Climate Has Changed Before

Some argue that because the Earth’s climate has changed many times during its history, we have no need to worry about changes taking place around us today. However, this is an inaccurate and misinformed picture of the changes during the past 250 years that is unprecedented in the changes that occurred in the previous hundreds of thousands of years. Scientists have ways to tell what the conditions were during those changes. One of these ways is to analyze the contents of ice cores drilled from the deep ice sheets in Antarctica and Greenland. These cores can be several kilometers deep and are made up of layers of ice that have built up over hundreds of thousands of years of snowfall. As the snow fell, air became trapped within the snow and ultimately ended up as tiny bubbles in the ice, like those shown in the sidebar photo. Samples of the ice from different layers, corresponding to different times before the present, can be melted to release the trapped gases for measurement.

It’s true that the climate has changed many times during Earth’s history and we have ways to tell what the conditions were during those changes. One of these ways is to analyze the contents of ice cores drilled from the deep ice sheets in Antarctica and Greenland. These cores can be several kilometers deep and are made up of layers of ice that have built up over hundreds of thousands of years of snowfall. As the snow fell, air became trapped within the snow and ultimately ended up as tiny bubbles in the ice, like those shown in the sidebar photo. Samples of the ice from different layers, corresponding to different times before the present, can be melted to release the trapped gases for measurement.

This graph shows the results from one of these analyses. The time axis on the graph starts about 650,000 years ago and extends to the present time. The red line is a plot of the amount of carbon dioxide in the air trapped in the ice core at a time before now. The blue and green lines are, respectively, the amounts of methane (CH₄) and nitrous oxide (N₂O), two other greenhouse gases, in the bubbles. There are peaks and valleys in the plots that correspond to higher and lower amounts of the carbon dioxide and methane in the trapped air. (The data for nitrous oxide are too sparse to see such past variations.)

The black line on the graph shows the variation in the atomic composition of the ice core itself. The peaks in this line represent higher temperature periods that are also denoted by the grey bars. These temperature variations show that less carbon dioxide and methane correspond to ice ages when the Earth was colder than it is now and that more carbon dioxide and methane is present in warmer periods, like now. These variations are consistent with the role of carbon dioxide and methane as greenhouse gases helping to keep the Earth warm—warmer when there are more of them in the atmosphere.

At the upper right-hand corner of the ice-core graph are red, blue, and green stars, each labeled with the amount of the corresponding greenhouse gas, carbon dioxide, methane, and nitrous oxide, respectively, in the atmosphere at the beginning of the twenty-first century. All three of the gases are now present in much larger amounts than at any time in the past 650,000 years. The blue line on the graph below shows data on this unprecedented increase in the greenhouse gas carbon dioxide during the last 1000 years.

You can see that the increase in carbon dioxide begins a little before 1800, in the early days of the Industrial Revolution, and climbs rapidly near the end of the 20th century as we burn more and more fossil fuel. The final values on the curve are direct measurements of the carbon dioxide beginning in the middle of the 20th century. For at least 650,000 years, until the 20th century, the amount of carbon dioxide in the air was never higher than 300 ppmv (parts per million by volume which is 0.0300% of the air). By the beginning of the 21^st century, about 116 ppmv of new carbon dioxide had been added to the air during the previous 200 years. This is about 925 billion (925 × 10⁹) extra metric tons of carbon dioxide (equivalent to the weight of about 2.3 billion fully loaded Boeing 747 airliners). This change is both far outside the range of natural variation for at least the last million years and enormously faster than the natural processes represented in the ice core data. This rapid infusion of carbon dioxide in unprecedented amounts can change the Earth’s climate in ways that are different from the past.

The temperature of the Earth is shown by the red line on the graph and is scaled to show the close correspondence of increasing Earth temperature to the rise of carbon dioxide in the atmosphere. Correlations like this are useful to guide investigations to determine if there is any cause-and-effect relationship between the quantities that seem to be correlated. To learn more about the relationship between greenhouse gases and Earth’s temperature, see the narrative, “What Is the Greenhouse Effect”. The narrative, “Can Humans Be Changing the Climate?,” provides one line of evidence that the increasing carbon dioxide is a result of our fossil fuel burning.

The Intergovernmental Panel on Climate Change (IPCC) periodically publishes an Assessment Report that brings together a consensus all the available evidence (based on the work of thousands of scientists from around the world) for the climate changes that are occurring, their sources, and estimates of future impacts. The findings by the second decade of the 21^st century are that the Earth has warmed by at least 1.6 degrees Fahrenheit above the pre-industrial level, that human activities are responsible for the rise, and, depending on the extent of mitigation efforts that are implemented, the estimated  increase will be 2.5 to 10 degrees Fahrenheit by the end of the century.