TAMPA, Fla. (WFLA) — You’ve probably heard the expression, “the climate has always changed.” That’s true, although not nearly as fast as it is now.

20,000 years ago – in the blink of a geological eye – ice blanketed the Northern Hemisphere, with glaciers from the North Pole to New York City. During that last ice age, sea levels were 400 feet lower than today.

Since then temperatures gradually warmed, so the Earth gradually thawed. And for the past 10,000 years temperatures have been reliably mild and steady allowing our modern human civilization to thrive.

But for much of Earth’s history, the climate has been turbulent, with cycles of cold glacial periods and warm interglacials.

So, what causes these natural fluctuations? A big part is driven by changes in the amount of sunlight reaching the Earth. And the rest is due to feedbacks, like ice, water vapor and carbon dioxide.

Over the course of tens of thousands of years, both Earth’s tilt -and- its orbital shape around the Sun, called eccentricity, change.

Image WFLA

Earth’s tilt varies by about 2-and-a-half degrees over 40,000 years. Earth’s orbit varies from a circle to an ellipse over 100,000 years. Combined, these effects can modulate the amount of sunlight being absorbed by the planet by up to 25%.

As a result of these changes in the heat absorbed by the planet, every 100,000 years the Earth cycles in -and- out of glacial cycles.

Image WFLA

You may find this surprising, the difference between an ice age and a warm interglacial period like today is not very much. Earth’s average temperature 20,000 years ago was 46 degrees. Today it’s 57. A difference of just 11 degrees.

But the warming over the past century is anything but natural. While the Sun’s radiation reaching Earth has slightly decreased, Earth’s temperatures have been rising at a pace 10 times faster than when the last ice age ended.

Image: Ed Hawkins and PAGES2K

Today’s warming is not driven by the Sun, which only has large-lasting impacts over the timescale of 1000s of years. Instead, our current warming is driven by carbon pollution from the burning of fossil fuels.