Saturday, January 31, 2015

Climate Change Part 15: Linear and Abrupt Models

Note: This program first aired January 31, 2015.

It may be no surprise to you that anthropogenic climate change is paying dividends to the world already. No longer is this something we can think of as impacting the future. The future, as they say, is now. World wide glaciers, both continental and alpine are in rapid retreat, phenological changes that include earlier dates of ice out on lakes and rivers and earlier timing of spring tree, flower and phytoplankton blooms are being documented everywhere, tropical diseases are spreading, along with their host insects to higher latitudes whose climates can now support them, the intensity and frequency of extreme heat waves are increasing, Arctic sea ice is getting smaller and thinner each summer, and last year was the warmest year on record.

When we think about the consequences of climate change, there are two models that are used to conceptualize what might happen (as well as for comparison with what IS happening). The first model is linear. In this world view, incremental increases in green house gasses yield incremental changes in climate, it’s a y equals mx plus b vision of how the climate system works. The carbon dioxide level and average global temperature have a direct relationship. As carbon dioxide levels increase, average global temperature increases at some set proportion described by the slope of the line, and that relationship remains a constant straight line regardless of how high carbon dioxide levels get. It is a reassuring, predictable relationship, in that the math is simple. If we know the X value, a carbon dioxide level, we can get a Y value, the resulting temperature. Everyone can interpret a straight line graph. It’s worth noting that this is the model that the Intergovernmental Panel on Climate Change, the world’s preeminent climate change research and policy body, uses to makes its predictions, predictions that direct the course of global policy actions.

The problem is that Earth’s climate system is not actually linear. The relationship between carbon dioxide and temperature may appear to have simple causal attributes, but it is just the tip of the iceberg. Climate is defined as the long term average of weather, and weather is acknowledged to be a chaotic system, chaos in this sense as a technical term, meaning distinctly non linear. As we increase atmospheric carbon dioxide and thus increase average temperatures as expected, all kinds of other related processes can get triggered, and lead to rapid and unexpected effects. Take for example, last year’s intensely cold winter in North America, not the kind of thing you would expect if you used the simplistic model of increasing carbon dioxide leading to warmer temperatures. In fact the increasing temperatures have disrupted typical atmospheric pressure arrangements and thus wind patterns, which led to changes in the jet stream. Instead of running primarily west to east last winter, the jet stream (which is simply the boundary between polar and mid latitude air masses) was able to take large north and south running loops, bringing very dry cold polar air to lower latitudes than we are used to. Chaos.

The ultimate expression of this non linear climate system, and the one that keeps climate scientist up at night is the model of abrupt climate change. It’s what many leading climate scientists see on the very near horizon. Essentially X leads to Y, which then leads to A, B, C or any other letter in the alphabet, or perhaps, all of them. A warming average global temperature pushes certain aspects of the climate system past their tipping points, which result in a rapid and more irreversible change in the functioning climate system. Those anticipating abrupt climate change are looking hard at what is happening in the Arctic. Polar amplification has long been recognized, in that the Arctic has seen warming temperatures several degrees higher than the global averages, due largely to feedback loops related to the loss of sea ice and therefore albedo. As less sunlight is reflected (because there is less highly reflective ice around), more light is absorbed, which then warms the Arctic and melts yet more ice. There are suggestions that the Arctic may be more intimately influential in global climate than was previously suspected, and if the Arctic continues its death march towards a tipping point, it may take the whole world with it, quickly. There is precendent for the mechanisms of abrupt climate change in the paleoclimate record, abrupt climate change is in fact what has led the Earth into glacial periods, as well as to rapidly emerge from those same ice ages over a matter of decades.  

Over the past many many weeks and months we’ve spent most of our climate change energy focusing on the carbon, because quite simply, this whole thing starts with the carbon, it’s the crux of what is driving the warming of the climate. The parallel story is that while carbon is simple, climate is wildly complex and the system is massively difficult to describe quantitatively in terms of predicting behavior. That is why climate scientists are a strange mix of computer geniuses who build super computing climate models, and field researchers who drill cores in glaciers and ocean sediments trying to reconstruct past climate patterns. We look at the past to predict the future, and the past tells us that it is likely an abrupt shift, rather than a gradual stepwise transition, that we should be looking forward to.


Current effects of climate change from NASA, including several really good graphical representations of various effects:

IPCC home page, read the projections for your self:

Maine Climate Change Institute on Abrupt Climate Change