  The demand for computing is continuing to grow exponentially. This growth
will translate to exponential growth in computing's energy consumption unless
improvements in its energy-efficiency can outpace increases in its demand. Yet,
after decades of research, further improving energy-efficiency is becoming
increasingly challenging, as it is already highly optimized. As a result, at
some point, increases in computing demand are likely to outpace increases in
its energy-efficiency, potentially by a wide margin. Such exponential growth,
if left unchecked, will position computing as a substantial contributor to
global carbon emissions. While prominent technology companies have recognized
the problem and sought to reduce their carbon emissions, they understandably
focus on their successes, which has the potential to inadvertently convey the
false impression that this is now, or will soon be, a solved problem. Such
false impressions can be counterproductive if they serve to discourage further
research in this area, since, as we discuss, eliminating computing's, and more
generally society's, carbon emissions is far from a solved problem. To better
understand the problem's scope, this paper distills the fundamental trends that
determine computing's carbon footprint and their implications for achieving
sustainable computing.
