Guest commentary by Loretta Mickley, Harvard University
Every summer over much of the United States, we brace ourselves for heat waves. During these periods, the air turns muggy and usually smoggy. After a few days, a cold front moves in, sweeping away the pollution and ending the heat. Given that we are on a path towards global warming, atmospheric chemists are asking how climate change could affect air quality. Will warmer temperatures mean more pollution during these episodes? Will episodes last longer? Most importantly, what effect will changes in air quality have on human health?
Recently the National Resource Defense Council (NRDC) released Heat Advisory, a report warning that surface air quality could suffer greatly as a result of climate change. In response, a group called the Pacific Research Institute (PRI), together with another group called United for Jobs, published Air Quality False Alarm, a detailed criticism of the NRDC forecast. PRI argues, among other things, that anthropogenic emissions in the U.S. will drop sharply in coming decades. In their view, air pollution will become a thing of the past, no matter what happens to climate.
What’s the story here? First, a little background on ozone and particulate matter (PM), two major components of smog. Surface ozone is formed from a mix of natural and anthropogenic precursors like nitrogen oxides and volatile organic carbon. We have measurements of surface ozone dating back to the late 1800s which imply that ozone in some regions has increased 2-5 times due to emissions of ozone precursors from cars, industry, and power plants. As for PM, there are many different kinds – e.g., organic carbon, soot, and sulfate-ammonium-nitrate. Some kinds of PM, like soot, are directly emitted into the air, but other kinds condense from gas-phase molecules. Like ozone, PM has both natural and anthropogenic ingredients.
Many factors govern the severity and timing of pollution episodes. An obvious factor is the magnitude of precursor emissions. But there are meteorological factors, like how stagnant the surface air is and whether it’s clear or cloudy, warm or cool. The summer of 1998, for example, saw a record number of ozone exceedances averaged over New England. That summer was also the warmest on record for that region. The hot summer that Europe endured in 2003 was also a summer of high pollution levels for that continent. But the cool summer in the U.S. that same year meant that the we saw low levels of pollution.
So how will pollution evolve over the coming decades as climate changes? The easy answer is: oh, the warmer temperatures mean greater pollution! But it’s more complicated than that. Then there are other meteorological factors to consider. As the surface temperatures rise, will the depth of the boundary layer increase, diluting the pollutants within it? Maybe stronger surface winds will carry all the pollution away. What about changes in cloud cover or rainfall?
To tackle issues of this complexity, modelers often turn to sensitivity studies. A sensitivity study is one in which you change just one or two variables, and keep everything else constant. By taking the problem apart in this way, you can isolate the effect of one or two factors at a time.
In one sensitivity study, Aw and Kleeman [2003] imposed a 5ºC increase in temperature over the Los Angeles basin, but kept all other meteorological variables (like windspeed) constant in their model. Ozone in the region increased by 10-15%, but concentrations of sulfate-ammonia-nitrate PM decreased by 10-15%. That’s because ammonia condenses less readily at high temperatures. This is an interesting result. But in the real world, stalled high pressure systems, like the one over the Midwest and Northeast last week (April 18-20), can lead to both warm temperatures and high PM. With clear skies and weak winds, PM can accumulate over the source regions. As the climate changes, not only could temperatures change, but also the behavior of these high pressure systems.
In my research group, we tried a different sensitivity study [Mickley et al., 2004]. We devised our model experiment to test just the effect of changing wind patterns on pollutant concentrations. What we found was that the severity summertime regional pollution episodes in the Midwest and Northeast U.S. increased significantly by 2050, relative to present. Also, the average length of an episode increased from 2 to 3-4 days. Why did this happen? Our model forecast a 20% decline in the frequency of cold fronts sweeping into the U.S., so stagnation events in the model persisted longer. That allowed both gas-phase and PM pollution to build to higher concentrations.
Another model study [Hogrefe et al., 2004] focused on the effect of climate change on just surface ozone. The authors found that even with emissions of ozone precursors in the model held at 1990s levels, the total number of “exceedance days” increased by about 60% over the eastern U.S. (An exceedance day is a day in which ozone averaged over 8 hours exceeds the EPA threshold of 84 ppb.) Because of the complexity of the study, Hogrefe et al. [2004] could not diagnose precisely all the meteorological changes (temperature? circulation patterns?) contributing to the increased surface ozone in their model. But they did find that one factor accounting for about half the increase was enhanced emissions of natural ozone precursors, which are temperature-sensitive.
