Ice ages come and go
Over the past 2.6 million years, ice ages have come and gone. Minor ice ages were first striking at intervals of 40,000 years, but then, about 800,000 years ago, the rhythm changed. Suddenly, the cold lasted for 100,000+ years, and the ice spread.
Spring is again long in coming this year. Months of snow has piled up along the roads in dirty heaps, while lakes and coastal bays are covered in thick ice. In spite of warm winter coats, woollen hats and thick gloves, the icy easterly wind is devastating for those who venture outside. Welcome to late 17th century Europe, at the height of a mini ice age.
Could this happen again? Astronomers predict that solar activity, and hence the Sun’s energy supply to Earth, will decrease over the next 10 years. The last time the Sun was this quiet, the Northern Hemisphere plunged into the minor ice age of 14501850. The coldest years werefrom 1645 to 1715, when Western Europe was held in an icy grip, with millions of people starving or freezing to death. Scientists still disagree about what exactly caused the minor ice age – and about what was happening here in Australia at the same time. Until recently it was believed that the Southern Hemisphere also cooled but to a lesser extent, with evidence that New Zealand’s glaciers grew to their greatest size at this time. But more recent re-dating of this evidence has indicated that this happened after the Northern Hemisphere’s cooling event. Deep-sea cores suggest that our southern oceans may actually have warmed during the period when things were cooling in the Northern Hemisphere, supporting a theory called the bipolar seesaw, where heat is trapped in one part of the world when the great ‘conveyor belts’ of deep ocean currents stop spreading warm water to the continents of the north.
All of which complicates the question of whether our human-made global warming may actually prevent a new minor ace age – or simply make it all even more unpredictable.
Growing ice caps
The Sun is vital for Earth’s climate, given it supplies 99.97% of the energy that our planet’s surface receives (heat from Earth’s interior accounts for the remaining 0.03%). The level of solar energy can affect temperatures drastically, and Earth has experienced ice ages several times over the past 2.6 million years.
Scientists have known about the major ice ages since the mid-1800s, and they have been trying to find the explanation ever since. The first theory was proposed in 1864 by self-made Scottish physicist James Croll.
He proposed that the ice’s migration back and forth was due to rhythmical changes in the quantity of sunlight – and hence energy – that influenced Earth at different times of year. Croll imagined that a period with extra cold winters laid the foundations of an ice age, subsequently intensified via a number of self-perpetuating effects. James Croll pointed out that extensive snow cover reflects the energy from the Sun back into space long into the spring, hence lowering temperatures with every year that passes by.
Subsequent studies have shown that Croll’s theory was correct regarding the fundamental cause, though the theory has been adjusted slightly and supplied with a few extra details. Ice ages in fact originate as a result of three minor variations of Earth’s orbit around the Sun. In combination, the variations cause Milankovitch cycles which lead, over millennia, to complex changes in the quantity of energy that influences Earth at different times of year and in different places. But Croll was wrong about one thing. It is not periods with cold winters that determine whether we get an ice age or not. On the contrary, most climatologists now agree that the shift into an ice age coincides with periods during which a zone around the 56th parallel north receives greatly reduced solar energy in the summer.
Spot-free periods cool the world
The most recent ice age, also known as the Weichselian glaciation, ended some 11,500 years ago. Scientists do not know exactly how many ice ages preceded the Weichselian glaciation, but have definite evidence for more than 10 previous such periods. Over
the past 800,000 years, the cold has come and gone under a rhythm in which the ice ages last some 100,000 years, interrupted by milder periods of 10-20,000 years – like the one we are experiencing right now. But even during the warm periods the climate is unstable and involves cold spells that last from a few decades to several centuries.
Again, the cause is the Sun – or more specifically what’s going on inside it. In 1645-1715, during the Maunder Minimum, the Sun was unusually quiet and its surface displayed hardly any sunspots – the dark splotches caused by knots in the magnetic field in the Sun’s upper atmosphere. In the late 1700s, German astronomer Gustav Spörer studied records to discover that between 1672 and 1700, the Sun produced a total of only 50 sunspots, very low compared with periods when solar activity is high and the Sun might produce 40,000-50,000 sunspots over a similar period.
According to the climatologists of the PAGES 2k project, the world became just 0.4 degrees colder during the Maunder Minimum in the minor ice age’s coldest decades. In Europe, however, the local temperatures drop of 1.5 degrees made glaciers grow and rivers freeze over so that no ships could navigate them, while crops failed over and over again. The result was famine, disease, and death. Volcanic eruptions also contributed to the cold, but at the same time, less energy was reaching Earth. Scientists from the University of Colorado estimate that the Sun supplied some 1360.25 watts/m2 back then, compared with an average of 1361.5 watts/ m2 during the 20th century.
Astrophysicist Irina Kitiashvili from NASA’s Ames Research Center studies variations in the Sun’s magnetic field. She has recreated developments in the Sun’s total magnetic field from 1976 to 2019, feeding the data into a magneto-hydrodynamic model that predicts how electrically conductive gases and liquids behave in a magnetic field, and thereby predicts the number of future sunspots.
