What better day to step back and take stock of the planet than Earth Day? Started in 1970 to raise awareness in the U.S. about the environmental state of the planet, Earth Day is now celebrated in more than 190 countries and has led to the creation of legislation in the U.S. aimed at protecting the environment. But one global trend has continued to alter the world—the rise of carbon dioxide and other greenhouse gas emissions, which have led to an ever-rising average global temperature.
It’s easy to get caught up in individual records or wondering what influence climate change has on extreme weather events. But to really understand climate change, the trends are what matter. Here are four that make it clear how our planet is changing.
The Number: 400 ppm
The Trend: Current level of CO2, up from pre-industrial levels of 280 ppm
Even though carbon dioxide doesn’t make up much of the Earth’s atmosphere, its heat-trapping ability helps prevent Earth from being cold and barren, like Mars. But there can be too much of a good thing: Since the beginning of the Industrial Revolution in the 19th century, the amount of carbon dioxide in the air has been steadily rising due to human activities like burning coal for electricity, with the excess CO2 trapping ever more heat and raising global temperatures. It is the trend that drives all of the others associated with global warming.
The concentration of CO2 in the atmosphere has risen from about 280 parts per million (ppm) in pre-industrial times to 400 ppm today, already appreciably raising Earth’s temperature. While that still makes CO2 a relatively small part of the atmosphere, it is extremely long-lived, so its heating power will hang around for thousands of years.
The inexorable rise of CO2 is clearly chronicled by the now-famous Keeling Curve—the daily measurements made atop Hawaii’s Mauna Loa volcano since 1958, first begun by the scientist Charles Keeling and now continued by his son, Ralph. CO2 levels go up by about 2 ppm each year, and first hit the 400 ppm milestone on May 2013. It was the first time in human history that CO2 levels were that high. The 400 ppm mark has been hit earlier each year since, and experts expect that CO2 levels will be above 400 ppm year-round within a year or two. The rise of the Keeling Curve and the heating it drives doesn’t look to end anytime soon.
The Number: 1.6°F
The Trend: Rise in global temperatures since 1880
Since the advent of modern recordkeeping in 1880, the global average temperature has risen 1.6°F. The trend is one of the hallmarks of global warming and tightly tied with the rise in human CO2 emissions.
Warming has accelerated since the first Earth Day in 1970. The global average temperature has risen by about 0.3°F per decade since then. In comparison, the rise up until that point was about 0.1°F per decade. The speed up in warming since 1970 has been tied to a number of factors, particularly a reduction in human aerosol emissions throughout the 1970s, which cool the planet.
Scientists have warned that warming should be limited to 3.6°F above pre-industrial levels—more commonly represented by its even Celsius counterpart of 2°C—to avoid severe consequences of climate change.
The Number: 361
The Trend: Consecutive months of above-normal temps
Every month, the story writes itself when it comes to the global average temperature: it’s above normal. Occasionally—and more frequently in recent years—it’s record-setting.
While any one month might reinforce the reality that the world is warming, it’s the trend that really stands out. As of March, the globe has had 361 consecutive months of above-normal temperatures according to the National Climatic Data Center (NCDC).
You’d have to go back to February 1985 to find the last cooler-than-average month. Which means if you’re younger than 30, you’ve never experienced a cooler-than-average month.
“It tells us pretty darn definitively we live in a changed world. We don’t live in a world that may change, we’re in a changed world and it will continue to change,” Deke Arndt, a scientist who runs NCDC’s Climate Monitoring Branch, said.
The Number: 10
The Trend: The 10 hottest years on record have occurred since 1998
If you take those monthly temperature numbers and average them over a year, another notable trend emerges. All 10 of the warmest years on record have come since 1998. What’s more, 13 of the 15 warmest years have come since 2000, including the exclamation point of 2014, the hottest year on record (though its rein may be short-lived as 2015 is on track to take the crown). The odds of that happening by chance alone? About 1-in-27 million.
Despite the spate of record-hot years—and the fact that the last record-cold year occurred in 1911—some have referred to the past 15 years as a global warming hiatus because surface temperatures haven’t risen as fast as they did in the previous decades. It’s more of a slowdown than the disappearance of global warming.
“We’re in a really warm neighborhood. We slowed down in a neighborhood we’ve never slowed down in before,” Arndt said.
A number of studies have pointed out that natural climate patterns have contributed to the slowdown, but that it could possibly end in the next 5-10 years as those patterns flip. And regardless of surface warming trends, oceans have continued to take up a whopping 93 percent of the extra heat that greenhouse gases emissions are trapping here on Earth. So calling it a slowdown is a bit of a misnomer; it’s more like a global warming shell game.