Japan

South Korea Taiwan

Haze Movement China

Satellite image of brown haze flowing across the East China Sea past the Korean Peninsula and northeastward toward Japan. Image courtesy of the SeaWiFS Project, NASA/Goddard Space Flight Center, and ORBIMAGE.

Global Warming Veerabhadran Ramanathan

These remarks were given at a meeting of the American Academy, held at the University of California, San Diego on November 21, 2005.

Veerabhadran Ramanathan is Victor C. Alderson study of global change. He demonstrated Why should we worry about this blanket of co Professor of Ocean Sciences and Director of the that the amount of in the added 2 and other greenhouse gases? Center for Atmospheric Sciences at the University atmosphere increased by about 20 percent The fundamental energy source for the planet of California, San Diego. He has been a Fellow of since the time he began his work in the mid- is sunlight. However, not all of the solar en- the American Academy since 1995. 1950s. If you take one million molecules of ergy is absorbed: about 30 percent is reflect- air, approximately 375 will be carbon dioxide. ed back to space by clouds, the atmosphere, Compare this with the fact that in the last and land and sea surface, including sea ice, he effect of greenhouse gases on global four hundred thousand years, the amount of ice sheets, and the like. The planet is warmed T co warming is, in my opinion, the most impor- 2 concentration has never been larger by the remaining 70 percent and, in turn, re- tant environmental issue facing the world than 290 parts per million. radiates the heat as infrared energy (also known as thermal energy or heat radiation). today. Our knowledge of the underlying The rapidity of the increase leaves little doubt Over the long term, climate is governed by causes of is growing, but the that human impact is the cause. What lies the balance between the incoming solar problem brims with uncertainties, raising behind such a signi½cant increase in a rela- heating and the cooling associated with the serious scienti½c and ethical questions. tively short time? The lifetime of carbon di- outgoing infrared energy. The added co oxide is over a century. If today you release a 2 In studies of global temperatures, one ques- upsets this balance by absorbing and re- can of co , roughly 25–35 percent of it will tion always arises at the outset: When did 2 emitting the infrared energy. In this process, humans become a major force in modifying still be with us a hundred years from now. co the blanket of 2 acts just like a wool the climate system? My own timetable begins co blanket on a cold night by trapping the What happens to this 2? The air carries it in the 1950s, when the world’s population in- around until it covers the entire planet like a outgoing infrared heat within the surface- creased by over 60 percent, resulting in a per- blanket of gases. In fact, no matter where we atmosphere system and causing the planet ceptible impact on many indices of change. measure–the Arctic, the Antarctic, the sur- to become warmer. With high-precision observations, my late face, or 20 miles above the surface–we detect The fact that added co can lead to a large colleague, Dave Keeling, produced the single the increase in co . Fossil-fuel combustion 2 2 global warming was estimated more than most important times-series data set for the and biomass burning are the major sources for this increase. 36 Bulletin of the American Academy Spring 2006 The effect of greenhouse periment (with P. J. Crutzen), which de- high-precision radiometers to discover that ployed six aircraft and two ships with several and other absorbing particles gases on global warming tens of instruments, I had the opportunity to in the brown haze over the Indian Ocean and observe from the C-130 aircraft the brown the Arabian Sea reduced sunlight by as much is, in my opinion, the most clouds spreading from South Asia and as 10–15 percent. The sunlight-reduction blanketing most of the North Indian Ocean. effect at the surface was larger by a factor of important environmental We then used satellite data to show that the two or more than estimated by climate South Asian brown haze occurs every year models. In terms of the ocean surface, black issue facing the world generally between November and May. It carbon in the brown haze reduces the average consists of a 3 km mixture of anthropogenic radiative heating by as much as 10 percent today. (human-produced) sulfates, nitrates, organics, and enhances atmospheric solar radiative black carbon, dust and fly ash particles, and heating by as much as 50–100 percent. 110 years ago by the Swedish Nobelist Svante Arrhenius. In the mid-1970s a series Composition of a brown cloud of complications began to unfold, leading to the realization that carbon dioxide was not Ash the only cause of global warming. Mario Dust Molina and Sherwood Rowland’s research Black Carbon on the impact of (cfcs) on the ozone hole led to my inves- tigations on the potential cfc of s. Our research revealed the unex- (includes all other Organics MISS pected result that adding one molecule of ions in the brown cfc to the atmosphere would have the same greenhouse effect as adding more cloud) than ten thousand molecules of co . The 2 -. fact that cfcs, which are relatively rare in 3 the atmosphere, could be such a powerful Sea-salt & NO force in global warming was initially met (Nitrate ion) with disbelief. But as other researchers NH (Ammonium+ ion) 4 reproduced our ½ndings, a Pandora’s box of greenhouse gases, including methane, + ozone, halocarbons used as refrigerants K (Potassium ion) and propellants, and nitrous oxide from = SO (Sulfate ion) fertilizer, began to open. 4

