The reason is that the sun may be entering an new minimum cycle similar to the
Current satellite data on the sun is providing much better quality data of the impact of the change, but seems to ask more questions than it answers. The total solar power or insulation TSI only changes about 1 W/m^2 during a minimum out of 1366 watts/m^2 average. That is not enough to make much change without some amplification of its change. There are theories a plenty of things than may amplify the sun's impact. I will let those lay while I stick with my train of thought. Do remember that 1 W/m^2 would only be felt as about 0.18 W/m^2 at the surface where a full one degree change in surface temperature would require and estimated decrease of 3.7 W/m^2
A.A. Tsonis has a few studies where he determines there have been climate shifts due to natural climate oscillation that can synchronize in warm of cool phases. His papers indicate one started around the year 2000, before the current solar minimum started to show itself. That shift appears to most likely caused by the cool phase of the Pacific Decadal Oscillation (PDO). The PDO seems to cause some changes in the El Nino / La Nina timing and intensities. Dr. Roy Spencer with the University of Alabama at Huntsville (UAH) has noted that there has been a change in the percentage of cloud cover in the tropics which may be due to the PDO shift. A 1% can in cloud cover results in nearly 3 W/m^2 or about 0.8 degrees possible temperature change which will likely be less than 0.4 due to atmospheric water vapor.
Clouds have a few impacts on climate that can cool or warm things. Cloud top reflectivity is one pretty important impact. In the the original post my rough numbers indicate the range and impact of albedo or reflectivity change. Since the climate appears to have two rough set points, it takes a little push to move from one to the other. Leif Svalgard, who is a scientist studying the sun, does not think the drop in TSI due to a minimum is enough. I completely agree, but I don't think the required push is as much as many may think. Combining Tsonis' method of determining climate shifts with past climate history and the newer solar TSI reconstructions, there may be part of the push available. The synchronizing of the solar minimum with a cooling PDO.
With the PDO shift, average temperatures have leveled off. The solar minimum has started in sequence with the PDO shift and atmospheric temperatures are still pretty level but some cooling of sea surface temperatures seems be be happening. Not enough to ring any alarm bells, but a slight drop. The La Nina cycled to neutral, but instead of a new El Nino, indications are that there may be a new La Nina on the way.
The record temperatures of 1998 have been attributed to the "Super" El Nino of the same year. With our moist atmosphere, it is easier to warm with an El Nino than it is to cool with a La Nina. Temperatures in general show more rapid warming than cooling due to atmospheric moisture. So it is possible that the new La Nina if it is fairly strong, will cause a slight decrease in temperature, maybe a little more with the solar minimum. Not enough for me to say, "Ha! Its the sun and natural variability!" Possibly enough for me to say, "Watch out if the Atlantic Multi-decadal Oscillation synchronizes with the PDO AND solar!" Which has a pretty fair possibility of happening in the next few years.
If those natural variations all synchronize, the result will be more than expected cooling. How much? I have no clue. I doubt as much cooling as the little ice age, that should take a little more pushing. That is where the other theories come into play.
One I consider a player is the reduction in UV intensity. UV has been recently found to vary more than expected. Most consider UV to be a minor player. That same "most" also underestimate, in my opinion, the impact on the deep ocean of the shorter wave lengths of light from the sun, UV being one. So we may have a variety of small impacts synchronizing to create a major impact. Not out of the realm of possibility for a system with dual set points, which has some level of instability. Interesting times may be heading our way.
Should the minor factors synchronize, albedo can amplify the cooling more than it can the warming. We have a tendency for ice ages historically, why should this Holocene be particularly special?
To hypothesize is easy, to theorize is not, scientifically speaking. So it will take more research on my part to flesh out this hypothesis. Anyone reading that cares to join in is welcome to help.