The human influence on Earth’s energy balance has made the planet warmer by adding heat-trapping gases to the air. This climate change can increase the intensity of some natural weather events such as hurricanes, floods and droughts. The most important human greenhouse gas is carbon dioxide (CO2). The fossil fuels we burn release CO2 into the atmosphere, and deforestation adds about seven billion metric tons of extra CO2 each year. The carbon in the atmosphere absorbs a lot of the solar radiation that would otherwise warm the air and cause warming.
Scientists measure how much of the planet’s surface is warming using thermometers and satellites. The historical record shows a clear trend of warming since the 1880s, when people began to record temperatures at weather stations and on ships as they traversed the world’s oceans. These records have been carefully checked for any possible errors or biases and they are confirmed by independent data, like satellite observations that cover the whole globe and other methods of measuring temperature changes.
Historically, there have been many factors that have led to climate changes, such as changes in solar output, ocean circulation and volcanic activity. Scientists can track these changes by studying fossils and other records from times when accurate thermometers were not available, such as tree rings, ice cores and marine sediments. By comparing modern thermometer data with these proxy records, scientists can “fingerprint” the source of the climate change. The results of fingerprinting indicate that the current global warming is mainly due to an increase in atmospheric CO2 concentrations.
The global average surface temperature has risen by more than two degrees Fahrenheit (1.1 degrees Celsius) since the late 1800s. The rise has been more rapid since the 1960s. The land area of the world has warmed by more than twice as much as the sea surface, and the Arctic has warmed even faster than the rest of the globe. These dramatic changes are already having profound impacts around the world.
Temperature increases are affecting water supplies, food production and health. Warmer water and air can cause droughts and more frequent flooding, especially in densely populated coastal areas. A warmer planet can also become more prone to extreme weather events such as heat waves, storms and wildfires.
Some individuals continue to question the scientific consensus on climate change, although most prominent skeptics have now changed their minds. Some skeptics have ties to the fossil fuel industry, but most have investigated their own assumptions and have come to the conclusion that humans are causing the planet to warm. Physicist Richard Muller is one such former skeptic who became a convert after carefully reviewing the evidence and examining the methods used by other researchers. He writes, “Call me a converted skeptic.”