The first three months of 2016 have been the hottest ever recorded, and by a large margin. Greenland’s massive ice sheet melted more this spring than researchers have ever seen. Warming seas are turning once-majestic coral reefs into ghostly underwater graveyards. And scientists are warning that sea levels could rise far faster than anyone expected by the end of the century, with severe impacts for coastal communities around the globe.

That grim drumbeat of news will loom over the United Nations on Friday — Earth Day — when officials from more than 150 countries gather to sign a landmark agreement aimed at slashing global greenhouse gas emissions and slowing the warming of the planet.

It simultaneously will be a moment of understandable celebration and sobering reality.

The agreement, forged late last year after intense negotiations in a Paris suburb, has been hailed as a crucial milestone in putting the world’s nations on course to reduce reliance on fossil fuels in favor of cleaner, more sustainable forms of energy. But in the four months since that rare moment of global accord, the near-constant reminders about shifts in the Earth’s climate have underscored that staving off the worst consequences of global warming may require increasingly ambitious actions.

“The strongest hurricane on record for both the Northern and Southern Hemisphere, unprecedented continuing drought in California, the warmest start to a year that we’ve ever seen, on the heels of what was the warmest full year on record for the globe,” ticked off Michael Mann, a climate researcher at Pennsylvania State University. Such developments, though driven in part by strong El Niño conditions, are a “reminder of how perilously close we are now to a permanent crossing into the global-warming danger zone,” he said. “We are at a critical juncture when it comes to preserving our climate.”

There is general consensus that the Paris accord is both a rare and important milepost — Friday is expected to draw the largest number of countries to sign a U.N. agreement on a single day — and, by itself, only a start to what must be a more aggressive effort over time. The overarching goal of the agreement is to bring down pollution levels so that the rise in global temperatures remains below 2 degrees Celsius (3.6 degrees Fahrenheit) above preindustrial averages. But meeting the global goal relies on each country fulfilling — and eventually increasing — its individual pledge.

Even with participants’ current pledges, the planet likely would still blow past the 2-degrees threshold scientists expect would trigger ever more drastic changes in Earth’s climate. Under the current pledges alone, there is little hope of hitting the more ambitious target of 1.5 degrees Celsius that was included in the Paris agreement at the behest of small island states and other particularly vulnerable nations.

“If you take Paris as the last word, of course it’s not enough,” said Michael Oppenheimer, a professor of geosciences and international affairs at Princeton University. “[But] no one who has followed the climate policy discussion for a significant amount of time thinks this is the last step. It’s merely the first step in a new direction. . . . It’s very important, and it’s certainly better than the alternative of countries having left Paris with their hands up in the air.”

Even after Friday’s ceremony, the agreement won’t take effect until at least 55 countries, representing 55 percent of the world’s emissions, ratify it. That means approvals in individual capitals based on myriad domestic political processes. It’s unclear how long that timeline will be, although the United States and China, the world’s two largest emitters, have pledged to move swiftly.

But the question remains: Can the world do more than lurch toward lower levels of greenhouse gas emissions?

Gavin Schmidt, the head of the NASA Goddard Institute for Space Studies, recently suggested that there’s a 99 percent chance 2016 will set a record for the warmest year since reliable measurements began in 1880. If accurate, that would mean a third consecutive year of record temperatures.

The news in recent months has only increased that sense of urgency.

The Greenland ice sheet, which contains the long-term potential for 20 feet of sea level rise, saw a startling April melt event. The result was that almost 12 percent of the ice sheet’s surface featured a layer of water at least a millimeter deep. The Antarctic ice sheet has long been considered more stable than Greenland’s, but research published last month suggested that it, too, could melt much faster than previously thought. Adding new physics to their computer simulations, a duo of scientists from Penn State and the University of Massachusetts at Amherst found that if countries continue to pour high levels of carbon dioxide into the atmosphere, Antarctica alone could raise seas by more than three feet in this century.

A group of scientists recently reported that far Arctic landscapes, filled with thick wedges of ice embedded in the frozen ground, also are thawing faster than expected. This could trigger new emissions of greenhouse gases into the atmosphere as once-frozen permafrost soil, which contains the preserved remains of long-deceased plants, warms and allows the material to decompose.

Scientists confirmed this week that much of the Great Barrier Reef is seeing what appears to be the worst coral bleaching ever recorded. Rising sea temperatures, worsened by El Niño, have put severe stress on many reefs, so much that the corals expel the algae that they need to survive and turn white. If corals don’t get a long enough reprieve from those temperatures, they can die in large numbers — and in some places they already have, according to accounts of extensive coral death around the island republic of Kiribati in the central Pacific.

Some scientists have even suggested that this bleaching event, which signals a warming of the seas at a rate too quick for a major planetary ecosystem to adapt to, may itself indicate that the world is crossing over into the realm of “dangerous” climate change — precisely what the Paris agreement hopes to prevent.

Paul Bledsoe, a former adviser on climate in the Clinton White House and now an independent energy and environmental consultant, said the drumbeat of troubling climate news should not overshadow the symbolic importance of Friday’s ceremony. But it should be a reminder, amid the pomp and circumstance of the moment, that the Paris accord is at once “essential and insufficient.”

“I can’t overestimate the political importance that every country is involved. That’s absolutely crucial,” he said. “It really is a testament to a profound political evolution. But we do have to recognize what it is and what it isn’t. It’s an incredibly important step that gets us halfway there, but it’s not in and of itself the ultimate solution.”

Researchers say 2015 was the hottest year on record, and that it "smashed" the previous record, which was 2014. The Post's Chris Mooney explains what that could mean for weather patterns, the Paris climate deal and 2016. (Gillian Brockell,Chris Mooney/TWP)

Read more:

As the climate changes, risks to human health will accelerate, White House warns

Atmospheric carbon dioxide levels are showing a startling increase

Scientists say a dramatic worldwide coral bleaching event is now underway

 5 things you should know about the historic Paris climate agreement

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