The Pacific Decadal Variation is a system that switches from more or less cold, nutrient-rich, deep ocean upwelling every 20 to 30 years. It includes both the Pacific Decadal Oscillation (PDO) and the El Niña-Southern Oscillation (ENSO). In a cool (ocean surface) mode – La Niña activity is much more intense and frequent. There was a warm (ocean surface) mode in the early 20th century, cool in the middle part and warm again at the end. This system is little understood by physicists and computer programmers – but has been known about by oceanographers and hydrologists for decades. The ocean surface warming and cooling precisely match warming and cooling in temperature records
Yes greenhouses gases cause warming – at an annual average rate 0.032Watts/meter squared (W/m2). In the late 20th century the Pacific system resulted in warming at a rate of 0.07 W/m2 over the Pacific – based on Earth Radiation Budget Experiment data.
We have as well high-resolution data on the history of this system. The 20 to 30 years shifts have persisted for at least 1000 years – and likely much longer. More salt in an Antarctic ice core is La Niña – and the rain is in Australia. There are, of course, global implications for temperature, hydrology and biology.
Source: Tessa Vance et al 2013
A cool Pacific in La Niña decrease infrared emissions from a cool atmosphere and increase cloud cover resulting in more sunlight being reflected back to space. The net effect is a planetary cooling. Within the large-scale variability are 20 to 30 year shifts in the mean and variance of ENSO activity. Over time these add up to millennial variability. It shows El Niño activity being higher in the 20th century than it generally was for a very long time. There is a simple statistical rule – reversion to the mean – that suggests that the energy gain in the 20th century from this system will be reversed over centuries to come.
The mechanism involves solar UV/ozone interactions at the poles. Cooling in the of the stratosphere during low solar activity increases surface level pressure at the poles – that pushes polar fronts into lower latitudes. The recent decline in solar activity – and the predicted future decline – means that the next climate shift – due in a 2018-2028 window – will be very interesting.