Butterfly effect limits climate models

National Center for Atmospheric Research's Clara Deser. Credit: NCAR

National Center for Atmospheric Research’s Clara Deser. Credit: NCAR

Natural chaos in our climate system creates uncertainty in predictions that can’t be removed, no matter how good scientists’ models get. Clara Deser from the US National Center for Atmospheric Research (NCAR) in Boulder, Colorado and her colleagues have shown these effects can be as strong as human-caused warming. “Over multiple decades intrinsic climate variability on a local and regional scale can be on a par with climate change due to greenhouse gas emissions,” she told me. “You’re not going to just see the result of the greenhouse gas increases – you’re going to see both. This simple message has been missing from the climate change literature.”

Climate scientists are working hard to improve the accuracy of their models’ predictions – perhaps so hard they haven’t yet looked at what their limits are. “We’ve been focussed on identifying how greenhouse gas changes and the like can affect the climate system,” Clara said.  “The uncertainties in climate projections have all been lumped together. There hasn’t been a set of runs that were designed the way that we have done them to really address this point.”

Anyone who’s had to run outside to rescue drying clothes from a rain shower knows that weather can be variable from day to day. Climate patterns also vary from year-to-year, like El Niño or the North Atlantic Oscillation, and some chaotic climate processes work over decades. Wanting to reduce model uncertainty, Clara previously tried to answer a series of detailed questions about these kinds of natural variability. Her team’s answers showed that they accounted for at least half of the disagreement between different climate model predictions. When she told this to two fellow climate scientists, Reto Knutti, from the Swiss Federal Institute of Technology, Zurich and Massachusetts Institute of Technology’s Susan Solomon, they were surprised. “They said, ‘Something very simple and illustrative is needed to get this important message across,’” Clara recalled.
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