One of the biggest unknowns, of course, is how anthropogenic emissions will evolve in the future. The Clean Air Act has led to tremendous improvements in air quality since the 1970s. But even if our emissions do decline, the consequences for air pollution are uncertain. Fiore et al. [2002] have shown that decreases in U.S. emissions may be offset by increases elsewhere in the world. Specifically, rising methane emissions elsewhere in the world could significantly enhance background levels of ozone over the U.S., leading to as much pollution in 2030 as we saw in the mid-1990s.
So there’s a lot more to be learned about the links between climate and pollution. Since both surface ozone and PM have adverse effects on human health, understanding the link is important.
References:
Aw, J., and M.J. Kleeman, Evaluating the first-order effect of intraannual air pollution on urban air pollution, J. Geophys. Res., 108, 4365, 10.1029/2002JD002688, 2003.
Fiore, A.M., D.J. Jacob, B.D. Field, D.G. Streets, S.D. Fernandes, and C. Jang, Linking ozone pollution and climate change: The case for controlling methane, Geophys. Res. Lett., 29, 1919, doi:10.1029/2002GL015601, 2002.
Hogrefe, C., B. Lynn, K. Civerolo, J.-Y. Ku, J. Rosenthal, C. Rosenzweig, S. Gaffin, K. Knowlton, and P.L. Kinney, Simulating changes in regional air pollution over the eastern United States due to changes in global and regional climate and emissions. J. Geophys. Res., 109, D22301, doi:10.1029/2004JD004690, 2004.
Mickley, L. J., D. J. Jacob, B. D. Field, and D. Rind, Effects of future climate change on regional air pollution episodes in the United States, Geophys. Res. Let., 30, L24103, doi:10.1029/2004GL021216, 2004.
Good piece Loretta, and the GRL paper looks good.
A question: many of the precursors to O3 formation are VOCs, and (my study area) trees contribute a significant fraction of VOCs to the atm. Tree canopies also can change the boundary layer mixing in cities, making it harder for wind to sweep out the goop in the air.
Were you able, in your work for this paper, to contact anyone and explore whether elevated CO2 levels decrease tree metabolic rates and thus decrease VOC emissions?
Best,
D
Unfortunately there isn’t yet much understanding of the large amount of interannual variability in blocking [high pressure systems] seen in the last few decades. So it is very hard to make a prediction of what might happen to blocking in a climate change experiment.
Whilst it is important to consider the possible effects of climate change on problems such as this I think it is best to emphasise that the dominant cause is the availability of the precursor pollution, and that reducing this pollution [by making changes to transport systems and industry] is the best way of tackling this problem.
Biogenic VOCs are highly reactive and the emissions are substantial especially in humid, warm seasons. I wonder in the case of blocking patterns leading to drought and well above normal temperatures if biogenic VOC emissions might actually decrease? Short of the vegetation actually dying from the drought of course.
I know this study focuses on ambient air pollution, but has anyone looked into trends to see if we (humankind in developed nations) stay indoors more during heat waves, thereby subjecting ourselves to increased levels of indoor air pollution ? Common sense tells me to stay in where its cool when its hot. Should we be asking if indoor air pollution will become a bigger issue as summers get hotter due to the effects of Global Warming?
DB
I have the impression that the prediction of future climate/pollution links is rather speculative.
In the not so long past, the worst pollution was during cold, calm, high humidity weather, at the time coal was used in open fire places, leading to the infamous “pea soup” smog in London, killing elderly people.
Since that time, SO2 and lead were reduced with over 80%, PM10 with over 60% and NOx with over 40% in all Western countries. As NOx is the main driver for low level ozone formation, it’s further reduction should have a large impact.
About biogenical VOC’s: these are mainly formed in summer, where high temperatures and secondly high light/photosynthesis are the primary drivers. Natural VOC’s exceed anthropogenic emissions with a factor 3-8
Some interesting literature about biogenical VOC’s:
http://boreal.fmi.fi/biphorep/report/chapter7.pdf and
http://ethesis.helsinki.fi/julkaisut/mat/kemia/vk/hakola/biogenic.pdf
Responses:
#1 and #3: Yes, the emissions of volatile organic carbon (VOCs) from vegetation play a role in ozone formation. In cities like Atlanta with lush vegetation, biogenic VOCs together with anthropogenic nitrogen oxides can have a significant impact on regional pollution.
