7 things we've learned about Earth since the last Earth Day

This year, Google is celebrating Earth Day, April 22, with five different Google Doodles illustrating the planet's major biomes: tundra, forest, grasslands, desert, and coral reefs. They're nifty and definitely worth checking out.

I always like marking Earth Day by looking through some of the best new discoveries that have been made about this breathing, seething, blue-and-green planet of ours — the only place in the universe where life is known to exist.

A lot has changed since the first Earth Day in 1970. Back then, America's most pressing environmental problems were air and water pollution. In the decades since, we've made huge progress cleaning that up, only to discover new challenges like global warming and ocean acidification. Even today, our understanding of the Earth itself keeps evolving with each passing year. We've uncovered brand-new geological features. We've brought endangered species back from the brink of extinction. We've transformed the atmosphere, for better and worse.

Here's a list of some of the most surprising, encouraging, and worrisome things we've learned about Earth since the last Earth Day:

1) Scientists just discovered an entirely new, 600-mile-long coral reef at the mouth of the Amazon

You'd think that, by now, we'd have mapped every last nook and cranny on this planet. Not so! We're still discovering new surprises all the time. This week, a team of Brazilian and American scientists told the Atlantic they'd discovered an entirely new coral reef spanning 3,600 square miles of ocean floor at the mouth of the Amazon River.

Robinson Meyer recounts the backstory. Ever since the 1970s, people had suspected there might be a coral reef lurking on the ocean floor there, beneath the turbid waters. But no one knew for sure. Then, recently, a team of researchers journeyed to the region to study CO2 absorption by the Amazon's freshwater plume. One of the scientists, Rodrigo Moura, brought along a dredge to check to see what was down beneath the surface. And jackpot: he pulled up coral, sponges, stars, and fish.

Mind you, not all the coral reef news this year has been quite so wondrous. In Australia, scientists recently announced that 93 percent of the Great Barrier Reef has succumbed to coral bleaching, a destructive phenomenon that occurs when ocean temperatures rise sharply. The coral expel their symbiotic algae, turning a ghastly white and becoming more vulnerable to disease. It's a real problem as global warming keeps boosting temperatures.

Coral reefs are often thought of as the rain forests of the ocean, home to 25 percent of marine species. So conservationists are racing desperately to save them before they vanish altogether. In the New Yorker, Elizabeth Kolbert recently wrote about scientists trying to breed hardier coral that can survive this new era of frequent bleaching.

2) We've discovered dozens of new species — as well as a few thought to be extinct

At this point, scientists have described about 1.5 million different species on the Earth. That sounds like a lot, but estimates suggest there are another 4 million species still waiting to be discovered.

Researchers managed to chip away at that number over the past year. New discoveries include a cartwheeling spider in Morocco, a giant walking stick insect 9 inches long in Vietnam, and a new species of pufferfish off the coast of Japan that create intricate geometric spawning nests in the sand on the ocean floor. You can see more photos of some recent discoveries here.

But one of the neatest discoveries last year wasn't even a new species. The Bouvier’s red colobus monkey, which lives in the Republic of Congo, was thought to have gone extinct back in the 1970s — a victim of heavy logging and hunting. Then, in 2015, primatologists Lieven Devreese and Gaël Elie Gnondo Gobolo stumbled upon a mother and infant Bouvier's red colobus monkey while exploring the swamp forests along the Bokiba River and took a photo. It was the first confirmed sighting in decades of a species once believed gone forever.

3) Earth has 3 trillion trees — far more than we thought

In September 2015, in a ground-breaking study in Nature, a team of scientists combined both satellite imagery and ground-based surveys to estimate that the planet has 3.04 trillion trees — way, way, way more than the previous estimate of 400 billion.

The researchers estimated that forest cover was most pervasive in the tropics and subtropics, which have some 1.39 trillion trees. Temperate forests have 610 billion trees and boreal forests in the far north have another 740 billion. Russia has by far the most trees — some 641 billion, much of them in Siberia. The United States has another 228 billion trees.

Oh, except here's the not-so-good news: The researchers also estimated that Earth now has 46 percent fewer trees than existed before humans first started cutting them down thousands and thousands of years ago. What's more, they estimated that deforestation has accelerated in recent years: logging, agriculture, wildfires, and pests contribute to the loss of about 15.3 billion trees each year, while about 5 billion end up growing back. (On the flip side, there have been a few notable forest conservation successes lately, so the story isn't entirely bleak.)

