The President of the United States, George W. Bush, recently voiced his opinions on the science of climate change:
We — first of all, there is — the globe is warming. The fundamental debate: Is it manmade or natural. Put that aside.
The first part is the silver lining: despite receiving novelist Michael Crichton in the White House recently, Bush obviously has not bought his theory that the globe is in fact not warming. Crichton is one of the last trend sceptics who deny the warming trend is real.
Rather, Bush adopts an attribution sceptic position: warming yes, but is it caused by humans? This position is equally out of step with science, where the debate over this question has also now been settled.
Data show that carbon dioxide levels are rising, they are now 30% higher than at any time during at least the past 650,000 years, and likely even the past several million years. This rise is caused entirely by human activities. This is also demonstrated beyond any reasonable doubt by data – for a start, we know how much CO2 we have emitted, and the observed rise is equal to 57% of this (the rest has been taken up by ocean and biosphere). That carbon dioxide acts as a greenhouse gas, trapping longwave radiation, is also a measured fact and well-established physics since the 19th Century. For a doubling of CO2 concentration, this amounts to a radiative forcing of 3.7 Watts per square meter, which in equilibrium would cause a warming of around 3 ºC (that’s the climate sensitivity discussed by us here). The rise of CO2, plus some other gases like methane, is already causing a forcing of 2.7 Watts per square meter. In equilibrium, you therefore expect a warming of 2 ºC based solely on the human-caused rise in greenhouse gas concentration. But there’s a time lag due to ocean heat uptake (“thermal intertia”), so that up to half the expected warming would still be in the pipeline and not here yet (this is shown by models and confirmed by oceanographic data, Hansen et al. 2005). That means: this rough calculation shows that the human-caused increase in greenhouse gases can explain at least 1 ºC of global warming. The observed warming is 0.8 ºC – this is less than what would be expected from greenhouse gases alone, because greenhouse gases are of course not the only factor that affects climate – there is a cooling effect by aerosols which counteracts part of the warming.
What about a “natural” explanation for the observed global warming? There is none. Indicators and measurements of solar activity show no increasing trend over the past 60 years. The orbital cycles, which cause the ice ages, would currently tend towards cooling, if anything. There is no remotely feasible alternative explanation for the observed warming published in the scientific literature. The “fundamental debate” postulated by Bush is a media phenomenon – to use the words of ABC News, a “con job” by special interest groups. It is not a debate that is ongoing in the scientific community. The numerous, often hair-raising arguments that have been brought forward as part of this “con job” have been thoroughly refuted many times.
In summary, the following scientific findings can no longer credibly be argued to be in dispute:
(1) The observed large-scale warming of the atmosphere and ocean is an entirely expected, and in fact well-predicted, consequence of the human-caused accumulation of greenhouse gases in the atmosphere.
(2) There is no other reasonable scientific explanation for the observed warming.