What is happening?
1. Climate change? Global warming? What do we call it?
Both are accurate, but they mean different things.
You can think of global warming as one type of climate change. The broader term covers changes beyond warmer temperatures, such as shifting rainfall patterns.
President Trump has claimed that scientists stopped referring to global warming and started calling it climate change because “the weather has been so cold” in winter. But the claim is false. Scientists have used both terms for decades.
2. How much is the Earth heating up?
Two degrees is more significant than it sounds.
As of early 2017, the Earth had warmed by roughly 2 degrees Fahrenheit (more than 1 degree Celsius) since 1880, when records began at a global scale. The number may sound low, but as an average over the surface of an entire planet, it is actually high, which explains why much of the world’s land ice is starting to melt and the oceans are rising at an accelerating pace. If greenhouse gas emissions continue unchecked, scientists say, the global warming could ultimately exceed 8 degrees Fahrenheit, which would undermine the planet’s capacity to support a large human population.
3. What is the greenhouse effect, and how does it cause global warming?
We’ve known about it for more than a century. Really.
In the 19th century, scientists discovered that certain gases in the air trap and slow down heat that would otherwise escape to space. Carbon dioxide is a major player; without any of it in the air, the Earth would be a frozen wasteland. The first prediction that the planet would warm as humans released more of the gas was made in 1896. The gas has increased 43 percent above the pre-industrial level so far, and the Earth has warmed by roughly the amount that scientists predicted it would.
4. How do we know humans are responsible for the increase in carbon dioxide?
This one is nailed down.
Hard evidence, including studies that use radioactivity to distinguish industrial emissions from natural emissions, shows that the extra gas is coming from human activity. Carbon dioxide levels rose and fell naturally in the long-ago past, but those changes took thousands of years. Geologists say that humans are now pumping the gas into the air much faster than nature has ever done.
5. Could natural factors be the cause of the warming?
In theory, they could be. If the sun were to start putting out more radiation, for instance, that would definitely warm the Earth. But scientists have looked carefully at the natural factors known to influence planetary temperature and found that they are not changing nearly enough. The warming is extremely rapid on the geologic time scale, and no other factor can explain it as well as human emissions of greenhouse gases.
6. Why do people deny the science of climate change?
Mostly because of ideology.
Instead of negotiating over climate change policies and trying to make them more market-oriented, some political conservatives have taken the approach of blocking them by trying to undermine the science.
President Trump has sometimes claimed that scientists are engaged in a worldwide hoax to fool the public, or that global warming was invented by China to disable American industry. The climate denialists’ arguments have become so strained that even oil and coal companies have distanced themselves publicly, though some still help to finance the campaigns of politicians who espouse such views.