sunset on ice
The gentle, much-appreciated warming of the second half of the 20th century could not have had carbon dioxide as its cause. Carbon dioxide's logarithmic heating effect is weak at 100 ppm, tuckered out well and truly at 200 ppm and beyond 300 ppm - well never mind. We are now above 400 ppm from which it is a monotonic 0.1°C for each 100 ppm increase.

That doesn't explain anything and in turn leaves the Sun as the only possible causative agent for the Modern Warm Period. Based on when we run out of rocks to dig up and burn, the atmospheric carbon dioxide concentration will peak out at around 560 ppm. We are now at 408 ppm so only another 0.15°C to go before the deep oceans, over the subsequent centuries, take almost all the extra carbon dioxide down into the Davy Deep and our plants go back to near-starvation levels of it.

CO2 warming effect

Figure 1: Logarithmic heating effect of atmospheric carbon dioxide
A version of this graph was first published on WUWT in 2006 and had its first peer-reviewed appearance in an Energy and Environment paper in 2007. While Figure 1 is diagrammatic it is accurate enough to base public policy on.

It has been argued by some here on WUWT that the Sun is so unvarying that it should be dismissed from consideration as our climatic benefactor. But along comes a Bulgarian-Russian paper that looks at the evidence from a different perspective - Georgieva et al.'s Solar Magnetic Fields and Terrestrial Climate from 2014. The Bulgarians are unequivocal in their message. The last sentence of the paper states:

Therefore, the expected decrease of aamin and increase of both predict future decrease of TSI which, added to the expected decreasing geomagnetic activity, will be an additional factor for the future global cooling.

Solar-driven global cooling is in our future. The paper starts by discussing TSI and notes that:

Reconstructions of TSI since the Maunder minimum in the second half of the 17th century, when the Sun was extremely inactive and Europe experienced the "Little Ice Age", estimate values of TSI from equal to the ones during the last 2008-2009 solar minimum to almost 6 W/m2 lower.

So there are a range of TSI reconstructions. Necessarily, some are wrong. The paper discusses how the ones that are wrong got it wrong:

Several wrong assumptions are made when reconstructing TSI from only the number of sunspots. The first one is that the sunspot number can be used instead of the sunspot area which actually determines the impact of sunspots reducing the TSI. Actually, the relation between sunspot number and area changes in time, and recently it was shown that the proportion of small to large spots has been increasing which means decreasing ratio of the total sunspot area to the number of sunspots.

sunspots

Figure 2: Ratio of small sunspots to large sunspots 1998 – 2011
The ratio of small sunspots to large sunspots increased dramatically near the end of Solar Cycle 23 which was also the end of the Modern Warm Period. TSI reconstructions overly reliant upon sunspot number will get it wrong.
sunspot chart

Figure 3: Sunspot magnetic field relative to sunspot area for Solar Cycles 20, 21, 22 and 23
There is another complication in that while the darkness of a sunspot important for its contribution to TSI depends on its magnetic field, which in turn is related to the spot's area, the relationship between the spots' area and magnetic field changes from cycle to cycle.

The paper makes some observations re the correlation between TSI and solar cycle amplitude:

The variations in TSI follow roughly the variations in the sunspot cycles amplitudes, however with important differences. The most intense cycle in this period as measured by the number of sunspots was cycle 19, but the most intense cycle as measured by TSI was cycle 21. Similarly, the weakest cycle in sunspot number was cycle 14, but the weakest cycle in TSI was cycle 12. The
explanation for these discrepancies is that the cycle averaged TSI is a result of the interplay between the variations of the darkening determined by the total area and magnetic field in sunspots, and the brightening determined by the total area and magnetic field in facular and ephemeral regions.

Katya Georgieva and her co-authors advance civilisation and push back against the darkness by contributing their own TSI reconstruction:
sunspot cycle

Figure 4: TSI since the beginning of the 17th century
Since the end of the Maunder minimum (Solar Cycle -4, 1698-1712), TSI has increased by about 3 W/m2, and since the deepest part of the Maunder minimum (not shown), the increase is about 7 W/m2.

There are many TSI reconstructions. The question is: Which to believe? As per Nir Shaviv's observation that the oceans are a giant calorimeter measuring solar output, so is the Earth from long term climate records. The Georgieva paper notes:

Total solar irradiance reconstructions, calculated taking into account the evolution of sunspot magnetic fields derived from geomagnetic data, support the TSI composites and reconstructions showing much higher long-term TSI variability, and consequently much bigger solar influences on climate variability than accounted for in popular models. Even for a very conservative value of climate sensitivity to TSI variations of 0.5 adopted by IPCC AR4, the estimated TSI increase since the early 18th century (the end of the Maunder minimum), and moreover since the deepest part of the Maunder minimum, demonstrate that TSI increase alone was responsible for ∆T of at least 1.5° and 3.5° K, respectively.

The Georgieva paper restores the Sun to its rightful place as the source of long term climatic variation. The erstwhile usurper, the pretender to the throne that is global warming theory, is left to agitating the minds of discredited elements and malcontents.

Shaunavon, Saskatchewan

Figure 5: Shaunavon, Saskatchewan April 8, 2018
Photo from Tim Lingenfelter who notes "Farmers in Saskatchewan should be in the fields by next week as a rule. Not this year." Operational practices are yet to adjust to the end of the Modern Warm Period.

David Archibald is the author of American Gripen: The Solution to the F-35 Nightmare