Additional Past Accomplishments:
The following are selected highlights of recent research supported by CCSP participating agencies (as reported in the fiscal year 2009 edition of the annual report, Our Changing Planet).
Climate-Relevant Properties of Aerosols
Aerosol Forcing Effects on Climate Change are Better Defined.1,2 Aerosols (atmospheric fine particles such as pollution, smoke, and desert dust) in sunlight directly heat the atmosphere if they absorb light, and cool the surface by absorbing and scattering light. Evaluating the net radiative effect of aerosols has been a key uncertainty in past Intergovernmental Panel on Climate Change (IPCC) assessments, partly due to the highly variable horizontal and vertical distribution of aerosol particles of differing chemical composition, size, and shape. This in turn hampers modeling efforts to understand the total amount and vertical distribution of solar radiation indicated by satellite observations. A recent field campaign studied the radiative forcing of atmospheric brown clouds—large pollution plumes that increasingly cover large regions of dry season southern Asia (see Figure 3). Critical measurements of solar heating profiles above, within, and below these pollution-dominated plumes over the Indian Ocean in the Northern Maldives were made with small unmanned aerial vehicles (UAVs) instrumented with aerosol and radiation detectors and positioned to make vertically aligned measurements. Findings indicated that continental air masses with higher aerosol particle concentrations, and in particular those containing the carbonaceous component of soot, exhibited increased aerosol absorption and heating by as much as 50% over background oceanic conditions. Other improvements in understanding have been made using the Aerosol Robotic NETwork (AERONET), a network of about 230 automated surface instruments that measure the optical properties of aerosols. Analysis of AERONET data collected in 2004 in the United Arab Emirates is leading to a better understanding of the dynamics of desert dust and pollution aerosols over a variety of environments in the Arabian Peninsula and over the Persian Gulf.
Current Trends in Arctic Haze and Implications for Climate Forcing.3 The long-range transport of anthropogenic pollution from North America, Europe, and western Asia creates the aerosols associated with so-called Arctic haze. U.S., Finnish, and Canadian researchers have recently compiled long-term data to determine trends in and climate impacts of Arctic haze. The analysis confirmed previously reported results of a decreasing trend in Arctic haze between the early 1980s and the mid-1990s. In addition, the analysis revealed evidence of increasing levels of aerosol scattering, black carbon, and nitrate over the past decade. Calculations of direct radiative forcing by Arctic haze for a representative case during the haze maximum (mid-April) resulted in an estimated 2 to 3 Wm-2 of additional heating to the atmosphere and approximately 1 Wm-2 of cooling at the surface. CCSP researchers, as part of the International Polar Year, returned to the Arctic in 2008 using aircraft and ship platforms to better characterize the direct and indirect climate impacts of the Arctic haze.
Cloud-Aerosol-Climate Feedbacks and Interactions
Improved Understanding of Connections Between Aerosol Chemistry, Clouds, and Climate.4,5,6,7 The increasing levels of aerosol from human activities affect cloud properties, cloud lifetime, and precipitation processes, and hence climate. However, the relationships are not well understood and the aerosol/cloud processes are one of the largest uncertainties in current understanding of climate change. Aerosols affect cloud properties by serving as cloud condensation nuclei (CCN) and thereby activating the formation of cloud droplets. The process of cloud drop activation is a function of both the size and composition of the aerosol particles, which, in turn, depend on the source of the aerosol and transformations that occur downwind. CCSP scientists conducted airborne and shipboard measurements of the aerosol number size distribution, aerosol chemical composition, and CCN concentration during the 2006 Gulf of Mexico Atmospheric Composition and Climate Study (GoMACCS). Aircraft measurements showed that the high organic emissions of the Houston Ship Channel region lead to a high organic acid content in the aerosol, and further revealed details of how the organic acid content evolves with increasing altitude within and above clouds to produce a ubiquitous layer of organic aerosol above cloud. Analysis of the GoMACCS shipboard data showed that when the organic content of the aerosol increases, the aerosol is less likely to form CCN and hence less likely to activate the formation of cloud droplets. These studies have led to an improved understanding of the links between aerosol composition and cloud drop formation. The work has yielded a simplified means of representing the processes in climate models, ultimately contributing to the development of an improved predictive capability. In the continental United States, 8 years of surface measurements at the Atmospheric Radiation Monitoring (ARM) Southern Great Plains site in Oklahoma have advanced understanding of how clouds, water vapor, and climate interact. The data have clearly shown that higher cloud fractions are the key factor in reducing the amount of solar radiation at the Earth’s surface, whereas the amount of precipitable water vapor in the clouds has a much greater impact on the cloud’s absorption of outgoing infrared radiation.
Atmospheric Aerosol Pollution Affects Structure and Reflectivity of Clouds.8 Viewed from space, stratocumulus clouds can have either an “open cell” or a “closed cell” appearance to their fine-scale structure (see Figure 4). The structure greatly affects the degree to which the clouds reflect light and, hence, the climate-relevant properties of the clouds. Recently it was hypothesized that precipitation may trigger the transition from closed to open cellular structure. Precipitation tends to occur in clean regions lacking in aerosol, thereby providing a potential link between the composition of the atmosphere and the organization of clouds. Polluted, non-precipitating clouds were shown to exhibit a closed cellular structure, whereas in clean conditions, open cells formed in response to drizzle. CCSP researchers have modeled the processes and confirmed the hypothesis. The transition from closed to open cells has dramatic implications for radiative forcing, essentially representing the transition from a highly reflective cloud to one of much lower reflectance.
