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What is global warming?

The world is getting hotter, and humans are responsible. That’s the short version.

When people say global warming, they’re typically referring to the rise in average temperature of the Earth’s climate system since the late 19th century. Temperatures over land and ocean have gone up 0.8° Celsius (1.4° Fahrenheit), on average, in that span:

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Many people also use the term “climate change” to describe this rise in temperatures and the associated effects on the Earth’s climate.

The consensus among climate scientists is that this temperature increase has been driven primarily by the extra greenhouse gases humans have put into the atmosphere since the Industrial Revolution. Greenhouse gases like carbon dioxide trap heat at the Earth’s surface, preventing that heat from escaping back out into space too quickly. So when we burn coal or oil for energy or cut down forests and add even more carbon dioxide to the atmosphere, the planet warms up.

Global warming also refers to what scientists think will happen in the future if humans keep adding greenhouse gases to the atmosphere. A 2013 report from the Intergovernmental Panel on Climate Change projects that temperatures could rise at least 2°C (3.6°F) by the end of the century under many plausible scenarios — and possibly 4°C or more.

Many experts consider 2°C of warming to be unacceptably high, increasing the risk of deadly heat waves, droughts, flooding, and extinctions. Rising temperatures will drive up global sea levels as the world’s glaciers and ice sheets melt. Further global warming could affect everything from our ability to grow food to the spread of disease.

Avoiding drastic global warming would likely require a complete overhaul of our energy system. Fossil fuels currently provide 87 percent of the world’s energy. To zero out emissions this century, we’d have to replace most of that with low-carbon sources like wind, solar, nuclear, geothermal, or carbon capture.

That’s a staggering task, and there are huge technological and political hurdles standing in the way. As such, the world’s nations have been slow to act on global warming — it’s a genuinely difficult issue to tackle, and efforts to revamp the energy system often encounter heavy opposition.