Posts Tagged ‘Earth’s orbit’
aspects of climate change – Milankovic cycles
from Wikipedia
I’m currently reading A brief history of the Earth’s climate, by Steven Earle, a Canadian geologist, who provides summaries of the various internal and external forces affecting our planetary atmosphere’s composition and temperature over its history. It’s all very sciencey, which is of course good, but not so good for dumb-funks like me, who have to put it into their own words to get a proper handle on it. So that’s why this piece is on Milankovic cycles, about which I know next to nothing.
In 1941 Milutin Milankovic completed a book entitled Canon of insolation and the ice-age problem, in which, according to Earle,
he argues how the natural variations in the shape of the Earth’s orbit around the sun and in the tilt of the Earth’s rotational axis played a critical role in the timing of glaciations over the past two million years.
A brief history of the Earth’s climate, p63
Insolation is defined as ‘the strength of sunlight shining on the various surfaces of the Earth’, bearing in mind that dark surfaces, such as the oceans and densely vegetated regions, absorb sunlight while ice and snow reflect it. Milankovic’ work went largely unrecognised in his lifetime but he was the first to ‘calculate the effects of insolation and to accurately determine the periods during which those changes would be most likely to contribute to the growth [or shrinkage] of glaciers’.
These periods have everything to do with the Earth’s eccentric (but not too eccentric) orbit, and its wobbling tilt, vis-à-vis its orbital plane. That orbit is elliptical, and the Sun is not at the centre, so the Earth’s distance from the Sun varies seasonally. Most people know this, I hope, but they may not know that the shape of Earth’s orbit varies over time, from slightly elliptical to even more slightly elliptical. But we’re talking about very long periods of time, many thousands of years, between the most and least elliptical orbits. When the orbit is most elliptical, the difference between the Sun at its closest and its farthest from Earth is, of course, greatest. It should be obvious, from what we know of the Sun as essentially our only heat source, that these differences will have a climatic impact.
Now to the wobbling tilt, or the Earth’s obliquity, relative to the plane of its orbit. This tilt is presently 23.5 degrees from vertical, and the degrees vary from 22.1 to 24.5 over a period of about 41,000 years. It basically defines our seasons, as the Northern Hemisphere tilts towards the Sun when the Southern Hemisphere tilts away from it, and vice versa. And the variation in that tilt, the wobble, creates greater or lesser variation between summer and winter seasons.
And now back to Milankovic. He, along with a few colleagues including Alfred Wegener of continental drift fame, made observations about the formation and growth of glaciers:
glaciers grow best at temperate latitudes – in fact at around 65 degrees north or south – and can start growing only on land.
There is in fact relatively little land at 65 degrees in the Southern Hemisphere, but plenty in the north, so that was where Milankovic focussed. He also focussed on the summer insolation, as cooler summers are more a factor in glacier growth than cold winters. As Earle explains, when the summers are cooler, there’s less melting of snow and ice, and when winters are colder, they’re also drier, and less snow falls.
So Milankovic based his cycles on three variables – eccentricity, tilt angle and tilt direction.
Eccentricity, which varies on a 100,000-year cycle, determines the distance between Sun and Earth. A high eccentricity (a greater distance), in conjunction with tilt direction, ‘provides a greater opportunity for the Earth to be pushed from a non-glacial state to a glacial state or vice versa’ (Earle).
Tilt angle, which has a 41,000-year cycle, affects seasonal differences. ‘A lesser tilt angle leads to cooler summers and warmer winters, and that favours the growth of glaciers’.
Tilt direction, which has a 23,000-year cycle, determines which hemisphere, north or south, points to the Sun when the Earth is farthest away from it. ‘Glaciation is favoured when the Earth-Sun distance is greatest during the northern hemisphere summer, leading to cool summers with less melting’.
When Milankovic died in 1958 his insolation theories were far from being accepted by mainstream science. This was largely because, though it was known that glaciers enlarged and reduced over millennia, the timing of these ebbs and flows was much of a mystery. Better measurement techniques were required to verify the Milankovic hypotheses. These came in the sixties and seventies with sea-floor and later ice core samples, as well as measurement of isotopic variations in the history of marine mammals, and their relation to temperature, culminating in a key paper published in 1976, at the end of which the authors wrote:
It is concluded that changes in the Earth’s orbital geometry are the fundamental cause of the succession of Quarternary ice ages.
It’s important to note that these orbital changes were not the cause of the ice ages, it simply explained their timing. The cause was a period of atmospheric cooling over 50 million years until recently, geologically speaking. That atmospheric cooling I’ll (try to!) explain in a follow-up post.
From the 70s onwards, ice core samples from Greenland and Antarctica have been able to be correlated with variations in surface temperatures over 250,000 years, based on measurements of the ratios of hydrogen isotopes in the water molecules from the ice at those sites. To quote Earle:
The correlation between the temperature record and the July insolation levels is reasonably clear. The third-last interglacial, extending from 245,000 to 235,000 years ago, corresponds with a period of high insolation. The following very low insolation initiated the beginning of the second-last glacial period. That was followed by a very high insolation period (at around 220,000 years ago, which led to significant warming but wasn’t enough to break the glacial cycle. Glacial conditions then intensified over the next 90,000 years.
Another period of very high insolation, culminating at around 120,000 years ago, was able to break the cycle, leading to the second interglacial, which lasted from about 127 to 90 thousand years ago. That was followed by a similar cycle of increasingly cold climates and strong glaciation until around 20,000 years ago, when the glacial cycle was again broken by a period of strong insolation.
As Earle further points out, methane levels from the same ice cores are even more closely correlated with the insolation pattern. And there are other positive feedback processes that ‘amplify Milankovic forcing’, as Earle puts it, including carbon dioxide levels and the albedo effect of accumulated ice and snow during cooling periods.
Our recent greater understanding of Milankovic cycles allows us to predict their effect on the future climate. We’re entering a period of low ellipticity in the Earth’s orbit, meaning that insolation levels won’t vary much for the next 50,000 years. This means we will have an ‘interglacial’ climate for a long long time to come. So, no cyclical glaciation will arrive any time soon to rescue us from anthropogenic global warming. Add that to the forlorn hopes about other processes touted by climate change skeptics/deniers, such as sunspots and a sudden upsurge of vulcanism….
References
Steven Earle, A brief history of the Earth’s climate: Everyone’s guide to the science of climate change, 2021