Sorry, but I don't really understand the premise of your question: the current issue is how fast excess man-made CO2 emissions will cause the planetary atmosphere to warm, not cool.
But you raise a deep question: in the absence of a particular cause, such as an increase in greenhouse gases, why does the atmosphere produce variations on a whole range of time-scales, including those of hundreds or thousands of years?
In the lab, or at school, we control all other factors, so far as possible, and try to establish the effect of a particular cause: heat an iron bar and it expands; close an electric circuit and a bulb comes on; and so on. In the atmosphere, there is no single cause. There are always many factors at work, though sometimes, one may be dominant. More usually, several compete for dominance. Obvious ones are diurnal variations in temperature, cloud and surface wind, caused by the rotation of the earth, and seasonal differences, caused by the earth's polar axis being tilted in relation to the plane in which it orbits the sun. On longer time scales there are other whole earth and astronomical factors such as changes in the angle of tilt and in the rotation of the major axis of the ellipse which the earth follows around the sun. (See, for example, Milankovitch cycles) These are believed to be the major factor in causing major ice-ages every hundred thousand years or so.
On shorter time-scales, from the hour to hour, day-to-day, season-to-season, year-to-year and decade-to-decade, changes are inherent in the irregular way in which the atmosphere flows, partly because of the distribution of the underlying sea and mountains, and random effects within the atmosphere itself (such as Ed Lorenz's flapping of a butterfly's wing). But also very important are effects of the atmosphere which happen on one time-scale, such as a day's snowfall, which can affect the air over it for a much longer time. Another instance is today's rough sea, from yesterday's strong wind, providing a rougher underlying surface for today's wind. Cloud and wind patterns during a month or so, and wind strength, determine the pattern of temperature of the sea surface underlying the atmosphere, as well as the profile of sea-surface-temperature in depth. And so on. On another scale, the varying extent of permanent snow- and ice-cover near the poles can be important over hundreds or thousands of years.
So a major effect, often the dominant one, producing variations in the atmosphere on a whole range of time-scales (and probably the 'cause' of your mini ice-age), is feedback in the combined system of variable atmosphere, oceans and cryosphere.