Short answer: no one really knows although theories abound, and the causes are probably interactions among multiple factors.
I’ve always found the history of earth’s ice ages to be fascinating, from the moment I discovered some maps as a child flipping through my old World Book Encyclopedia (those of you of a certain age will know what I’m talking about). I’ve also always found it to be very very mysterious, and that hasn’t changed.
A few excerpts from the vast number of articles out there [emphasis mine]:
During the present ice age, glaciers have advanced and retreated over 20 times, often blanketing North America with ice. Our climate today is actually a warm interval between these many periods of glaciation. The most recent period of glaciation, which many people think of as the “Ice Age,” was at its height approximately 20,000 years ago.
Although the exact causes for ice ages, and the glacial cycles within them, have not been proven, they are most likely the result of a complicated dynamic interaction between such things as solar output, distance of the Earth from the sun, position and height of the continents, ocean circulation, and the composition of the atmosphere.
Between 52 and 57 million years ago, the Earth was relatively warm. Tropical conditions actually extended all the way into the mid-latitudes (around northern Spain or the central United States for example), polar regions experienced temperate climates, and the difference in temperature between the equator and pole was much smaller than it is today. Indeed it was so warm that trees grew in both the Arctic and Antarctic, and alligators lived in Ellesmere Island at 78 degrees North.
Then earth cooled, then warmed, then cooled…well, you get the idea.
The Earth was once more released from the grip of the big chill between 5 and 3 million years ago, when the sea was much warmer around North America and the Antarctic than it is today. Warm-weather plants grew in Northern Europe where today they cannot survive, and trees grew in Iceland, Greenland, and Canada as far north as 82 degrees North.
We are still in the midst of the third major cooling period that began around 3 million years ago, and its effect can be seen around the world, perhaps even in the development of our own species. Around 2 and a half million years ago, tundra-like conditions took over north-central Europe. Soon thereafter, the once-humid environment of Central China was replaced by harsh continental steppe. And in sub-Saharan Africa, arid and open grasslands expanded, replacing more wooded, wetter environments. Many paleontologists believe that this environmental change is linked to the evolution of humankind.
Plate tectonics are part of the causal explanations as well. But only part.
Another theory explaining these changes in climate involves the opening and closing of gateways for the flow of ocean currents. This theory suggests that the redistribution of heat on the planet by changing ocean circulation can isolate polar regions, cause the growth of ice sheets and sea ice, and increase temperature differences between the equator and the poles.
Also carbon dioxide, but not manmade carbon dioide:
One mechanism proposed as a cause of this decrease in carbon dioxide is that mountain uplift lead to enhanced weathering of silicate rocks, and thus removal of carbon dioxide from the atmosphere.
Here’s another article I think worth quoting [emphasis mine]:
Throughout the Quaternary period, high latitude winters have been cold enough to allow snow to accumulate. It is when the summers are cold, (i.e., summers that occur when the sun is at its farthest point in Earth’s orbit), that the snows of previous winters do not melt completely. When this process continues for centuries, ice sheets begin to form. Finally, the shape of Earth’s orbit also changes. At one extreme, the orbit is more circular, so that each season receives about the same amount of insolation. At the other extreme, the orbital ellipse is stretched longer, exaggerating the differences between seasons. The eccentricity of Earth’s orbit also proceeds through a long cycle, which takes 100,000 years. Major glacial events in the Quaternary have coincided when the phases of axial tilt, precession of equinoxes and eccentricity of orbit are all lined up to give the northern hemisphere the least amount of summer insolation.
See this as well [emphasis mine]:
Today’s ice age most likely began when the land bridge between North and South America (Isthmus of Panama) formed and ended the exchange of tropical water between the Atlantic and Pacific Oceans, significantly altering ocean currents…
Records show that ice ages typically develop slowly, whereas they end more abruptly. Glacials and interglacials within an ice age display this same trend.
On a shorter time scale, global temperatures fluctuate often and rapidly. Various records reveal numerous large, widespread, abrupt climate changes over the past 100,000 years. One of the more recent intriguing findings is the remarkable speed of these changes. Within the incredibly short time span (by geologic standards) of only a few decades or even a few years, global temperatures have fluctuated by as much as 15°F (8°C) or more.
For example, as Earth was emerging out of the last glacial cycle, the warming trend was interrupted 12,800 years ago when temperatures dropped dramatically in only several decades. A mere 1,300 years later, temperatures locally spiked as much as 20°F (11°C) within just several years. Sudden changes like this occurred at least 24 times during the past 100,000 years. In a relative sense, we are in a time of unusually stable temperatures today—how long will it last?
I’m not a scientist, but I can see why at least some scientists buck the current political trends and advance the idea that natural processes account for much or even most of our current tendency towards global warming, and/or climate change in general. My favorite climate scientist, Judith Curry, has this to say on the subject:
She tells me, for example, that between 1910 and 1940, the planet warmed during a climatic episode that resembles our own, down to the degree. The warming can’t be blamed on industry, she argues, because back then, most of the carbon-dioxide emissions from burning fossil fuels were small. In fact, Curry says, “almost half of the warming observed in the twentieth century came about in the first half of the century, before carbon-dioxide emissions became large.” Natural factors thus had to be the cause. None of the climate models used by scientists now working for the United Nations can explain this older trend. Nor can these models explain why the climate suddenly cooled between 1950 and 1970, giving rise to widespread warnings about the onset of a new ice age. According to a group of scientists, we faced an apocalyptic environmental scenario—but the opposite of the current one.
This brings us to why Curry left the world of the academy and government-funded research. “Climatology has become a political party with totalitarian tendencies,” she charges. “If you don’t support the UN consensus on human-caused global warming, if you express the slightest skepticism, you are a ‘climate-change denier,’ a stooge of Donald Trump, a quasi-fascist who must be banned from the scientific community.” These days, the climatology mainstream accepts only data that reinforce its hypothesis that humanity is behind global warming. Those daring to take an interest in possible natural causes of climactic variation—such as solar shifts or the earth’s oscillations—aren’t well regarded in the scientific community, to put it mildly…
What could lead climate scientists to betray the very essence of their calling? The answer, Curry contends: “politics, money, and fame.”…Among climatologists, Curry explains, “a person must not like capitalism or industrial development too much and should favor world government, rather than nations”; think differently, and you’ll find yourself ostracized. “Climatology is becoming an increasingly dubious science, serving a political project,” she complains. In other words, “the policy cart is leading the scientific horse.”