UC Riverside

Black carbon and tropospheric ozone, two pollutants typically associated with urban smog, may be key drivers in the advance of the northern tropics.

The northern tropics are on a march toward the pole. Over the last thirty years, the warm, moist belt around the equator has expanded by between 2-and-8 degrees northward.

When the phenomenon was first described five years ago, it was thought to be fueled primarily by carbon dioxide emissions. But a report, published recently by University of California at Riverside researchers in the journal Nature, has proposed a new driver of the expanding tropics: soot and ozone pollution generated largely by wood burning and diesel combustion in the rapidly developing nations of Southeast Asia.

Of particular concern for residents of California and other arid regions of the West is that the growth of the tropical belt may be forcing storm tracks farther north, in turn, causing the margins of the world’s subtropical deserts — including those of the American Southwest — to advance northward as well.

According to Robert Allen, an earth science professor at UC Riverside and lead author of the report, the study examined the effect of two short-lived atmospheric pollutants: black carbon, the soot seen emanating from diesel exhaust pipes or household chimneys; and tropospheric ozone, a product formed when carbon monoxide and nitrogen oxides react with organic compounds in the presence of sunlight.

Both are easily recognized when they are lingering in a “brown cloud” above a large city, but it turns out that smog also plays an important role in the dynamics of the global climate. “The prevailing theory was that this expansion of the tropics was due to greenhouse gases and a more uniform warming,” Allen told me.

While the bulk of the world’s black carbon comes from the burning of forests and grasslands, the greatest anthropogenic sources are the burning of wood and animal dung for cooking and heating homes and the combustion of diesel engines. Pound-for-pound, so to speak, black carbon is far more potent than CO2 in terms of its warming effects on the atmosphere.

Previous computer models, which only considered carbon dioxide “forcing,” showed about 11% of the observed expansion. By adding black carbon and tropospheric ozone into the equation, Allen says his team’s model revealed nearly half the expansion observed in nature. “It may be that the amount of emissions is being underestimated, and therefore so are the trends.”

A local consequence here in the western U.S. is that tropical expansion seems to be influencing precipitation patterns, pushing storm tracks farther north. “For example, the southern portions of the United States may get drier if the storm systems move further north than they were 30 years ago,” said Allen in a UC Riverside press release. “Indeed, some climate models have been showing a steady drying of the subtropics, accompanied by an increase in precipitation in higher mid-latitudes.”

If the pattern holds, in the not too distant future, cities at the northern margins of subtropical deserts could find themselves thrust into full-fledged deserts, says Allen – with one caveat, that is. “There is no way to know if the current emission levels will hold,” he said. If China, India and other large emitters can substantially cut back on the black carbon and tropospheric ozone being put into the air, the effect could be mitigated rapidly. This is because unlike carbon dioxide, which remains in the atmosphere for centuries after it is emitted, these pollutants have short “residence” times in the atmosphere.

The tropics of the Southern Hemisphere are also expanding. But unlike the expansion of the northern tropics, that process has been largely attributed to the depletion of the stratospheric ozone layer along with greenhouse gas emissions.

U.S. Chemical Safety Board

Methane can escape from mines, power plants, farms, and landfills.

Carbon dioxide is the primary driver of climate change, but it’s not the only one. Methane also contributes to warming. In fact, a single molecule of methane causes more warming than a single molecule of carbon dioxide does. But it doesn’t stay in the atmosphere as long, so a new study from NASA affirms what others have suggested for years: that cutting methane emissions would show quicker results than cutting CO2 emissions. The same goes for soot, also known as black carbon. Plus, cutting back on soot would put a damper on the respiratory diseases it causes, and capturing more methane, which is basically natural gas, would save money.

NPR’s Christopher Joyce reports on the study for All Things Considered. He talked to Durwood Zaelke, who runs the Institute for Governance and Sustainable Development in Washington, D.C.

Zaelke is a grizzled veteran of the climate wars: he was in Kyoto in 1997 when the world’s nations drafted a treaty promising to curb warming, and he has watched that promise fizzle while the planet’s temperature continues to rise.

Zaelke says the Kyoto treaty focused too much on the main greenhouse gas: carbon dioxide.

“I mean, it’s like picking a fight with the biggest bully in the schoolyard,” he says with a note of lament. “You know, you get your lunch money stolen, you get your pants pulled down, and you get sent home humiliated. We’ve made about that much progress with CO₂.”

Cutting down on methane and soot alone won’t solve climate change, but the scientists in the study say they expect they could “reduce projected global mean warming [by about] 0.5°C by 2050.”

The article, “Simultaneously Mitigating Near-Term Climate Change and Improving Human Health and Food Security,” by lead author Drew Shindell, was published in Science (the full article is available for a fee).

Lisa Collins, a lecturer at the USC Dornsife College, spent four years studying samples from floating sediment traps in the San Pedro Basin as a way to determine what’s falling through the water column and how deep it’s getting.