We could no longer ignore the effect of greenhouse gases and our sense that global natural such as sea salt and mineral Aerosols also produce more cloud drops, warming would occur much earlier than we dust. Measurements from aircraft, ships, which increase the reflection of solar radi- might have imagined. My work with clima- and surface stations involved in the experi- ation by clouds, adding to the surface-cooling tologist Roland Madden some twenty-½ve ment found that biomass burning and fossil- effect; and decrease the size of cloud drops years ago revealed that the impact of global fuel combustion contribute as much as 75 and suppress precipitation. percent to the observed . warming would become discernible by the The link between aerosols and precipitation year 2000. Meeting in 2001, the Intergov- represents an added complication. As emis- ernmental Panel of Climate Change, con- As emissions from fossil sions from fossil fuel and black carbon have sisting of a group of over a thousand sci- increased, monsoonal rainfall and surface entists, con½rmed our prediction. fuel and black carbon have sunlight have decreased. Modeling research Coincident with the warm- increased, monsoonal conducted by us with the climate-system ing is the appearance of atmospheric brown model developed by the National Center for clouds. If greenhouse gases are the ultimate rainfall and surface sun- Atmospheric Research in Boulder, Colorado, co indicates that three factors may contribute end product of fossil fuel and 2, then particulates in the air represent an interme- light have decreased. to the drying effect. First, sunlight provides diate phase. A brown haze is generally asso- the energy to evaporate water from the ocean, ciated with urban areas, but in 1999, the Black carbon is probably the most insidious causing rainfall. With reduced sunlight the Indian Ocean Experiment (indoex), in- component of the haze as far as health is evaporation from the ocean decreases and, volving more than two hundred scientists, concerned; it is also the most important fac- as a result, the rainfall decreases. Second, as tor in terms of climate change. During the solar absorption by black carbon heats the focused on a brown cloud that spans an entire indoex continent and ocean. As coleader of this ex- campaign we deployed a suite of atmosphere, it cools the surface by shielding it from the sun. This redistribution of sunlight

Bulletin of the American Academy Spring 2006 37 causes warmer air to overlie the colder sur- by the increase in greenhouse gases from the the regional level, affecting most severely faces, suppressing rainfall. Finally, the brown late nineteenth century to the present time the poorest people on Earth. International haze appears to slow summer monsoonal has committed the planet to a global warming cooperation among scientists and among circulation, leading to a reduction in precipi- in the range of 1°C to 3°C. We have realized nations will be essential if we are to effec- tation over South Asia. These ½ndings are only a fraction (25–50 percent) of this warm- tively address the formidable political, social, important reasons for reducing air pollution. ing. Some of this warming has been masked and ethical consequences of global warming. by the dimming due to brown clouds, and Among the many obvious reasons for the the remaining heat is stored in the depths of urgent need for international collaboration We continue to face a the ocean to be released later. Through the is the fact that the developed nations are the process of convective overturning, oceans major contributors to the greenhouse gases number of uncertainties transfer infrared energy to their deepest lay- while the developing nations are the major ers and hold the heat, delaying the impact of sources for particles in the brown clouds. in our efforts to predict global warming. Whether this stored heat will warm the atmosphere in a few decades climate change. © 2005 by Veerabhadran Ramanathan or a few centuries is unknown. The delay of the warming by decades to centuries by the Lest you think that air pollution is a con½ned flywheel effect of ocean mixing, when com- problem, I want to emphasize that long- bined with the century or more lifetime of range wind transport means that pollution co (and molecules of other greenhouse on the East Coast of the United States is go- 2 gases) in the atmosphere, presents policy- ing across the Atlantic to Europe, European makers with the central moral dilemma of pollution is traveling to Asia, and Asian pol- the global-warming problem. Every decade lution is coming back to America. To better we delay in taking action, we are commit- understand how atmospheric brown clouds ting the planet to additional warming that impact the environment, climate, and the future generations have to deal with. quality of life, we have launched a project to integrate scienti½c ½ndings with impact assessment. With support from the United Every decade we delay in Nations Environment Program and the Na- tional Oceanic and Atmospheric Adminis- taking action, we are tration, we are working with scientists from Sweden, Germany, China, , Japan, and committing the planet to other Asian countries to develop a system of strategically located ground-based observa- additional warming that tories in the Indo-Asian and Paci½c regions to monitor atmospheric pollution. Together future generations have to with satellite data and periodic aircraft mea- surement, these observatories will document deal with. changes in aerosol content, pollutant gases, and some greenhouse gases, enhancing the The masking effect is equally troubling. We predictive capacities of climate models. We now know that the surface-cooling effect of hope that in time this work will revolutionize aerosols may have masked as much as 50 how we look at the atmosphere. percent of the global warming caused by greenhouse gases, presenting a serious di- However, we continue to face a number of lemma for the global community: If we at- uncertainties in our efforts to predict climate tempt to reduce air pollution because of its change. First, by the beginning of the next effect on health, we may see an ampli½ca- century, the global population will reach tion of global warming. At the same time, if about nine billion, and people in the devel- greenhouse gases are curbed because of our oping world will be striving to achieve West- concerns about global warming, the brown ern standards of living. Their efforts will result clouds may weaken the Earth’s water cycle, in enormous amounts of atmospheric pol- particularly the monsoonal rainfall in Asia, lutants and other stresses on the environ- leaving us with conflicting options involving ment. those regions negatively impacted by global Second, there are the unsolved questions warming and those negatively impacted by surrounding the rate and masking of global air pollution. warming. The extent of global warming is Although we talk about global averages in not fully reflected in the Earth’s surface discussions of global warming, we cannot temperatures. The additional heat trapped forget the marked changes that also occur at

38 Bulletin of the American Academy Spring 2006