A 4.5 C decrease would be a tough pill to swallow considering the "Little Ice Age" was only an estimated 1C drop. So lets say Astro's forecast does not materialize and we only get a 2+ degree drop over the next 15 years. Instead of a 700 mile grow line movement south it only migrates 350 miles. North America, Europe and Asia would all suffer shortened growing seasons and much, much less food production. We would not be able to sustain current populations.
I'm not even sure the world is ready for this glass is 1/2 full scenario. 4.5 C would be apocalyptic. Governments and social systems would collapse under the stress...
Ice Ages represent periods of cooling that can see global average air temperatures drop as much as 5° C below the global average.
For instance, look at the Canadian provinces where this graphic shows the current regions of Canadian wheat production and the reduction that would result from even a small drop in average surface temperature.
There are some findings that show there are very serious consequences that are significantly greater and more persistent than those that resulted from the greatest volcanic eruption of the past 500 years.
That was the Tambora eruption in Indonesia in 1815 as Tambora spewed enormous amounts of volcanic smoke particles into the stratosphere. All that soot and volcanic smoke blocked and scattered enough sunlight to cause the following 'Year Without Summer, in 1816.
It helped to cause frosts that killed plant life and greatly disrupted agriculture every month of the summer of 1816 in New England along with widespread harvest failure and famine in Europe.
Our climate under solar-forced global warming has seen record crop growth as agriculture today had the present warm climate able to then yield record crops.
But with the Sun's Grand Minimum, and global cooling, all of that will be severely disrupted by a rapid average temperature decline of just a few degrees Celsius.
Just a few degrees Celsius decline within grain growing regions of Eurasia and North America mean that a major disaster is ahead.
The climate change to global cooling means in reality a corresponding shortening of growing seasons by up to 30 days or more and a 10 percent reduction in average world precipitation. Of course that will have major impacts on global food supplies.
Already world average temperatures have been falling for 10-11 years and the 'pause' in global warming has now reached its 18th year. What is happening is that we are nearing a minima in the 80 to 90-year Gleissberg cycle of the Sun's activity.
This definitely coincides with periods of a colder climate on Earth. Astronomically it is linked to an 83-year cycle in the change of rotary force that drives the Sun's oscillatory motion about the center of mass of our solar system.
In making astronomic climate forecasts for the Earth, what is calculated are the irregular oscillation of the Sun about the center of mass of the solar system from a heliocentric view.
The position of the center of mass relative to the Sun's own center show strong variations in the physical quantities measuring the Sun's orbital motion that then form climatic cycles of different lengths on Earth and the other planets of our solar system.
The late astrometeorologist Theodor Landscheidt stated,
"The dynamics of the sun's motion about the centre of mass can be defined quantitatively by the change in its orbital angular momentum L. The time rate of change in L is measured by its first derivative dL/dt. It defines the rotary force, the torque T driving the sun's motion about the CM.
Variations in the rotary force defined by the derivative dT/dt are a key quantity in this connection as they make it possible to forecast Gleissberg extrema for hundreds of years and even millennia.
A cycle of 166 years and its second harmonic of 83 years emerge when the time rate of change in the torque dT/dt is subjected to frequency analysis (Landscheidt, 1983.)
Cycles of this length, though not well known, were mentioned in the literature before.
Brier (1979) found a period of just 83 years in the unsmoothed cosine transform of 2148 autocorrelations of 2628 monthly sunspot numbers. Cole (1973) confirmed this result when he investigated the power spectrum of sunspot data covering 1626 - 1968.
He found a dominant peak at 84 years. Juckett (2000) derived periods of 165 and 84 years from his model of spin-orbit momentum exchange in the sun's motion. As the wave length of the Gleissberg cycle is not far from the second harmonic of the 166-year cycle, it suggests itself to see whether the Gleissberg cycle and the dT/dt-cycle have synchronized minima and maxima. This is actually the case.
Gleissberg (1958) found the cycle named after him by smoothing the length of the 11-year sunspot cycle, a parameter that is only indirectly related to the sunspot number R measuring the intensity of sunspot activity.
