The Earth is a complicated planet. There is plenty we don't understand about it and a lot more we'd need to know to manage its climate. Nevertheless, we have acquired a lot of confidence regarding the key elements of climate science. As far back as the late 1800s, for example, scientists knew that a minute increase in the amount of carbon dioxide in the atmosphere can have a tremendous influence on the average global temperature. Carbon dioxide is only 0.04 percent of the atmosphere, yet without it and similar greenhouse gases the global average temperature of 15 °C would be at least 20°C colder! Human-induced global warming occurs on top of this already substantial natural effect.

The basic theory behind the greenhouse effect is simple. The sun's energy—about 344 watts per square meter on average—arrives largely in the visible portion of the electromagnetic spectrum. The Earth's surface and atmosphere absorb this energy and reradiate it, largely in the far-infrared part of the spectrum. In thermal equilibrium, the power level of the incoming radiation matches what is going out; Earth's temperature rises or falls until it reaches this radiation balance. Carbon dioxide, which is at its highest atmospheric concentration in at least 650 000 years, modifies this simple balance. It is a strong absorber of infrared wavelengths, so it traps energy that would otherwise escape to space and nudges upward the temperature at which the radiation balance occurs.

In reality, of course, the situation is more complicated. Warmer air holds more water vapor, which is itself an important greenhouse gas. If we add carbon dioxide to the atmosphere, water vapor becomes more abundant and amplifies the temperature increase that would result from the carbon dioxide alone. Indeed, somewhat more than half of any human-induced global warming comes from this and other feedback processes. (One critical difference between the effect of carbon dioxide and the effect of water vapor is that carbon dioxide lingers for decades to centuries in the atmosphere, but water vapor readily rains out when conditions change.) Scientists are working to further refine the theory through better understanding of positive and negative feedback loops introduced by clouds and aerosols, the impact of other greenhouse gases such as methane, and the role of land-cover changes such as deforestation, among other things.

Indeed, these feedback loops are what makes it challenging to determine how rapidly climate is changing and what is causing the change. We know that the atmospheric abundance of carbon dioxide has increased by a third since scientists first made direct measurements in the late 1950s. By the basic greenhouse theory, this fact alone would make us expect changes in climate. But how do we conclusively relate changing temperature and rising sea levels to carbon dioxide or any other cause, given all of the feedback mechanisms involved? And, even more important, how do we reliably distinguish human impacts from those that occur naturally?

As with any field of science, the answers lie in finding the theory that best fits the evidence. Basic physics tells us that added carbon dioxide must warm the atmosphere and oceans unless something else changes to counteract the additional trapped heat. It is entirely possible that natural global warming, due to solar variation or other mechanisms, could be occurring as well—but that doesn't let us off the hook—our emissions and other activities must still be trapping heat and affecting the climate.

Here's where things get challenging. The many feedback mechanisms, including absorption of atmospheric carbon into the ocean and changes in cloud patterns, mean that our seemingly simple question about additional heat turns rapidly into a complex question about how the climate responds. Computer models provide the only means to bring together all of the interactions and feedback mechanisms to provide an answer. What scientists find is that they can account for the observed patterns of temperature change around the world only if they build in human carbon dioxide emissions—natural variations such as those of solar output, volcanism, and cosmic rays just don't do it. It is possible to find situations for which the greenhouse theory fails. But it succeeds far more than any competing theory and its success rate is only improving.