A search for human influences on the thermal structure of the atmosphere

Abstract

Changes in the vertical structure of atmospheric temperature have been proposed as a possible "fingerprint" of greenhouse-gas-induced climate change4-'. Until recently, most of our information about the structure of such a fingerprint has been derived from equilibrium COa doubling experiments performed with atmospheric General Circulation Models ( AGCMs) coupled to mixed-layer oceansgJo. These experiments yielded a fingerprint pattern characterized by stratospheric cooling, tropospheric warming, a warming maximum in the tropical upper troposphere, and (for annual mean zonally-averaged changes) an approximate hemispheric symmetry of the temperature response (see Fig. la).One recent study '' has compared such model-predicted patterns of temperature change with observed latitude-height temperature-change profiles. The latter were 2 obtained from the radiosonde analyses of Oort, and span the period 1963-8712*13. The conclusion reached by this work was that the observed data showed an increasing expression of the equilibrium temperature-change signal predicted by two different AGCMs in response to C02 doubling. This time-increasing similarity was judged to be significant, and it was further concluded that the individual pattern signatures of El Nifio-Southern Oscillation (ENSO) events and stratospheric ozone reduction were spatially dissimilar to the searched-for COa fingerprint.Although suggestive of a causal relationship between increasing levels of atmospheric C 0 2 and the vertical structure of atmospheric temperature changes, this investigation did not claim attribution of all or even part of the observed changes to the specific cause of changes in CO2. The principal uncertainties were related to the quality and short record length of the radiosonde data, the lack of a dynamic ocean in the model experiments, the neglect of other anthropogenic forcings (such as changes in sulphate aerosol loadings) and concerns regarding the estimation of significance by resampling of the observed data1'J4. One further concern was whether natural climatic variability could mimic the model-predicted greenhouse fingerprint, as preliminary analyses of observations and model control runs had s u g g e~t e d '~-~~. All of these factors hampered more confident statements regarding detection of a significant change, and attribution of (some fraction of) that change to increasing C02.Our investigation differs from this earlier work in three ways. First, we examine the relative detectability of vertical temperature-change signals from recent experiments with individual and combined changes in atmospheric C02 and anthropogenic sulphate aerosolslg. Second, we consider how a combined C02+S04 vertical temperature-change signal might be modified by observed changes in stratospheric ozone. It is highly likely that recent reductions in stratospheric ozone are in part attributable to industrial production of halocarbons2'. These changes may have a complex signature in the thermal structure of the atmosphere, varying as a function of latitude, alt...

Detectability of the impacts of ozone-depleting substances and greenhouse gases upon stratospheric ozone accounting for nonlinearities in historical forcings

“…External influences include human-caused changes in well-mixed greenhouse gases (GHGs), ozonedepleting substances (ODSs), and other radiative forcing agents, as well as natural fluctuations in solar irradiance and volcanic aerosols. Past D&A studies have found that each of these external influences has a unique fingerprint in the detailed zonal-mean latitude-altitude pattern of temperature change (Hansen et al, 2005;Karoly et al, 1994;Santer et al, 1996a;Tett et al, 1996;Thorne et al, 2002;Vinnikov et al, 1996). The use of such profiles of atmospheric temperature change has proved particularly useful in separating human, solar, and volcanic influences on climate, and in discriminating between externally forced signals and internal variability.…”

Identification of human-induced changes in atmospheric moisture content

“…Such work has been influential in shaping the ''discernible human influence'' conclusions of national and international scientific assessments (2-4). Most fingerprint studies have focused on temperature changes at the earth's surface (5, 6), in the free atmosphere (7,8), or in the oceans (9), or have considered variables whose behavior is directly related to changes in atmospheric temperature (10).Despite a growing body of empirical evidence documenting increases in moisture-related variables (11,12), and climate model evidence of a number of robust hydrological responses to global warming (13,14), there have been no formal fingerprint studies involving changes in the total amount of atmospheric water vapor, W. Other aspects of moisture changes have received attention in recent fingerprint work, with identification of an anthropogenic signal in observed records of continental river runoff (15), zonal mean rainfall (16), and surface specific humidity (17).Warming induced by human-caused changes in well mixed greenhouse gases (GHGs) should increase W (11,12). Under the assumption that relative humidity remains approximately constant, for which there is considerable empirical support (13,18,19), the increase in W is estimated to be Ϸ6.0-7.5% per degree Celsius warming of the lower troposphere (13, 18).…”

“…Such work has been influential in shaping the ''discernible human influence'' conclusions of national and international scientific assessments (2-4). Most fingerprint studies have focused on temperature changes at the earth's surface (5, 6), in the free atmosphere (7,8), or in the oceans (9), or have considered variables whose behavior is directly related to changes in atmospheric temperature (10).…”

Science prizes: Best in class

“…One of his key studies showed that an anthropogenic 'fingerprint' on climate change predicted by computer models could be detected in weather-balloon data 2 , and was cited as "the most convincing demonstration yet" of a human contribution to changing global air temperatures 3 .…”