The emissions of biogenic VOCs increase rapidly as temperatures increase. But plants also respond to changing CO2. While some plants may flourish in an enriched CO2 atmosphere, VOC emissions may decrease. And as Dan points out below, vegetation may also suffer from heat or water stress in a changing climate. It’s a complicated picture.
I did not include any of these biogenic effects in my simple sensitivity study. I kept emissions of pollution precursors constant. All I wanted to see was this:
if I increase the long-lived greenhouse gases like CO2 in the model and let the
climate respond, what happens to the patterns of air circulation? What I found
was that stagnation events lasted longer in the future model atmosphere.
On the other hand, Hogrefe et al. [2004] did include the biogenic VOCs that Dano and Dan are talking about. Of the effects listed above, Hogrefe et al. [2004] considered only the temperature effect on biogenic VOC emissions. They found a 10-50% increase in these emissions with climate change over the eastern U.S. Unfortunately there isn’t yet much understanding of the large amount of interannual variability in blocking [high pressure systems] seen in the last few decades. So it is very hard to make a prediction of what might happen to blocking in a climate change experiment.
#2. I agree with Tim that more work is needed to understand what controls cyclone (and anticyclone) variability in the observation record. A number of model studies, such as our own, have found a decline in cyclone number in a future atmosphere. The model trends can be explained with mechanisms such as the flattening of the temperature gradient from equator to pole. Improving our understanding of present-day variability will give us greater confidence in what these models say.
I also agree that tackling emissions of pollution precursors is important. But so long as emissions of pollution precursors remain in a kind of middle range (i.e., above natural levels), we can expect that daily weather patterns will play a role in whether or not we have a bad air day.
A changing climate will influence the daily weather and could have consequences for air quality.
Loretta
Along with the potential influences of changes in temperature and moisture on biogenic VOC emissions from individual plant species there are likely to be changes in VOC emissions based on longer-term changes in composition of vegetation [for example, Kellomäki et al. 2001] because of a wide range in VOC emissions among species [for example, Kesselmeier and Staudt, 1999; Kesselmeier et al. 2002]. If climate change results in the large-scale changes in forest species composition as vegetation adapts to a warmer climate as some models suggest [Iverson and Parsad, 1998], these species shifts could influence the rates of emission of biogenic precursors and thus the frequency and intensity of air pollution episodes. The lengthening of the growing season in northern temperate latitudes also suggests a longer period where conditions could increase air pollution risk. I am curious what replacement of spruce-fir and northern hardwood forests with oak and pine-dominated forests in the far northeastern US would mean for ozone attainment in the future.
Iverson, L.R., and Prasad, A.M., 1998, Predicting abundance of 80 tree species following climate change in the eastern United States: Ecol. Monogr., v. 68, no. 4, p. 465�485.
http://www.fs.fed.us/ne/delaware/4153/iverson18.pdf
Kellomäki, S.,et al.., 2001, Impact of global warming on the tree species composition of boreal forests in Finland and effects on emissions of isoprenoids: Global Change Biology, v. 7, no. 5, p. 531-544.
http://www.blackwell-synergy.com/links/doi/10.1046/j.1365-2486.2001.00414.x/abs/
Kesselmeier, J.K., and Staudt, M., 1999, Biogenic volatile organic Compounds (VOC): An overview on emission, physiology and ecology: J. Atmos. Chem., v. 33, no. 1, p. 23 – 88.
Kesselmeier, J.K.et al.., 2002, Concentrations and species composition of atmospheric volatile organic compounds (VOCs) as observed during the wet and dry season in Rondônia (Amazonia): J. Geophys. Res., v. 107(D20), p. 8053, http://www.agu.org/pubs/crossref/2002/2000JD000267.shtml
In addition to GW contributing to increased pollution, it is also important for laypersons to keep in mind that most human activities that generate GHGs, also generate other forms of pollution to air, land, & water, & many non-environmental harms/costs, when we consider the entire life cycle of products from resource extraction, shipping, manufacture, shopping, consumption, disposal, plus military/government costs associated with protecting/procuring resources/products.
I wish there were better ways to internize a least a few more of the real costs of products and compensate victims better.