Steps and Cycles

Climate Change, Anthropocene, & Associated Issues

Date posted: 07/15/15

Andy Chadwick and Paul Williamson, of the British Geological Survey, explain how interaction between the continuous trend with natural climate cycles produces the observed stepped pattern of global warming. So-called climate-change sceptics have made great mileage out of the current hiatus in observed global warming. Atmospheric temperatures have not really increased since the turn of the 21st Century, a fact frequently cited as incompatible with, or even disproving, global warming. These same commentators draw a discreet veil over the fact that temperatures also did not rise between 1940 and 1970 - an inconvenient truth for them, since that pause was followed by 30 years of rapid warming. However, the truth is that a long term rising temperature trend, steepening with time, has lifted average global temperatures by around 0.9 degrees C since the early 20th Century. This long term trend correlates closely with the rise in atmospheric CO2 and with its expected greenhouse warming effects. But anthropogenic emissions are why the observed temperature variation is more complex. The basic picture of long term trend warming is in fact characterized by thirty year ramps (where temperatures rise relatively quickly), alternating with thirty year flats, where temperatures either fall slightly, or remain roughly constant. The latest flat commenced around the year 2000. It is relatively easy to set out, simply and without complex modelling, the way in which global temperatures and atmospheric CO2 concentrations have changed over the past century and more. By identifying the different natural, shorter-term variations that affect the atmosphere, they show quite clearly why one should not expect temperature increases to be as smooth as the rise in CO2 content, and how it is that the interference between the steady curve and the different cyclicities produces the stepped graph that is observed. A good read ...