Climate Forcing

[Launching an ozonesonde] To slow the rate of anthropogenic-induced climate change in the 21st century and to minimize its eventual magnitude, societies will need to manage the climate forcing factors that are directly influenced by human activities, in particular greenhouse gas and aerosol emissions. For effective management of these species, a solid scientific understanding of their natural cycles and the processes that influence those cycles is necessary. Atmospheric measurements are the touchstone of theories or models describing these cycles. Providing a sound basis for important societal decisions requires a global effort, one that involves studying numerous gases, particles, and atmospheric radiation on appropriately designed spatial and temporal scales.

Greenhouse Gases

NOAA measurements of climatically important gases began on an expanded scale in the mid-1970s for carbon dioxide (CO2), nitrous oxide (N2O), chlorofluorocarbons (CFC's), and ozone (O3). Over the years a number of other gases have been added, including methane (CH4), carbon monoxide (CO), hydrogen (H2), hydrochlorofluorocarbons (HCFC's), hydrofluorocarbons (HFC's), methyl halides, and sulfur hexafluoride (SF6).

In collaboration with the Institute for Arctic and Alpine Research (INSTAAR) of the University of Colorado, GMD now routinely monitors carbon isotopic ratios of CO2 and CH4. GMD measures the evolving distributions of these gases through its global cooperative air sampling network (see inside front cover). Understanding the interactions of the atmosphere with the land and ocean is key to understanding the natural cycles of atmospheric gases and requires going beyond the earth's surface. Thus, in addition to widespread, frequent, fixed surface-level measurements, GMD also studies the processes driving greenhouse gas cycles from very tall communication towers, planes, trains, and ships.

Aerosols

The original objective of "baseline" aerosol measurements was to detect a response to changing conditions on a global scale. Since the inception of the program (1960s) our understanding of the behavior of atmospheric aerosols has improved considerably. In response to the finding that anthropogenic aerosols create a significant perturbation in the earth's radiative balance on regional scales, GMD expanded its aerosol research program (1992) to include aerosol monitoring stations in regions where significant aerosol forcing was expected.

The goals of this regional-scale monitoring program are to characterize the means, variabilities, and trends of climate-forcing properties of different types of aerosols, and to understand the factors that control these properties. An important aspect of this sampling strategy is linking chemical measurements to physical measurements. GMD's aerosol measurements also provide ground-truth data for satellite measurements and inputs for global models.

Radiation

The combined effects of climate forcing lead ultimately to alteration of the earth's radiation budget. Broadband irradiance, as routinely measured by GMD, is intended primarily to provide benchmarks of climatic processes. Forced changes in irradiance are not only affected by changing concentrations of constituents or other external sources, but also by changes in water vapor and clouds.

Ancillary measurements of the atmospheric thermodynamic state and composition are necessary to resolve sources of irradiance variability. Data records of sufficient duration are expected to reveal the extent of irradiance variations over time that reflect a combination of cause and effect of climate change.

With reference to the following graphic from the IPCC 2001 report, GMD conducts monitoring and research in all radiatively important aspects of climate forcing except land-use albedo.

The global mean radiative forcing of the climate system for the year 2000, relative to 1750.

[Global, annual-mean radiative forcings (Wm-2) due to a number of agents from the period from pre-industrial (1750) to present (2000)]

Scientific Questions Related to Climate Forcing

  • What is the response of the carbon cycle to human perturbations?
  • What are the long-term trends and year-to-year variations in the terrestrial and oceanic sources and sinks of greenhouse gases?
  • What is the contribution of noncarbon gases and aerosols to climate forcing?
  • How will future climate change affect the fluxes of climatically important gases?
  • What are the direct effects of climate change on the radiation budget?
  • What controls aerosol radiative and cloud-nucleating properties?
  • What drives the changing growth rate of gases such as methane?

Actions and Impacts

GMD plans the following actions:

  • Continue climate-related observations at GMD observatories and cooperative sampling sites.

    Impact: The records of climate forcing agents, some extending back 44 years, will continue to be available for models that project future climate and to assess progress on climate change mitigation actions.

    [Net solar radiation at Mauna Loa Observatory, relative to 1958, showing the effects of major volcanic eruptions. Annual variations are due to transport of Asian dust and air pollution to Hawaii.]
    Net solar radiation at Mauna Loa Observatory, relative to 1958, showing the effects of major volcanic eruptions. Annual variations are due to transport of Asian dust and air pollution to Hawaii.

  • Establish a carbon-observing network over North America.

    Impact: From atmospheric measurements, the North American continent has been estimated to be a major carbon sink in some years but the uncertainty is high and surface carbon inventory assessments are not in agreement. The new observing network would reduce the uncertainties and provide information useful for policy makers.

    [Vertical profiles of carbon cycle gases are conducted from communications towers and aircraft.] [Vertical profiles of carbon cycle gases are conducted from communications towers and aircraft.]
    Vertical profiles of carbon cycle gases are conducted from communications towers and aircraft.

  • Continue conducting measurements from ships and expand measurements to ocean buoys to obtain a better understanding of carbon gases and oceanic gas fluxes.

    Impact: An understanding of oceanic sources and sinks of major climate forcing gases is critical in understanding and projecting future global climate. Net solar radiation at Mauna Loa Observatory, relative to 1958, showing the effects of major volcanic eruptions. Annual variations are due to transport of Asian dust and air pollution to Hawaii.

  • Collaborate with other laboratories in establishing new measurement sites.

    Impact: The global climate observing system will be expanded, providing regional fluxes of climatically important data, while building scientific capacity.

  • Add perfluorocarbons (PFC's), including CF4 and C2F6, to the observing system.

    Impact: There is little information on the global distribution of PFCs. These gases have potential as long-term contributors to climate forcing because of their high per-molecule radiative forcing and long atmospheric lifetimes.

  • Replace aging surface ozone monitoring equipment at the Baseline Observatories and increase the number of monitoring sites.

    Impact: Replacing old ozone analyzers at existing stations and adding new sites on the west and east coast of the U.S. would increase the reliability and scope of surface ozone measurements (which may be impacted by industrial activity in Asia).

  • Maintain and improve the accuracy and representativeness of radiation data, expand the ancillary data collection, and extend analysis of existing and newly acquired data.

    Impact: A greater understanding of radiation observations in the absence of model calculations, and vice versa, can be established to fill gaps in the climate record. Vertical profiles of carbon cycle gases are conducted from communications towers and aircraft.