Over a period of around 11 years, the number of sunspots varies from a few to many, and back to a few again. Astronomers have numbered the cycles, with cycle 24 beginning in 2008 and ending in late 2019. Irina Kitiashvili’s model predicted cycle 24 very accurately, causing much interest in her predictions for cycle 25, which is expected to be the weakest in 200 years. The sunspot activity is expected to halve compared to cycle 24, which was itself only half as active as cycles 21 and 22 from 1976 to 1996. But despite this, temperatures on Earth have been notably rising, not falling.
Cooling versus warming
The rising temperatures are due to the fact that the atmosphere’s CO2 content has risen by 45% since the Industrial Revolution gained momentum in the mid-1800s. Quantities of methane, an even more powerful greenhouse gas, have increased by no less than 150%. Such greenhouse gases are the cause of the present global warming – and it is an irony of
MICHAEL SANDSTROM PALEOCLIMATOLOGIST We pump so much carbon dioxide into the atmosphere that we will probably not have another ice age for the next 100,000 years.
climate change that they are proving a potent weapon in the fight against the cold from space. If all greenhouse gases were to disappear from the atmosphere, the global average temperature would fall to a scary minus 18 degrees. Currently we enjoy a more pleasant plus 15 degrees.
In 2017, three American scientists from the University of California in San Diego analysed 20 years of astronomical data from 33 sun-like stars to establish how much their star radiation has varied over time. Their studies showed that it is energy in the shape of ultraviolet radiation that decreases most when a star enters a Maunder Minimum. Based on the 33 stars, the scientists estimate that the decrease is between 5.5% and 8.4%. On Earth, then, a Maunder Minimum should result in a natural temperature reduction of only 0.5°C.
Lecturer Michael Brown from Monash University in Melbourne has taken a closer look at the struggle between heat and cold. According to him, a repetition of the Maunder Minimum would no longer cause a temperature reduction on Earth, primarily because the atmosphere is supplied with ever more of the greenhouse gases that have heated the world by about 1°C since 1850. Data from the International Energy Agency show that Earth’s total energy requirement was 2.3% higher in 2018 than in 2017, the increasing requirement primarily met by coal, oil and gas, not by green energy sources such as wind and sunlight. Nothing indicates that the temperature rise will halt any time soon.
New ice age paused
So it seems a new ice age is not something about which we should worry – at least for the next 1500 years. We might then expe
rience a new ‘real’ ice age – not just a brief temperature drop for a few decades, but tens of thousands of years with a climate that is at least five degrees colder than today.
Until then, it’s warming that requires our urgent attention. Paleoclimatologist Michael Sandstrom from Columbia University in New York has examined how much carbon dioxide the air includes during warm periods and ice ages. The quantity of CO2 is measured in millionths of air molecules, ppm. For an ice age to materialise, the atmosphere’s CO2 content must drop to some 170 ppm – 170 CO2 molecules per million air molecules. The quantity of CO2 molecules must reach 280 ppm to cause a warm period such as the present. Before the burning of fossil fuels gathered momentum in the mid-1800s, the atmosphere’s contents of CO2 was just that: 280 ppm. But since then the concentration has risen to 410 ppm.
The last time Earth was subjected to similar warming was some 56 million years ago during the Paleocene-Ecocene Thermal Maximum (PETM). Over 10,000 years, the world became 5-8 degrees warmer, with undersea volcanoes melting methane ice on the ocean floor, releasing large quantities of greenhouse gases. After these emissions stopped, it took 170,000 years for temperatures to fall back to their earlier levels.
Such slow cooling is because Earth’s integrated thermostat is inefficient, and its systems slow to remove CO2 from the atmosphere. Many of the processes triggered by heating go forwards much faster than backwards. The melting of ice caps in Antarctica and Greenland takes place far faster than the formation of new ice as temperatures fall again. Melted ice also has a self-perpetuating effect. When ice melts, darker sea and land appear, and these absorb more sunlight than white reflective ice, heating the planet more. This self-reinforcing heating makes it harder to reverse rising temperatures.
The Sun can buy us time
The emission of greenhouse gases not only leads to present-day global warming, it also disturbs the world’s natural swings between cold and heat. If astrophysicists such as Irina Kitiashvili are right that we are entering a new period of very low solar activity, we may be lucky enough to have this mitigating factor to reduce some effects of our self-inflicted climate change. While greenhouse gases drive temperatures up, the Sun’s decreasing activity may dampen their effect on the climate, giving us valuable extra time to switch to green energy and find alternatives to concrete, steel and other practices which emit huge quantities of CO2.
But these issues must be tackled, and tackled now. After all, any delay is merely temporary. Once the Sun and its sunspots cycle back to normal and then high levels of activity, their effect will begin to add to, rather than subtract from, the effects of our self-induced global warming – unless we have by then brought it under control.