Scientists say greenhouse gases are causing global warming. Greenhouse gases are released when we burn fossil fuels. Some fossil fuels are oil, natural gas, and coal.
The gases act like the glass walls of a greenhouse. The gases get trapped in the atmosphere. Then they trap heat from the sun. This makes the world hotter.
Carbon dioxide (CO2) is the main greenhouse gas.
An instrument in the U.S. state of Hawai'i records CO2. On May 9, 2013, it recorded an important event. The amount of CO2 in the air was higher than 400 parts per million (ppm). It could be the first time CO2 reached this level in human history. The last time it was that high was three million years ago.
Back then, sea levels were at least 9.1 meters (30 feet) higher. Today, oceans that high would flood many cities around the world.
The "Greenhouse Age"
The planet was also hotter three million years ago. It was about 3.6 to 5.4 degrees Fahrenheit warmer. However, Earth then was near the end of a long "greenhouse age." CO2 levels eventually headed down.
This time, 400 ppm is a new beginning. It is unlikely to go down again. What will happen now is uncertain.
The instrument that measures CO2 is on a mountain in Mauna Loa, Hawai'i. Teams of scientists measure CO2 there.
Charles David Keeling started the Mauna Loa measurements in 1958. His "Keeling curve" is a measurement. It marks CO2 levels. It has become a marker of climate change.
When Keeling started measuring CO2, the level was at 315 ppm. When he died in June 2005, it was at 382.
Plant Life and CO2
CO2 peaks in May every year. By June the level begins falling. This is when spring starts in the Northern Hemisphere. Most of Earth's land is there.
Plants draw CO2 out of the air. They use it to make food and grow. By November, the CO2 level drops five or six ppm lower than in May.
In winter, plants stop making new food. Instead, they burn energy. This puts CO2 back into the air.
Think of it as the forests breathing. Now, their breath is climbing each year. This is because of all the CO2 being put in the air.
In 2016, the average CO2 level was 400 ppm for the whole year. We might stay above 400 forever. What difference will that make?
Taking a Swing At CO2 Levels
In some ways, 400 ppm is just a number. In 1941, baseball player Ted Williams had a batting average of .400. That means he got a hit four out of every 10 chances.
In some ways, 400 ppm is like this batting average. It is just a marker. It's not much different from 399 or 401. Still, no one has batted .400 since Williams did in 1941.
Still, this says something important about baseball. The same is true for CO2 in Earth's atmosphere.
Lawmakers around the world want to burn less fossil fuels. They have tried to come to an agreement. However, countries have not been able to agree.
A Cause for Concern
Many scientists argue that the CO2 levels must be stopped at 450 ppm. They say that would avoid the worst effects of climate change. Some activists argue for 350 ppm. That would be a more difficult goal. A CO2 reading below 350 ppm has not been read at Mauna Loa since October 1988.
The last time the CO2 level hit 400 ppm was probably between 2.6 million and 5.3 million years ago. Scientists can measure CO2 by looking at ancient air bubbles. They are trapped in ice in Antarctica. But that only goes back 800,000 years. CO2 levels before that are estimated by scientists.
However, millions of years ago, CO2 must have been much higher than now. There's no other way to explain how warm Earth was then.
About 50 million years ago, alligators lived in Greenland. Today, Greenland is mostly covered in ice. Then, it was covered in swampy forests. It was much like swampy forests in the southeastern United States today. CO2 may have been anywhere from two to 10 times higher back then than it is today.