As it could be that the smaller or greater values of the positive and negative extrema of the dT/dt cycle have a similar parametric function, the amplitudes of these maxima and minima are taken to constitute a smoothed time series covering 2000 years. The interval is from A. D. 300 to 2300. The data were subjected to moving window Gaussian kernel smoothing (Lorczak) with a bandwidth of 60.
This graphic shows the result for the sub period 300 - 1200.
Smoothed time series (A. D. 300 – 1200) of extrema in the change of the sun's orbital rotary force dT/dt forming a cycle with a mean length of 166 years.
Up to the phase reversal around 1120, set off by an arrow, zero phases in the cycle, marked by empty circles, coincide within a relatively narrow margin with observed maxima in the Gleissberg cycle indicated by filled triangles. Minima in the Gleissberg cycle, marked by empty triangles, go along with extrema in the 166-year cycle.
The phase reversal explains the outstanding Medieval sunspot maximum. The secular maximum around 1100 was followed by another maximum around 1130 without an intermittent minimum.
As Gleissberg maxima coincide with warm climate and minima with cool climate, the Medieval sunspot maximum was related to exceptionally warm climate.
Up to the phase reversal around 1120, indicated by an arrow, zero phases of the 166-year cycle, marked by empty circles, coincide within a relatively narrow margin with maxima in the Gleissberg cycle, indicated by filled triangles.
Only close to the phase reversal the deviation of the secular maximum from the zero phase is wider.
The epochs of Gleissberg minima are indicated by empty triangles. Up to the phase reversal, they consistently go along with extrema in the 166-year cycle.
It makes no difference whether the extrema are positive or negative. This is reminiscent of the 11-year sunspot cycle with its exclusively positive amplitudes though the complete magnetic Hale cycle of 22 years shows positive and negative amplitudes indicating different magnetic polarities in consecutive 11-year cycles.
An even more difficult question is whether future Gleissberg minima will be of the regular type with moderately reduced solar activity as around 1895, of the type of very weak activity like the Dalton minimum around 1810, or of the grand minimum type with nearly extinguished activity like the nadir of the Maunder minimum around 1670, the Spoerer minimum around 1490, the Wolf minimum around 1320, and the Norman minimum around 1010 (Stuiver and Quay, 1981).
This graphic depicts a heuristic solution.
It shows the time series of unsmoothed dT/dt-extrema for the interval 1000 – 2250.
A consistent regularity attracts attention. Each time when the amplitude of a negative extremum goes below a low threshold, indicated by a dashed horizontal line, this coincides with a period of exceptionally weak solar activity.
Time series of the unsmoothed extrema in the change of the sun's orbital rotary force dT/dt for the years 1000 – 2250. Each time when the amplitude of a negative extremum goes below a low threshold, indicated by a dashed horizontal line, a period of exceptionally weak solar activity is observed.
Two consecutive negative extrema transgressing the threshold indicate grand minima like the Maunder minimum (around 1670), the Spoerer minimum (around 1490), the Wolf minimum (around 1320), and the Norman minimum (around 1010), whereas a single extremum below the threshold goes along with events of the Dalton minimum type (around 1810 and 1170) not as severe as grand minima.
So the Gleissberg minima around 2030 and 2200 should be of the Maunder minimum type.
As climate is closely linked to the Sun's activity, conditions around 2030 and 2200 should approach those of the nadir of the Little Ice Age around 1670.
Without exception, the outstanding negative extrema coincide with periods of exceptionally weak solar activity and vice versa. So there are good reasons to expect that the coming Gleissberg minimum around 2030 will be a deep one.
As there are three consecutive extrema below the quantitative threshold, there is a high probability that the event will be of the Maunder minimum type.
Landscheidt closed by stating,
"It has been shown that there is a close relationship between deep Gleissberg minima and cold climate. So the probability is high that the outstanding Gleissberg minima around 2030 and 2201 will go along with periods of cold climate comparable to the nadir of the Little Ice Age."
As to the minimum around 2030, there are additional indications that global cooling is to be expected instead of global warming. The Pacific Decadal Oscillation (PDO) will show negative values up to at least 2016 and La Niñas will be more frequent and stronger than El Niños.