The first thing to know about the U.N. Climate Summit is that it's not a part of the formal global climate treaty negotiations.

Except it sort of is. By bringing together more than 100 heads of state in New York on Sept. 23, along with CEOs and leaders from civil society organizations, U.N. Secretary-General Ban Ki-moon is hoping that it serves as diplomatic jumper cables to revitalize the stagnated U.N. climate treaty talks as they head into a crucial year.

EU Commisioner for Climate Action Connie Hedegaard (R) with Alice Akinyi Kaudia from Kenya at the UN Climate Change Conference in Warsaw, Poland, in 2013. Image: Alik Keplicz/Associated Press

Climate scientists and many political leaders have warned that, because of the inertia of the climate system, tackling global warming is becoming more urgent with each passing day, and that if the world does not enact major emissions cuts by 2020, all hope of limiting global warming to at or below dangerous levels will be lost.

To put it another way, the negotiations that informally kick off in New York this month could make or break the fate of many countries, species and communities worldwide.

You can’t negotiate in here. This is a momentum room.

After attempts to craft a top-down global climate treaty faltered in Copenhagen, Denmark, in 2009, negotiators are now focusing on a way to back into a bottom-up agreement, starting with the non-summit summit. One way to think of the gathering is as the cocktail party ahead of a formal dinner. Participants will get to know one another, and become more familiar with their negotiating positions and points of view, before heading into the more crucial negotiations during the next 15 months.

According to the U.N. Secretary-General's office, the climate summit is meant to catalyze action on climate change in part by announcing a series of commitments from the private sector and governments.

"We believe that the Climate Summit is an opportunity to approach climate change differently,” Eri Kaneko, associate spokesperson for Ban Ki-moon, told Mashable. “Instead of a negotiation, the Summit will be an opportunity for world leaders — from governments, businesses, and civil society — to announce what actions they will be embark on to reduce emissions and to live with the impacts of climate change.”

She said the summit “will be different because it is about action and ambition. It is not about words and rhetoric.”

According to Peter Ogden, a senior fellow at the Center for American Progress who was part of the U.S. climate negotiating team during the Copenhagen Summit, the New York meeting is part of a much longer process.

“Anyone expecting a big kind of breakthrough at the summit has mismatched expectations,” he said in an interview.

2009 feels like forever ago.

Climate science has advanced a great deal since 2009, with the consensus that global warming is largely manmade growing stronger, and predictions becoming more dire in most cases.

Scientists have warned that the world is almost out of time to prevent manmade global warming from exceeding the dangerous threshold of 3.6 degrees Fahrenheit above pre-industrial levels. If that threshold is exceeded, which some scientists think is almost guaranteed at this point anyway, small island states such as Kiribati and the Maldives could be overtaken by the sea and wiped off the map.

Global warming under a low emissions scenario (left) and a high emissions scenario (right). Image: National Climate Assessment

Here is a sampling of some of the latest scientific findings:

The rise in global average sea level since the late 1800s has been much greater than at any other time in the past 2,000 years. Since 1992, sea level has risen at twice the rate it had during the past century, decreasing the amount of time that residents of U.S. coastlines have to prepare for coastal flooding.

The world is on course to see an increase in global average surface temperatures of up to 9 degrees Fahrenheit by the end of this century.

Climate change will reduce median global crop yields by up to 2% per decade through 2100, when compared to a world without climate change. At the same time as crop yields will be challenged, crop demand will be rising by about 14% per decade until 2050, which will greatly increase food stress.

If global warming reaches 4.5 degrees Fahrenheit above pre-industrial levels, it could lead to “global aggregate economic losses” of between 0.2% and 2.0% of GDP. (This is considered to be a conservative estimate.)

Manmade global warming is responsible for nearly 70% of global glacier mass loss between 1991 to 2010.

The amount of carbon dioxide in the air surpassed 400 parts per million, making it the highest in all of human history.

Trends in carbon dioxide levels in the atmosphere. Image: National Climate Assessment

About half of all cumulative manmade carbon-dioxide emissions between 1750 and 2010 happened during the past 40 years. Since carbon dioxide remains in the atmosphere for as many as 1,000 years, the cumulative emissions are what determines how much warming we’re ultimately in store for, and we’ve already burned about half of the carbon budget that would keep warming to less than 3.6 degrees Fahrenheit.

The most recent report from the U.N. Intergovernmental Panel on Climate Change (IPCC) found that global average sea level is likely to increase by 10.2 to 32 inches by the year 2100, with a highest emissions scenario showing a sea level rise of between 21 and 38 inches by the end of the century if greenhouse gas emissions continue to rise unabated. Many scientists now consider this to be an underestimate.

By giving countries incentives to slash their global warming emissions, the next climate treaty may be the decisive factor in setting how high sea levels will rise, how many species will go extinct, and how severe heat waves and droughts will get, among other climate change-related impacts.

