Remember the O16-O18 story of several days ago. As one drills down into the ocean sediments -- and therefore into the past -- the ratio of the isotopes changes. More heavy O18 is in the sea when more of the lighter O16 is stored on land in glacial ice. So more O18 in core fossils is a proxy for a colder Earth, and less O18 indicates a warmer Earth.
The O18 record in any one core is not always the best it could be. In 2005, Mo and her colleague Lorraine Lisiecki combined the data from 57 of the best core records from drill holes worldwide into one data set that goes back 5 million years (horizontal scale on graph below). It is called the Lisiecki-Raymo stack, and is the gold standard of climate change. The oscillating light blue line represents actual changes in temperature. The dark blue line is an average. Click to enlarge.
The black horizontal line is the O18 concentration today, so you can see that we are relatively warmer than at any time in the past 3 million years. The last 3 million years are the so-called Ice Ages, with the ice advancing and retreating with the rising and falling temperature indicated by the light blue lines, with roughly a dozen ice ages every million years. We are sitting at the warm top of one of those temperature oscillations, and one might reasonably expect -- all else being equal -- that as more thousands of years pass temperatures will drop and the glaciers will advance again across the northern continents, sweeping our great cities and all else before advancing ice.
All else being equal.
The apparently wild oscillations in global temperature (light blue) can be attributed to well understood and precisely calculable wobbles in the Earth's orbit that affect how much sunlight falls on the Earth. For example, you will observe a major oscillation approximately every 100,000 years, which is assumed to be related to a well-known 100,000 year variation in the eccentricity of the Earth's orbit, a so-called Milankovitch cycle. These astronomical effects are totally independent of anything that happens on Earth, and is something over which we have no control. It's the dark blue line, the average temperature, that concerns us.
What astronomy can't explain is the steady cooling of the Earth over the past 3 million years, from a time when the average temperature of the planet was much warmer than today.
More on why this might have happened tomorrow, and why it is important to each of us, and especially to our great-grandchildren.