“We have a pretty good idea of how much biomass is produced in the ocean, but we don’t have a great idea of how much of that biomass actually gets down through the water column and ultimately to the sediment,” said Collins.

One reason that matters, she says, is that phytoplankton, which make up much of the biomass, live and grow by taking up sunlight and carbon dioxide, just like plants on land do. When the phytoplankton die, they sink, taking that stored carbon down the water column with them. If they make it all the way to the mud at the bottom of the ocean, Collins says, that carbon will be sequestered there for hundreds or thousands of years or more.

“San Pedro Basin is a sink for carbon,” said Collins.

She said there’s more carbon getting sequestered there than in other areas of the world where similar studies have been conducted. The reason, she said, is the relatively cool water temperatures in the region.

“The microbes that are chewing up and eating up that carbon are utilizing that carbon as particles are sinking,” said Collins. “In the warmer waters, they can work faster, and so are able to eat up more of the carbon. So the net result is that less carbon makes it to the bottom.”

Once microbes eat the carbon, they then “breathe” it out said Collins, releasing it back in the water and the atmosphere.

Within the San Pedro Basin, Collins found differences in the amount of carbon making it to the sea floor depending on the season and the weather across the four years. She said she’s hoping this kind of detailed information can help climate modelers make more accurate predictions about how the ocean is going to respond to climate change.

“When we think generally about how the world is warming up and is predicted to continue warming, it’s something we should pay attention to,” said Collins, “because not only is it going to affect us on land, but if we’re talking about temperatures getting warmer in the ocean as well, what we’ll probably see is that the oceans won’t be taking up as much carbon.”

The study is published in the August issue of Deep-Sea Research I.

(Photo: David McNew/Getty Images)

By Andrew Freedman

Pollution reduction measures that were aimed primarily at reducing California’s notorious smog problem and improving public health, also helped cut emissions of black carbon — a key global warming agent — according to a new study published Tuesday.

Black carbon, more commonly referred to as soot, is an atmospheric particulate that scientists have shown to be a significant contributor to global warming. It is an attractive target for emissions reductions because relatively cost effective technologies to reduce it already exist, such as diesel particulate filters for trucks, and because unlike carbon dioxide (CO2), which stays in the air for decades to millennia, black carbon only remains airborne for days to weeks.

According to the study, published in the journal Atmospheric Environment, scientists from the Scripps Institution of Oceanography and Argonne National Laboratories found that in California, statewide annual average black carbon concentrations declined by nearly 50 percent between 1989 and 2008. These reductions occurred in direct proportion to a decline in fossil fuel emissions, mainly from diesel engines, during that period. The researchers say the reductions were largely the result of laws enacted to reduce diesel pollution, instituted to benefit public health and alleviate California’s smog.

Speaking at the annual meeting of the American Geophysical Union in San Francisco, coauthor V. Ramanathan, distinguished professor at Scripps and a prominent figure in black carbon research, said the California emissions reductions offer a demonstration of how reducing black carbon emissions can influence the climate on a regional level. “For a scientist it’s a spectacular experiment, because we can see the result of [mitigation] policies,” he said.

One of the complications facing scientists and policy makers seeking to address black carbon is that many of the processes that emit black carbon also emit other particulates, such as sulfate aerosols, that can help cool the climate. So ideally, policies aimed at reducing global warming would selectively cut the warming agents without taking away the cooling influences. Otherwise, they may cause a warming rebound.

However, hard data about what would happen if only the heat-trapping aerosols were slashed, without reducing the cooling agents, has until now been in short supply. This study, Ramanathan said, changes that. The study estimates that the decrease in black carbon led to a cooling of 1.4 watts per square meter, which it states is “sufficiently large to have had an impact on regional heat and water budget[s]” in California.

Ramanathan said this study demonstrates that black carbon emissions can be tackled state by state or country by country, rather than trying to get the entire world agree to an agreement which, as seen by the case of CO2, can be a nearly impossible task. “The whole world need not agree [to black carbon reductions],” Ramanathan said. “Local benefits nicely interface with global goods.”

The study was funded in part by the California Air Resources Board (ARB), which regulates California’s air quality in conjunction with the U.S. EPA. “This study demonstrates that ARB’s efforts to cut air pollution, whether by promoting cleaner cars or controlling agricultural burning, have significantly reduced threats to public health while also helping address climate change,” said ARB chairwoman Mary Nichols in a press release.

Ramanathan’s daughter Nithya, an assistant research professor of computer science at UCLA, has also been working to develop cellphone-based technology to monitor concentrations of black carbon. Such observations could be used to verify the effects of emissions reduction measures, among other uses, they said.

