D8 Monitoring of Trace Gases and Forcings

Topic

Monitoring trace gases and forcings is essential for quantifying anthropogenic disturbance of the climate system; only by continuously measuring the concentration of radiation-absorbing trace gases and the magnitude of the various forcings acting on the climate system is it possible to distinguish, over time, how much of an observed change is attributable to human activity versus natural causes.

The record of carbon dioxide concentrations at Mauna Loa, Hawaii, is the longest and most iconic continuous record in trace gas monitoring; initiated in 1958 by scientist Charles David Keeling, it has since shown a long-term upward trend overlaid by a seasonal cycle with an amplitude of approximately six parts per million. This seasonal cycle is driven by photosynthesis and respiration in Northern Hemisphere vegetation: when Northern Hemisphere vegetation photosynthesizes most intensely during spring and summer, it removes carbon dioxide from the atmosphere, causing the concentration at Mauna Loa to drop; conversely, when that same vegetation respires more than it photosynthesizes during autumn and winter, it releases carbon dioxide into the atmosphere, causing the concentration at Mauna Loa to rise.

A global network of monitoring stations extends this trace gas surveillance beyond carbon dioxide and the Mauna Loa site; it also covers methane, nitrous oxide, sulfur hexafluoride, and halocarbons—trace gases that, like carbon dioxide, absorb radiation and contribute to total anthropogenic radiative forcing, yet originate from different sources and have atmospheric lifetimes that differ from that of carbon dioxide itself. In addition to trace gases themselves, the monitoring of trace gases and forcings also characterizes atmospheric aerosols via their optical depth—a quantity that measures the amount of solar radiation scattered or absorbed by aerosols throughout the atmospheric column. The greater the aerosol optical depth at a given location and time, the more solar radiation those aerosols scatter or absorb before the radiation reaches the surface. A network of ground-based solar photometers measures this aerosol optical depth at multiple wavelengths simultaneously, making it possible to infer the size distribution and optical properties of the aerosols; a satellite equipped with a high-precision laser instrument complements this network by providing high-resolution vertical profiles of extinction caused by aerosols and clouds at various altitudes. Despite this instrumentation, the effective radiative forcing of aerosols remains the most uncertain component of the total anthropogenic radiative budget, precisely because aerosol optical depth varies significantly from place to place and from day to day.

Total column ozone is another quantity continuously tracked through trace gas and forcing monitoring, which combines ground-based Dobson spectrophotometers with specialized satellite instruments; both types of instruments measure the amount of ultraviolet radiation absorbed by ozone throughout the atmospheric column and use this absorption data to infer the total column ozone present above a specific surface location.

Trace gas and forcing monitoring also tracks total solar irradiance, which has been measured continuously since 1978 by a succession of specialized satellite instruments, each replacing the previous one as it reached the end of its operational life. The precision of this continuous measurement of total solar irradiance is critical precisely because it allows us to answer the question that originally motivated the monitoring of trace gases and forcings: if total solar irradiance were to show a long-term upward trend similar to that of carbon dioxide at Mauna Loa, some of the observed warming could be attributed to the Sun rather than to human activity. The evidence provided by this continuous measurement indicates that this is not the case: total solar irradiance has not changed significantly since 1980, whereas warming of the climate system has continued throughout that same period; this rules out total solar irradiance as the cause of the observed warming and reinforces—by elimination—the attribution of that warming to the anthropogenic disturbance that the monitoring of trace gases and forcings aims to quantify.

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