What Is the Scientifically Accurate Response to Claims That Global Warming Is Only Related to Natural Changes in Earth's Climate?
Ask a Scientist - September 2010
C. Ortiz from Ivyland, PA, asks "What is the scientifically accurate response to those who claim that global warming is not human-caused, but is only related to natural changes in Earth's climate?" and is answered by Dr. Brenda Ekwurzel, Assistant Director of Climate Research and Analysis
The climate can be influenced by a number of factors from both natural causes to human activities. Natural phenomena that can affect the climate include the sun's energy output, particulate matter from volcanic activity, and changes in snow and ice cover or vegetation that influence the reflectivity of the surface of Earth. Human effects include heat-trapping emissions from cars and power plants and particulate matter and soot from pollution. But how do scientists know which of these factors are causing the climate to change?
One way is to tease apart the atmospheric concentration of carbon dioxide (CO2)—one of the main heat-trapping gases responsible for global warming—to see how much is from natural sources and how much is from human activity like combusted fossil fuel sources.
This is actually my area of research—isotopes. An isotope is an element (those things on the periodic table we all study in school) that has a different number of neutrons. For example, each carbon atom has six protons in the nucleus, but there are several different carbon isotopes with varying numbers of neutrons in the nucleus. It’s these carbon isotopes that climate scientists like me look at when trying to determine where the carbon is coming from. Fossil fuel emissions have a mixture of different carbon isotopes than natural atmospheric carbon does. But that’s what we’ve found is changing. Over time, we’ve seen more and more carbon isotopes from human-caused sources in our atmosphere. That’s a really big red flag and clearly shows that the increases we’ve seen in carbon emissions have human fingerprints all over them.
Another way to find out whether natural or human factors are causing global warming is to track which factors match the changes in global temperature that we are seeing. We use computer models to study the climate, both to help us analyze the past and, for various emissions scenarios, project the future. These models are subjected to numerous tests before they are used to ensure they produce accurate results. For instance, in one test, researchers use the model to reconstruct what’s happened to the climate over the course of the twentieth-century. They start the computer model with data from 1850 and run it to the present, accounting for changes in heat-trapping emissions, volcanic activity, and a host of other variables. The model has to be accurate enough to reproduce the climate features of the twentieth century before scientists use it to project into the future.
What’s most revealing about this exercise is that when these computer models include only recorded natural effects on the climate—such as the sun’s intensity or large volcanic eruptions—the models cannot accurately reproduce the observed warming of the past half century. While the rate of energy coming from the sun changes slightly day to day, including sun spot activity and solar flares, and, over the time-scale of millions of years, is a critical factor influencing climate (e.g. ice ages), changes in solar energy over the last century don’t match the magnitude and distribution of the rise in global mean temperature we’ve observed during that same time period. So it’s clear to scientists that solar activity isn’t the cause of global warming.
These are just a few examples (you can find more about the fingerprints that humans have left on Earth’s climate here); the critical point is that no natural changes alone explain the temperature changes we’ve seen. But when human-related factors are also included in the computer models, then they can accurately capture the recent temperature increases we’ve seen in both the atmosphere and the oceans. When all the natural and human-induced factors are compared to one another, the dramatic accumulation of heat-trapping emissions from human sources is by far the largest contributor to climate change over the past half century. That, along with multiple other lines of evidence, is why scientists are clear that, if we are to avoid the worst effects of global warming, we need to take action now to make swift and deep reductions in these emissions
Brenda Ekwurzel is a climate scientist who leads UCS's climate science education work aimed at strengthening support for sound U.S. climate policies. She holds a Ph.D. in isotope geochemistry from the Department of Earth Sciences at Columbia University's Lamont-Doherty Earth Observatory and has studied climate variability in places as disparate as the Arctic Ocean and the desert Southwest.
