Is the Earth Headed Toward Warming or Cooling?

by Ingrid Zabel

Originally posted February 4, 2020; last updated April 24, 2023.

A visitor to the Museum of the Earth recently noticed a sign in the Museum’s Ice Age room that states that “Ithaca will become glaciated again within 80,000 years unless global warming ends the Ice Ages and leaves Ithaca with a warmer climate.” He wondered why the Earth might be heading toward a cooler state when we hear so much about a warming climate, and that the current warming might be irreversible. It’s a good question!

Part of the confusion might be about the term “Ice Ages.” The Earth has had at least five ice ages—defined as times when the poles were covered with ice. The last ice age started around 3 million years ago, and we’re still in it today! We can see today that Antarctica and Greenland are covered by huge ice sheets, and sea ice exists in both the Arctic and Antarctic. 

Much of Earth’s history has consisted of long periods of time when the Earth was not in an ice age, and the planet was so warm that there was no ice at the poles.  An example of this would be during the Triassic period (255–205 million years ago), when dinosaurs first appeared.

Within the current ice age, there have been warmer and colder times, called “interglacial” and “glacial” periods. Figure 1 (below) shows the temperature in Antarctica over the last 800,000 years. These data come from chemical analysis of ice cores from Antarctica, one of the most exquisite data sets in Earth science; very recent data come from a weather station at the ice core site. Ice cores are cylinders of ice extracted from the ice sheet, and the deeper down one drills, the older the ice. In Figure 1, one can see a pattern of transitions from glacial to interglacial periods approximately every 100,000 years. Ice cores collected from other parts of the world show similar temperature patterns, indicating that these patterns were global. The changing temperature is initiated by periodic changes in three things: Earth’s orbit around the sun, the tilt of Earth’s axis, and the wobble of the Earth around its axis. The temperature changes are amplified by changes in the physics, chemistry, and biology of the ocean that affected the exchange of carbon dioxide between the ocean and the atmosphere.

In Figure 1 we see some very rapid increases in Antarctic surface temperature. For example, starting around 136,000 years ago the temperature rose over 20° F in under 8,000 years. This rate of change, around 0.003°F/year, is very fast on geological time scales, but not nearly as fast as the rate of global surface temperature increase in the last 50 years. The global rate of increase over the last 50 years is around 0.03°F/year—about ten times as fast! Comparing the rate of temperature increase today to the rate of temperature increase during very rapid times of change in the last million years is like comparing the speeds of a sprinter and a turtle.

The last glacial period, which ended around 18,000 years ago, is what people commonly refer to as “the Ice Age” (this is what’s depicted in the animated “Ice Age” movies for kids). Around 18,000 years ago, what is now Ithaca was covered by about a mile of ice. While we should call the period around 18,000 years ago the end of “the last glacial period,” very few of us are going to do this in our everyday language. Figure 2 (below) shows the Antarctic temperature record from this time to 1860. Note how the temperature has been relatively stable for about the last 10,000 years, compared with the last few hundred thousand years. It was during this time that agriculture and modern societies developed. But that’s a different story.

Imagine what would happen if humans were not changing the climate by burning oil, coal, and natural gas and releasing carbon dioxide (CO₂), methane, and other gases that heat the atmosphere. The natural cycle that one sees in Figure 1 would continue, and we would expect the Earth’s temperature to decrease over time, ending in a glacial period about 80,000 years from now. However, we are disrupting the natural cycle. Figure 3 (below) shows CO₂ in the atmosphere over the last 800,000 years. This data set comes from the ice core record and from direct measurements of atmospheric CO₂ since 1958. It is evident that today’s levels of CO₂ are much higher than the natural fluctuations over the last 800,000 years. CO₂ concentrations have been increasing rapidly over the last 175 years (since the start of the Industrial Revolution), and have risen sharply in the last 30 years or so. Excess CO₂ in the atmosphere is warming the planet.

Is today’s climate change irreversible? In some sense, even if we could magically cut all CO₂ emissions tomorrow, the atmosphere would still heat up in response to the CO₂ we’ve already put in. But if we reduce CO₂ emissions quickly we can reduce future change, and we can potentially bring the Earth’s temperature down. In its 2021 report on climate science, [1] the Intergovernmental Panel on Climate Change (IPCC) concluded that “under the lower-emissions scenarios considered in this Report, the increase in atmospheric carbon dioxide concentrations would slow visibly after about five to ten years, while the slowing down of global surface warming would be detectable after about twenty to thirty years.”[2]

The IPCC also concluded that sea level will continue to rise for at least hundreds of years because of the changes we’ve already made to the planet. This is because the ocean responds very slowly to the enormous amount of heat that’s been added to it since the Industrial Revolution. We will need to prepare for this rising sea level at the same time that we are reducing carbon dioxide emissions to prevent more warming. Taking rapid action to reduce our carbon dioxide emissions will also make it easier for us to adapt to other future climate changes, because it’s generally easier to adapt to smaller, more gradual change than to rapid, drastic change. And if we can find ways to actively pull carbon out of the air, we might be able to reverse climate change. Trees and phytoplankton already pull CO₂ out of the air through photosynthesis, so we have examples from Nature of one way to do it.

See a video on how trees take in carbon dioxide and how we can measure this phenomenon.

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