The Nelson response

In case Tom Nelson asks:

Minoan Warm Period
http://www.dandebat.dk/eng-klima7.htm
“Not much is known about the Minoan warm period beyond what can be gauged from cores from boreholes in the ice sheet. That the climate really was warmer then may be derived from that in the Minoan warm period, which occurred during the bronze age, millet was grown in southern Scandinavia.”

So what caused it?   Simply put:  Natural variability.   https://blogs.ei.columbia.edu/2019/07/24/climate-epochs-that-werent/  “A new study puts together the evidence on a global scale for the first time. Based on this, the authors say that the supposed warm and cold epochs may represent, more than anything, regional variations that can be explained by random variability. … “It does not suggest that the periods of high or low temperatures observed during the named epochs did not exist in certain places; rather that they did not exist everywhere at the same time, and thus probably were not caused by some kind of planetary driver. That said, the study does find one very coherent period: an unprecedented warm one extending over 98 percent of the globe, starting in the 20th century. This is almost certainly caused by us.”

The article is based on interpretation of, and an interview regarding, this paper:

No evidence for globally coherent warm and cold periods over the preindustrial Common Era, https://www.nature.com/articles/s41586-019-1401-2

Both the Roman Warm Period (ca. 100-300 AD) and the Dark Ages Cold Period (ca. 400-800 AD) are thus also ascribed to natural variability.  Also note these are all Euro-centric periods, because that’s where most of the data and observations were being recorded. 

But if an actual mechanism is still desired, the most likely candidate is variability in ocean circulation, particularly deep-water formation and El Niño/La Niña events.  Deep-water formation rates in the North Atlantic can cause European cooling (when the rate decreases, so that more cold remains in the atmosphere) and warming (when the rate increases, sequestering more cold water in the ocean depths and thus allowing an increase in the influence of the Gulf Stream’s warm waters).  If El Niño frequency increases, this will cause increased warming signals particularly on the eastern side of the Pacific Ocean, especially the tropics, and increased La Niña frequency will do the reverse.  The influence of the water temperature is not as strong on the western side, but there would be a tendency for cooler temperatures with more El Niño events, and again the reverse for La Niña events – showing the regional asychronicity.   The influence of either is felt globally, as discussed below.

Medieval Warm Period (~800-1200 AD)

Global Signatures and Dynamical Origins of the Little Ice Age and Medieval Climate Anomaly, https://science.sciencemag.org/content/326/5957/1256

“Relative warmth in the central North Pacific MCA is consistent with the expected extratropical signature of the strong observed La Niña–like pattern in the tropical Pacific (strong cooling in the east and warming in the west).  [As described above.] Certain regions, such as central Eurasia, northwestern North America, and (with less confidence) parts of the South Atlantic, exhibit anomalous coolness.”

Little Ice Age   

“LIA pattern is characterized primarily by pronounced cooling over the Northern Hemisphere continents, but with some regions—e.g., parts of the Middle East, central North Atlantic, Africa, and isolated parts of the United States, tropical Eurasia, and the extratropical Pacific Ocean—displaying warmth comparable to that of the present day.” There is an influence of solar radiative forcing during the LIA, equivalent to a difference of about 0.37 W/m2 at the tropopause between the MCA and the LIA.  This difference occurs during the Maunder Minimum in sunspot numbers.

Early 20th century warming

The early 20th century warming: Anomalies, causes, and consequences
https://onlinelibrary.wiley.com/doi/10.1002/wcc.522

“Attribution studies estimate that about a half (40–54%; p > .8) of the global warming from 1901 to 1950 was forced by a combination of increasing greenhouse gases and natural forcing, offset to some extent by aerosols. Natural variability also made a large contribution, particularly to regional anomalies like the Arctic warming in the 1920s and 1930s.”

https://judithcurry.com/2019/01/23/early-20th-century-global-warming/

“Hegerl et al. [the paper above] focus their arguments regarding internal variability associated with large-scale ocean circulations on the Atlantic Multidecadal Oscillation (AMO) and the Pacific Decadal Oscillation (PDO). Warm phases of the both the AMO and PDO contributed to warming particularly during the 1930’s and 1940’s.”

https://www.gfdl.noaa.gov/early-20th-century-global-warming/

“Since the model includes no forcing from interdecadal variations of volcanic emissions or solar irradiance, this suggests that the observed early 20th century warming could have resulted from a combination of human-induced increases of atmospheric GHG and sulfate aerosols, along with internal variability of the ocean-atmosphere system.”

Mid-20th century cooling
The primary cause of the slight mid-20^th century cooling was increased sulfate aerosols from post WWII industrialization.

Anthropogenic global cooling, https://tamino.wordpress.com/2010/08/23/antrhopogenic-global-cooling/

“The cooling effect of man-made sulfates also helps explain the hemispheric asymmetry in temperature history. Most industrial activity is in the northern hemisphere, so most of the anthropogenic sulfate cooling should be there too. The northern hemisphere has warmed faster than the southern because there’s more land in the north than the south, and land has far less thermal inertia than ocean. But if sulfates are mostly in the northern hemisphere, that means that there should have been a stronger mid-century cooling effect in the north than in the south — and that’s exactly what we observe:” [See data plot at the link]

The linked article also includes plots that show sulfur emission estimates are in line with sulfate concentration measurements in ice cores.

We need to rethink everything we know about global warming
https://www.sciencedaily.com/releases/2019/01/190122104611.htm


“Rosenfeld and his colleagues were able to more accurately calculate aerosols' cooling effects on the Earth's energy budget. And, they discovered that aerosols' cooling effect is nearly twice higher than previously thought.”