As has already been posted the Milankovitch Cycles provide a very good explanation for why the temperatures dropped in the past producing the recent glacial periods.
The Earth circles the Sun in a flat plane. It is as if the spinning Earth is also rolling around the edge of a giant, flat plate, with the Sun in the center. The shape of the Earth’s orbit—the plate—changes from a nearly perfect circle to an oval shape on a 100,000-year cycle (eccentricity). Also, if you drew a line from the plate up through the Earth’s North and South Poles—Earth’s axis—the line would not rise straight up from the plate. Instead the axis is tilted, and the angle of the tilt varies between 22 and 24 degrees every 41,000 years (obliquity). Finally, the Earth wobbles on its axis as it spins. Like the handle of a toy top that wobbles toward you and away from you as the toy winds down, the “handle” of the Earth, the axis, wobbles toward and away from the Sun over the span of 19,000 to 23,000 years (precession). These small variations in Earth-Sun geometry change how much sunlight each hemisphere receives during the Earth’s year-long trek around the Sun, where in the orbit (the time of year) the seasons occur, and how extreme the seasonal changes are.
In the early 1900s, a Serbian mathematician named Milutin Milankovitch meticulously calculated the amount of sunlight each latitude received in every phase of Earth’s orbital variations. His work culminated in the 1930 publication of Mathematical Climatology and the Astronomical Theory of Climate Change. He theorized that the ice ages occurred when orbital variations caused the Northern Hemisphere around the latitude of the Hudson Bay and northern Europe to receive less sunshine in the summer. Short, cool summers failed to melt all of the winter’s snow. The snow would slowly accumulate from year to year, and its shiny, white surface would reflect more radiation back into space. Temperatures would drop even further, and eventually, an ice age would be in full swing. Based on the orbital variations, Milankovitch predicted that the ice ages would peak every 100,000 and 41,000 years, with additional “blips” every 19,000 to 23,000 years.
The paleoclimate record shows peaks at exactly those intervals. Ocean cores showed that the Earth passed through regular ice ages—not just the 3 or 4 recorded on land by misplaced boulders and glacial loess deposits—but 10 in the last million years, and around 100 in the last 2.5 million years.
Evidence supporting Milankovitch’s theory of the precise timing of the ice ages first came from a series of fossil coral reefs that formed on a shallow ocean bench in the South Pacific during warm interglacial periods. As the ice ages came, more and more water froze into polar ice caps and the ocean levels dropped, leaving the reef exposed. When the ice melted, the ocean rose and warmed, and another reef formed. At the same time, the peninsula on which the reefs formed was steadily being pushed up by the motion of the Earth’s shifting tectonic plates. Today, the reefs form a visible series of steps along the shore of Papua New Guinea. The reefs, the age of which was well-defined because of the decaying uranium in the coral, measured out the millennia between ice ages. They also defined the maximum length of each ice age. The intervals fell exactly where Milankovitch said they would.
This comes from genuine researchers working in the real world collecting massive amounts of data that are then formulated into coherent theories that give us a high confidence of what is occurring in regards to changes in the global climate due to natural and human created radiative forcings.
The deniers like JohnH and Mike Yohe start from the assumption that human emitted carbon dioxide must be omitted from these calculations because the corporations who fund them make their money by selling products that when burned now emit more than 100 times the CO2 as all tectonic activity on the Earth.
I’ll leave it up to people with any sort of critical thinking skills at all to decide who is more likely to be providing a rational explanation for global warming and climate change.
The choices are;
1. People who devote their lives to the objective study of natural phenomena with the intent to present genuine explanations that can then be used to formulate effective polices that can allow us to navigate through extremely dangerous conditions that will result if the evidence is ignored.
2. People working at what are in fact public relations firms heavily funded by corporations that make incredible profits selling products that we know play a central role in moderating the Earth climate when burned emitting billions of tons of CO2 a year.
Hint- It’s probably not a very wise choice to believe the people we know beyond any reasonable doubt are being paid very well to lie.