Climate Science Comes of Age - The Hockey Stick and the Climate Wars - page 13

warming. But other factors, such as how clouds change under warmer conditions and to what effect,
were highly uncertain, and remain so still.
Most models indicate a tendency for more low clouds in a warmer climate; low clouds mimic
the effect of surface ice, for example, reflecting solar radiation back to space. Thus, in these models,
clouds play the role of a negative feedback, diminishing the warming. Yet in other credible climate
models, clouds behave differently, effectively enhancing the overall greenhouse effect and acting as
yet another positive feedback. Representing cloud effects is perhaps the most daunting challenge for
climate models, because they occur at scales too small to capture explicitly in the models, and their
effects must therefore be represented only through approximations.
What's All the Fuss About?
By the mid-1990s, larger questions regarding the potential societal and environmental impacts of
climate change were beginning to receive more attention as well. Did climate change pose a threat to
the future welfare of our civilization, and even possibly to our species? And if so, what, if anything,
should we do about it, and when? Science alone could not, of course, answer many of these questions.
They are as much matters of policy (and risk management, economics, and ethics) as they are matters
of science. The science could, however, inform matters of policy.
There was increasing recognition by the mid-1990s that another 2°C (3.5°F) warming beyond
current levels (for a total of 3°C or 5°F warming relative to preindustrial times) could represent a
serious threat to our welfare. 4 Precisely what limitations in global greenhouse gas emissions would
be required to avoid that amount of warming remained uncertain, and still does, because of the spread
of predictions among models. If we choose to take the midrange model estimates as a best guess,
avoiding another 2°C of warming would require stabilizing atmospheric CO 2 concentrations at no
higher than about 450 parts per million (ppm).
Preindustrial levels were about 280 ppm, reflecting a long-term balance between natural
processes that produce (sources) and those that take up (sinks) CO 2 from the atmosphere. Humans,
through extensive fossil fuel burning and other practices, have upset that natural balance, causing CO 2
concentrations to rise steadily. Indeed, those concentrations will continue to rise until human
emissions are brought essentially to zero. 5 The carbon we emit into the atmosphere today has an
extended legacy; it will potentially reside there for centuries.
Levels, as of 2011, are nearly 390 ppm and are increasing by 2 to 3 ppm per year as a result of
annual carbon emissions. The average American, through various actions and activities, emits roughly
20 tons—the weight of two very large adult male African elephants—of carbon per year. Globally,
human beings emit the equivalent of more than 400 million of those elephants—roughly 8.5 billion
tons—of carbon per year. A 450 ppm stabilization target would require greenhouse gas emissions be
brought to a peak of no more than about 9 billion tons (450 million elephants) per year within the next
decade, be lowered to mid-twentieth-century levels of roughly 1 billion tons (50 million elephants)
per year by midcentury, and brought to near zero by the end of the century, to avoid breaching 450
ppm. That is a daunting task, as global population continues to increase, developing nations such as
China and India continue to ramp up their own emissions, and industrial nations like the United States
continue with business as usual. Given the enormity of the challenge, it was convenient for some to