The differences are because the impacts of greenhouse gases accumulate and intensify over time, and because they persist in the atmosphere for such long periods," said Unger. "In contrast, aerosols rain out after a few days and can only have a short-term impact."

Factoring in Clouds

For each sector of the economy, Unger's team analyzed the effects of a wide range of chemical species, including carbon dioxide, nitrous oxide, methane, organic carbon, black carbon, nitrate, sulfate, and ozone.

The team also considered how emissions from each part of the economy can impact clouds, which have an indirect effect on climate, explained Surabi Menon, a coauthor of the paper and scientist at the Lawrence Berkeley National Laboratory in Berkeley, Calif.

Some aerosols, particularly sulfates and organic carbon, can make clouds brighter and cause them to last longer, producing a cooling effect. At the same time, one type of aerosol called black carbon, or soot, actually absorbs incoming solar radiation, heats the atmosphere, and drives the evaporation of low-level clouds. This process, called the semi-direct aerosol effect, has a warming impact.

The new analysis shows that emissions from the power, biomass burning, and industrial sectors of the economy promote aerosol-cloud interactions that exert a powerful cooling effect, while on-road transportation and household biofuels exacerbate cloud-related warming.

More research on the effects of aerosols is still needed, Unger cautions. "Although our estimates of the aerosol forcing are consistent with those listed by the International Panel on Climate Change, a significant amount of uncertainty remains."

Unger's analysis is one of the first of its kind to incorporate the multiple effects that aerosol particles can have on clouds, which affect the climate indirectly.
Credit: NASA's Johnson Space Center
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Related Links
Related Q & A with Nadine Unger
› http://www.nasa.gov/topics/earth/features/unger-qa.html

Nadine Unger Bio
› http://www.giss.nasa.gov/staff/nunger.html

Attribution of Climate Forcing to Economic Sectors
› http://pnas.org/content/early/2010/02/02/0906548107.abstract

Nadine Unger Bio
› http://giss.nasa.gov/staff/nunger.html

Other Research by Nadine Unger
› http://pubs.giss.nasa.gov/authors/nunger.html

Clean the Air, Heat the Planet
› http://sciencemag.org/cgi/content/short/326/5953/672

NASA Scientist Nadine Unger Discusses Which Sectors of the Economy Impact the Climate

Nadine UngerCredit: NASA's Goddard Institute for Space Studies › Larger imageNadine Unger
Credit: NASA's Goddard Institute for Space Studies
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Nadine Unger, a climatologist with NASA’s Goddard Institute for Space Studies in New York City, spoke with NASA's Earth Science News Team about her recent study that analyzed how different human activities impact climate. The study appeared in the Proceedings of the National Academy of Sciences in February.

NASA's Earth Science News Team: Your research suggests that the climate science community ought to shift its focus from looking at the impacts of individual chemicals to economic sectors. Why?

Nadine Unger: There's nothing "wrong" with dividing climate impacts up by chemical species, but it's not particularly useful for policy makers. They need to know which human activities are impacting the climate and what the effect will be if they attempt to curb emissions from a particular sector. Also, there's a great deal of complexity in our emissions that they need to be mindful of if we want to mitigate climate change efficiently.

NASA: What sort of complexity?

Nadine Unger: Some sectors of the economy produce a mixture of pollutants -- particularly aerosols -- that cause cooling rather than warming in the short term. Since warming can accelerate as we remove aerosols, we've been inadvertently geoengineering for decades with aerosol emissions.

Take the heavy industry and shipping sectors, for example. These sectors burn a great deal of coal and bunker fuel, which releases carbon dioxide, which causes greenhouse warming. But they also release sulfates, which cause cooling by blocking incoming radiation from the sun and by changing clouds to make them brighter and longer-lived. In the short term, the cooling from sulfates actually outweighs the warming from carbon dioxide, meaning the net impact of the shipping and heavy industry sectors today is to cool climate.

Compare that to cars and trucks, which emit almost no sulfates but a great deal of carbon dioxide, black carbon, and ozone -- all of which cause warming and happen to be very bad for human health. Cutting transportation emissions would be unambiguously good for the climate in the short term, while cutting heavy industry emissions would have less of an impact right now.

NASA: You keep mentioning "short-term" impacts. Could the climate impacts of some sectors of the economy change over longer time periods?

Nadine Unger: Yes. Greenhouse gases have a much longer lifespan -- or residence time -- in the atmosphere than aerosols, which typically rain out after a few days or weeks. This means that the impact of greenhouse gases can accumulate and intensify over time, while the aerosol effects become comparatively less important on longer time scales due to the accumulation of carbon dioxide.

NASA: You've mentioned industry, shipping and on-road transportation. What other sectors of the economy did you analyze?

Nadine Unger: Aviation, household fossil fuels, railroads, household biofuels (mainly wood and dung used for home cooking and heating), animal husbandry, the electric power sector, waste and landfills, agriculture, biomass burning...

NASA: What is biomass burning?

Nadine Unger: Mainly tropical forest fires, deforestation and savannah and shrub fires. We also looked at agricultural waste burning, which relates to seasonal clearing of the fields common in many countries in Africa and South America.

NASA: So, does this mean that pollution from industry and biomass burning is good for the climate?

Nadine Unger: No, not at all. Both of those sectors contribute to warming over the long term, so we'll have no choice but to reduce our emissions over time. But these sectors do mask warming from greenhouses gases in the short term. Just because an activity causes cooling in the short-term does not mean that it is ‘good’ for the climate. The emissions might disturb other aspects of the climate system including the amount of rainfall in a region and therefore the water supply to humans.

NASA: Where did you get all the information about emissions?

Nadine Unger: We used emission inventories assembled by colleagues. For instance, a colleague from the University of Illinois -- Tami Bond -- has some of the best information on some types of aerosols, such as black carbon.

NASA: But how can you estimate the impacts of emissions that haven't happened yet?

Nadine Unger: We used a computer model at GISS to look at future at climate impacts if we continued emitting pollutants at today's rate. Using this approach, we looked specifically at two snapshots in time: 2020 and 2100.

NASA: What can we do if we want to minimize climate change in the near term?

Nadine Unger: Well, our analysis suggests that on-the-road transportation and household biofuels are very attractive sectors to target. We can reduce human warming impacts most rapidly by tackling emissions from these sectors. In order to protect climate in the longer term, emissions from power and industry must be reduced.

NASA: Are there any uncertainties in your results?

Nadine Unger: There are. There's a large amount of uncertainty about how aerosols affect climate, especially through the indirect effects on clouds. Hopefully, NASA's Glory mission will help reduce the uncertainties associated with aerosols.

NASA: What direction do you see your research going next?

Nadine Unger: Our focus has been on global climate so far, but in future work we'll assess regional climate impacts, as well as other disturbances to the climate system, such as effects on the water supply and land ecosystems.

In addition, we plan to investigate many of the sectors in greater detail. In the power sector, for example, we might look specifically at power stations that operate with coal or natural gas. And in the on-road transportation sector, we might break out heavy- from light-duty vehicles.

Finally, we're planning to partner with environmental economists to determine the damage costs of emissions from all the sectors due to both climate and air quality impacts, results that we can use to develop alternative mitigation scenarios.

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