20 November 2007

Natural variability vs. climate change

Global changes in weather and climate are undoubtedly being observed. Are these signs of climate change, or of some other natural (interdecadal) variability? Does it have to be an either/or proposition? Here are two examples that highlight the importance of these questions.

A team of NASA and university scientists has detected an ongoing reversal in Arctic Ocean circulation triggered by atmospheric circulation changes that vary on decade-long time scales. The results suggest not all the large changes seen in Arctic climate in recent years are a result of long-term trends associated with global warming.

That's the lede of the story, something to give those who think it is all natural variability (i.e. skeptics) hope. But the key words are 'not all'. The authors are not claiming that global warming is not part of the picture, regardless of changes in the Arctic Oscillation. The relationship between interdecadal variability and climate change remains unclear. In fact the changes in the AO could be related to climate change.

Further highlighting this uncertainty is a report of a talk from a climate change conference in Australia.

...[S]cientists should reassess the use of El Nino as a forecasting tool because climate change seems to be altering the way weather processes like El Nino work.

..."The leverage that El Nino exerts on Australia is principally through its La Nina phase, principally through the flood phase of the El Nino cycle,"

Despite the on-going La Nina, much of Australia remains mired in drought, going on 11 years in some regions of the southeast. From a historical perspective, this should be a time of abundant rain in Australia. Is climate change having an impact on ENSO, or is this some heretofore unobserved pattern in its behaviour? Since the early-90s, when we began to observe unusual behaviour in ENSO, this debate has raged. The IPCC expects little change in ENSO, but there is low confidence in that projection.

We are undoubtedly doing something to our atmosphere and our planet on quite large scales. For example, observations show we have seriously altered the carbon cycle of the planet. In some areas, we have also changed the amount of insolation reaching the surface, due to smog and pollution. It seems inconceivable that such significant changes would not have some effect on the climate. With our relatively limited observations, we also know there are coherent patterns on very long time scales.

How much is due to each factor? Perhaps alterations in the patterns of interdecadal variability is how climate change manifests itself, or maybe climate change just exacerbates the natural patterns. We just don't know. We've never scientifically observed climate changes of this magnitude and scale before. So surprises in the behaviour of the climate system should be expected. After all, climate models are not without their flaws, but they are correct to the first order, and they do represent one of the best tools we have in our arsenal to help prevent a greater calamity for mankind.

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