Geoscience Reference
In-Depth Information
What do the sceptics say?
One of the best ways to summarize the evidence for climate change is to review what the
global warming sceptics or climate change deniers say against the current state-of-the-art
science. Though I must stress I dislike the term sceptic because it seems to co-opt or steal a
fundamental of science. All great scientists are sceptical about the state of current know-
ledge, and this drives them forward into making new observations and new experiments to
allow them to develop new testable theories. The fundamental principle of 'weight of evid-
ence' within science is a way of testing our sceptics about new ideas and data. So the term
'climate (change) deniers' is probably better term technically, as they are denying the sci-
entific weight of evidence.
(1) Ice-core data suggest atmospheric carbon dioxide (CO
2
) responds to global temperature, therefore atmospheric
CO
2
cannot cause global temperature changes.
At the end of the last ice age as the Earth warmed up, we now know from ice cores from
Greenland and Antarctica that the Northern and Southern Hemispheres warmed up at dif-
ferent times and at different rates. On top of this there are millennial-scale climate events,
when huge amounts of ice broke off from the North American ice sheet flooding the North
Atlantic Ocean with freshwater changing ocean circulation and in essence trying to push
global climate back into much colder conditions. One of these events called Heinrich event
1 occurred about 15,000 years ago and the other was the Younger Dryas, which occurred
about 12,000 years ago. Because of the wonderfully named 'bipolar climate seesaw',
whenever the Northern Hemisphere cools down heat is exported southwards and the South-
ern Hemisphere warms up. So if you compare an individual ice-core temperature record
with reconstructed atmospheric CO
2
levels then there will be times when the relationship
seems to swap. To really understand the relationship between global temperatures and CO
2
,
Dr Jeremy Shakun of Harvard University and colleagues created a master stack of all the
temperature records across the end of the last ice age (see
Figure 10
). This shows that at-
mospheric CO
2
leads global temperatures adding to our confidence that it is contributing to
the warming of the Earth as we exited the last great ice age.
