"Non-natural" external forcings are of interest, as well as concern. While these include factors such as changes in stratospheric ozone related to the use of CFCs, changes in tropospheric ozone, changes in land use (altering albedo) and even the effects of jet contrails, most of which are still poorly understood, the major concern is that loading of atmospheric greenhouse gases and of black carbon due to burning of fossil fuels and biomass will cause warming. At least part of the increase in global mean SAT since the mid 1800s of about 0.7 °C (Jones et al., 1999; Jones and Moberg, 2003), is attributed by the majority (not all) of the science community to trace gas loading. While projections of climate through the twenty-first century range widely between different climate models, models are in general agreement that the effects of trace gas loading will be amplified in northern high latitudes (e.g., Holland and Bitz, 2003). Two fundamental issues are involved. First, warming due to higher greenhouse gas concentrations will initiate further warming and thinning and retreat of the sea ice cover due to ice-albedo feedbacks and associated processes. Second, the strong stability of the Arctic's lower troposphere (see Chapter 5) will tend to focus heating near the surface. If models are correct in their depictions of change, the observed buildup of greenhouse gas concentrations (an equivalent increase in carbon dioxide by about 50% since the mid eighteenth century, with present-day concentrations well outside the range of the Pleistocene glacial-interglacial cycles) should by now arguably be producing detectable climate signals.
But detection of an anthropogenic signal in climate is a difficult task. Quality time series of temperature and other variables are of short duration (with few exceptions 150 years at best and generally much shorter in the Arctic). Hence it can be difficult to separate trends associated with human impacts from low-frequency variability of natural origin. Furthermore, while there have been great advances in climate modeling, many climate processes, interactions and feedbacks are still inadequately represented or poorly understood (see Chapter 10). Along these lines, we are beginning to recognize that anthropogenic forcing may alter the frequency and persistence of natural atmosphere modes.
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