The Pacific Decadal Oscillation, the Arctic Oscillation, the Antarctic Oscillation, the El Niño-Southern Oscillation, the Atlantic Multidecadal Oscillation, the Indian Ocean Dipole, the Interdecadal Pacific Oscillation. A proliferation of oscillations it seems – changing abruptly between states on decadal to millennial scales. Science shows these oscillations ‘synchronising’ – just like the metronomes in this video – before diverging again in a new pattern.
Climate shifts abruptly every two to three decades. Shifts can be relatively minor or can involve major change in surface temperature, hydrology or biology in as little as a decade. To understand why requires a little knowledge of nonlinear dynamics and complexity science. Complexity arises from the interactions of simple components. The global climate system is composed of a number of internal components – atmosphere, biosphere, cryosphere, hydrosphere and lithosphere – each of which changes over periods of days to millennia. They form a global system in which the climate state emerges from spontaneous changes in the balance of the system components.
It can be seen surface temperature data. An increasing trend from 1912 to 1944, decreasing to 1976, increasing to 1998 and decreasing since.
Figure 1: HadCRUT4 surface temperature with trend (source: Wood for Trees)
The El Niño/Southern Oscillation (ENSO) is the most important coupled ocean-atmosphere phenomenon to cause global climate variability. The multivariate ENSO index combines sea-level pressure, wind, sea surface temperature, surface air temperature and total cloudiness in a single index of the Pacific state. Blue (La Niña) dominant to 1976, red (El Niño to 1998) and blue again since.
Figure 2: MEI (source: NOAA ESRL)
There are interesting implications for the near term evolution of climate. The current state of non-warming may persist for a decade or two yet. Neither the degree nor the direction of the next climate shift is knowable. Climate is likely to shift several times this century. There is thus no way to make credible climate predictions.
Complexity theory suggests that the system is pushed by such things as solar intensity and Earth orbital eccentricities – past a threshold at which stage the components start to interact chaotically in multiple and changing negative and positive feedbacks – as tremendous energies cascade through powerful subsystems. Some of these control variables have a regularity within broad limits and the planet responds over decades to millennia with a broad regularity in changes of ice, cloud, Atlantic thermohaline circulation and ocean and atmospheric circulation.
The US National Academy of Sciences (NAS) defined abrupt climate change as a new climate paradigm as long ago as 2002. A paradigm in the scientific sense is a theory that explains observations. A new science paradigm is one that better explains data – in this case climate data – than the old theory. The new theory says that climate change occurs as discrete jumps in the system. Climate is more like a kaleidoscope – shake it up and a new pattern emerges – than a control knob with a linear gain.
The critical policy response then becomes one of building societal resilience to whatever the planet throws at us. This requires optimum trade and aid policy – the Copenhagen Consensus Post 2015 Millennium Development Goals provide a worthwhile starting point. This brings with it reduced population pressures, enhanced environmental conservation and restoration of agricultural soils. Ironically these are the effective ways of mitigating the broad range of human pressures on the climate system – sequestering carbon dioxide in the landscape, reducing methane, nitrous oxide, tropospheric ozone, black carbon and CFC emissions. The world needs as well new, cheaper and easily deployed energy sources for a high energy future.