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Atlantic Multidecadal Oscillation (AMO)
Sunspot correlation with the Pacific (multi) Decadal Oscillation
El Nino Southern Oscillation
For info on the Arctic Oscillation and N. Atlantic Oscillation:
When the oscillations synchronize, we get amplified warming or cooling. When that happens and other factors coincide greater amplification results. Other factors? Dimming due to aerosols e.g. from volcanic eruptions, ozone volume increase in the stratosphere, low solar spot activity (as an indicator of reduced TSI, reduced UV, reduced proton emission, reducing magnetic field) increased cloudiness, planet wobble, distance to the Sun.
The oscillations are synchronizing in their cool phase. The Arctic Oscillation is particularly relevant as its phase governs the southerly extent of its influence. From the 1970s it stayed in positive, low pressure phase causing higher than normal temperatures in much of the United States and northern Eurasia. It is now in its negative phase, high pressure that causes cool Arctic air to extend its influence further south. Were CO2 to be in any way influential to climate temperature, a prudent leadership would be rewarding its emission and encouraging ever greater volumes. Unfortunately its influence is no greater than a bum burp in a gale. E.g.
Annual average minimum temperature for Tombstone (a desert region). No water vapour means no warming.
Why did we see warming from the late 1970s to 2002? The length of time should be a clue, ~30 years. It was the upside of a cycle. The AMO has a ~60 year full cycle. The only factor that could cause these oscillations to go negative at the same time is the Sun.
How? The level of UV and protons emitted by the Sun, the strength of the solar magnetic field determining the volume of protons from space (GCR) determine the volume of ozone from the top down, water vapour volume in the stratosphere determines the volume from the bottom up. Ozone volume determines how much UV reaches the ocean and that in turn "tweaks" the underlying evaporation level governed by TSI and air current strength. Evaporation levels control cloudiness and water vapour volume that regulate underlying surface air temperature. Variation in the underlying air temperature is caused by ocean mood and Arctic air pressure. ENSO appears to be the trigger for ocean temperature direction change and that is governed by trade winds. The direction and strength of trade winds is strongly influenced by lower stratospheric air current direction and strength that is governed by temperature that in turn is strongly influenced by ozone volume. (See this essay)
The southern hemisphere has seen virtually no warming and can be discounted from the equation. Warming was a northern hemisphere phenomenon, mostly occurring in cities with a little seen in rural and oceanic areas. Discounting "alarming" warming due to urbanisation of thermometers (UHI) the temperature increase seen over the last century is ~0.5C and that number has been consistent since cooling stopped during the LIA. Without this moderate, natural increase we would still be in the Little Ice Age and if present indications bear fruit, that appears to be the climate regime towards which we are headed. Dramatic cooling can happen in less than a decade.