20.11.14

A MODEL IS ONLY AS GOOD AS ITS EQUATIONS.

Last month, I noted that climate modelling that leaves out the equations of motion for heat flows in the ocean might produce dubious forecasts.  Calibration is like that.  And thus, there are opportunities for further research.
The biggest mystery in climate science today may have begun, unbeknownst to anybody at the time, with a subtle weakening of the tropical trade winds blowing across the Pacific Ocean in late 1997. These winds normally push sun-baked water towards Indonesia. When they slackened, the warm water sloshed back towards South America, resulting in a spectacular example of a phenomenon known as El Niño. Average global temperatures hit a record high in 1998 — and then the warming stalled.

For several years, scientists wrote off the stall as noise in the climate system: the natural variations in the atmosphere, oceans and biosphere that drive warm or cool spells around the globe. But the pause has persisted, sparking a minor crisis of confidence in the field. Although there have been jumps and dips, average atmospheric temperatures have risen little since 1998, in seeming defiance of projections of climate models and the ever-increasing emissions of greenhouse gases. Climate sceptics have seized on the temperature trends as evidence that global warming has ground to a halt. Climate scientists, meanwhile, know that heat must still be building up somewhere in the climate system, but they have struggled to explain where it is going, if not into the atmosphere. Some have begun to wonder whether there is something amiss in their models.
There's nothing wrong with something being amiss in the models, as long as the models provide a better explanation of reality than a coin flip, and as long as the models aren't misused.  There is nothing wrong with a scientific tool having large out-of-sample forecast errors, as long as that tool isn't being used as a guide to public policy.

And thus, it is likely that a fair amount of what the article characterises as "climate scepticism" is actually "policy scepticism."  One ought not impose a costly, socialist programme on the developed world as the sole response to what might be a change in the heat balance of the atmosphere.  Yet more than a few climate scientists have used their status in that field to make non-scientific, populist appeals to change public policy.  (Yes, I know, athletes and entertainers also do this, but at least they don't hide behind "settled science" arguments).
Now, as the global-warming hiatus enters its sixteenth year, scientists are at last making headway in the case of the missing heat. Some have pointed to the Sun, volcanoes and even pollution from China as potential culprits, but recent studies suggest that the oceans are key to explaining the anomaly. The latest suspect is the El Niño of 1997–98, which pumped prodigious quantities of heat out of the oceans and into the atmosphere — perhaps enough to tip the equatorial Pacific into a prolonged cold state that has suppressed global temperatures ever since.
A model of atmospheric mixing that doesn't accurately capture heat exchanges between ocean and air is likely to forecast poorly out-of-sample, as the new dynamics can't possibly have been calibrated into the model.
On a chart of global atmospheric temperatures, the hiatus stands in stark contrast to the rapid warming of the two decades that preceded it. Simulations conducted in advance of the 2013–14 assessment from the Intergovernmental Panel on Climate Change (IPCC) suggest that the warming should have continued at an average rate of 0.21 °C per decade from 1998 to 2012. Instead, the observed warming during that period was just 0.04 °C per decade, as measured by the UK Met Office in Exeter and the Climatic Research Unit at the University of East Anglia in Norwich, UK.
And capturing the anthropogenic component might be difficult. The task of the scientist, however, is to most accurately identify the primary causes, not set up a model that allows the Moral Equivalent of War types to hijack the results for policy purposes.
The simplest explanation for both the hiatus and the discrepancy in the models is natural variability. Much like the swings between warm and cold in day-to-day weather, chaotic climate fluctuations can knock global temperatures up or down from year to year and decade to decade. Records of past climate show some long-lasting global heatwaves and cold snaps, and climate models suggest that either of these can occur as the world warms under the influence of greenhouse gases.
That's a call for additional research, completely independently of whether policies to limit the use of the atmosphere as a sink for industrial processes make economic sense.
But none of the climate simulations carried out for the IPCC produced this particular hiatus at this particular time. That has led sceptics — and some scientists — to the controversial conclusion that the models might be overestimating the effect of greenhouse gases, and that future warming might not be as strong as is feared. Others say that this conclusion goes against the long-term temperature trends, as well as palaeoclimate data that are used to extend the temperature record far into the past. And many researchers caution against evaluating models on the basis of a relatively short-term blip in the climate. “If you are interested in global climate change, your main focus ought to be on timescales of 50 to 100 years,” says Susan Solomon, a climate scientist at the Massachusetts Institute of Technology in Cambridge.
Read the fine print: the long-term data suggest climate cycles in the absence of industrialization.  That noted, core samples from Antarctic ice show evidence of carbon being laid down from the end of the 18th century on, corresponding with the beginning of the Age of Steam.
But even those scientists who remain confident in the underlying models acknowledge that there is increasing pressure to work out just what is happening today. “A few years ago you saw the hiatus, but it could be dismissed because it was well within the noise,” says Gabriel Vecchi, a climate scientist at the US National Oceanic and Atmospheric Administration’s Geophysical Fluid Dynamics Laboratory in Princeton, New Jersey. “Now it’s something to explain.”
If we knew what we were doing, it wouldn't be research. But my colleagues who are still active in the academy ought to have some thinking time free of the distractions of rubrics or meetings or, Odin forbid, weeknight football, simply to be able to think, and to come up with clean explanations for those refractory empirical phenomena.

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