4) We discovered homo naledi, a new species of ancient human that once roamed the Earth

This one was a major revelation: In September 2015, an international team of researchers announced that they'd discovered fossils of a new species of ancient human that, they believe, lived some 2 to 3 million years ago. It's name? Homo naledi.

The story actually started years ago, when researchers discovered mysterious bone fragments — hundreds of them — in a cave in South Africa. Eventually, they realized this was a brand-new hominin, similar to Homo erectus, albeit with smaller brains. There's also some evidence that H. naledi may have buried its dead.

My colleague Joseph Stromberg wrote about the discovery here: "When it comes to the study of human evolution, all this is a really big deal. To date, we know of only a handful of other species similar enough to us to fit in to our genus: Homo. Scientists will debate this designation for H. naledi — as they do for all newly discovered species — but the bottom line is that these fossils, detailed in a pair of new papers in the journal eLIFE, give us a fascinating glimpse into a new part of our ancient history."

5) Unicorns and humans once coexisted. (Albeit really ugly unicorns.)

The Elasmotherium sibiricum is a creature often described as the "Siberian unicorn." For a long time, scientists thought it went extinct some 350,000 years ago — well before humans ever came to the Siberian plains. Until recently, that is.

In March 2016, a study in the American Journal of Applied Science described a skull discovered in Kazakhstan that suggests that a subpopulation of, uh, "unicorns" still roamed the Earth just 29,000 years ago. They may have even coexisted with humans. Scientists are now trying to figure out just how the animals in this particular region survived for so much longer than the rest of their bretheren.

That said, "unicorn" seems like a bit of a misnomer. Here's how Ellen Brait describes it in the Guardian: "It was about 6 feet tall, 15 feet long, and weighed about 9,000 pounds, making it more comparable to a woolly mammoth than a horse."

6) West Antarctica's ice sheet could melt faster than anyone thought

Icebergs, Paradise Harbour, Antarctic Peninsula, Antarctica.

Not all of the discoveries about our planet over the last year have been inspiring. Some have been downright unnerving.

In March 2016, climate scientists took a fresh look at the West Antarctic Ice Sheet, which has been melting in dribs and drabs for decades as humans have been burning fossil fuels and increasing global warming. If this ice sheet were to slide entirely into the ocean, it would raise sea levels worldwide by 12 feet on average. Most experts had, however, long assumed this wouldn't happen for hundreds or maybe even thousands of years.

Now this new study, published in Nature, has thrown that complacency into question. The researchers used sophisticated computer models to look at a variety of plausible processes by which the ice sheet could disintegrate as temperatures keep rising — including some that had not been incorporated into previous models, such as "hydrofractures" and "cliff collapses." This new model was much better able to explain the Antarctic ice sheet's behavior in Earth's distant past, during periods when it has also been much warmer.

But here's where things got hairy: This model also spit out worrisome results for the future. The researchers found that the West Antarctic Ice Sheet could start destabilizing rapidly within decades, not centuries, if greenhouse gas emissions kept soaring. When added to projections for melting ice elsewhere, that could push global sea levels up 5 to 6 feet by 2100 — nearly double past projections. That worst-case scenario could force a retreat from many coastal cities.

"We are not saying this is definitely going to happen," David Pollard, a co-author of the new paper, told the New York Times. "But I think we are pointing out that there’s a danger, and it should receive a lot more attention."

7) We also learned that humans are capable of cooperating on climate change

I'm going to end this list on a somewhat hopeful note. I do think we learned something quite new about the planet — and the people living on it — in December 2015, when the world's nations gathered in Paris and struck a landmark new deal to tackle global warming.

This is not to take a starry-eyed view of the deal itself. The Paris climate agreement didn't save the planet, and it hasn't solved global warming. Not by a long shot. The deal mainly adds structure and momentum to efforts already underway to reduce greenhouse gas emissions. That's a worthwhile goal in itself, but there's no guarantee it will succeed in averting drastic temperature increases. We'll see.

So why is it mentioned here? Because the deal marked the first time since global climate talks began 20 years ago that world leaders were able to craft a treaty committing every single country to climate action. Rather than the bitter divisions and endless backfighting that marked past conferences, cooperation reigned. It was, as I've written, a radically new approach.

We'll have to see how everyone actually follows through. But if countries do manage to ratchet up their efforts to curtail global emissions in the decades ahead, then 2015 may end up being the year we learned that humans are capable of coming together and addressing one of the most devilishly difficult environmental problems we've ever faced.

Further reading: 15 before-and-after images that show how we're transforming the planet