Saturday, July 01, 2017

Climate change: How fast does it happen?

A story about climate change.
A true story.

     The science department head at W C Eaket Secondary School was a member of the American Association for the Advancement of Science. She received their weekly journal, and placed it in the school library, where I read it most weeks.
     Every so often, there were papers on weather and climate modelling, which was developing quickly as computer power increased. The models were based on weather data laid down in ice-cores, tree-rings, layers of silt on lake bottoms and swamps, and so on. These data are good for many thousands of years in the past, but obviously not for millions for years. The geological record supplies data for those long range climate changes.
     Small changes in climate such as the Little Ice Age in the 1500s-1600s (which killed off the Viking Greenland colony) were used to test the models. These were strong models because they were based on large amounts of data. If they worked well, they were run backwards beyond the range for which there was much data, to see if they described the climate as known from geology. They were also run forward, to see what could happen if the CO2 continued to increase to the levels known to have existed millions of years ago.
     The tests were designed to guide further development of weather and climate models. The models varied in the weighting of different factors known to affect the weather, estimated and known rates at which the effects occurred, and different ideas about the feedback loops between these factors. As better data became available, the models were tweaked. Because of their differences, the models were in fact tests of different theories of how weather and climate change. Weather prediction models are so powerful now that we expect a three- to four-day forecast to be accurate. Back then, one day was considered good. When I was a child, we expected weather forecasts to be updated between morning and evening.
     The results of the climate models were, as they say, interesting. The authors reported on and discussed the successful models, the ones that closely described the known history of the weather and climate. Most of these models predicted continuing slow changes in climate like the ones known from the past.
     A handful of models in the early to mid-70s predicted very sudden changes in climate. Changes that didn’t take thousands or tens of thousands of years, but a few hundred years, or even less. The authors were uncertain what to do with those. It wasn’t clear how to decide whether these models were any better or worse than the ones that predicted slow changes. Their conclusions were cautious.  As I recall the theme of their discussions, it was along the lines of “If these models that predict fast changes are accurate, then the rapidly rising levels of CO2 in the atmosphere could cause rapid changes in climate.”
     Those papers have stayed with me. Some time later, I learned about the mathematics of chaos. A chaotic system cycles through a series of changes with minor variations from one cycle to the next. Think of the seasonal cycle of weather.  But if one variable exceeds some critical value, the system shifts into another state. This new state will cycle through a different series of changes.
      Climate is a chaotic system. As with any chaotic system, a fundamental question is how quickly the shift can occur. Some chaotic systems change so fast that we speak of a “tipping point”. There is increasing evidence that climate is such a system.

Global Warming

1 comment:

pferk said...

Interesting comment