There is no real debate about whether climate change is occurring. The only dissent comes from the fringes, and generally from those whose research institutions or blogs are devoted, for ideological or other reasons, to attempting to debunk the notion that human activities are altering the planet's climate. But for many, the discussion, such as it is, can seem confusing. Is the Arctic Ocean predicted to be ice-free by the summer of 2100, or 2050, or 2030? And what exactly does ice-free mean? Are hurricanes supposed to become more frequent, or less frequent but more intense?
For scientists studying the impacts of climate change, such questions - and answers - are constantly being revised and refined as more information is gathered, models are fine-tuned, and feedbacks are better understood. But even as they focus their forecasts, those scientists are increasingly seeing the evidence of global warming happening right now, many of them in line with predictions and some of them even more severe and more rapid than anticipated. The following list provides a sampling of some of the key pieces of evidence that climate change is not just a prediction, it is already underway.
1. Carbon Dioxide Concentrations in the Atmosphere Are Increasing
This is the first, key point. By analyzing air bubbles trapped in the ice of Antarctica and Greenland, scientists have been able to determine that over the past 650,000 or so years, atmospheric concentrations of carbon dioxide (CO2) varied between 180 and 300 parts per million (ppm), and in the years immediately prior to the Industrial Revolution in the mid-eighteenth century stood at approximately 280 ppm. Since then, however, that figure has steadily increased; by the time continuous monitoring began at Mauna Loa Observatory in Hawaii, it had climbed above 310 and is now closing in on 400.
Because we know that carbon dioxide is a greenhouse gas, we can reasonably infer that increasing the amount of CO2 that enters the atmosphere, particularly at the level of 90 million tons a day, will increase the greenhouse properties of the atmosphere and thus lead to warming. (And while, as skeptics often like to point out, water vapor is a more powerful greenhouse gas than CO2, we are not emitting vast amounts of water vapor every day; indeed, the atmosphere can only hold a certain amount of atmosphere at a time. But, by warming the atmosphere, increased CO2 levels enable the retention of greater amounts of water vapor, thus enhancing warming.
Furthermore, scientists know, from analyzing the isotopes of the carbon in the atmosphere, that the increase in atmospheric CO2 is the result of burning fossil fuels and forests, and not the result of natural processes. Accordingly, a National Research Council study was able to point out back in 2001 that, "Greenhouse gases are accumulating in Earth's atmosphere as a result of human activities, causing surface air temperatures and sub-surface ocean temperatures to rise." Because, indeed, as that study noted, "Temperatures are, in fact, rising."