Atmospheric Constituents other than CO2, including Water Vapor, and Implications for Earth’s Energy Balance
Lightning and Pollution Combine to Cause Ozone Enhancements in the Summer Upper Troposphere.10,11,12,13,14 In the upper troposphere, ozone acts as a greenhouse gas and hence is relevant to climate. Analyzing data from a summer 2004 study, CCSP researchers found unexpectedly high levels of ozone in the upper troposphere 10 to 11 km above eastern North America during summer that were not attributable to either the high amounts of ozone pollution at Earth’s surface or the higher ozone levels in the stratosphere. The researchers investigated the cause of the increased ozone, which can nearly double the amount of upper-tropospheric ozone above the region. It was found that a natural factor—the emission of nitrogen oxides from lightning—acts in concert with the generally higher background levels of ozone precursor compounds in today’s polluted atmosphere to produce much of the upper-tropospheric ozone enhancement (see Figure 5). This ozone enhancement with a strong natural component contributes to the radiation budget at the regional scale. Other data gathered in the study demonstrate that the influence of lightning and convection on upper-tropospheric ozone has been previously underestimated. Lightning may have been underestimated by a factor of four, and faster convection rates may be needed to accurately model this region of the atmosphere. Large differences remain between observed and modeled levels of free radicals in the upper troposphere, an indicator that a major uncertainty remains in the understanding of how lightning-produced nitrogen oxides affect ozone in the upper troposphere.
Field Mission Investigates Atmospheric Composition, Clouds, and Climate in the Tropical Atmosphere. CCSP researchers carried out a field mission that gathered chemical and meteorological data for the cold, dry conditions of the upper tropical tropopause (an important transition region between the troposphere and stratosphere). This region of the Earth’s atmosphere between 14 and 18 km plays a key role in both climate change science and depletion of the stratospheric ozone layer. The Tropical Composition, Clouds, and Climate Coupling (TC4) experiment, based in San Jose, Costa Rica, involved dozens of scientists from U.S. agencies and academia and used a unique combination of coordinated observations from satellites, ground stations in the inter-tropical convergence zone, and instrumented aircraft during July and August 2007 (see <www.espo.nasa.gov/tc4/>). One of the specific goals of TC4 was to study the composition, formation, and radiative properties of clouds (cirrus and sub-visible cirrus) in this region, thereby assessing the contributions of such clouds, aerosols, and water vapor to climate forcing. Other aspects of the campaign focused on understanding the convective processes that control the transport of trace gases and aerosols from the lower atmosphere into the tropical tropopause and thence into the stratosphere, where they can influence stratospheric ozone. This mission gathered data that CCSP researchers are using to expand the scientific understanding of climate-cloud-chemistry interactions in the highly active and climate-relevant region of the tropical tropopause. The measurements also will be used to test retrieval algorithms for several instruments on the Aura satellite that measure trace gases (High Resolution Dynamic Limb Sounder, Microwave Limb Sounder, and Thermal Emission Spectrometer).
Global Transport of Pollution from Satellite Observations of Carbon Monoxide.15,16 Carbon monoxide (CO) in the Earth’s atmosphere is formed by the incomplete combustion of fossil fuels and biomass burning, and is primarily emitted at the surface. It can be lifted into the atmosphere by convection and transported around the globe by the prevailing winds. Its relatively long lifetime enables CO to be a good tracer of transport processes, such as the trans-Pacific transport of Asian pollutants to North America. Two years of observations from the Microwave Limb Sounder on the Aura satellite have provided the spatial distribution, temporal variation, and long-range transport of atmospheric CO and have shown the close relationship of concentrations of this gas to surface emissions, deep convection, and horizontal winds. The transport of CO from Southeast Asia across the Pacific to North America occurs most frequently during the Northern Hemisphere summer when deep convection associated with the Asian monsoon is clustered over the strong anthropogenic emission regions. Measurements of the global distribution of CO over time provide a strong indicator of the connections between changes in air quality due to increased industrialization and climate.
Regional Pollution and Global Climate Change
Impact of Global Change on U.S. Regional Air Quality.17,18,19,20 Emerging study results show that climate change has the potential to increase ground-level ozone concentrations in many areas of the United States and to lengthen the season of elevated ozone events. These increases may be beyond the envelope of natural interannual variability. Planned and future emissions controls will lower U.S. ozone concentrations, but the impacts of global change may necessitate further reductions to meet national air quality standards.
Recovery of the Stratospheric Ozone Layer
A New Formulation for Gauging the Effects of Ozone-Depleting Substances on the Ozone Layer.21 Equivalent effective stratospheric chlorine (EESC) is a convenient parameter to quantify the effects of halogens (chlorine and bromine) on the depletion of the stratospheric ozone layer. EESC has been extensively used to evaluate future scenarios of ozone-depleting substances (ODS) in the stratosphere. CCSP research has led to a new formulation of EESC that provides revised estimates of ozone layer recovery. The work shows that ODS levels will recover to 1980 levels in the year 2041 in the mid-latitudes, and 2067 over Antarctica, assuming adherence to international agreements that regulate the use of ODS. The researchers also assessed the uncertainties in the estimated recovery times. The 95% confidence interval associated with the mid-latitude recovery is the time period from 2028 to 2049, while it is from 2056 to 2078 for Antarctic ODS recovery.
Additional Past Accomplishments:
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