What we know about the climate

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Just two years ago, 62 percent of people in the United States agreed that global warming is largely the result of well-documented man-made increases in carbon dioxide and other greenhouse gases. There was also considerable support—67 percent—for taking steps to reduce our carbon footprint.

But a poll in June showed a drop of nearly 10 percentage points in the number of people who accept that the globe is warming. Recent controversies, such as the debate over e-mails stolen from a British researcher, have contributed to public skepticism.

But to climate scientists, myself included, and, indeed, to most members of the scientific community, the general picture has never been more clear. Anthropogenic climate change is real; alleviating the cause is a pressing issue.

Since the start of the Industrial Revolution, the concentration of atmospheric carbon dioxide, the chief greenhouse gas, has risen from about 280 parts per million to nearly 390 ppm, accompanied by increases in other greenhouse gases, such as methane and nitrous oxide. Reliable temperature records that go back to about 1850 show that over the last 160 years the global average surface temperature has risen by about 1.5 degrees F.

Several lines of evidence demonstrate that most of this warming results from human activities and not, as skeptics have suggested, because of natural changes in such factors as solar intensity and volcanic eruptions. To be sure, natural factors have always contributed to the way the climate behaves, particularly over the long term, but their impact does not account for what we observe today.

How do we know that the warming is related to the increase in greenhouse gases? First, we know that greenhouse gases trap heat, preventing it from escaping to space. Second, there is a general link in time between the increases in greenhouse gases and in temperature. Third, there is a link in space as well. If warming were the result of natural processes like an increase in solar radiation, it should affect the entire atmosphere more or less uniformly. But this is not what is observed. Instead, the lower parts of the atmosphere have warmed and the upper parts have cooled. This is precisely what we expect with greenhouse gases. They act like a blanket in the atmosphere, retaining heat from the Earth’s surface—thus warming the lower part of the atmosphere while preventing this heat from warming the upper part of the atmosphere.

If you look, as I have, at the chemical components of the greenhouse gases, human fingerprints are clearly visible. Take, for example, the stable carbon isotope known as carbon-13, or13C. The carbon dioxide emitted by burning fossil fuels contains less 13C than does the atmosphere. We are now seeing a gradual decrease of 13C in atmospheric carbon dioxide—best explained by the burning of fossil fuels. Similar evidence from the radioactive isotope 14C, which is completely absent in fossil fuel, corroborates this finding.

Many scientists, working independently and, I should add, competitively, have published similar results. The general understanding we already have is sufficient to attribute the observed climate changes to human actions. But even though the general picture is clear, it is important to acknowledge that there are details we still do not know that well. This results in the disagreements we see among various computer models used to predict future climate.

Many of the uncertainties are related to feedback loops in the climate system—when a change in one climate factor, such as temperature, leads to changes in other factors, such as ice cover, vegetation, or clouds. These, in turn, may induce further changes in temperature. Such effects, rippling throughout the system, make precise prediction difficult; research aimed at understanding them is critical to enable scientists to gauge how large future changes will be and how they will be distributed around the world.

One way to help clear up such uncertainties is by studying the climate of the past. We can do this by analyzing ancient chemical and biological materials that hold clues about the conditions in which they were formed. These “climate proxies” allow us to deduce climatic conditions beyond the time period for which we have direct measurements.

Of particular interest to me is a new proxy nicknamed “clumped isotopes”—a cluster of the isotopes 13C and 18O, found in carbonate minerals, like clam shells in the ocean or stalagmites on land. When the temperature is colder, these isotopes tend to cluster together; when it’s warmer, they have less of a tendency to bond. So the concentration of this isotopic cluster in a sample gives us a measure of the temperature at the time it was formed.

We use this proxy to study changes in ocean temperatures as much as 50 million years ago, examining the responses to high levels of greenhouse gases. We also reconstruct temperatures on land during the last ice age, which might help us to understand and predict changes in rainfall patterns.

To increase public confidence, scientists need to acknowledge the limitations of our current understanding of the climate system and the fact that we can predict its behavior only in general terms. But those terms all point in one direction. The problem of climate change is real, and, while research must continue, we must start acting now to reduce the impact of greenhouse gases. We cannot wait until we have complete certainty about every detail. By then, it may be too late.