This week begins the United Nations Climate Change conference in Copenhagen, following the controversy of stolen emails from the University of East Anglia's Climate Center. Both events have inspired a flood of news stories, editorials and blogs.
Although climate change remains one of our most urgent contemporary issues, it is wise to step back and view the big picture of what scientists do and do not understand about the Earth's climate system and future climate change.
By its nature, scientific inquiry is highly competitive. "Scientific consensus" describes a state of shared knowledge in which each researcher is skeptical of the work of all other researchers. All ideas, interpretations, and data are aggressively cross-examined, debated and dissected to a degree rarely encountered in other professions. Scientists challenge prevailing ideas and concepts, and are often motivated to reveal the fallacy of commonly held perceptions.
Climate science is no exception, and given the high-stakes nature of the topic, results are more highly scrutinized by scientists and policy makers than in many other scientific disciplines. There is a massive international community competing over and examining climate data and models — the principal results and interpretations are not generated by a few groups in isolation.
Our broad understanding of Earth's climate system is based on observations, experiments and models by atmospheric chemists, meteorologists, glaciologists, solar and planetary physicists, oceanographers, geologists, geochemists, biologists, paleontologists, paleoclimatologists, paleoecologists and climate dynamicists.
Finding agreement among these diverse groups is often as fractious as peace negotiations between warring nations. And yet, when it comes to the major issues of climate change, agreement exists.
We understand that Earth's climate and temperature results from a complex interaction between solar heating, rocks, oceans, sea- and land-ice, water vapor, clouds, aerosols, biological processes and greenhouse gases. Revealing the nature of these interactions and determining the direction of climate change as these variables are altered has required an unusually broad collaboration between scientists from a wide range of fields.
We know that the rise in carbon dioxide since the start of the Industrial Revolution has been the result of human activity, mainly from the combustion of fossil fuels. We know that carbon dioxide is a greenhouse gas, which traps heat and warms our planet. For an example of greenhouse warming, one need only look at our sister planet Venus, whose surface temperature makes rocks glow at night. Although Venus is closer to the Sun than is Earth, its extremely high surface temperature is mostly a result of its carbon dioxide-rich atmosphere.
An increase of carbon dioxide on Earth triggers other effects, called feedbacks, in response to increasing temperature. As surface temperature rises, more water evaporates from the ocean. Water vapor is another important greenhouse gas that causes more warming. Warmer oceans also do a poorer job of absorbing carbon dioxide from the atmosphere, while the melting of permafrost releases more carbon dioxide and methane, yet another potent greenhouse gas. All of these feedbacks lead to more surface warming.
Warming also leads to a reduction of our "heat mirrors" — sea and land ice — which causes more sunlight to be absorbed by the surface of the Earth. This drives more warming and more ice melting, and so on. Positive feedbacks amplify the temperature effects of relatively small variations in carbon dioxide or other changes in the climate system, and it is for this reason that Earth experiences large swings in climate. It also warns us that the full effects of rising carbon dioxide levels do not occur overnight.
There are always complexities that are less well understood, like the role that clouds and aerosols (volcanic sulfates, bacteria, dust, soot) play in heating or cooling Earth's temperature. These are important effects and are intensely studied.
We know that Earth's surface temperature will continue to rise even if we immediately stop increasing carbon dioxide. The oceans are massive and take a long time to warm and catch up with surface temperatures, and as they do they absorb less and less carbon dioxide, while releasing more water vapor. However, there is no consensus as to how fast and how much warming will occur over the coming decades and century. This depends, to a large extent, on how much carbon is added to the atmosphere, as well as the "climate sensitivity to carbon dioxide," which is the response of global temperature to the various feedbacks mentioned above. Determining the climate sensitivity is key to understanding how much warming we can expect. Magnitudes of climate sensitivity range from small to large and are different among the different computer models used to project future climate changes.
However, Earth's history has something to say about climate sensitivity and the role of carbon dioxide, as well.
The reconstruction of Earth's history reveals a story of slow and rapid climate change and clear evidence for immense variations in temperature. While most discussions in the popular press focus on the past 100 to a few 100,000 years and the precise relationship between carbon dioxide and temperature, it is informative to examine the full range of climate variations over millions of years.
Earth was, in fact, ice-free for most of its history. For example, Earth was much warmer and had no significant polar ice between 65 to about 34 million years ago. Fifty-five million years ago, rapid and massive releases of carbon acidified the oceans and warmed Earth's surface about 5 degrees Celsius above what was already a warm planet. At peak warming, about 50 million years ago, crocodiles roamed the Arctic amongst subtropical flora and fauna, even though the Sun's intensity was lower than today. Much higher carbon dioxide during this time is revealed by various paleoclimate reconstructions, and subsequent global cooling is shown to have followed carbon dioxide decline.
Earth's history tells us that the leading driver of climate change is the concentration of atmospheric carbon dioxide. Not the only driver, but the leading one. It also reveals that climate sensitivity to carbon dioxide is possibly much higher than discussed in policy-making circles. About five million years ago, carbon dioxide was as high or only slightly higher than 2009 values, and Earth reached temperatures 4 degrees Celsius warmer than now, with sea levels tens of meters higher. The present-day location of Yale University was underwater.
Many lines of evidence and study tell us about the effects of carbon dioxide release. In the past, large increases in carbon dioxide corresponded to major warming events. It is unwise to think that today's increase in carbon dioxide will, for some reason, produce a different outcome.
Mark Pagani is an associate professor of geology and geophysics, and a member of the Yale Climate and Energy Institute executive committee; John Wettlaufer is the A.M. Bateman Professor of Geophysics, Physics and Applied Mathematics; Jeffrey Park is a professor of geology and geophysics, and the director of the Yale Institute for Biospheric Studies; David Bercovici is a professor and chair of the Department of Geology and Geophysics, and the deputy director of the Yale Climate and Energy Institute.