Global warming is the rising average temperature of Earth's atmosphere and oceans since the late 19th century and its projected continuation. Warming of the climate system is unequivocal, and scientists are more than 90% certain that most of it is caused by increasing concentrations of greenhouse gases produced by human activities.
The United States is one of the largest contributors of global warming pollution, despite a recession-related drop in U.S. carbon dioxide emissions in 2009. Carbon dioxide in Earth's atmosphere increased in 2009, and projections are for continued increases in global CO2 emissions.
Couldn't global warming just be a natural occurrence?
It's true that Earth's climate has always been in flux. Periods of warming have followed ice ages. A decade of warmer temperatures can be followed by a decade of cooler temperatures. But the current warming trend has been documented over a period of more than one hundred years. And it's occurring much faster than previous episodes of global warming.
The most respected scientific bodies have stated unequivocally that global warming is occurring, and people are causing it by burning fossil fuels and cutting down forests.
Is wildlife already being affected by global warming?
There is mounting scientific evidence that global warming is already having profound effects on birds, broader biodiversity, wildlife habitat, and ecosystems.
Some places are getting more rainfall and others are getting less. Nearly everywhere is experiencing more heavy rainfall events, as warmer air is able to hold more water vapor.
Global warming causes sea-level rise in two ways: (1) Ocean water is expanding as it warms. (2) Land-based ice in glaciers and ice sheets is melting.
The pollutants that contribute to global warming are commonly known as greenhouse gas emissions. Carbon dioxide is probably the best known greenhouse gas, but methane, nitrous oxide and fluorinated gases also play a role in driving climate change.
The Nobel Prize-winning chemist Svante Arrhenius first proposed the idea of global warming in 1896. Carbon dioxide, he knew, traps heat in the Earth's atmosphere. He also knew that burning coal and oil releases carbon dioxide (CO2).
Arrhenius speculated that continued burning of coal and oil would increase concentrations of CO2 in the Earth's atmosphere, making the planet warmer. It's called the greenhouse effect.
The widespread burning of fossil fuels began with the Industrial Revolution, when humankind discovered that the energy from burning fossil fuels like coal could power machinery that performed work faster and more efficiently than reliance on human labor. According to the IPCC, burning fossil fuels is the leading cause of the greenhouse gas emissions that cause climate change.
Global warming refers to the recent and ongoing rise in global average temperature near Earth's surface. It is caused mostly by increasing concentrations of greenhouse gases in the atmosphere.
Rising global temperatures have been accompanied by changes in weather and climate. Many places have seen changes in rainfall, resulting in more floods, droughts, or intense rain, as well as more frequent and severe heat waves. The planet's oceans and glaciers have also experienced some big changes - oceans are warming and becoming more acidic, ice caps are melting, and sea levels are rising.