Most of the discussion regarding the highly anticipated Senate energy and climate change legislation, which Senators John Kerry (D-MA) and Joseph Lieberman (I-CT) introduced last week following months of negotiations, has focused on the bill’s provisions pertaining to offshore oil and gas drilling, incentives for renewable energy, and cap on carbon emissions for certain economic sectors.

Although the bill’s carbon dioxide (CO2) emissions reduction targets–an 80 percent emissions cut by 2050 compared to 2005 levels–would yield significant long-term climate benefits, the bill also addresses man-made climate change in the shorter term.

Stack emissions from a bulk freighter in San Francisco Bay. Photo: Craig Miller

A little-noticed portion of the bill concerns short-lived air pollutants such as black carbon (otherwise known as soot) and tropospheric ozone. These pollutants disrupt the climate on far shorter timescales than CO2, which scientists consider the most important greenhouse gas and the main villain in the climate change story.

Once emitted by the burning of fossil fuels, the use of solid-fuel cooking stoves or biomass burning, among other sources, black carbon only stays aloft for days to a few weeks before being washed out of the atmosphere by precipitation. This means that once black carbon emissions are reduced, there would be almost immediate climate benefits.

The Kerry-Lieberman bill would direct the US EPA to use its existing authority under the Clean Air Act to reduce black carbon emissions from diesel engines, using devices called diesel particulate filters which trap soot emissions before they escape from a vehicle’s tailpipe.

It would also call upon the EPA to publish a report on black carbon “sources, impacts, and reduction opportunities,” including an examination of how foreign assistance programs could help reduce emissions in other nations. In addition, the bill would establish an inter-agency process to facilitate “fast mitigation strategies” that focus on non-CO2 warming agents. This process would involve agencies such as the EPA and the Energy Department (DOE).

How big a climate player is black carbon?

Black carbon is thought to be a powerful warming agent in many regions, particularly snow and ice-covered areas such as the Himalayas and the Arctic. As its name suggests, black carbon particles are dark in color, and are therefore strong absorbers of incoming solar radiation. They warm the atmosphere and alter cloud characteristics, and when they land on brightly colored snow and ice, they darken the surface, causing a large uptick in the absorption of solar radiation, which hastens melting.

In the Arctic, black carbon contributes to a feedback loop that has helped cause a rapid melting of sea ice cover and drive temperatures upward at nearly twice the rate of the rest of the world. The decade from 1999-2008 was the warmest ten-year period in the Arctic of the past 2,000 years, according to a study published in the journal Science in 2009.

In addition to Arctic warming, black carbon has been shown to alter regional climate patterns such as the Indian monsoon, and human inhalation of soot particles is known to be a major health hazard worldwide.

In recent years numerous scientists, most prominently V. (Ram) Ramanathan of the Scripps Institution of Oceanography and James Hansen of NASA have called for significant cuts in short-lived air pollutants as a way to reduce climate change in the near term, while efforts continue to address CO2 emissions in the long run. Ramanathan’s studies have shown that black carbon may be the second largest contributor to global climate change.

In March testimony before the House Select Committee for Energy Independence and Global Warming, Ramanathan stated that the current global warming effect of black carbon “may be as much as 60 percent” of the CO2 warming effect. He noted, however, that there are significant uncertainties about black carbon’s role in the climate system.

Ramanathan told House lawmakers that reducing black carbon emissions “may provide a possible mechanism for buying time to develop and implement effective steps for reducing CO2 emissions.”

Bill is aligned with recent scientific advice

The Kerry-Lieberman bill’s inclusion of rapid mitigation strategies is consistent with advice contained in a new paper from an interdisciplinary panel of scholars, published on May 11 by the University of Oxford in the UK. The paper argues that non-CO2 drivers of climate change have been overlooked “for reasons of convenience in framing policy” rather than due to scientific concerns, and it presents a vision for an overhaul of climate policy that would include a much more prominent role for addressing emissions of short-lived air pollutants.

“Since action on these non-CO2 ‘forcers’ may have quicker impact and large, immediate primary benefits, we would give them priority, now. In contrast to long and arduous tasks, these can be ‘quick hits’,” the report states.

The bill’s provisions are also consistent with the findings of a scientific panel that examined options to address rapid Arctic climate change. In a 2008 report, the panel strongly endorsed pursuing emissions reductions of black carbon and other short-lived air pollutants. “…Curbing short-lived climate forcing agents, through rapid international action and Arctic nation leadership, may prove to be the best and perhaps only viable strategy for slowing Arctic warming in the time frame of years to a decade,” the report stated.

Considering that the Kerry-Lieberman bill itself faces a highly uncertain future, with significant resistance in both political parties, it may yet take even longer to address what many experts consider to be a ripe, low hanging fruit of the climate challenge. This does not bode well, given the much more difficult work that lies ahead to reduce CO2 and other longer-lasting greenhouse gases.

In April, Molly Samuel reported on the effects of black carbon and snow albedo on the California’s water forecasting efforts.