World leaders are gathered in Paris for , or COP21, to try create international legislation to help solve the world's greatest climate change issues.

Here are five things you should know about COP21 as it kicks off in Paris today:

Protests and Presidents

Hundreds of thousands of people across several countries marched in a collective call to action Sunday with the goal of putting political pressure on world leaders to collectively lower emissions and reduce long-term global warming. According to NBC News, marched in the so-called Global Climate March.

Among those, violence broke out between protestors and police in Paris, with protestors reportedly hurling objects at police and police pepper spraying and , BBC reports.

And it's easy to see why so many marched. Heads of state from 150 countries are in attendance – including the heads of China, the U.S., Russia and India all major emitters of greenhouse gases.

Roughly 45,000 delegates from 195 countries, those that make up the U.N. Framework Convention on Climate Change, will be in attendance at the conference.

This Could be Historic

Global leaders are coalescing in Paris with the goal of agreeing on the first ever universally, legally binding accord on climate change. Simply put: A universally legally binding agreement of the sorts has never been accomplished before.

Previous agreements, have been non-binding and missing key players like the United States, CNN notes.

'Never have the stakes been so high because this is about the future of the planet, the future of life.'

Those were the words French President Francois Hollande , but what specifically is at stake at COP21?

Countries are trying to limit the rise of global temperatures to 3.6 °F (or 2 °C) above pre-industrial levels by the year 2,100. Doing so would help reduce the potentially catastrophic impacts of global climate change, including global sea level rise and an increase in extreme weather events, among others.

If the world continues on its current emissions pace, the global temperature would , the New York Times reports.

Current individual pledges to COP21 by 181 member-states would likely only lower the potential rise in global temperatures by just over 6°F, the New York Times adds.

So the meetings are about trying to increase pledges and make current pledges more enforceable. There's also an ongoing battle between developed nations and undeveloped ones about how much money developed countries should commit to supporting the clean development of undeveloped nations.

China vs. the United States

The United States and China are in the world, combining for an estimated as of 2011, the EPA reports.

last November in the months leading up to COP21 in an effort to escalate the tone for the conference.

Here are some of the major pledges both countries have made so far:

United States agreed to:

by 26 to 28 percent compared to 2005 levels by 2025. sources by 2030.

China agreed to:

Reach peak greenhouse gas emissions by 2030.of emissions by up to 65 percent by significantly increasing its dependency on renewable energy resources.

What's Enforceable

The sticking point, as is the case with all international agreements, is what parts of it will be punishable, or enforceable, by the United Nations.

Many nations submitted individual pledges, called , ahead of COP21, each with varying degrees of commitment to measures to curb climate change.

There was no universally objective measure by which all of the INDCs must abide by, and as the New York Times reports, and not by anything more concrete and with more bite like global sanctions.

So, unless something drastic changes in Paris, the extent by which countries will be held to their word remains to be seen.

MORE ON WEATHER.COM: Photos From the Global Climate March

Policemen detain an activist during a protest ahead of the 2015 Paris Climate Conference, in Paris, France, on Nov. 29, 2015.