The international geopolitical landscape has also shifted dramatically. In 2009, the economic crisis was still battering the globe, particularly the U.S. and Europe. This constrained the ambition of these countries, due to fear of incurring high costs. In addition, in Copenhagen, America had a new, untested president in Barack Obama.

President Barack Obama addresses delegates at the Copenhagen Climate Summit in 2009. Image: UN Photo

In 2014, President Obama will arrive at the summit just a few months after announcing the most far-reaching climate change measures ever to be undertaken by any U.S. president.

In addition, since 2009, advances in production techniques (i.e. hydraulic fracturing, also known as "fracking"), and economic conditions, the U.S. has shifted its electricity generation mix from coal to mainly natural gas, which has lowered the country’s emissions significantly. This also gives the country more credibility on the international stage.

According to a report from the White House Council on Economic Advisers, from 2005 to 2011, the U.S. reduced its "total carbon pollution" more than any other country, and now comprises about 15% of global carbon emissions.

Other major players are also acting on climate change. China recently announced a regional cap and trade program to cut emissions, partly due to their widely-publicized struggles with air pollution. Narendra Modi, the new prime minister of India, has indicated his desire to move forward with expanding renewable energy programs to expand the country’s electricity network.

According to David Waskow, director of the international climate action initiative for the World Resources Institute, a Washington think tank, “countries have actually deepened what they are doing in very concrete ways" since 2009. In India, for example, Waskow said that Modi “is very committed to renewable energy as a key lever — if not the key lever — to increasing energy access in that country.”

Other countries, most notably Australia, have backtracked on climate action since 2009, dismantling programs that were put in place to limit carbon emissions or announcing that they would not meet their emissions reduction targets.

Has anyone seen Xi Jinping and Prime Minister Modi? Could've sworn they were invited.

There will be a few glaringly noticeable no-shows at the summit. According to news reports, China’s President Xi Jinping will not attend the summit, which is significant considering China is the world’s top emitter of global warming pollutants. In addition, India's Modi is not expected to attend, although he will meet with Obama in Washington just days later — and climate change may come up.

“Just 'cause you’re not in New York for the summit doesn’t mean you’re not talking climate change,” Ogden, of the Center for American Progress, said.

The leaders of Canada and Australia, the two industrialized countries that are the current international pariahs on the climate issue (Canada for developing its oil sands resources and missing its Kyoto targets, and Australia for backtracking on its emissions regulations), are also skipping the meeting.

Don’t expect a resolution to the "what form of treaty?" debate.

The U.S. has signaled that it does not want the next climate agreement to be a top-down, binding treaty that would force the administration to submit it to the Senate for approval. Considering that the Senate has been hostile to treaties of nearly all kinds, including a U.N. treaty on protecting the disabled, the Obama administration knows the chances of getting a climate change agreement to the required two-thirds majority vote will be nearly impossible.

This will especially be the case if Republicans take the Senate in 2016, which many political prognosticators consider to be the most likely outcome of midterm elections.

Therefore, the U.S. negotiating team seems to be aiming for an agreement under which each country commits to taking certain actions and making specified contributions, such as climate adaptation assistance to developing countries. The U.S. is also seeking an agreement that involves the entire world, including rising developing countries that are responsible for a growing share of global warming emissions.

Surface temperature trend since instrument records began in 1850. Image: National Climate Assessment

“If you look at it,” Ogden said, “The Copenhagen Accord is sort of an example of an agreement that was not a new treaty or protocol, but instead harnessed a set of nationally self-determined set of targets.” That agreement didn’t require senate ratification. “There is precedent here,” he added.

The Copenhagen Accord established emissions reduction goals for industrialized and developing countries, and enshrined into international law the goal of limiting global warming to 3.6 degrees Fahrenheit or less compared to pre-industrial conditions.

Nigel Purvis, the president and CEO of the consulting firm Climate Advisers, told Mashable that most agreements the U.S. agrees to on the international stage don’t need Congress’ buy-in anyway.

“It’s a false choice between legally binding and not legally binding,” he said.

One of the main concerns about a treaty that follows a bottom-up approach, with each country deciding for itself how much it will reduce emissions, is that at the end of the day emissions will still be high enough to take the world over the climate cliff. It's like a large group that goes out for happy hour after work, leaving one person stuck with the bill at the end of the evening. Inevitably, someone underpays, resulting in a gap between the money on the table and the check.

An agreement like this, Purvis said, “has the potential to enshrine a gap in between what the science requires and what nations are doing, and that gap could actually grow.”

“There is a real need for advocacy and engagement to convince countries to be as ambitious as they need to be.”

Speaking of advocacy...

In association with the summit, environmental groups are organizing what they are billing as the largest climate march in history, which will take place in New York on Sept. 21.