By Caroline Alden, BURN Contributor

Does it matter when nature offers up round numbers? Maybe not, but for the same reasons that we attach special significance to anniversaries and birthdays ending in zero, humans treat big tick marks and bold milestones with gravitas.

For Earth’s climate, a very significant round number milestone was reached last week, when NOAA measured an atmospheric concentration of CO2 of 400 ppm at the Mauna Loa Observatory in Hawaii for the first time in modern history.

PPM – or parts per million – is a measure of concentration. 400 ppm means that for every one million parts dry air in the atmosphere (water is excluded because its concentration is variable), 400 of those parts are CO2. These ‘parts’ are moles: a chemist’s unit of measurement to keep track of molecules.

Think of the atmosphere as a big pot of soup with lots of finely chopped vegetables. Carbon dioxide is the carrots. Prior to the Industrial Revolution, if you filled a ladle with 1,000,000 really finely chopped vegetables, then you’d have found that 280 of those veggies in any given ladle-full of soup that you scooped would be carrots, or carbon dioxide.

Now, today, after we have been dumping extra chopped carrots into the soup (i.e. burning fossil fuels) for a couple hundred years, a ladle-full of 1,000,o00 veggie bits would include 400 carrot chunks (carbon dioxide). For the last few years, we have diluted the soup by about 4-5 carrot bits every year.

There are many places across the globe that measure atmospheric concentrations of carbon dioxide, but the measure of atmospheric COon Mauna Loa (Long Mountain in Hawaiian) is an important and historically significant indicator for two reasons.

First, because of the remote and high altitude location (measurements take place at a height of 2 miles above sea level), measurements of atmospheric CO2 at Mauna Loa generally come very close to representing the global mean concentration of that gas.

Keeling measuring CO2 at Mauna Loa in 1988. Photo: Scripps Institution of Oceanography/UCSD

Second, the record of CO2 at Mauna Loa represents the longest, continuous monitoring of carbon dioxide on Earth. In 1958, Charles Keeling, a scientist employed by the Scripps Institution of Oceanography in La Jolla, California, began regularly collecting samples of air from the atmosphere and measuring the concentration of CO2.

Within a few years, Keeling not only observed remarkable seasonal variability in CO(from large swaths of northern hemisphere plants breathing CO2 in and out, summer to winter), he also clearly showed – for the first time – that atmospheric CO2 was steadily increasing each year.

The canonical time history of Mauna Loa atmospheric CO2 concentrations, which scientists have relied on for 50 years, is, as a result, called the Keeling Curve.

Now. How big of an impact does a change from 280 ppm to 400 ppm have on the Earth’s climate?

To answer this question, it is best to peer back into Earth history to see what the world looked like the last time the atmosphere had 400 little carrots pieces for every million-chopped-veggie ladle full. Scientists have tried to do just that by looking at various types of ancient rocks and sediments, and even bubbles in ancient ice.

One good estimate of when atmospheric CO2 was last 400 ppm was produced by Yale researcher Mark Pagani and fellow scientists, who looked at the chemical properties of ancient ocean sediment.

What these scientists found is that the last time atmospheric CO2 reached 400 ppm was likely somewhere around 4.5 million years ago.

At that time, temperatures on the planet were an average of 4° Celsius (7.2° Fahrenheit) higher than today and sea level was about 22 meters (72 feet) higher. Because of a phenomenon known as Arctic Amplification, northern climes were even warmer – likely 19° Celsius (34.2° Fahrenheit) warmer than today.

Since the Earth’s climate system takes a little bit of time to adjust to atmospheric greenhouse gas concentrations, perhaps these are changes we might expect to see coming down the climate pipeline.

Caroline Alden is a graduate student at the Institute of Arctic and Alpine Research in the Department of Geology at the University of Colorado at Boulder.