” Š the likelihood of global warming even beyond the 2.4°C margin in the
21st century is frustratingly high.”
” My conclusion is that we are still left with a fair chance to hold the 2°C line,
yet the race between climate dynamics and climate policy will be a close one.”
“This requires an industrial revolution for sustainability starting now.”
Proceedings National Academy of Sciences USA
September 23, 2008
vol. 105 no. 38 14239 -14240
Global warming: Stop worrying, start panicking?
Hans Joachim Schellnhuber
In their excellent Perspectives article in this issue (1), Ramanathan and Feng (R&F) sound a
harsh wake-up call for those concerned about anthropogenic climate change: the authors
maintain that the greenhouse gas (GHG) emissions of the past have already loaded the
Earth System sufficiently to bring about disastrous global warming. In other words, the
ultimate goal of climate protection policy, as stipulated by the United Nations Framework
Convention on Climate Change (UNFCCC) (2), appears to be a delusion. So should we stop
worrying and rather start panicking now?
Our planet is already committed to anthropogenic warming in the range of 1.4 – 4.3°C, where
2.4°C is the most likely amount. The main reason why only roughly a quarter (actually 0.76°C
since the latter half of the 1800s) of that equilibrium temperature response to the current
atmospheric GHG concentrations has been observed is the (predominantly) cooling effect of
various aerosols that often accompany GHG emissions. Large scientific uncertainties remain
regarding the forcing potential of the various aerosol species.
There is certainty, however, that GHG concentrations (in particular, CO2 levels) will rise further
in the medium-term future and that clean-air policies will remove that accidental antidote against
global warming in the decades to come. Thus, the likelihood of global warming even beyond the
2.4°C margin in the 21st century is frustratingly high.
Venturing into the 2 realm is risky, as the authors rightly emphasize in their second point, because
large-scale nonlinear responses of the planetary machinery are likely to be triggered then (8, 9).
My conclusion is that we are still left with a fair chance to hold the 2°C line, yet the race between
climate dynamics and climate policy will be a close one.
However, the quintessential challenges remain, namely bending down the global Kyoto-GHG output
curve in the 2015-2020 window (further procrastination would render necessary reduction gradients
too steep thereafter) and phasing out carbon dioxide emissions completely by 2100. This requires
an industrial revolution for sustainability starting now.