Take note: For better or worse, Earth Day 2016 is going down in history.
On April 22, at least 170 heads of state and diplomats from around the globe signed the world's first international climate change accords, known as the Paris Agreement.
The pact was agreed upon by 195 countries (that's almost all of them) in December 2015, after two weeks of tough negotiations.
According to Reuters, the agreement will have the highest number of signatories in history for any treaty's opening day.
The 31-page accord marked the first successful international effort to fight climate change, which 97% of climate scientists agree is driven by human activity.
After today, it will be open for signing until April 21, 2017. And even after that, additional states can still decide to adopt the agreement through accession, which is more or less United Nations code for "coming late to the party."
The list of countries who agreed to the treaty but might not be signing today includes the Organization of the Petroleum Exporting Countries (OPEC) and other large-scale oil producers.
Think tank Third World Network has also urged developing nations to hold off joining until they see concrete plans from wealthy nations, pointing to a history of empty promises to combat climate change.
Signatories have pledged to create climate plans aiming to limit global warming to 2.7 degrees Fahrenheit, with the more realistic (but perhaps arbitrary) goal of keeping global temperatures no more than 3.6 degrees above pre-industrial levels. (The pact is big on goals, but short on the details of how to achieve them.)
The agreement requires countries to meet every five years with new emissions-reductions targets, though it allows them to set their own benchmarks.
The US, for example, has pledged to reduce emissions 26% below 2005 levels by 2015; the European Union is aiming for 45% below 2005 levels by 2030. China hopes to reach the same target by 2020.
It also includes provisions designed to protect the global poor, who are almost certainly going to bear the brunt of the warming sparked by the rapid industrialization of other, richer countries.
This includes allocating funding from wealthy states to developing ones as well as committees tasked with exploring solutions to issues like the inevitability of climate refugees.
It's worth remembering that December's victory was the first after a long string of mostly ineffective climate conventions and talks spanning two decades, starting with the first United Nations Framework Convention on Climate Change (UNPCCC) in 1992 that gave rise to the failed 1997 Kyoto Protocol.
These past failures should guide world leaders as they pursue the Paris Agreement, which like its predecessors is not legally binding — yet.
The multi-step process is long and complicated, and there are lots of junctures where it could be derailed.
In Paris, 195 states agreed that they would sign the accord on Earth Day 2016 — but even then, individual countries have to approve or ratify the agreement at the national level before they've officially "joined."
Once 55 countries, whose collective emissions must add up to at least 55% of the global total, have reached that step, the accord becomes legally binding and members states will be expected to fulfill their commitments.
The US and China, both of which have announced their intent to become official member states, account for 40% of global emissions. Both will likely ratify their country's participation through executive action.
This could fast-track the agreement's formal start date by four years: Instead of 2020, the pact could go into effect by the end of 2016.
Progress could be delayed if not enough countries ratify the agreement. Some states, mostly island nations already being affected by rising sea levels, came to the signing ceremony with domestic approval in hand.
And then there's the US presidential election. While President Barack Obama has pledged to follow through on US commitments, the next president could, in theory, pull out of the deal, effectively halting progress on reducing emissions.
Democratic presidential candidates Hillary Clinton and Bernie Sanders both support continued action on climate change.
John Kasich is the only Republican candidate willing to entertain the notion that climate change is established fact, but he shies away from committing to a path to mitigation.
The two Republican front-runners, Ted Cruz and Donald Trump, (incorrectly) contend that climate change itself is still a matter of debate.
In February, the BBC reported that Obama's current climate envoy, Todd Stern, has warned that the US leaving the climate deal could lead to "diplomatic consequences" on the international stage.
No matter who the next president is, however, the sad fact is that the Paris Agreement may have come too late.
This year, 2016, is set to be the hottest on record. Earth's climate is already on track to blow past the 2.7-degree target — the agreement's more hopeful goal — and perhaps even the more arbitrary 3.6 degree mark.
But hope springs eternal this Earth Day: Peabody Energy, the largest publicly held coal company in the US, became the most recent of an estimated 50 coal companies to file for bankruptcy in the last few years; and renewable energy sources are quickly eclipsing fossil fuels in several countries. Coal accounts for 44% of global CO2 emissions, so this kind of progress is significant — and a sign that real change may be ahead.
While far from perfect, the Paris Agreement remains a promising development for world action on climate change, and the April 22 signing ceremony has world leaders promising to pursue climate-change solutions at home and abroad.
Despite decades of fossil fuel industry-funded denial and snowball-throwing senators here in the US, world leaders have managed to put aside their differences to solve humanity's greatest challenge to date.
And that's worth celebrating.
